X-ray Pulsations from the region of the Supergiant Fast X-ray Transient IGR J17544-2619

Abstract

Phase-targeted RXTE observations have allowed us to detect a transient 71.49
\pm 0.02 s signal that is most likely to be originating from the supergiant
fast X-ray transient IGR J17544-2619. The phase-folded light curve shows a
possible double-peaked structure with a pulsed flux of ~4.8*10^-12 erg cm^-2
s^-1 (3-10 keV). Assuming the signal to indicate the spin period of the neutron
star in the system, the provisional location of IGR J17544-2619 on the Corbet
diagram places the system within the classical wind-fed supergiant XRB region.
Such a result illustrates the growing trend of supergiant fast X-ray transients
to span across both of the original classes of HMXB in Porb - Pspin space.

Full text

Page 1

Astronomy & Astrophysics manuscript no. IGRJ17544˙pulsations
January 12, 2012
c ? ESO 2012
X-ray pulsations from the region of the Supergiant Fast X-ray
Transient IGR J17544−2619
S. P. Drave∗1, A. J. Bird1, L. J. Townsend1, A. B. Hill1, V. A. McBride1,2,3, V. Sguera4,5, A. Bazzano5, and D. J.
Clark6,7
1School of Physics and Astronomy, University of Southampton, University Road, Southampton, SO17 1BJ, UK
2Astronomy, Gravity and Cosmology Centre, Department of Astronomy, University of Cape Town, Rondebosch, 7701, South Africa
3South African Astronomical Observatory, PO Box 9, Observatory, 7935, South Africa
4INAF-IASF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Gobetti 101, Bologna, Italy
5INAF-IASF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
6Centre d’Etude Spatiale des Rayonnements, CNRS/UPS, BP 4346, 31028 Toulouse, France
7CREATEC, Unit 8, Derwent Mill Commercial Park, Cockermouth, Cumbria, CA13 0HT, UK
Recieved − 25/08/2011 / Accepted − 06/01/2012
ABSTRACT
Phase-targeted RXTE observations have allowed us to detect a transient 71.49±0.02s signal that is most likely to be originating from
the supergiant fast X-ray transient IGR J17544−2619. The phase-folded light curve shows a possible double-peaked structure with a
pulsed flux of ∼4.8×10−12erg cm−2s−1(3−10keV). Assuming the signal to indicate the spin period of the neutron star in the system,
the provisional location of IGR J17544−2619 on the Corbet diagram places the system within the classical wind-fed supergiant XRB
region. Such a result illustrates the growing trend of supergiant fast X-ray transients to span across both of the original classes of
HMXB in Porb− Pspinspace.
Key words. X-rays: binaries - X-rays: individual: IGR J17544−2619 - stars: winds, outflows - stars: pulsars
1. Introduction
Supergiant Fast X-ray Transients (SFXTs) are a new class of
high mass X-ray binary (HMXB) system that have been un-
veiled over the lifetime of the INTEGRAL mission (Winkler
et al. 2003). These sources are characterised by rapid X-ray out-
bursts, with durations of the order of tens of minutes to tens of
hours (Sguera et al. 2005), and an association with OB super-
giant companion stars (Negueruela et al. 2006b). To date there
are 10 confirmed SFXTs clustered along the Galactic plane (see
individual papers for details: Pellizza et al. 2006, Negueruela
et al. 2006a, Romano et al. 2007, Masetti et al. 2008, Nespoli
et al. 2008, Rahoui & Chaty 2008, Zurita Heras & Walter 2009,
Romano et al. 2009). There are also several candidate SFXTs
which exhibit the same X-ray flaring behaviour but for which
an optical/IR counterpart is yet to be determined (e.g. Sguera
et al. 2006). With the determination of source distances from
optical/IR spectroscopy (Rahoui et al. 2008), peak outburst lu-
minosities of 1036− 1037erg s−1have been deduced (Grebenev
et al. 2004). Combined with the observation of quiescence states
at 1032erg s−1(Bozzo et al. 2008, in’t Zand 2005) this illus-
trates a very high X-ray dynamic range of 104−105within these
systems. X-ray pulsations have been detected in four confirmed
SFXTs implying there are accreting neutron stars in these sys-
tems (for individual sources see: Lutovinov et al. 2005, Sguera
et al. 2007, Sidoli et al. 2007a, Sidoli et al. 2008).
IGR J17544−2619 was first discovered as a hard X-ray tran-
sient source on 2003 September 17 (Sunyaev et al. 2003) with
the IBIS/ISGRI (Ubertini et al. 2003/Lebrun et al. 2003) instru-
ment aboard INTEGRAL. Two short outbursts, 2 and 8 hours
∗sd805@soton.ac.uk
long respectively, were observed on the same day, indicating
fast and recurrent transient behaviour. A subsequent detection on
2004 March 8 (Grebenev et al. 2004) further illustrated the re-
current nature of the X-ray outbursts in IGR J17544−2619. After
the INTEGRAL detection, IGR J17544−2619 was associated
with the soft X-ray source 1RXS J175428.3-262035 (Wijnands
2003, Voges et al. 2000) and discovered in archival Beppo-
SAX (in’t Zand et al. 2004) and XMM-Newton data (Gonz´ alez-
Riestra et al. 2004). A Chandra observation (in’t Zand 2005)
precisely located the source with a positional accuracy of 0.6”
(RA = 17:54:25.284, DEC = -26:19:52.62, J2000.0), confirming
the association of IGR J17544−2619 with 2MASS J17542527-
2619526. Pellizza et al. (2006) classified the companion as an
O9Ib star with a mass of 25−28M?at a distance of 2−4kpc.
Subsequently Rahoui et al. (2008) performed SED fitting to the
mid-IR spectrum and refined the distance estimate to the system
as ∼3.6kpc.
Using long baseline IBIS/ISGRI light curves, Clark et al.
(2009) identified the orbital period of IGR J17544−2619 as
4.926 ± 0.001d, one of the shortest orbital periods observed in
an SFXT.
ThespectralpropertiesofIGRJ17544−2619havebeenstud-
ied at all levels of emission. The outburst spectra are often well
fit with powerlaw models that show variations in column den-
sity, 1.1−3.3×1022cm−2, and photon index, 0.75−1.3, between
outbursts (Rampy et al. 2009, Sidoli et al. 2009, Romano et al.
2008). The quiescence spectra are far softer than those of out-
bursts with photon indices between 2.1 and 5.9 (Sidoli et al.
2008, in’t Zand 2005). The softness of these spectra has led to
the conclusion that the compact object in IGR J17544−2619 is
most likely a neutron star (in’t Zand 2005, Pellizza et al. 2006).
1
arXiv:1201.2284v1 [astro-ph.HE] 11 Jan 2012

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Drave et al.: X-ray pulsations from the region of the SFXT IGR J17544−2619
In this paper we outline a new study performed on the IGR
J17544−2619 system. Our data set is described in Sect. 2, fol-
lowed by temporal and spectral results in Sect. 3. A discussion
of these results is given in Sect. 4 followed by conclusions in
Sect. 5.
2. Data Set and Analysis
Using the orbital ephemeris of Clark et al. (2009) three ob-
servations of the region around IGR J17544−2619 were per-
formed through one half of the compact object orbit using the
Proportional Counter Array (PCA) instrument aboard RXTE
(Jahoda et al. 2006, Swank 1994 respectively). The observations
were performed on 2010 May 15, 16 and 17 at 04:47, 16:54 and
03:53 UTC with each observation having an exposure of ∼ 10ks.
In all subsequent analysis we now use an updated orbital pe-
riod determination and periastron ephemeris of 4.9278±0.0002d
and MJD 53732.632 respectively, placing periastron at an orbital
phase of 0.0. These improved values were determined through
utilising the analysis methods outlined in Clark et al. (2009) with
an INTEGRAL/IBIS data set of 13.3Ms, representing an ∼66%
increase in exposure compared with the original study. The or-
bital phase of the three observations are then 0.412, 0.720 and
0.811 respectively.
The data were analysed using the standard tools within
HEASOFT v6.9. The analysis was performed on both the sci-
ence array (standard mode one) and science event (good xenon)
data. PCU 2 was the only detector active throughout all three
observations, with PCU 4 active for a small percentage of each,
hence extraction was performed on PCU 2 data only. The GTIs
of the observations were generated with the ftool MAKETIME
using the observation filter files and the standard event selec-
tion criteria described in the RXTE data analysis manual. Light
curves of the standard mode one data, which covers the full en-
ergy response of the PCA at 0.125s time resolution, were ex-
tracted using the ftool SAEXTRCT. The light curves were back-
ground subtracted using the most recent PCA faint background
model and barycentered using the FXBARY ftool. Similarly the
event mode data were treated in the same manner, whereby the
light curves were extracted using SEEXRCT at 0.125s resolu-
tion. Three light curves were extracted covering the full PCA en-
ergyresponse,3−10keVand10−120keVwithalllayersofPCU
2 used in each case. Again the light curves were barycentered
using FXBARY. The science event data was also used to create
energy spectra of the observations. The spectra, background and
response files were generated for each science event file and for
every combination of active detectors using the standard meth-
ods. As the majority of the exposure in each observation was
achieved whilst PCU 2 was the only active detector, these spec-
tra, and backgrounds, were summed using SUMPHA and the
responses combined using ADDRMF.
3. Results
Figure 1 shows the PCA standard mode one light curves of the
observations at 500s binning. In this mode PCA data has no en-
ergy resolution and hence these represent the emission detected
across the full 2−120keV PCA energy range. We see that steady,
low level emission at an intensity of ∼7 counts s−1PCU−1is be-
ing detected across all three observations. This corresponds to
a flux of ∼ 5.0−5.5×10−11erg cm−2s−1in the 3−10keV energy
range using the spectral models outlined in Table 2. The orbital
phase of the centre point of each observation is also indicated
55331.0055331.50 55332.0055332.5055333.0055333.50
MJD
0
2
4
6
8
10
counts sec!1 PCU!1
Obs 1
Phase 0.412
Obs 2
Phase 0.720
Obs 3
Phase 0.811
Fig.1. Background subtracted, binned standard mode one light
curves of the RXTE/PCA observations (bin time: 500s). The or-
bital phase of each observation is shown using the zero phase
ephemeris MJD53732.632 and orbital period 4.9278d where
periastron occurs at orbital phase 0.0.
Table 1. Statistical properties of the 0.125s resolution 2
−120keV standard mode one light curves.
ObsAverage Flux
counts s−1PCU−1
1 7.02 ± 0.06
27.19 ± 0.06
3 6.87 ± 0.06
ˆ χ2
Exposure
s
9102
9722
8872
12.1
10.9
12.2
and it is seen that the first observation (Obs 1) occurs just before
apastron whilst the second and third observations (Obs 2 and 3)
were performed as the compact object is approaching periastron.
The shape and intensity of the emission in the three observa-
tionswasfirstcharacterisedbyassessingthestatisticalproperties
of the finely binned (0.125s) standard mode one light curves.
The mean fluxes are seen to vary by a maximum of ∼5% be-
tween the observations. The level of variation within each ob-
servation was also investigated by means of the goodness of fit
to a constant flux at the average count rate. All three observa-
tions showed a poor fit, indicating variations in excess of sta-
tistical noise within the light curves. These statistics are out-
lined in Table 1. The properties of the observed emission suggest
that there are no sources undergoing bright, fast outbursts within
these observations.
3.1. Periodicity Analysis
To search for pulsations in the data, the background subtracted,
0.125s resolution standard data mode one light curves of each
observation were subjected to a Lomb-Scargle analysis (Lomb
1976, Scargle 1982). The analysis of Obs 2 and 3 showed no
significant signals within their periodograms. However, a signif-
icant peak at 71.49s was observed with a power of 22.039 in the
periodogram of Obs 1, as shown in Fig. 2. This peak does not
correspond to a beat frequency between any characteristic time
scales within the data and is present when the analysis is per-
formed on light curves with a wide range of different time bin-
nings. The 99.99% and 99.999% confidence levels were calcu-
lated at Lomb-Scargle powers of 20.133 and 22.227 respectively
using a randomisation test as outlined in Hill et al. 2005 whilst
2

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Drave et al.: X-ray pulsations from the region of the SFXT IGR J17544−2619
0100200
Period (Seconds)
300 400500
0
5
10
15
20
25
Power
99.99% Confidence Level
99.999% Confidence Level
Fig.2. Lomb-Scargle periodogram of the background subtracted
Obs 1 0.125s resolution standard mode one lightcurve showing
a peak at a period of 71.49s. This period is calculated to have a
significance of 4.37σ.
also taking into account the extra trials resulting from the anal-
ysis of the light curves from the three observations. By linearly
interpolating between these confidence levels we calculate the
significance of the 71.49s period as 4.37σ. A similar randomi-
sation based test was used to estimate the error on this period
as ±0.02s (see Drave et al. 2010 for details). Figure 3 shows
the phase-folded light curve of Obs 1 using the 71.49s period.
The shape is dominated by a large peak on top of an underlying
flux at ∼6.4counts s−1PCU−1, representing a pulse fraction of
∼13%, where the pulse fraction is defined as (Cmax− Cmin)/(Cmax
+ Cmin). A possible second peak is also observed at lower signif-
icance and is offset from the larger peak by a phase of ∼0.3−0.4.
A discussion of this shape and the non-detections in Obs 2 and
3 is given in Sect. 4
For consistency the event mode data were also investigated
for periodicities using the extracted 0.125s resolution back-
ground subtracted light curves. Using the full energy response
light curve (2−120keV) the only significant feature in the peri-
odogram is detected at 71.52s with a power of 20.37, in agree-
ment with the result from the standard mode one data. The event
mode data light curves in the 3−10 and 10−120keV energy
bands were also searched for pulsations (Note: When using re-
stricted energy ranges 3keV is used as the low energy cut-off
point due to the degrading of the PCA energy calibration below
this value). In both cases a peak at ∼71.5s is observed in the pe-
riodogram but it is not at a significant level. The sum of these
periodograms does however, produce a single significant peak at
the correct periodicity. We take this to illustrate that due to the
faintness of the emission a significant detection can only be ob-
tained when the full energy range is included in the data set and
hence the periodicity is present (to some extent) over a broad
range of energies.
Variations in the hardness ratio as a function of pulse phase
have been investigated. Figure 3 shows the 2−120keV phase-
folded light curve (top) above the phase-folded 10−120 to
3−10keV hardness ratio (bottom). It can be seen that the emis-
sion hardens during the two pulse phase regions that are coin-
cident with increased flux in the phase-folded light curve. This
suggests a physical origin for both the high and lower signifi-
cance peaks seen in the phase-folded light curve shown in Fig. 3
(top).
0.00.51.0
Phase
1.52.0
6.0
6.5
7.0
7.5
8.0
8.5
Counts s!1 PCU!1
0.0 0.51.0
Phase
1.52.0
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Hardness Ratio 10 ! 120 / 3 ! 10 keV
Fig.3.Top:Obs1lightcurvephase-foldedonthe71.50speriod.
The profile shows a pulse fraction of ∼13%. The dashed line
shows the relative exposure of each phase bin (a relative expo-
sure of 1 equates to a count rate of 8.3 and 0 to 0 within this scal-
ing).Bottom:Hardnessratiobetweenthe3−10and10−120keV
energy bands showing a hardening of the observed emission dur-
ing the peaks in the above phase folded light curve. Both curves
posses the same phase binning and arbitrary zero pulse phase
ephemeris.
3.2. Spectral Analysis
The energy spectra derived from the total data in each observa-
tion were investigated to characterise the spectral shape of the
emission and aid in the identification of its source. The spectra
werefitwithmodelsinthe3−20keVenergyrangeusingXSPEC
v12.4. All reported errors from the spectral fits are quoted at the
90% confidence level. The spectra were initially fit with sim-
ple absorbed models (e.g. phabs(bremss)), however the large
reduced Chi-Squared values (6.8−12.8 for 37 degrees of free-
dom) showed a need for an additional Gaussian component in-
terpreted as an iron emission line. It was found that absorbed
power-law models again with this additional Gaussian compo-
nent, gave the best fits across all three observations. Power-laws
with high energy cut-offs and thermal bremsstrahlung models
were also found to produce statistically similar fits for some in-
dividual observations, but neither could produce good fits across
all three observations.
The results of the power-law fits are outlined in Table 2. It is
seen that the spectra are quite highly absorbed and do not show
a significant variation in column density across the observations.
They possess photon indices that vary from 2.37+0.09
−0.09to 2.69+0.13
−0.13
3

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Drave et al.: X-ray pulsations from the region of the SFXT IGR J17544−2619
between Obs 1 and 3. The inferred 3−10keV fluxes also show
some variation across the three observations, declining signif-
icantly from 5.43+0.08
cm−2s−1in Obs 3. The flux in Obs 2 lies between these two
values, suggesting a decline throughout the observations. The
spectra of the individual observations were also summed and the
total spectrum fitted. The best fit parameters are shown Table 2
and a similar shape is again observed.
−0.10×10−11in Obs 1 to 5.13+0.10
−0.12×10−11erg
The large equivalent width of the iron line component in
each spectrum, varying from 0.78 to 0.88keV, is of interest.
These values are consistent with those quoted as resulting from
Galactic Ridge emission (Koyama et al. 1986), suggesting that
GalacticRidgeemissioncouldaccountforpartoralloftheemis-
sion detected in these observations. To investigate this we fit
the spectra with the model of the central ridge region presented
in Table 3 of Valinia & Marshall (1998). The Raymond-Smith
plasma temperature and power-law photon index were fixed to
kT=2.9keV and Γ =1.8 respectively, whilst the absorption and
normalisations were left as free parameters. As Table 3 shows,
Obs 2 and 3 are well fit by this model. However Obs 1 is not
well fit by this model, indicating that Galactic Ridge emission
does not fully describe the spectral shape of this observation. A
decreasingunabsorbedfluxtrendisalsoobservedbetweenObs1
and 3. Combining the poor fit given by the Galactic Ridge model
in Obs 1 and the decreasing flux trend seen across the three ob-
servations with the fact that a pulsation is only seen during Obs
1, is taken to show that in this observation there is an additional
source of X-ray emission that is generating the pulsed signal that
is observed in addition to the Galactic Ridge emission.
To further investigate the possible nature of the additional
X-ray source we used pulse phase resolved spectroscopy to ex-
tract spectra during the pulse on, phase 0.4−0.7 in the top panel
of Fig. 3, and pulse off, 0.7−1.0, phase regions. The spectrum
collected from the pulse off region was then subtracted from the
pulse on region, this calculation was performed in ‘count rate’
space to compensate for the different exposure times accumu-
lated for each phase region. However, whilst there were resid-
ual counts in the subtracted spectrum the signal-to-noise was
not sufficient to allow the fitting of spectral models to charac-
terise the pulsed emission and provide a direct estimate of the
residual flux. Figure 3 (bottom) does show a hardening of the
emission during the pulse on phase region however, suggesting
the presence of an additional active, pulsing X-ray source within
the FOV as the Galactic Ridge emission does not vary on these
time scales.
We can make a refined estimate of the pulsed flux in Obs
1 by using the phase-folded light curve shown in the top panel
of Fig. 3. Under the assumption that the minimum count rate in
the phase-folded light curve corresponds to zero pulsed emis-
sion, which is supported by the consistency of the count rate
between phases 0.70 and 1.0 in Fig. 3, we calculate the percent-
age excess above this minimum count rate in each phase bin.
Taking the average of the excesses observed in each phase bin
then produces an estimate of the pulsed flux fraction as 8.9%.
Using the Obs 1 flux value obtained from the spectral fits out-
lined in Table 2 this corresponds to a flux of 4.8×10−12erg cm−2
s−1(3−10keV) originating from the pulsed signal with the re-
mainder resulting from the constant Galactic Ridge emission.
As the Galactic Ridge emission cannot generate a pulsed signal
or spectral variations on a time scale of tens of seconds we there-
fore attribute this pulsed flux component to another active X-ray
source within the FOV during Obs 1.
4. Discussion
Following the spectral analysis presented in Sect. 3.2 we have
concluded that the emission observed in Obs 2 and 3 is most
likely originating from the diffuse Galactic Ridge emission.
However in Obs 1 there appears to be evidence for an additional
flux component that is generating a periodic signal. This emis-
sion is attributed to a further active X-ray source within the FOV
during this observation. As no significant periodic signals were
observed during Obs 2 and 3, and no structure was seen when
these light curves were folded on the known 71.49s period, we
conclude that the additional X-ray source was not active during
these observations. This interpretation is supported by the signif-
icant decrease in 3 − 10keV flux observed between Obs 1 and
3, see Sect. 3.2. We note however that this interpretation repre-
sents the simplest situation whereby we only use two flux com-
ponents that we can definitively identify, namely the pulsed flux
in Obs 1 and a contribution from the Galactic Ridge emission
in all three observations. In fact it may be that there are addi-
tional faint, non-pulsating sources within the PCA FOV during
all three observations that contribute towards the detected flux in
each. Changes in the flux emitted by or the number of any such
sources could cause the variations in the average 2 − 120keV
flux detected in each observations as outlined in Table 1. Due
the non-imaging nature of the PCA however it is not possible to
perform identification of any non-pulsating source other than the
Galactic Ridge emission, identified by the large equivalent width
of the iron line component in the spectral fits, and hence we use
the most simplistic interpretation for the remainder of this paper.
A second consequence of the non-imaging nature of the PCA is
that we are also required to give further consideration as to the
source of the excess, pulsed emission seen in Obs 1.
Figure 4 shows the fourth INTEGRAL/IBIS survey signifi-
cance map of the region in the 18−60keV energy band (Bird
et al. 2010). Overlaid are the PCA half and zero collimator re-
sponse contours, at 0.5oand 1orespectively, for the pointing
used, along with the sources in the INTEGRAL general ref-
erence catalog (squares) (v.31, Ebisawa et al. 2003) and fur-
ther X-ray detections (circle). Similarly the ROSAT all sky sur-
vey photon map of the region in the 0.1−2.4keV energy range
is shown in Fig. 5 (Voges et al. 1999). It can be seen that
the only two sources significantly detected within the zero re-
sponse contour by INTEGRAL are IGR J17544−2619 and IGR
J17507−2647. The latter of these sources, which is at the edge
of the FOV, was characterised using Chandra observations by
Tomsick et al. (2009) as a weak, persistent source with a flux
of 4.5×10−12erg cm−2s−1(0.2−10keV) that is most likely
a distant HMXB at ∼8.5kpc. The Chandra spectrum of the
source reported showed a high level of absorption with nH =
1.34×1023cm−2and the source was not detected in the ROSAT
map. As IGR J17507−2647 is located at the edge of the PCA
FOV it is in a region of low collimator response (< 5%) and
would therefore require a flux of at least a factor of 20 greater
than that reported by Tomsick et al. 2009 to generate the ob-
served pulsed flux. As this is a persistent source with no reported
outbursts we conclude that the emission observed by RXTE is
unlikely to be contaminated by IGR J17507−2647.
The soft X-ray source 1RXS J175454.2-264941 (Voges et al.
1999) is reported in the ROSAT bright source catalog and is lo-
cated near the half response contour; it is the only bright soft X-
ray source detected within the PCA FOV by ROSAT. The hard-
ness ratio reported suggests the source is moderately hard (0.82
for the 0.5−2/0.1−0.4keV energy bands) and could be detected
by the PCA. There are also four sources from the ROSAT faint
4

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Drave et al.: X-ray pulsations from the region of the SFXT IGR J17544−2619
Table 2. Spectral fits to the total and three individual observations using the model: Phabs(Powerlaw + Gaussian). Error are quoted
at the 90% confidence level
Obsˆ χ2/d.o.f.
Γ
nH
Line Energy
keV
6.54+0.05
Line Sigma
keV
0.03+0.19
Line Equivalent
Width keV
0.82
Flux (3−10keV)
erg cm−2s−1
5.43+0.08
1022cm−2
4.4+1.3
−1.2
5.1+1.7
−1.7
5.9+1.6
−1.6
5.2+0.8
−0.8
1 0.95/342.37+0.09
−0.09
−0.05
−0.03
−0.10×10−11
5.28+0.10
20.39/342.40+0.13
−0.13
6.56+0.07
−0.07
0.15+0.18
−0.15
0.78
−0.11×10−11
5.13+0.10
3 0.78/342.69+0.13
−0.13
6.67+0.07
−0.06
0.14+0.17
−0.14
0.88
−0.12×10−11
5.32+0.06
Total 1.01/342.48+0.06
−0.06
6.58+0.03
−0.03
0.11+0.11
−0.10
0.80
−0.06×10−11
Table 3. Spectral fits to the three individual observations using
the model Phabs(Raymond + Powerlaw) of Valinia & Marshall
(1998)
Obsˆ χ2
Flux (3−10keV)
erg cm−2s−1
5.41+0.09
11.735
−0.08×10−11
5.26+0.10
20.700
−0.08×10−11
5.13+0.07
30.869
−0.09×10−11
source catalog (Voges et al. 2000) within the FOV, however we
would not expect a detection of these sources using the PCA.
Finally the ASCA sources shown in Fig. 4 are reported as X-ray
point sources by Sugizaki et al. (2001). AX J1753.5−2538 was
detected at 3σ in the 0.7−2keV band but was not found in the
2−10keVbandindicatingthatitisasoftsource,hencewewould
not expect a detection with PCA. However the non-detection
in Fig. 5 does suggest a transient nature for this source. AX
J1754.0−2553 is not detected in the 0.7−2keV band but does
have a 3.8σ detection in the 2−10keV energy range. This sug-
gests the source is harder and, as for 1RXS J175454.2−264941,
could be detected in the PCA data.
Of all the sources discussed above, IGR J17544−2619 is the
most active showing a large number of outbursts that have been
detectedbyalargevarietyofmissions(Clarketal.2009).Taking
into account the nature of the known sources within the PCA
FOV makes IGR J17544−2619 the most likely known source of
the emission detected by the PCA instrument. For the remainder
of this paper we assume this to be the case, although we can-
not rule out the possibility that the emission is coming from one
of the other known sources, 1RXS J175454.2−264941 and AX
J1754.0−2553 in particular, or a new unknown source within the
FOV.
If we consider the 71.50s signal as a pulsation from the IGR
J17544−2619 system, then this confirms that the compact object
in the system is a neutron star, as has been suggested from qui-
escence spectra (in’t Zand 2005, Pellizza et al. 2006). Assuming
a source distance of ∼3.6kpc this makes the estimated source
flux equivalent to an unabsorbed luminosity of ∼1×1034erg s−1
(3−10keV), indicating that IGR J17544−2619 was observed in
a low X-ray state as opposed to during one of its large outbursts
(such as the 1036erg s−1event reported in Grebenev et al. 2004).
X-ray pulsations have also been detected in other SFXT sys-
tems observed during similar low luminosity states, ∼1×1034erg
s−1(0.5−10keV) in IGR J18483−0311 (Giunta et al. 2009)
and 2.3×1034erg s−1(2−10keV) in AX J1841.0-0536 (Bamba
et al. 2001) for example. Additionally the hardness ratio has
Fig.4. The IBIS survey 18−60keV significance map of the IGR
J17544−2619 region, exposure ∼8Ms, with the PCA FOV half
and zero response contours overlaid (Bird et al. 2010). The
sources in this region that are contained in the INTEGRAL gen-
eral reference catalog are shown as square points, whilst further
X-ray sources are shown as circles.
also been seen to vary as a function of pulse phase in the SFXT
IGR J11215−5952 (Sidoli et al. 2007b), showing a hardening of
emission during the pulse-on phase region as is also observed
for IGR J17544−2619 in the lower panel of Fig. 3.
The detection reported here, combined with past observa-
tions by other observatories, indicates that the pulsation signal
produced by IGR J17544−2619 is not always observable. IGR
J17544−2619 has had snapshot observations taken by XMM-
Newton (three ∼10ks exposures, Gonz´ alez-Riestra et al. 2004)
and Chandra (19.6ks, in’t Zand 2005) along with a monitoring
campaign by Swift/XRT (Sidoli et al. 2008), none of which show
detections of the 71.49s signal. However the observational prop-
erties of SFXTs make the detection of pulse periods difficult.
The biggest obstacle is the X-ray flaring time scale observed at
soft X-ray energies, that is similar to likely pulsation periods (i.e.
tens to hundreds of seconds). Flares dominate many of the soft
X-ray data sets and mask pulsation signals due to the larger flux
variations they induce. Many of the remaining data sets that are
not dominated by flares are short Swift/XRT exposures that have
too short baselines or insufficient statistics for accurate timing
5

Page 6

Drave et al.: X-ray pulsations from the region of the SFXT IGR J17544−2619
Fig.5. The ROSAT all sky survey photon map of the IGR
J17544−2619 region in the 0.1−2.4keV energy range (Voges
et al. 1999). The annotations are the same as those in Fig. 4.
analysis to be performed (e.g. Sidoli et al 2008). Given the na-
ture of the previous soft X-ray observations of this source it is
plausible that the pulsation of IGR J17544−2619 has not been
detected prior to these observations, which show a steady flux
at the 1034erg s−1level. These fluxes are consistent with those
during which previous SFXT pulsations have been detected (e.g.
Giunta et al. 2009).
Furthermore, the observation was performed as the neutron
star was approaching apastron in the system which could also
help explain why the pulsations were detected here. Clark et al.
(2009) showed that at this orbital phase there is still a non-
zero probability of stellar wind clump interaction in this sys-
tem (Clark et al. 2009, Fig. 7). Additionally under the model of
Ducci et al. (2009) the stellar wind clumps expand as they move
out from the companion supergiant star, likely becoming more
homogeneous in density as they travel. As a result we may ex-
pect that the X-ray emission generated during the interaction of
the neutron star and the expanded stellar wind clump would also
be smoother and less prone to undergo the fast flares that dom-
inate other soft X-ray observations of this source. Until a larger
number of detections have been achieved however, formal con-
clusions on any link between the detection of the pulsations and
the orbital phase of the observations cannot be drawn.
Combining this detected pulsation with the 4.926d orbital
period of Clark et al. (2009) allows the placement of IGR
J17544−2619 on the Corbet diagram (Corbet 1986), Fig. 6. We
seethatIGRJ17544−2619islocatedclosetotheclassical,wind-
fed SgXRBs. Under the ‘clumpy wind’ model of SFXTs (in’t
Zand 2005) the difference in behaviour seen when compared to
the classical systems is explained by an enhanced eccentricity
which results in the compact object spending only a fraction
of its orbit within a dense stellar wind environment (Walter &
Zurita Heras 2007). In this respect SFXTs can be considered as
an extension of the classical SgXRBs that result from varying or-
bital parameters. However, some SFXTs have longer orbits and
show orbital emission profiles that could be explained by the
presence of a disk-like structure within the stellar wind of the
10 0
10 1
10 2
10 3
Orbital Period (d)
10!2
10!1
10 0
10 1
10 2
10 3
10 4
Pulse Period (s)
IGR J11215!5952
IGR J16418!4532
IGR J17544!2619
IGR J18483!0311
IGR J16479!4514
SAX J1818.6!1703
XTE J1739!302
IGR J16465!4507
AX J1841.0!0535
SFXTs
BeXRBs
Wind!fed SgXRBs
RL Filling SgXRBs
Fig.6. The Corbet Diagram showing the locations of the SFXT
systems with at least one known period (Orbital or Pulse)
(Corbet 1986). IGR J17544−2619 lies in the region of param-
eter space populated by the classical SgXRBs.
supergiant companion, for example IGR J11215−5952 (Sidoli
et al. 2007b) and XTE J1739−302 (Drave et al. 2010). Such
characteristics are more akin to the BeXRB class of HMXBs.
Currently the SFXT class is split into classic and intermediate
SFXTs via the X-ray luminosity dynamic range observed from
the system: > 100 for an ‘intermediate’ and > 1000 for a ‘clas-
sic’ SFXT. It is now becoming apparent that a distinction can
also be drawn from the level of similarity of individual SFXTs
to each of the classic HMXB family members and such similar-
ities may be visualised by the Corbet diagram. This distinction,
and the level to which some systems may again be ‘intermedi-
ate’ between the two classes, should become more apparent as
the number of SFXTs that can be placed on the Corbet diagram
increases. Such a distinction could be an indication of a vari-
ety of stellar wind geometries present in SFXT systems and/or
of varying evolutionary paths followed in their creation (see Liu
et al. 2011 for further details).
5. Conclusions
Using observations from RXTE we have detected a transient
71.49±0.02s signal that is most likely from the spinning neu-
tron star in the SFXT IGR J17544−2619. The phase-folded
light curve shows a double peaked structure where the emis-
sion is observed to harden during the peaks in the profile.
The source was observed in a steady state of emission with a
luminosity of ∼1×1034erg s−1(3−10keV). This pulse period
places IGR J17544−2619 in the same region as the classical
wind-fed SgXRBs on the Corbet diagram. The candidate SFXT
IGR J16418−4532 also occupies this region, however other sys-
tems appear to be in intermediate (e.g. IGR J18483−0311) and
BeXRB like positions (e.g. IGR J11215−5952) suggesting that
SFXTs either span the gap between the classical types of HMXB
or represent extreme examples of the known classes. A greater
population of SFXTs is required on the Corbet diagram to in-
vestigate this further. We encourage further observations of the
IGR J17544−2619 system using focusing X-ray telescopes to
remove the current uncertainty in the origin of the detected pul-
sations and definitively prove that this is the pulsation of IGR
J17544−2619. Phase-targeted, high-cadence observations of the
source will also allow for a better understanding of the physi-
cal processes that could be causing the transient nature of the
6

Page 7

Drave et al.: X-ray pulsations from the region of the SFXT IGR J17544−2619
pulsation detection. Further observations of all SFXTs with one
known periodicity (pulsation or orbital) are also vital to allow
the largest possible sample to be placed on the Corbet diagram,
furthering our knowledge of the nature of the class as whole.
Acknowledgements
The authors wish to thank the anonymous referee for their help-
ful comments and suggestions. The authors wish to thank R. H.
D. Corbet and the PCA instrument team for their discussions on
the time scales of systematic effects within the PCA background
model. The authors also wish to thank J. J. M. in’t Zand for his
helpful discussion on the Galactic Ridge emission.
S. P. Drave acknowledge support from the Science and
Technology Facilities Council, STFC. L. J. Townsend is sup-
ported by a Mayflower scholarship from the University of
Southampton. A. Bazzano and V. Sguera acknowledge sup-
port from ASI/INAF contract n.I/009/10/0. This research
has made use of the SIMBAD database, operated at CDS,
Strasbourg, France. This research has made use of the IGR
Sources page maintained by J. Rodriguez & A. Bodaghee
(http://irfu.cea.fr/Sap/IGR-Sources/).
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