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The populations of hard X-ray and γ-ray sources: A correlation study and new possible identifications


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We present the results of our analysis devoted to the research of sources emitting in the energy bands surveyed by both the Swift-BAT and the Fermi-LAT telescopes. We cross-correlate the Fermi-LAT 1-year point source catalogue (1FGL) of {\gamma}-ray sources and the second Palermo BAT catalogue (2PBC) of hard X-ray sources, establishing a correspondence between sources when their error boxes overlap. We also extract the significance value in the BAT 15-150 keV map, obtained using a dedicated software for the reduction of BAT data, in the direction of the 1FGL sources and take into account those above the significance threshold {\sigma} = 3. We obtain a sample of common sources emitting in both the hard X- and the {\gamma}-ray energy bands and evaluate its content in galactic and extragalactic objects. We assess the fraction of unidentified sources and describe in greater detail the properties of two of them, 1FGL J0137.8+5814 and 1FGL J2056.7+4938, supporting their classification as blazars after the analysis of their broad-band spectral energy distribution. We discuss the blazar content of the collected 1FGL-2PBC sources: we build its redshift distibution and compare it with that of the whole blazar population as reported in the second edition of the BZCAT blazar catalogue.
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arXiv:1104.3006v1 [astro-ph.HE] 15 Apr 2011
Astronomy & Astrophysics
manuscript no. batlat˙astroph
ESO 2011
April 18, 2011
The populations of hard X- and γ-ray sources:
a correlation study and new possible identifications
A. Maselli
, G. Cusumano
, E. Massaro
, A. Segreto
, V. La Parola
, A. Tramacere
, I. Donnarumma
INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, Via U. La Malfa 153, I-90146 Palermo, Italy
Dipartimento di Fisica, Universit`a La Sapienza, Piazzale A. Moro 2, I-00185 Roma, Italy
INTEGRAL Science Data Centre, CH-1290 Versoix, Switzerland
INAF IASF Roma, via Fosso del Cavaliere 100, I-00133 Roma, Italy
Received 18 March 2011 / accepted 09 April 2011
We present the results of our analysis devoted to the research of sources emitting in the energy bands surveyed by both the Swift-BAT
and the Fermi-LAT telescopes.
We cross-correlate the Fermi-LAT 1-year point source catalogue (1FGL) of gamma-ray sources and the second Palermo BAT
catalogue (2PBC) of hard X-ray sources, establishing a correspondence between sources when their error boxes overlap. We also extract the
significance value in the BAT 15–150 keV map, obtained using a dedicated software for the reduction of BAT data, in the direction of the
1FGL sources and take into account those above the significance threshold σ = 3.
We obtain a sample of common sources emitting in both the hard X- and the γ-ray energy bands and evaluate its content in
galactic and extragalactic objects. We assess the fraction of unidentified sources and describe in greater detail the properties of two of them,
1FGL J0137.8+5814 and 1FGL J2056.7+4938, supporting their classification as blazars after the analysis of their broad-band spectral energy
distribution. We discuss the blazar content of the collected 1FGL-2PBC sources: we build its redshift distibution and compare it with that of
the whole blazar population as reported in the second edition of the BZCAT blazar catalogue.
Key words. galaxies: active - galaxies: BL Lacertae objects - radiation mechanisms: non-thermal
1. Introduction
The present generation of space observatories for high energy
astrophysics is characterised by large area and wide field in-
strumentation. This is the case of the Burst Alert Telescope
(BAT, Barthelmy et al. 2005) and Large Area Telescope (LAT,
Atwood et al. 2009) onboard Swift and Fermi-GST, respec-
tively. One of the main throughputs of these instruments is the
discovery of a thousand of hard X- and γ-ray sources and the
possibility of performinginvestigationson the population prop-
erties and evolution much more accurate than in the past.
The first catalogue of γ-ray point sources (1FGL,
Abdo et al. 2010a), based on data obtained in the 11 months af-
ter the beginning of scientific operation (2008, August 4) con-
tains 1451 entries, and a fraction of about 40 % of them does
not have a reliable counterpart.A rather similar situation is also
occurring with the hard X-ray sources detected by BAT: the
second Palermo BAT catalogue (Cusumano et al. 2010b, here-
after 2PBC), that coversthe observation period from November
2004 to May 2009 and is the richest one in this energy range,
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also includes 177 not associated sources, over a total num-
ber of 1256. A multifrequency approach based on the cross-
correlation of catalogues in dierent energy bands from the ra-
dio to the γ rays can be very usesul, if not essential, to unravel
the nature of unidentified sources.
In this paper we use the 1FGL and the 2PBC catalogues
to compare the populations of hard X- and γ-ray sources and
to search for possible correspondences between them. The pa-
per is organised as follows: we report the details of the cross-
correlation between the two catalogues in Section 2 and of the
cross-correlation of the 1FGL catalogue with the BAT 15–150
keV all-sky significance map in Section 3. We discuss our re-
sults in Section 4 giving a particular emphasis on the blazar
content of the collected group of sources. In Section 5 we de-
scribe the properties of two unidentified objects in the 1FGL
catalogue, at low Galactic latitude, with a significant emission
in the hard X-ray band: we collect all the available data in the
literature and analyse in detail their broad band spectral energy
distribution(SED) to supporttheir classification as blazars. The
main results of our analysis are summarised in Section 6.
2 A. Maselli et al.: The populations of hard X- and γ-ray sources
Fig.1.Details ofthe BAT significance map at the position of a) 1FGL J1103.72329(left panel), b) 1FGL J2056.7+4938(middle
panel) and c) 1FGL J0238.36132 (right panel). A dashed line represents the position of the 1FGL (black ellipse) and the 2PBC
(white circle) sources with its uncertainty. For each source, the position of the associated counterpart is indicated by a cross of
the corresponding colour. Each square map is 60
× 60
; the colour scale is optimised for each hard X-ray source.
2. Correspondences between 1FGL and 2PBC
The 2PBC catalogue has been obtained from the reduc-
tion of the BAT data collected in 54 months since the
launch of the Swift mission using the dedicated software
BatImager (Segreto et al. 2010). It contains 1256 hard X-ray
sources detected at a significance level higher than σ
= 4.8;
their coordinates are given with a 95% confidence level radius
. A counterpart was associated to 1079 sources ( 86%);
for 26 of them a double association was found, and in two cases
three possible counterparts were proposed; sources without any
associated counterpart are 177.
We match the 2PBC catalogue with the 1FGL catalogue to
search for sources emitting in both the energy bands surveyed
by the Swift-BAT and the Fermi-LAT telescopes. The positions
of the 1451 1FGL γ-ray sources, determined by means of a
maximum likelihood algorithm, are given with an uncertainty
ellipse corresponding to the 95% probability of locating the
source; 58 objects are reported without any positional uncer-
tainty. We adopt, when available, the mean value of the two
axes of the 95% confidence ellipse as the radius r
of the
error circle for a 1FGL source. We calculate the angular dis-
tance d among the centroids of 1FGL and 2PBC sources and
establish a correspondence when the two error circles overlap,
adopting the condition d (r
): applyingthis criterion
we obtain 77 correspondences.We define a simple parameter to
estimate the quality of the correspondence and use the expres-
sion Q = d/r
, where r
is the higher value between r
and r
. The parameter Q < 1 implies that the wider error
circle includes both centroids: this value occurs for 64 corre-
spondences, while in four cases we find Q > 1.4. We consider
the possibility that the choice of the mean value of the axes of
the 95% uncertainty ellipse, adopted to estimate the 1FGL un-
certainty region, may be not firmly reliable. This aspect raises
particularly in cases of high eccentricity values, in which the
orientation of the 1FGL ellipse with respect to the 2PBC circle
must be taken into account. In order to gain confidence about
the established correspondence between sources we compute
the axial ratio of the uncertainty ellipse and verify in the sky-
map the 25 cases ( 1/3) in which we find values higher than
1.2. We find that in all but four cases not only the centroid of
the 2PBC source, but also the associated counterpart, is inside
the 1FGL ellipse.
Further correspondences have been established for 9 of the
58 1FGL sources with no positional uncertainty as the posi-
tion of the 1FGL source is within the error circle of the 2PBC
source. The remaining 49 cases are all unambiguously dis-
carded as no 2PBC source is found in the proximity of these
1FGL sources. The final list includes 86 1FGL-2PBC corre-
spondences for a total of 84 1FGL sources: two of them have
in fact a possible correspondence with two dierent 2PBC
sources. Moreover,we find also a correspondencein which two
possible counterparts,bothhigh mass X-raybinaries, havebeen
associated to the same 2PBC source in the Small Magellanic
Cloud. The list of correspondences has been splitted in two
parts reported in Table 1 and Table 2 according to the Galactic
latitude, with 63 correspondences at |b| > 10
and 23 at
|b| < 10
. For each correspondence we report the 1FGL and
2PBC identifiers, the associated counterparts and their clas-
sification. For a few AGNs the counterpart, missing in the
1FGL catalogue, has been found in the first catalogue of ac-
tive galactic nuclei detected by the Fermi Large Area Telescope
(Abdo et al. 2010b): these cases have been marked with an as-
terisk (
) in Table 1. The value of the Q parameter and the
agreement between the corresponding counterparts is reported
in the last two columns. We mark with a colon the Q values of
the four correspondences for which the 2PBC counterparts are
outside the 1FGL uncertainty ellipse. We find also two cases
(marked with “c”) having a possibility of multiple association:
this confusion is due to crowded fields, one of them close to
the Galactic centre direction. In 62 cases (marked with “y”) the
counterpart is the same, while in 12 cases (marked with “n”)
A. Maselli et al.: The populations of hard X- and γ-ray sources 3
Table 5. A statistical descriptionof the blazar contentof sources showinga simultaneousemission both in the BAT and in the LAT
energy ranges. Left column: all the blazars classified in the BZCAT. Middle column: blazars obtained by the cross-correlation
of 1FGL with the 2PBC catalogues. Right column: blazars with a significance higher than σ
= 3. The mean redshift value is
reported for each subsample. Values in brackets refer to firm redshift estimates, or dierent from zero.
type BZCAT hzi
1FGL-2PBC hz
i σ > σ
BZB 1164 (531) 0.33 ± 0.01 16 (14) 0.17 ± 0.06 24 (19) 0.21 ± 0.05
BZQ 1660 (1638) 1.40 ± 0.02
27 1.39 ± 0.17 41 1.23 ± 0.12
BZU 262 (215) 0.42 ± 0.04
7 0.32 ± 0.14 8 0.32 ± 0.12
total 3086 (2384) 50 (48) 73 (68)
the counterparts associated with the high-energy sources are
dierent: in these cases the correspondence is supposed to be
due to chance. We find also 13 correspondencesfor which both
sources, or just one of them, lack the associated counterpart.
Assuming that the 1FGL-2PBC correspondence is the result of
high-energy emission from a single source, this could be con-
sidered a hint toward the association of a counterpart to some
of these 1FGL sources; all these correspondences, with only
one exception, are characterised by a Q parameter lower than
We plot in Fig. 1 the details of the BAT 15–150 keV sig-
nificance map for some of these correspondences. In the left
panel of Fig. 1 we report the case in which the 1FGL and
the 2PBC sources correspond to the same object, the BL Lac
1H 1100230. In the middle panel we report the case of
the source 1FGL J2056.7+4938 that has not been identified
in the 1FGL catalogue: the correspondence with the 2PBC
source suggests the X-ray source RXJ2056.6+4940 as possi-
ble counterpart. The properties of this source have been inves-
tigated in greater detail and discussed in Section 5.2. Finally,
in the right panel of Fig. 1 we report the case of a cor-
respondence probably due to chance: two close objects, the
galaxy IRAS F023746130 and the flat spectrum radio quasar
PKS 0235618, are responsible for the hard X- and the γ-ray
emission, respectively.
We compare our results with those reported in Abdo
et al. (2010b, their Table 7) and find that all the 50 sources
provided in their list are included in Table 1, with the excep-
tion of 1FGL J1938.23957. Hard X-ray emission in this di-
rection was revealed by Integraland reported in the fourth IBIS
catalogue (Bird et al. 2010): the counterpart associated to this
1FGL source, PKS 1933400, is also reported in the BZCAT
as a blazar with uncertain classification. The value σ 3.5 that
we find in the BAT 15–150 keV significance map at the posi-
tion of PKS 1933400 is compatible with hard X-ray emission
from this source, but it is lower than σ
3. Correspondences of 1FGL sources with the
54-month hard X-ray maps
The 2PBC catalogue includes sources with a significance
threshold σ
= 4.8. Reasonably, a remarkable number of
fainter objects are imaged in the 54-month BAT all-sky maps
at a significance level lower than σ
. In the eort of increas-
ing the list of objects emitting both in the BAT and in the
LAT energy ranges we consider the adoption of a lower sig-
nificance threshold. Taking into account the results obtained
by Maselli et al. (2010) in the cross-correlation of the BZCAT
Blazar Catalogue (Massaro et al. 2009) with the 39-month 15–
150 keV BAT map (Cusumano et al. 2010a) we adopt σ
3. From their analysis, carried out at |b| > 10
, Maselli
et al. (2010) found that the adoption of σ
follows in a fraction
of 3% of related spurious associations; this value is presum-
ably underestimated at |b| < 10
due to the higher density of
A total of 1043 and 408 objects are found in the 1FGL cat-
alogue at high (|b| > 10
) and low Galactic latitude, respec-
tively. We extract the significance value in the 54-month 15-
150 keV BAT all-sky map at their positions and find σ σ
for 80 objects at |b| > 10
and 49 objects at |b| < 10
. In a few
of these objects the σ value may be strongly biased by some
close, very bright hard X-ray sources. Following the same cri-
terion adopted in Maselli et al. (2010), we exclude the 1FGL
sources whose position is found within 36
from these bright
BAT sources and no overlap, even marginal, is found between
the corresponding error regions. After this screening we obtain
a sample of 75 (|b| > 10
) and 29 (|b| < 10
) 1FGL sources.
All the sources listed in Table 1 and Table 2 are included in
this sample with nine exceptions, all of them at high Galactic
latitude. They concern 1FGL sources for which the 95% confi-
dence ellipse is particularly wide and the 2PBC source is at a
considerable distance from the centre of the ellipse. The final
list of additional sources with respect to those obtained from
the cross-correlation between the 1FGL and 2PBC catalogues
has been splitted according to the Galactic latitude and reported
in Table 3 and Table 4.
4. Properties of the resulting samples of sources
The firm correspondences that we find cross-correlating the
1FGL and 2PBC catalogues and verifying the agreement be-
tween the identified counterparts lead to 62 sources. This num-
ber raises to 104 ( 7% of all the 1FGL objects) considering all
the sources with a significance down to σ
= 3. Therefore, the
number of sources emitting in both the energy ranges covered
by the BAT and the LAT instruments aboard Swift and Fermi,
respectively, is small. This is a clear indication that the emis-
sion in thehard X- and in theγ-ray sky is dominated by sources
with a dierent nature. This result is not unexpected: among
extragalactic sources, the main contribution in the 1FGL cat-
4 A. Maselli et al.: The populations of hard X- and γ-ray sources
0 0.2 0.4 0.6 0.8
1.2 1.4 1.6 1.8 2
redshift (z)
Number of sources
4.5 5 5.5
redshift (z)
Number of sources
Fig.2. Histograms of the redshift distributions for BL Lac objects (left panel) and flat spectrum radio quasars (right panel). The
contribution of 1FGL sources with a significance down to σ
= 3 in the BAT 15–150 keV significance map (filled columns) is
compared with that of the whole corresponding population reported in the BZCAT (empty columns). The contribution of sources
included in the 2PBC catalogue is emphasised by a thicker colour.
alogue is given by blazars, while in the 2PBC catalogue it is
given by Seyfert galaxies. Considering the 1FGL extragalactic
sources provided with an association, the blazar contribution
is given by 295 BL Lacs and 274 FSRQs: active galaxies with
uncertain classification are 92, while non-blazar active galaxies
are only 28. Conversely, in the 2PBC catalogue there are 307
Seyfert 1 and 165 Seyfert 2 galaxies. Even if considerable, the
blazar contribution (97 objects) is less relevant.
The group of 104 objects that we have collected is made
up of 83 extragalactic and 15 galactic sources; 6 objects are
unidentified. The largest part of extragalactic sources is given
by blazars with only a very few exceptions: the Seyfert 1.2
galaxy ESO 32377, the FR II radiogalaxy 3C 111, the flat
spectrum radio source PKS 0336177 and the starburst galax-
ies M 82 and NGC 4945. At low Galactic latitude we find 6
pulsars (including Crab and Vela), 4 high mass X-ray binaries
(including Cyg X-3) and a low mass X-ray binary, the super-
nova remnant Cas A, two cataclismic variable stars and the
peculiar object Eta Carinae. We briefly report on the results
obtained considering the lower significance threshold value
σ = 2: we find further 74 sources, the largest fraction of which
are blazars (16 BZB, 18 BZQ and 9 BZU), followed by other
AGNs (among which the well known FR I radiogalaxy M87)
the starburst galaxy NGC 253 and a few pulsars; 14 sources are
unidentified.We remark that, accordingto Maselli et al. (2010),
the number of spurious correspondences for such a low signif-
icance threshold is 20%.
We focus on the blazar content of the 1FGL-2PBC sources
and investigate about their distance. We follow the clas-
sification of the second edition of the BZCAT catalogue
(Massaro et al. 2010) which includes 1164 BL Lac objects (in-
dicated in the followingwith the sux “B”), 1660 flat spectrum
radio quasars (sux “Q”) and 262 blazars with uncertain clas-
sification (sux “U”). The number of BL Lacs without redshift
estimate (541) is considerable: moreover, in our computation
we consider only firm estimates, for a total of 531 BL Lacs.
We compute the mean redshift value of the dierent classes
of 1FGL-2PBC blazars: the value hz
i refers to the group of
blazars obtained from the cross-correlation of the 1FGL and
2PBC catalogues while hz
i refers to the larger group obtained
adopting the lower threshold value σ
= 3 in the BAT 15–
150 significance map. We note that 5 among the 24 BL Lacs
detected above σ
do not have any redshift estimate, two of
which are included in the 2PBC catalogue. We compare these
results with the values which characterise the blazar subclasses
as a whole, computed considering the totality of sources clas-
sified in the BZCAT: all these values are reported in Table 5.
Moreover,we plot in Fig. 2 thehistogramsof the redshift distri-
butions for BL Lacs (left panel) and flat spectrum radio quasars
(right panel) for the considered groups of sources.
Our results show that the subsample of blazars emitting
both in the BAT and in the LAT energy bands is made of
sources relatively closer than the average to the observer. In
fact, hz
i is lower than the mean redshift value computed for all
the dierent blazar subclasses, as reported in Table 5. The anal-
ysis of the plot in the left panel of Fig. 2 shows that the largest
part of BL Lac objects coming from the cross-correlation of
1FGL and 2PBC catalogues has a redshift z
< 0.2, while
the modal value of the BL Lac distribution is in the range
0.2 < z
< 0.3. The addition of sources with significance down
to σ
= 3 confirms this result: the largest part of them have
< 0.1, with the remaining sources more or less equally dis-
tributed at higher redshift up to z
= 0.7. An analogous re-
sult is true for flat spectrum radio quasars (Fig. 2, right panel):
high-energy sources emitting both in the BAT and in the LAT
energy bands have redshift z
< 3.2, with a peak in the range
0.5 < z
< 1. The addition of sources with significance down
to σ
= 3 increments this peak with further 7 sources, and
A. Maselli et al.: The populations of hard X- and γ-ray sources 5
none of them has a redshift higher than z
= 2. Conversely,
the modal value of the distribution of all the FSRQs catalogued
in the BZCAT is at higher values than this peak, in the range
1.25 < z
< 1.5.
5. New possible associations of sources in the
BAT and LAT surveys
The high-energy emission revealed by the BAT and the LAT
telescopes is very helpful in addressing a more correct classi-
fication of already known sources. Moreover, it can lead to the
discovery of new blazars in regions where their identification
is complicated by the belt surrounding the Galactic plane inter-
cepting the line of sight towards them. We focus our attention
on two 1FGL unidentified sources, 1FGL J0137.8+5814 and
1FGL J2056.7+4938, detected at low Galactic latitude and in-
cluded in Table 2 and Table 4. For each source we build the
spectral energy distribution (SED) by adding the data obtained
from our reduction of Swift and XMM-Newton pointed obser-
vations to all the data that we have found in the literature.
Swift-XRT observations, carried out using the most sen-
sitive Photon Counting readout mode (see Hill et al. 2004
for a description of readout modes), are available for both
1FGL J0137.8+5814 and 1FGL J2056.7+4938. The highest
detected count rate for each source is in any case lower than
the pile-up threshold (0.5 cts s
). We reduce the data with the
HEASOFT 6.8 package distributed by the NASA High Energy
Astrophysics Archive Research Center (HEASARC). All the
files necessary for the spectral analysis are obtained using the
xrtpipeline and the xrtproducts tasks. For each source two
Swift observations, characterised by very similar values of the
count rate, are available: therefore we decide to sum the two
event files using the xselect task and the exposure maps to cor-
rect for vignetting, CCD hot and damaged pixels. We use the
xrtcentroid task to detect the centroid of the source and the
corresponding positional error; a nearby source-free region is
chosen for the extraction of the background spectrum. Spectral
data of sources are extracted in circular regionssurrounding the
centroid, adoptinga radius of 20 pixels (1 pixel = 2.36
) for the
source spectrum and of 50 pixels for the background spectrum.
A pointed observation from XMM-Newton is available for
1FGL J0137.8+5814: we restrict our analyses of this obser-
vation to MOS1 and MOS2, discarding PN data because the
source is located at the edge of the instrument FoV. We use
the standard analysis software SAS 10.0 to extract high level
science products from the ODF files. The source spectrum is
extracted by selecting all the events with PATTERN 12 (re-
stricting the patterns to single and doubles) and FLAG = 0
within a circle of 40
radius centered on the source. Similarly,
the background is extracted collecting all the counts within an
annulus, centered on the source, with inner and outer radii of
and 85
, respectively. We combine the instrumental chan-
nels in the spectral files to include at least 20 counts in each
new energy bin for the Swift observation and 25 counts in the
case of XMM-Newton; the spectral analysis is carried out using
XSPEC 12.5.1n.
The flux density of both sources in the hard X-ray domain
(20 keV) has been obtained converting their count rate in the
15–30 keV map of the 54-month BAT survey. The conversion
factor is calculated from the count rate ofCrab and its spectrum
used for calibration purposes, as explained in the BAT calibra-
tion status report
The measurements available for these objects at dierent
frequencies are mostly non-simultaneous and their number is
not sucient to allow the possibility of analysing in detail their
intrinsic variability at dierent epochs. We evaluate their gen-
eral emission properties by selecting opportune energy ranges
and, whenever possible, by fitting the corresponding data with
analytical models to estimate some relevant parameters. The
data in the radio band are well fitted by a power-law model
S(ν) ν
to derive their spectral index α
. We adopt a log-
parabolic model to fit the curvature of energy distribution of
the synchrotron and inverse Compton components that charac-
terise the SED of blazars. This model, expressed by the analyt-
ical formula
νF(ν) = ν
) × 10
β (Log ν/ν
, (1)
provides the peak frequency ν
of the component and the pa-
rameter β that describes its curvature at the peak. This law re-
producescurvedspectra with a small number of parameters and
has been verified to fit well the broad band spectra of blazars
(Landau et al. 1986, see also Massaro et al. 2004a,b for an in-
terpretation in terms of statistical acceleration).
We note that the Galactic latitude of these sources is very
low. For this reason the flux density in the optical band may be
severely aected from uncertainties of the Galactic extinction
values which are supposed to be not fully reliable. As regards
the γ-ray data, derived from the 1FGL catalogue, we consider
the possibility of contamination due not only to the γ-ray back-
groundbut also to theoccasionalpresenceof neighbour sources
in the field as for the case of1FGL J0137.8+5814,with a pulsar
at an angular distance of 12
5.1. 1FGL J0137.8+5814
We assume from the 1FGL catalogue the posi-
tion of 1FGL J0137.8+5814 (RA=01
.1) with an error radius 7
.5; the Galactic
latitude is b = 4
The analysis of the 54-month BAT significance map
shows a considerable hard X-ray emission at this posi-
tion but no source has been included in the 2PBC cata-
logue (Cusumano et al. 2010b) because its detection signifi-
cance (3.4 σ) is below the catalogue threshold σ
. In this
case, establishing the position of the source is a delicate
task; we report the coordinates of the pixel with the locally
higher significance value (3.5 σ) that are RA=01
and Dec=+58
.1. An hard X-ray detection in this re-
gion of the sky by Integral has been first included in the
all-sky survey by Krivonos et al. (2007). It is also re-
ported in the 4th IBIS/ISGRI catalogue (Bird et al. 2010) with
.8, Dec= +58
.6 and with an error radius
of 5
, thus shrinking the region for the search of a counter-
part to the Fermi γ-ray source. The variability of this object in
6 A. Maselli et al.: The populations of hard X- and γ-ray sources
Fig.3. The BAT 15–150 keV significance map (60
× 60
) in
the field of 1FGL J0137.8+5814. The positions of the Fermi-
LAT and the Integral-IBIS detections with the corresponding
uncertainty regions are plotted with a black and a white dashed
line, respectively. The positions of the soft X-ray detections
(ROSAT: yellow cross; XMM-Newton : green cross; Swift-
XRT: blue cross) are very close to each other and practically
coincident to that of the optical counterpart.
the 20-40 keV band has been recently assessed by Telezhinsky
et al. (2010): it is still classified as “unidentified and as a tran-
sient source detected at the intrinsic variance map but not at the
significance map.
At lower energies,a source in the LATand IBIS errorboxes
has been reported in the 1RXS catalogue (Voges et al. 1999)
at RA=01
.0, Dec=+58
.5 with an error radius
of 9
. Later on, an X-ray observation performed by XMM-
Newton on January 16, 2003 and aimed at observing the pulsar
PSR 0136+57has revealed a serendipitous source that has been
included in the 2XMMi catalogue(Watson et al. 2009) with co-
ordinates RA=01
.4, Dec=+58
and an error ra-
dius of 1
. We analyse this XMM-Newton observation, char-
acterised by a long exposure time t
= 8439 s, considering
the results obtained by the combined analysis of MOS1 and
MOS2 detectors. We fit the spectrum with a power-law model
and obtain a photon index Γ = (2.31 ± 0.06). The value of the
hydrogen column density N
= (4.96 ± 0.18) × 10
we obtain leaving this parameter free to vary is very similar to
the Galactic one (N
= 4.01 × 10
) as reported in the
Leiden/Argentine/Bonn (LAB) Survey (Kalberla et al. 2005).
The source flux is 1.04 × 10
erg cm
in the 2–10 keV
band and is 1.5 × 10
erg cm
in the 0.2–12 keV band.
We note the lower value of the flux 1.8 × 10
erg cm
in the 2–10 keV band reported by Stephen et al. (2010) and at-
tribute it to the dierent value of the photon index Γ = 1.7 that
they adopted in their fit.
Two Swift observations have been recently obtained in this
region: the XRT exposure of the first observation (September
4, 2010) is 1153 s, while a longer exposure of 3393 s is avail-
able for the second observation (October 22, 2010). Across this
period the source has not shown appreciable variations of ac-
tivity, with a stable count rate around 1.7 × 10
cts s
The position of the XRT source is RA=01
.37 and
.5 with an error radius 3.61
. We fit the ob-
tained spectrum with a power-law model and find χ
/ d.o.f.
= 0.96 / 35. As for the XMM-Newton observation, also in
this case the obtained value for the hydrogen column density
= (4.75 ± 0.49) × 10
can be considered consistent
with the Galactic one. The value of the photon spectral index is
Γ = (2.21 ± 0.14), while the obtained value for the 2–10 keV
flux is 5.67 × 10
erg cm
that is nearly half the value
measured by XMM-Newton in 2003.
Possible interesting radio counterparts are in the 87GB and
NVSS (Condon et al. 1998) catalogues (RA=01
.2) with flux densities of F
5 GHz
= 136 mJy
and F
1.4 GHz
= 170 mJy, respectively, from which we derive a
spectral index α
0.28. The image in the latter survey shows
a marginally extended source with a very compact core.
Optical observations (Bikmaev et al. 2008) aimed at the
identification of ve Integral sources reported by Krivonos
et al. (2007), performed with the Russian-Turkish 1.5-m RTT-
150 and the 6-m BTA telescopes, led to the discovery of an ob-
ject having a continuum without emission or absorption lines,
located at RA=01
.45 and Dec=+58
.6, fully
compatible with the radio and X-ray position and at a sep-
aration of 0.8
from the γ-ray centroid. Photometric data
of the object in the field are available from the Sloan Digital
Sky Survey (SDSS-DR8). The morphological classification is
that of a starlike source and the magnitudes are r = (18.12 ±
0.01) mag, g = (19.05± 0.01)mag and u = (20.04± 0.05)mag,
with a reddening in the r band equal to 1.47 mag. This implies
a correction to the u r colour index of 1.2 mag and the
intrinsic colour index would be 0.7 mag, corresponding to a
spectral distribution with a strong excess at blue wavelengths,
typical of BL Lac objects.
Considering all these available data we obtain the spectral
energy distribution of this source shown in Fig. 4. It looks very
similar to a SED for a LAT blazar (Abdo et al. 2010c) with
the two broad bumps associated with the synchrotron and in-
verse Compton emission. We estimate the peak frequency ν
and the curvature parameter β of both components fitting a log-
parabolic law to the data. The observations of the X-ray tele-
scopes, monitoring the region close to the synchrotron peak
in two dierent epochs, have shown a relevant variability of
emission from this source. The results of the log-parabolic fit
including Swift data are ν
= 1.66 × 10
Hz for the peak fre-
quency with a corresponding maximum of synchrotron emis-
sion ν
) = 6.25 × 10
erg cm
; the estimated value
of the curvature parameter is β
= 0.08. The log-parabolic fit
including XMM-Newton data provides a higher value (ν
6.69 × 10
Hz) for the synchrotron peak frequency and a
corresponding maximum of synchrotron emission ν
) =
8.30× 10
erg cm
; a very similar curvature (β
= 0.07)
was found also in this case. As regards the inverse Compton
component,we obtain ν
= 1.78×10
Hz with a correspond-
ing ν
) = 7.78 × 10
erg cm
; the curvature pa-
A. Maselli et al.: The populations of hard X- and γ-ray sources 7
ν (Hz)
ν F(ν) (erg cm
Fig.4. The Spectral energy distribution of
1FGL J0137.8+5814. From lower to higher frequencies
we report radio data up to 5 GHz (red circles) and data
from 2MASS (orange circles), SDSS-DR8 (yellow circles),
Swift-XRT (light green circles), XMM-MOS (dark green
circles), Swift-BAT (cyan square), Integral-IBIS (blue squares)
and Fermi-LAT (magenta squares). Data are fitted with a
power-law model in the radio band; a log-parabolic model
has been used to emphasise the synchrotron and inverse
Compton component of this high-energy synchrotron peak
(HSP) BL Lac object.
rameter is β
= 0.07. Both the flux at the peak frequency and
the curvature parameter that we derive from our log-parabolic
models are therefore very similar for the synchrotron and the
inverse Compton components.
A BL Lac classification for this source, first suggested by
Bikmaev et al. (2008), has been recently confirmed by Stephen
et al. (2010). Following the classification scheme reported in
Abdo et al. (2010c) we can conclude from our analysis that this
object is anhigh-energysynchrotron peak (HSP) BL Lac object
with a ν
> 10
Hz in dierent detected states of source’s
5.2. 1FGL J2056.7+4938
The position of 1FGL J2056.7+4938 (RA=20
.3, corresponding to the Galactic latitude b =
.74), can be assumed with an error radius r = 2
.7 from the
1FGL catalogue.
In the 2PBC there is the source 2PBC J2056.5+4938 close
to this location at RA=20
.66, Dec=+49
.3 with
an error radius 3
.5 at 95% confidence level. The significance
of the detection is 5.5σ in the 15–150 keV map and 6.3 σ in the
15–30 keV map. Earlier than Swift-BAT, a detection in the hard
X-ray band in the proximity of 1FGL J2056.7+4938 was ob-
tained by Integral-IBIS. The source, first reported by Krivonos
et al. (2007) with the name IGR J20569-4940, was left with-
out classification; the detection was later confirmed by Bird
et al. (2010) in their 4th IBIS/ISGRI soft gamma-ray survey
The counterpart of this high-energy emission can be
searched by shrinking the region of the sky with circles
centered at the positions of Fermi-LAT, Swift-BAT and
Integral-IBIS centroids with a radius proportional to their
errors, respectively. The radio source 4C +49.35, classified
as symmetric double in NED, is found within the inter-
section of these circles. Radio measurements at 1.4 GHz
reported in the NVSS catalogue (Condon et al. 1998) re-
solve two components separated by 3
: a North-East
component (RA=20
.69, Dec=+49
.6) with a
flux density F
= 167 mJy and a South-West compo-
nent (RA=20
.45, Dec=+49
.0) with F
124 mJy.
A soft X-ray emission was detected in this region for the
first time by the Ariel V satellite and reported by Warwick
et al. (1981) with the name 3A 2056+493. Later detections by
ROSAT and, more recently, by Swift and XMM-Newton are
characterised by adequate precision to associate the soft X-ray
emission with the NE component. Two detections from XMM-
Newton have been reported in the slew survey clean source
catalogue (Saxton et al. 2008). Their positions are very similar
and separated by 2.5
from each other: RA=20
.8 for the first one and RA=20
.3 for the second one; the error on the posi-
tion is 8
at 1 σ confidence. Despite the two slews were carried
out along the same day (November 03, 2007) flux variations in
the 0.2–12 keV band have been reported in the catalogue, drop-
ping from (1.85 ± 0.31)to (0.83 ± 0.25) ×10
erg cm
a few hours.
Two soft X-ray pointed observations have been performed
by Swift on February26, 2009 and March 03, 2009;the first one
has a much longer exposure (8377 s) than the other (1439 s).
We stack the two observations and obtain RA=20
.69 for the position of the X-ray detection,
with a precision of r = 3.54
. We carry out a spectral analysis
adopting a power-law model: leaving all the parameters free to
varywe obtain N
= (1.62±0.09)×10
, Γ = (2.43±0.08)
and χ
/ d.o.f. = 0.94 / 97; the 2–10 keV flux is 1.19 × 10
erg cm
. We note that, at variance with our result, Landi
et al. (2010) report a value of the hydrogen column density
= (0.53 ± 0.18) × 10
lower than the Galactic con-
tribution (N
= 1.0 × 10
) quoted in the LAB Survey
(Kalberla et al. 2005). We repeat the fit fixing the N
ter to the Galactic one but we find a not acceptable variation in
the χ
value (χ
/ d.o.f. = 1.61 / 96). A broad band spectral fit
of data from radio to the X-rays with a log-parabola provides
a curvature parameter β
= 0.07 (Fig. 5). Therefore we repeat
the fit of the X-ray spectrum with a log-parabolic model fix-
ing β = 0.07 and also the hydrogen column density at a value
= 1.2 × 10
) moderately higher with respect to the
Galactic one: in this way we obtain the acceptable result χ
d.o.f. = 1.14 / 94.
The association between the radio and the soft X-ray emis-
sion can be considered firmly established by the short dis-
tance between the Swift and the XMM-Newton centroids on
8 A. Maselli et al.: The populations of hard X- and γ-ray sources
ν (Hz)
ν F(ν) (erg cm
Fig.5. The Spectral energy distribution of
1FGL J2056.7+4938. From lower to higher frequencies
we report radio data up to 22 GHz (red circles) and data
from 2MASS (orange circles), Swift-XRT (light green circles),
Swift-BAT (cyan square), Integral-IBIS (blue squares) and
Fermi-LAT (magenta squares). Data arefitted with a power-law
model in the radio band; a log-parabolic model has been used
to emphasise the synchrotron and inverse Compton component
of this high-energy synchrotron peak (HSP) BL Lac object.
the one hand and the NE component of 4C +49.35 on the
other hand. These are very close to the bright star SAO 50269
.966; Dec=+49
.55; R
= 8.39 mag in
USNO-B1, Monet et al. 2003). The UVOT image in the UV-
W2 filter does not show evidence of sources in proximity of the
star, for which we obtain a magnitude W2 = (13.12±0.05)mag.
Both images acquired in the U filter are saturated in corre-
spondence to the star. In a study of the low latitude sample
area in Cygnus of the ROSAT galactic plane survey, Motch
et al. (1997) associated the X-ray source RX J2056.6+4940
with SAO 50269 with a probability for a random associa-
tion of about 10%. The distance between the X-ray and the
star position is 13
within the estimated ROSAT accu-
racy that is 25
. The origin of the X-ray emission was
related to an active corona surrounding this star, as for the
85% of sources with a PSPC count rate higher than 3 × 10
cts s
belonging to their sample. Recently, Haakonsen &
Rutledge (2009) reported a low probability of association be-
tween 1RXS J205644.3+494011 and the star in their statisti-
cal cross-association of the ROSAT Bright Source Catalogue
(Voges et al. 1999) and the 2MASS Point Source Catalogue
(Cutri et al. 2003). As reported by Landi et al. (2010) the coun-
terpart of this high-energy source is, with high probability, an
object located at RA=20
.719 and Dec=+49
at only 12
.9 from SAO 50269. It has been detected in
the 2MASS infrared images retrieved from the NASA/IPAC
infrared Science Archive. The magnitude in the J filter is
13.69 mag and is uncertain, whereas in the H and K filters they
are (14.30 ± 0.10) mag and (13.74 ± 0.08) mag, respectively.
The emission of this object in the optical as well as in the UV
band is most likely overwhelmed by that originating from the
star. This source is separated from the γ-ray centroid by 1
Paredes et al. (2002) cited the ROSAT source
1RXS J205644.3+494011 in a search for microquasar
candidates at low galactic latitudes obtained by means of
a cross-identification between the ROSAT bright source
catalogue (RBSC, Voges et al. 1999) and the NVSS catalogue.
These authors gave a priority to sources for which the oset
between the X-ray and the radio position was within the
1σ RBSC position error: unfortunately, this was not the
case for 1RXS J205644.3+494011. Recently, the hypothesis
that the nature of this source might be extragalactic was
taken into account by several authors (Landi et al. (2010);
Voss&Ajello (2010); Stephen et al. (2010)). We collect for
the first time all the available data in literature to build the
SED of this object, that is reported in Fig. 5, to investigate the
broad-band emission properties of 1FGL J2056.7+4938.
We exclude the radio measurements reported by surveys
with a low resolution to avoid the risk of including a spurious
contribution from the SW component of 4C +49.35. We esti-
mate a radio spectral index α
0.24 from the fit of data
up to 22 GHz (Petrov et al. 2007): this value of α
is indeed
typical of flat spectrum radio sources. An evaluation of cor-
rect fluxes both at optical and at infrared wavelengths is dif-
ficult not only for the near star but also for the high value of
the reddening E(B V) = 2.85 mag (Schlegel et al. 1998), that
may be uncertain for such a low value of the Galactic latitude.
We fit with a log-parabola the data from radio to X-ray fre-
quencies and obtain the peak frequency ν
= 3.86 × 10
the corresponding maximum of synchrotron emission ν
= 1.28 × 10
erg cm
and the curvature parameter
= 0.07. In carrying out our fit we force the log-parabola
profile to agree with the power-law fit to the radio data and
assume E(B V) = 1 mag, consistently lower than the value
quoted by Schlegel et al. (1998); the uncertain infrared mea-
surement in the J filter has been omitted. We also carry out a
fit of high-energy data from the hard X- to the γ-ray band and
obtain ν
= 6.46× 10
Hz with a correspondingν
) =
erg cm
and a curvatureparameter β
= 0.04.
More precise measurements are requested to characterise
both emission components of this source. Nevertheless, the
analysis of its broad-band properties such as the radio spectral
index, the low Compton dominance and the synchrotron peak
frequencyhigherthan 10
Hz support for1FGL J2056.7+4938
the classification as a blazar and in particular as a HSP BL Lac
object, but only the analysis of the optical spectrum can con-
firm this interpretation.
6. Conclusions
We have reported the results of our analysis devoted to the re-
search of the sources showing high-energyemission in both the
Swift-BAT and the Fermi-LAT telescopes, according to the data
collected by LAT across the first11 months of operation and by
BAT across a much longer period of 54 months. As expected,
we have found a low number of sources: only 7% of those in-
A. Maselli et al.: The populations of hard X- and γ-ray sources 9
cluded in the 1FGL catalogue are characterised by a significant
emission (σ > 3) as shown by the 54-month BAT 15-150 keV
map. The larger fraction of them is given by extragalactic ob-
jects, and the dominant part is represented by blazars. We have
investigated with greater detail their redshift distribution mak-
ing a distinction among its subclasses: the comparison with the
distribution of the whole population classified in the second
edition of the BZCAT catalogue shows that they are mainly
closer than the average to the observer.
Driven by the detection from both the BAT and the LAT
instruments we have focused on a couple of objects with a very
low Galactic latitude. Our detailed analysis of their broad-band
spectral energy distribution supports the classification of these
objects as blazars, and in particular as high synchrotron peaked
(HSP) BL Lac Objects. Analyses of the same kind can be car-
ried out with success on other sources that are expected in the
forthcoming future with the release of the second Fermi-LAT
point source catalogue, based on two years of γ-ray data, and
also a new Palermo BAT catalogue obtained from the analysis
of six years of BAT data.
Acknowledgements. The authors are grateful to the referee for the
suggestions and comments that helped to improve the manuscript.
They acknowledge financial support by ASI/INAF through contract
I/011/07/0. Part of this work is based on archival data, software or on-
line services provided by the ASI Science Data Center (ASDC) and by
the SIMBAD database which is operated at CDS, Strasbourg, France.
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10 A. Maselli et al.: The populations of hard X- and γ-ray sources
Table 1. The list of correspondences between the 1FGL and 2PBC catalogues at galactic latitude |b| > 10
. For each high energy
source the counterpart associated in the corresponding catalogues, and the relative classification, is reported: their agreement is
marked by a code in the last column. In a few cases marked with an asterisk (
) the counterpart of the 1FGL source comes from
Abdo et al. (2010b). The parameter Q gives an estimate of the reliability of the association based on the overlap between the
error circles: it lacks for 1FGL sources with no error estimate.
1FGL source
1FGL counterpart type 2PBC source 2PBC counterpart type Q code
1FGL J0101.37257 SMC gal 2PBC J0102.77241 XTE J0103728 HXB 0.86 n
RX J0104.17244 HXB 0.86 n
1FGL J0217.8+7353
1ES 0212+735 bzq 2PBC J0217.4+7349 1ES 0212+735 BZQ 0.47 y
1FGL J0238.36132
PKS 0235618 bzq 2PBC J0238.36116 IRAS F023746130 G 0.94 n
1FGL J0242.7+0007
RXJ0241.9+0009 (
) unk 2PBC J0242.70000 NGC 1068 Sy2 0.88 n
1FGL J0319.7+4130
NGC 1275 agn 2PBC J0319.7+4129 NGC 1275 BZU 0.41 y
1FGL J0325.0+3403
B2 0321+33B agn 2PBC J0324.7+3409 1H 0323+342 Sy1 0.46 y
1FGL J0334.2+3233
NRAO 140 (
) bzq 2PBC J0336.5+3219 NRAO 140 BZQ 1.27: y
1FGL J0339.11734
PKS 0336177 agn 2PBC J0339.21742 1RXS J033913.4173553 X 1.49 y
1FGL J0405.61309
PKS 040313 bzq 2PBC J0405.61308 RX J0405.51308 BZQ 0.43 y
1FGL J0437.24715
PSR J04374715 PSR 2PBC J0437.84713 RBS 0560 Sy1 n
1FGL J0440.6+2748
B2 0437+27B bzb 2PBC J0440.8+2739 1RXS J044046.9+273948 X 1.38: n
1FGL J0522.83632
PKS 052136 bzb 2PBC J0523.03626 RBS 0644 BZU 1.01 y
1FGL J0531.0+1331
PKS 0528+134 bzq 2PBC J0530.9+1333 PKS 0528+134 BZQ 0.52 y
1FGL J0538.84404
PKS 0537441 bzb 2PBC J0538.94406 PKS 0537441 BZB 0.46 y
1FGL J0539.12847
PKS 0537286 bzq 2PBC J0539.82839 PKS 0537286 BZQ 0.56 y
1FGL J0557.63831
CRATES J05583838 bzb 2PBC J0558.03821 H 0557385 Sy1 1.21 n
1FGL J0636.17521
PKS 063775 bzq 2PBC J0635.47514 PKS 0637752 BZQ 0.80 y
1FGL J0710.6+5911
BZB J0710+5908 bzb 2PBC J0710.2+5909 1H 0658+595 BZB 0.58 y
1FGL J0746.6+2548
B2 0743+25 bzq 2PBC J0746.4+2548 87GB 074322.5+255639 BZQ 0.31 y
1FGL J0750.6+1235
PKS 0748+126 bzq 2PBC J0750.6+1231 OI +280 BZQ 0.43 y
1FGL J0806.2+6148
CGRaBS J0805+6144 bzq 2PBC J0805.4+6146 GB6 J0805+6144 BZQ 0.87 y
1FGL J0842.2+7054
4C +71.07 bzq 2PBC J0841.4+7053 S5 0836+71 BZQ 0.37 y
1FGL J0929.4+5000
CGRaBS J0929+5013 (
) bzb 2PBC J0928.5+4959 0.96 -
2PBC J0930.6+4954 RBS 0782 BZB 1.29: n
1FGL J0949.0+0021
CGRaBS J0948+0022 agn 2PBC J0948.9+0021 RX J0948.8+0022 BZQ 0.27 y
1FGL J0956.5+6938
M 82 sbg 2PBC J0955.7+6941 M 82 IG 0.64 y
1FGL J1048.7+8054
CGRaBS J1044+8054 bzq 2PBC J1044.1+8054 S5 1039+81 BZQ 1.07 y
1FGL J1103.72329
CRATES J11032329 bzb 2PBC J1103.62329 1H 1100230 BZB 0.37 y
1FGL J1104.4+3812
Mkn 421 bzb 2PBC J1104.4+3813 Mrk 421 BZB 0.41 y
1FGL J1130.21447
PKS 112714 bzq 2PBC J1130.11449 OM 146 BZQ 0.37 y
1FGL J1136.2+6739
BZB J1136+6737 bzb 2PBC J1137.2+6735 RBS 1004 BZB 0.18 y
1FGL J1221.3+3008
B2 1218+30 bzb 2PBC J1221.3+3008 1RXS J122121.7+301041 BZB 0.12 y
1FGL J1222.5+0415
4C +04.42 bzq 2PBC J1222.3+0415 4C 04.42 BZQ 0.62 y
1FGL J1224.7+2121
4C +21.35 bzq 2PBC J1224.8+2122 4C +21.35 BZQ 0.40 y
1FGL J1227.94852
2PBC J1228.04854 XSS J122704859 CV* 0.22 -
1FGL J1229.1+0203
3C 273 bzq 2PBC J1229.1+0202 3C 273 BZQ 0.25 y
1FGL J1256.20547
3C 279 BZQ 2PBC J1256.10547 3C 279 BZQ 0.37 y
1FGL J1305.44928
NGC 4945 agn 2PBC J1305.44928 NGC 4945 Sy2 0.10 y
1FGL J1307.04030
ESO 32377 (
) agn 2PBC J1306.54025 ESO 32377 Sy2 0.74 y
1FGL J1320.14007
2PBC J1320.24014 0.72 -
1FGL J1325.64300
Cen A agn 2PBC J1325.44301 Cen A BZU 0.61 y
1FGL J1331.90506
PKS 1329049 bzq 2PBC J1332.00510 PKS 1329049 BZQ 0.91 y
1FGL J1417.8+2541
2E 1415+2557 bzb 2PBC J1417.9+2543 7C 1415+2556 BZB 0.37 y
1FGL J1428.7+4239
1ES 1426+428 bzb 2PBC J1428.6+4239 H 1426+428 BZB 0.58 y
1FGL J1442.8+1158
1ES 1440+122 bzb 2PBC J1442.8+1202 RBS 1420 BZB 0.90 y
1FGL J1512.80906
PKS 151008 BZQ 2PBC J1512.80906 PKS 151008 BZQ 0.42 y
1FGL J1517.82423
AP Lib bzb 2PBC J1517.72419 Ap Lib BZB 0.20 y
1FGL J1555.7+1111
PG 1553+113 bzb 2PBC J1555.5+1109 PG 1553+113 BZB 0.73 y
1FGL J1626.22956 PKS 162229 bzq 2PBC J1626.02952 PKS 162229 BZU 0.85 y
1FGL J1642.5+3947 3C 345 (
) bzq 2PBC J1643.0+3951 4C 39.48 BZQ 1.52 y
A. Maselli et al.: The populations of hard X- and γ-ray sources 11
Table 1. continues
1FGL source
1FGL counterpart type 2PBC source 2PBC counterpart type Q code
1FGL J1653.9+3945 Mkn 501 bzb 2PBC J1653.8+3945 Mrk 501 BZB 1.16 y
1FGL J1829.8+4845
3C 380 agn 2PBC J1829.6+4845 3C 380 BZU 0.24 y
1FGL J1835.33255
NGC 6652 glc 2PBC J1835.73259 XB 1832330 LXB 0.80 n
1FGL J1925.22919
PKS B1921293 bzq 2PBC J1924.42913 OV 236 BZQ 1.47 y
1FGL J2000.0+6508
1ES 1959+650 bzb 2PBC J1959.8+6509 1ES 1959+650 BZB 1.10 y
1FGL J2011.42903
2PBC J2010.82910 1.71 -
1FGL J2148.5+0654
4C +06.69 bzq 2PBC J2148.0+0657 4C +06.69 BZQ 0.41 y
1FGL J2202.8+4216
BL Lac bzb 2PBC J2202.7+4217 BL Lac BZB 0.24 y
1FGL J2229.70832
PKS 222708 bzq 2PBC J2229.60831 PKS 222708 BZQ 0.65 y
1FGL J2232.5+1144
CTA 102 bzq 2PBC J2232.4+1144 4C +11.69 BZQ 0.11 y
1FGL J2253.9+1608
3C 454.3 BZQ 2PBC J2253.9+1609 3C 454.3 BZQ 0.46 y
1FGL J2327.7+0943
PKS 2325+093 bzq 2PBC J2327.4+0939 PKS J2327+0940 BZQ 1.22 y
1FGL J2359.03035
1H 2351315 bzb 2PBC J2359.13035 H 2356309 BZB 0.28 y
Table 2. The list of correspondences between the 1FGL and 2PBC catalogues at galactic latitude |b| < 10
1FGL source
1FGL counterpart type 2PBC source 2PBC counterpart type Q code
1FGL J0035.9+5951 1ES 0033+595 bzb 2PBC J0035.8+5951 1ES 0033+59.5 BZB 0.49 y
1FGL J0240.5+6113
LS I+61 303 HXB 2PBC J0240.6+6114 GT 0236+610 HXB y
1FGL J0419.0+3811
3C 111 agn 2PBC J0418.3+3801 3C 111 Sy1 0.86 y
1FGL J0534.5+2200
PSR J0534+2200 PSR 2PBC J0534.5+2201 Crab PSR y
1FGL J0730.31141
PKS 072711 bzq 2PBC J0730.41142 PG 072711 BZQ 0.37 y
1FGL J0835.34510
PSR J08354510 PSR 2PBC J0835.34511 Vela Pulsar PSR y
1FGL J1045.25942
2PBC J1044.85942 V* eta Car V* 0.70 -
1FGL J1124.65916
PSR J11245916 PSR 2PBC J1124.95919 SNR G292.0+01.8 SNR y
1FGL J1632.74733c
2PBC J1632.74727 IGR J163284726 gam 0.87 -
1FGL J1656.23257
2PBC J1656.23303 SWIFT J1656.33302 QSO 0.85 -
1FGL J1724.03611c
2PBC J1725.23616 IGR J172523616 HXB 0.69: -
1FGL J1738.52656
2PBC J1738.22700 SLX 1735269 LXB 0.74 -
1FGL J1746.42849c
PWN G0.130.11 pwn 2PBC J1746.82845 CXOGCS J174621.05284343.2 X 1.00c n
1FGL J1747.22958
PSR J17472958 PSR 2PBC J1747.43000 1RXS J174726.8300008 LXB n
1FGL J1747.62820c
2PBC J1747.62820 CXOGCS J174742.4282228 X 0.94 -
1FGL J1826.21450
LS 5039 HXB 2PBC J1826.31450 V* V479 Sct HXB y
1FGL J1833.51034
PSR J18331034 PSR 2PBC J1833.51033 SNR 021.500.9 PSR y
1FGL J1833.62103
PKS 183021 bzq 2PBC J1833.72102 PKS 1830211 BZQ 0.34 y
1FGL J2015.7+3708
2PBC J2015.9+3712 RX J2015.6+3711 CV* 0.95 -
1FGL J2032.4+4057
Cyg X-3 MQO 2PBC J2032.4+4057 Cyg X-3 HXB y
1FGL J2056.7+4938
2PBC J2056.5+4938 RX J2056.6+4940 X 0.43 -
1FGL J2323.4+5849
spp 2PBC J2323.3+5849 Cas A SNR 0.21 -
12 A. Maselli et al.: The populations of hard X- and γ-ray sources
Table 3. The list of 1FGL sources at Galactic latitude |b| > 10
obtained considering a significance threshold σ
= 3 in the BAT
15–150 keV all-sky map. For each source the association and the classification type indicated in the 1FGL catalogue is reported,
together with BZCAT blazar classification.
1FGL source
1FGL counterpart type BZCAT source
1FGL J0137.52428 PKS 0135247 bzq BZQ J01372430
1FGL J0144.92732
PKS 0142278 bzq BZQ J01452733
1FGL J0334.24010
PKS 0332403 bzb BZB J03344008
1FGL J0403.93603
PKS 0402362 bzq BZQ J04033605
1FGL J0455.64618
PKS 045446 bzq BZQ J04554615
1FGL J0507.36103
CRATES J05076104 bzq BZQ J05076104
1FGL J0507.9+6738
1ES 0502+675 bzb BZB J0507+6737
1FGL J0639.9+7325
CGRaBS J0639+7324 bzq BZQ J0639+7324
1FGL J0721.9+7120
S5 0716+714 bzb BZB J0721+7120
1FGL J0739.1+0138
PKS 0736+01 bzq BZQ J0739+0137
1FGL J1031.0+5051
1ES 1028+511 bzb BZB J1031+5053
1FGL J1159.4+2914
4C +29.45 bzq BZQ J1159+2914
1FGL J1354.91041
PKS 1352104 bzq BZU J13541041
1FGL J1604.3+5710
CGRaBS J1604+5714 bzq BZQ J1604+5714
1FGL J1617.97716
PKS 161077 bzq BZQ J16177717
1FGL J1637.9+4707
4C +47.44 bzq BZQ J1637+4717
1FGL J1725.0+1151
CGRaBS J1725+1152 bzb BZB J1725+1152
1FGL J1800.4+7827
CGRaBS J1800+7828 bzb BZB J1800+7828
1FGL J1849.3+6705
CGRaBS J1849+6705 bzq BZQ J1849+6705
1FGL J1923.52104
OV 235 bzq BZQ J19232104
1FGL J2143.4+1742
OX 169 bzq BZQ J2143+1743
1FGL J2158.83013
PKS 2155304 bzb BZB J21583013
Table 4. The list of 1FGL sources at Galactic latitude |b| < 10
obtained considering a significance threshold σ
= 3 in the BAT
15-150 keV all-sky map.
1FGL source
1FGL counterpart type BZCAT source
1FGL J0137.8+5814
1FGL J0849.63540
VCS2 J08493541 agu
1FGL J1329.25605
PMN J13295608 agu
1FGL J1420.16048
PSR J14206048 PSR
1FGL J1714.53830c
1FGL J2021.0+3651
PSR J2021+3651 PSR
1FGL J2347.1+5142
1ES 2344+514 bzb BZB J2347+5142
From Abdo et al. (2010a): gal = normal galaxy; agn = non blazar active galaxy; sbg = starburst galaxy; bzb = BL Lac object; bzq = flat spectrum radio quasar; agu = active galaxy
of uncertain type; psr = pulsar; pwn = pulsar wind nebula; spp = potential association with a supernova remnant or a pulsar wind nebula; glc = globular cluster; mqo = microquasar
object; hxb = other X-ray binary. Designations in capital letters correspond to firm identifications. From Abdo et al. (2010b): unk = AGN of unknown type. The nomenclature adopted
in Cusumano et al. (2010b) is derived from SIMBAD online services, with the exception of blazars: LXB = low mass X-ray binary; HXB = high mass X-ray binary; G = normal galaxy;
Sy1 = Seyfert 1 galaxy; Sy2 = Seyfert 2 galaxy; X = X-ray source; IG = interacting galaxy; CV* = cataclismic variable star; PSR = pulsar; V* = variable star; SNR = superNova
remnant; gam = gamma-ray source; QSO = Quasi Stellar object. The nomenclature of the BZCAT (Massaro et al. 2009) has been used for blazars: BZB = BL Lac objects; BZQ = flat
spectrum radio quasars; BZU = blazars with uncertain classification.
XSS J12270-4859 is the only low mass X-ray binary (LMXB) with a proposed persistent gamma-ray counterpart in the Fermi-LAT domain, 2FGL 1227.7-4853. Here, we present the results of the analysis of recent INTEGRAL observations, aimed at assessing the long-term variability of the hard X-ray emission, and thus the stability of the accretion state. We confirm that the source behaves as a persistent hard X-ray emitter between 2003 and 2012. We propose that XSS J12270-4859 hosts a neutron star in a propeller state, a state we investigate in detail, developing a theoretical model to reproduce the associated X-ray and gamma-ray properties. This model can be understood as being of a more general nature, representing a viable alternative by which LMXBs can appear as gamma-ray sources. In particular, this may apply to the case of millisecond pulsars performing a transition from a state powered by the rotation of their magnetic field, to a state powered by matter in-fall, such as that recently observed from the transitional pulsar PSR J1023+0038. While the surface magnetic field of a typical NS in a LMXB is lower by more than four orders of magnitude than the much more intense fields of neutron stars accompanying high-mass binaries, the radius at which the matter in-flow is truncated in a NS-LMXB system is much lower. The magnetic field at the magnetospheric interface is then orders of magnitude larger at this interface, and as consequence, so is the power to accelerate electrons. We demonstrate that the cooling of the accelerated electron population takes place mainly through synchrotron interaction with the magnetic field permeating the interface, and through inverse Compton losses due to the interaction between the electrons and the synchrotron photons they emit. We found that self-synchrotron Compton processes can explain the high energy phenomenology of XSS J12270-4859.
Pulsars are remarkable objects that emit across the entire electromagnetic spectrum, providing a powerful probe of the interstellar medium. In this study, we investigate the relation between dispersion measure (DM) and X-ray absorption column density N H using 68 radio pulsars detected at X-ray energies with the Chandra X-Ray Observatory or XMM-Newton. We find a best-fit empirical linear relation of , which corresponds to an average ionization of %, confirming the ratio of one free electron per 10 neutral hydrogen atoms commonly assumed in the literature. We also compare different N H estimates and note that some N H values obtained from X-ray observations are higher than the total Galactic H I column density along the same line of sight, while the optical extinction generally gives the best N H predictions.
Full-text available
We searched for gamma-ray blazar candidates among the 382 unidentified hard X-ray sources of the 3rd Palermo BAT Catalog (3PBC) obtained from the analysis of 66 months of SWIFT-BAT survey data and listing 1586 sources. We adopted a recently developed association method based on the peculiar infrared colors which characterize the gamma-ray blazars included in the second catalog of active galactic nuclei detected by the Fermi Large Area Telescope (2LAC). We used this method exploiting the data of the all-sky survey performed by the Wide-Field Infrared Survey Explorer (WISE) to establish correspondences between unidentified 3PBC sources and WISE gamma-ray blazar candidates located within the BAT positional uncertainty region at 99% confidence level. We obtained a preliminary list of candidates for which we analysed all the available data in the SWIFT archive to complement the information in the literature and in the radio, infrared and optical catalogs with the information on their optical-UV and soft X-ray emission. Requiring the presence of radio and soft X-ray counterparts consistent with the infrared positions of the selected WISE sources, as well as a blazar-like radio morphology, we finally obtained a list of 24 gamma-ray blazar candidates.
Full-text available
Despite the large number of discoveries made recently by Fermi, the origins of the so called unidentified gamma-ray sources remain unknown. The large number of these sources suggests that among them there could be a population that significantly contributes to the isotropic gamma-ray background and is therefore crucial to understand their nature. The first step toward a complete comprehension of the unidentified gamma-ray source population is to identify those that can be associated with blazars, the most numerous class of extragalactic sources in the gamma-ray sky. Recently, we discovered that blazars can be recognized and separated from other extragalactic sources using the infrared (IR) WISE satellite colors. The blazar population delineates a remarkable and distinctive region of the IR color-color space, the WISE blazar strip. In particular, the subregion delineated by the gamma-ray emitting blazars is even narrower and we named it as the WISE Gamma-ray Strip (WGS). In this paper we parametrize the WGS on the basis of a single parameter s that we then use to determine if gamma-ray Active Galactic Nuclei of the uncertain type (AGUs) detected by Fermi are consistent with the WGS and so can be considered blazar candidates. We find that 54 AGUs out of a set 60 analyzed have IR colors consistent with the WGS; only 6 AGUs are outliers. This result implies that a very high percentage (i.e., in this sample about 90%) of the AGUs detected by Fermi are indeed blazar candidates.
Full-text available
Blazars constitute the most enigmatic class of extragalactic gamma-ray sources, and their observational features have been ascribed to a relativistic jet closely aligned to the line of sight. They are generally divided in two main classes: the BL Lac objects (BL Lacs) and the Flat Spectrum Radio Quasars (FSRQs). In the case of BL Lacs the double bumped spectral energy distribution (SED) is generally described by the Synchrotron Self Compton (SSC) emission, while for the FSRQs it is interpreted as due to External Compton (EC) emission. Recently, we showed that in the [3.4]-[4.6]-[12] micron color- color diagram the blazar population covers a distinct region (i.e., the WISE blazar Strip, WBS), clearly separated from the other extragalactic sources that are dominated by thermal emission. In this paper we investigate the relation between the infrared and gamma-ray emission for a subset of confirmed blazars from the literature, associated with Fermi sources, for which WISE archival observations are available. This sample is a proper subset of the sample of sources used previously, and the availability of Fermi data is critical to constrain the models on the emission mechanisms for the blazars. We found that the selected blazars also lie on the WISE blazar Strip covering a narrower region of the infrared color-color planes than the overall blazars population. We then found an evident correlation between the IR and gamma-ray spectral indices expected in the SSC and EC frameworks. Finally, we determined the ratio between their gamma-ray and infrared fluxes, a surrogate of the ratio of powers between the inverse Compton and the synchrotron SED components, and used such parameter to test different emitting scenarios blazars.
Blazars constitute the most interesting and enigmatic class of extragalactic gamma-ray sources dominated by non-thermal emission. In this Letter, we show how the WISE infrared data make possible to identify a distinct region of the [3.4]-[4.6]-[12] micron color-color diagram where the sources dominated by the the thermal radiation are separated from those dominated by non-thermal emission, in particular the blazar population. This infrared non-thermal region delineated as the WISE Blazar Strip (WBS), it is a powerful new diagnostic tool when the full WISE survey data is released. The WBS can be used to extract new blazar candidates, to identify those of uncertain type and also to search for the counterparts of unidentified gamma-ray sources. We show one example of the value of the use of the WBS identifying the TeV source VER J 0648+152, recently discovered by VERITAS.
We selected a sample of 437 BL Lac objects, taken from the RomaBZCat catalogue, for which spectroscopic information and SDSS photometry is available. We propose a new classification of BL Lacs in which the sources' type is not defined only on the basis of the peak frequency of the synchrotron component in their Spectral Energy Distribution (types L and H), but also on the relevance of this component with respect to the brightness of the host galaxy (types N and G, for nuclear or galaxy dominated sources). We found that the SDSS colour index u-r=1.4 is a good separator between these two types. We used multiband colour-colour plots to study the properties of the BL Lac classes and found that in the X-ray to radio flux ratio vs u-r plot most of the N (blue) sources are located in a rather narrow strip, while the G-sources (red) are spread in a large area, and most of them are located in galaxy clusters or interacting systems, suggesting that their X-ray emission is not from a genuine BL Lac nucleus but it is related to their environment. Of the about 135 sources detected in the gamma-rays by Fermi-GST, nearly all belong to the N-type, indicating that only this type of sources should be considered as genuine BL Lac nuclei. The J-H, H-K plot of sources detected in the 2MASS catalogue is consistent with that of the "bona fide" BL Lac objects, independently of their N or G classification from the optical indices, indicating the existence in G-type sources of a K-band excess possibly due to a steep, low frequency peaked emission which deserves further investigations. We propose to use these colour plots as a further tool for searching candidate counterparts of newly discovered high-energy sources.
Full-text available
A catalogue is presented of 109 X-ray sources at low galactic latitude (|b| < 10°) observed by the Leicester Sky Survey Instrument on Ariel V. The catalogue is based on observations extending over a 5½-yr period and gives details of the position (in some cases including an improved error box), 2–10 keV flux, X-ray variability and, where possible, the proposed optical identification of each source. Nine of the sources have not been previously reported. The flux calibration and sky coverage of the survey are described and a comparison is made with the 4U catalogue. The distribution, log N–log S relation, X-ray variability and optical identifications of the sources are then briefly discussed.
Full-text available
IRAS, IUE, and ground-based optical, NIR, mm and submm, and radio observations obtained mainly on Apr. 9-23, 1983, are reported for 19 active extragalactic sources and eight control sources. The overall spectra of the compact active sources are shown to be well represented by continuous-curvature functions such as parabolas. The spectra are found to be consistent with models involving continuous particle injection (with synchrotron losses) or first-order Fermi acceleration (with escape and synchrotron losses), but not with models using relativistic Maxwellian electron distributions.
Full-text available
We have conducted a detailed investigation of the broadband spectral properties of the γ-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi γ-ray spectra with Swift, radio, infra-red, optical, and other hard X-ray/γ-ray data, collected within 3 months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous spectral energy distributions (SED) for 48 LBAS blazars. The SED of these γ-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual log ν-log ν F ν representation, the typical broadband spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SED to characterize the peak intensity of both the low- and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broadband colors (i.e., the radio to optical, αro, and optical to X-ray, αox, spectral slopes) and from the γ-ray spectral index. Our data show that the synchrotron peak frequency (νS peak) is positioned between 1012.5 and 1014.5 Hz in broad-lined flat spectrum radio quasars (FSRQs) and between 1013 and 1017 Hz in featureless BL Lacertae objects. We find that the γ-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron-inverse Compton scenarios. However, simple homogeneous, one-zone, synchrotron self-Compton (SSC) models cannot explain most of our SED, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving external Compton radiation or multiple SSC components are required to reproduce the overall SED and the observed spectral variability. While more than 50% of known radio bright high energy peaked (HBL) BL Lacs are detected in the LBAS sample, only less than 13% of known bright FSRQs and LBL BL Lacs are included. This suggests that the latter sources, as a class, may be much fainter γ-ray emitters than LBAS blazars, and could in fact radiate close to the expectations of simple SSC models. We categorized all our sources according to a new physical classification scheme based on the generally accepted paradigm for Active Galactic Nuclei and on the results of this SED study. Since the LAT detector is more sensitive to flat spectrum γ-ray sources, the correlation between νS peak and γ-ray spectral index strongly favors the detection of high energy peaked blazars, thus explaining the Fermi overabundance of this type of sources compared to radio and EGRET samples. This selection effect is similar to that experienced in the soft X-ray band where HBL BL Lacs are the dominant type of blazars.
Full-text available
The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view (FoV), high-energy γ-ray telescope, covering the energy range from below 20 MeV to more than 300 GeV. The LAT was built by an international collaboration with contributions from space agencies, high-energy particle physics institutes, and universities in France, Italy, Japan, Sweden, and the United States. This paper describes the LAT, its preflight expected performance, and summarizes the key science objectives that will be addressed. On-orbit performance will be presented in detail in a subsequent paper. The LAT is a pair-conversion telescope with a precision tracker and calorimeter, each consisting of a 4 × 4 array of 16 modules, a segmented anticoincidence detector that covers the tracker array, and a programmable trigger and data acquisition system. Each tracker module has a vertical stack of 18 (x, y) tracking planes, including two layers (x and y) of single-sided silicon strip detectors and high-Z converter material (tungsten) per tray. Every calorimeter module has 96 CsI(Tl) crystals, arranged in an eight-layer hodoscopic configuration with a total depth of 8.6 radiation lengths, giving both longitudinal and transverse information about the energy deposition pattern. The calorimeter's depth and segmentation enable the high-energy reach of the LAT and contribute significantly to background rejection. The aspect ratio of the tracker (height/width) is 0.4, allowing a large FoV (2.4 sr) and ensuring that most pair-conversion showers initiated in the tracker will pass into the calorimeter for energy measurement. Data obtained with the LAT are intended to (1) permit rapid notification of high-energy γ-ray bursts and transients and facilitate monitoring of variable sources, (2) yield an extensive catalog of several thousand high-energy sources obtained from an all-sky survey, (3) measure spectra from 20 MeV to more than 50 GeV for several hundred sources, (4) localize point sources to 0.3-2 arcmin, (5) map and obtain spectra of extended sources such as SNRs, molecular clouds, and nearby galaxies, (6) measure the diffuse isotropic γ-ray background up to TeV energies, and (7) explore the discovery space for dark matter.
Full-text available
The NRAO VLA Sky Survey (NVSS) covers the sky north of J2000.0 delta = -40 deg (82% of the celestial sphere) at 1.4 GHz. The principal data products are (1) a set of 2326 4 deg x 4 deg continuum "cubes'' with three planes containing Stokes I, Q, and U images plus (2) a catalog of almost 2 x 10^6 discrete sources stronger than S ~ 2.5 mJy. The images all have theta = 45" FWHM resolution and nearly uniform sensitivity. Their rms brightness fluctuations are sigma ~ 0.45 mJy beam^-1 ~ 0.14 K (Stokes I) and sigma ~ 0.29 mJy beam^-1 ~ 0.09 K (Stokes Q and U). The rms uncertainties in right ascension and declination vary from <~1" for the N ~ 4 x 10^5 sources stronger than 15 mJy to 7" at the survey limit. The NVSS was made as a service to the astronomical community. All data products, user software, and updates are being released via the World Wide Web as soon as they are produced and verified.
Full-text available
A catalogue is presented of 109 X-ray sources at low galactic latitude (|b| < 10°) observed by the Leicester Sky Survey Instrument on Ariel V. The catalogue is based on observations extending over a 5 -yr period and gives details of the position (in some cases including an improved error box), 2-10 keV flux, X.ray variability and, where possible, the proposed optical identification of each source. Nine of the sources have not been previously reported. The flux calibration and sky coverage of the survey are described and a comparison is made with the 4U catalogue. The distribution, log N-log S relation, X-ray variability and optical identifications of the sources are then briefly discussed.
Full-text available
We present a new catalogue of blazars based on multi-frequency surveys and on an extensive review of the literature. It is useful for the identification of the extragalactic sources that will be detected by present and future experiments for X and gamma -ray astronomy, like Swift, AGILE, GLAST and SIMBOL-X. An electronic version is available from the ASI Science Data Center web site at The catalog presently includes objects between R.A.= 0h and 16h; the full sky is expected to be available by the end of 2007.
Full-text available
We present a catalog of high-energy gamma-ray sources detected by the Large Area Telescope (LAT), the primary science instrument on the Fermi Gamma-ray Space Telescope (Fermi), during the first 11 months of the science phase of the mission, which began on 2008 August 4. The First Fermi-LAT catalog (1FGL) contains 1451 sources detected and characterized in the 100 MeV to 100 GeV range. Source detection was based on the average flux over the 11 month period, and the threshold likelihood Test Statistic is 25, corresponding to a significance of just over 4 sigma. The 1FGL catalog includes source location regions, defined in terms of elliptical fits to the 95% confidence regions and power-law spectral fits as well as flux measurements in five energy bands for each source. In addition, monthly light curves are provided. Using a protocol defined before launch we have tested for several populations of gamma-ray sources among the sources in the catalog. For individual LAT-detected sources we provide firm identifications or plausible associations with sources in other astronomical catalogs. Identifications are based on correlated variability with counterparts at other wavelengths, or on spin or orbital periodicity. For the catalogs and association criteria that we have selected, 630 of the sources are unassociated. Care was taken to characterize the sensitivity of the results to the model of interstellar diffuse gamma-ray emission used to model the bright foreground, with the result that 161 sources at low Galactic latitudes and toward bright local interstellar clouds are flagged as having properties that are strongly dependent on the model or as potentially being due to incorrectly modeled structure in the Galactic diffuse emission.
Aims: We present the results from a study of the blazar content in the 15-150 keV map of the entire sky obtained analysing 39 months of data collected by the BAT telescope aboard the Swift satellite. Methods: We performed a cross-correlation of the significance map, obtained with a dedicated highly efficient algorithm for data processing and image reconstruction of the BAT survey data, with the blazar population of the Roma-BZCAT. After corrections for source confusion and spurious detections, we found significance excesses higher than two standard deviations for 304 sources, and the corresponding fraction of expected spurious associations is ~20%. We selected hard X-ray blazars according to their significance level and carried out a statistical analysis to characterise their emission properties. Results: A sample of 121 blazars detected at a significance level σ > 3 is discussed in greater detail. The fraction of blazars with uncertain classification in this sample is considerable (~23%), more than twice the percentage obtained considering all the blazars classified in the Roma-BZCAT (~9%). We attribute the X-ray flux of the majority of selected BL Lac objects to the synchrotron emission. Table 2 is only available in electronic form at the CDS via anonymous ftp to ( or via