X-ray and optical observations of M55 and NGC 6366 : evidence for primordial binaries

Abstract

We present Chandra X-ray Observatory ACIS-S3 X-ray imaging observations and VLT/FORS2 and Hubble Space Telescope optical observations of two low-density Galactic globular clusters; NGC 6366 and M55. We detect 16 X-ray sources with 0.5–6.0 keV luminosities above LX = 4 × 1030 erg s−1 within the half-mass radius of M55, of which 8 or 9 are expected to be background sources, and 5 within the half-mass radius of NGC6366, of which 4 are expected to be background sources. Optical counterparts are identified for several X-ray sources in both clusters and from these we conclude that 3 of the X-ray sources in M55 and 2 or 3 of the X-ray sources in NGC 6366 are probably related to the cluster. Combining these results with those for other clusters, we find the best fit for a predicted number of X-ray sources in a globular cluster μc = 1.2Γ + 1.1 Mh, where Γ is the collision number and Mh is (half of) the cluster mass, both normalized to the values for the globular cluster M4. Some sources tentatively classified as magnetically active binaries are more luminous in X-rays than the upper limit of LX= 0.001 Lbol of such binaries in the solar neighbourhood. Comparison with XMM and ROSAT observations lead us to conclude that the brightest X-ray source in M55, a dwarf nova, becomes fainter in X-rays during the optical outburst, in accordance with other dwarf novae. The brightest X-ray source in NGC6366 is a point source surrounded by a slightly offset extended source. The absence of galaxies and Hα emission in our optical observations argues against a cluster of galaxies and against a planetary nebula, and we suggest that the source may be an old nova.

Full text

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Astronomy & Astrophysics manuscript no. 9350
July 2, 2008
c ? ESO 2008
X-ray and optical observations of M55 and NGC6366:
evidence for primordial binaries
C. G. Bassa1,2, D. Pooley3, F. Verbunt1, L. Homer4?, S. F. Anderson4, and W. H. G. Lewin5
1Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
2Physics Department, McGill University, Montreal, QC H3A 2T8, Canada; e-mail: bassa@physics.mcgill.ca
3Astronomy Department, UC Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411, USA
4Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195, USA
5Kavli Institute for Astrophysics and Space Research, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
Received / Accepted
ABSTRACT
We present Chandra X-ray Observatory ACIS-S3 X-ray imaging observations and VLT/FORS2 and Hubble Space Telescope optical
observations of two low-density Galactic globular clusters; NGC6366 and M55. We detect 16 X-ray sources with 0.5–6.0keV lumi-
nosities above LX= 4 × 1030ergs−1within the half-mass radius of M55, of which 8 or 9 are expected to be background sources, and
5 within the half-mass radius of NGC6366, of which 4 are expected to be background sources. Optical counterparts are identified
for several X-ray sources in both clusters and from these we conclude that 3 of the X-ray sources in M55 and 2 or 3 of the X-ray
sources in NGC6366 are probably related to the cluster. Combining these results with those for other clusters, we find the best fit for
a predicted number of X-ray sources in a globular cluster µc= 1.2Γ + 1.1Mh, where Γ is the collision number and Mhis (half of)
the cluster mass, both normalized to the values for the globular cluster M4. Some sources tentatively classified as magnetically active
binaries are more luminous in X-rays than the upper limit of LX? 0.001Lbolof such binaries in the solar neighbourhood. Comparison
with XMM and ROSAT observations lead us to conclude that the brightest X-ray source in M55, a dwarf nova, becomes fainter in
X-rays during the optical outburst, in accordance with other dwarf novae. The brightest X-ray source in NGC6366 is a point source
surrounded by a slightly offset extended source. The absence of galaxies and Hα emission in our optical observations argues against
a cluster of galaxies and against a planetary nebula, and we suggest that the source may be an old nova.
Key words. Globular clusters: individual (NGC6366 and M55)
1. Introduction
All stars emit X-rays, but some emit more than others. Thanks
to the Chandra X-ray Observatory the study of X-ray sources in
globular clusters now includes sources down to luminosities of
LX ∼ 1029−30ergs−1in the 0.5–2.5keV band. At these low lu-
minosities, most sources are low-mass main-sequence stars that
rotate rapidly, which in old stellar clusters is the case only when
they have been spun up by tidal forces or by accretion in close
binaries. At the high luminosity end, LX ∼>1032ergs−1, most
sources are low-mass X-ray binaries in which a neutron star ac-
cretes matter from a companion. At intermediate luminosities,
most sources are cataclysmic variables (CVs). The clusters best
studied at low X-ray luminosities are 47Tuc (Heinke et al. 2005
and references therein) and M4 (Bassa et al., 2004).
Whereas the binaries of main-sequence stars are primordial,
i.e. formed as binaries when the component stars formed, the
low-mass X-ray binaries in globular clusters are thought to be
formed in close stellar encounters that bring a previously sin-
gle neutron star into a binary (see the review by Verbunt &
Lewin 2006 and references therein). For cataclysmic variables
both formation mechanisms, evolution from a primordial binary
and capture of a previously single white dwarf in a close en-
counter, are viable, depending on the circumstances. The pro-
genitor binary of a cataclysmic variable must be wide enough
?Current address: Abbey College Cambridge, 7 Station Rd,
Cambridge, CB1 2JB, United Kingdom
to allow the more massive star to evolve into a fairly big gi-
ant, before it reaches its Roche lobe. Such wide binaries are de-
stroyed (‘ionized’) in cores of globular clusters with densities
ρ0 ∼>103M?pc−3(Davies, 1997).
One would naively expect that all cataclysmic variables in
these cores were formed in stellar encounters. However, it has
been pointed out that cataclysmic variables that evolved in the
low-density outskirts of globular clusters can sink towards the
core at late times. Thus, the population of cataclysmic variables
in dense cores can be a mixture of locally produced products of
stellar encounters and recently arrived products of the evolution
of primordial binaries (Ivanova et al. 2006; see also Hurley et al.
2007).
The number of stellar encounters in a globular cluster scales
roughly with the collision number Γ ≡ ρ02rc3/v, where ρ0is the
central density of the cluster, rcthe core radius, and v the central
velocity dispersion. Through the virial theorem v ∝ rc√ρ0and
thus Γ ∝ ρ01.5rc2(Verbunt & Hut, 1987; Verbunt, 2003). The
mass M of a cluster can be estimated from the total luminosity
of a cluster, with use of a mass-to-light ratio appropriate for a
cluster star population, and if necessary a bolometric correction.
The number of X-ray sources in a globular cluster then may be
expected to depend on the collision number (dynamical origin)
and on the cluster mass (primordial origin); the form of the de-
pendence depends on the type of binary, and therefore on the
luminosity range that is studied.
arXiv:0807.0411v1 [astro-ph] 2 Jul 2008

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2C. G. Bassa et al.: X-ray and optical observations of M55 and NGC6366: evidence for primordial binaries
Table 1. Parameters of M55 and NGC6366 used in this pa-
per; core radius rc, half-mass radius rh, distance d and red-
dening EB−V = AV/3.1 (taken from Harris 1996, version Feb
2003). The interstellar column NH is computed from NH =
1.79 × 1021cm−2AV, see Predehl & Schmitt (1995).
Cluster
M55
NGC6366
rc(?)
2.83
1.83
rh(?)
2.89
2.63
d (kpc)
5.3
3.6
EB−V
0.08
0.71
NH(cm−2)
4.44 × 1020
3.94 × 1021
The brightest low-mass X-ray binaries, with LX
1035ergs−1, are sufficiently rare that a cluster contains either
noneorone;inonecase(M15)two.Theprobabilitythatacluster
contains such a source scales with Γ (Verbunt & Hut, 1987). The
fainter low-mass X-ray binaries, with 1032∼<LX ∼<1035ergs−1,
are more numerous; the number of them in a cluster also scales
with Γ (Heinke et al., 2003; Gendre et al., 2003; Pooley et al.,
2003). These results appear to confirm the theoretical idea that
neutron stars enter binaries in globular clusters via stellar en-
counters. Studies of globular clusters within other galaxies com-
plicate the issue, because they indicate that the probability that a
cluster contains a bright source depends on its metallicity, being
higher for metal-rich clusters (e.g. Kundu et al. 2002), and fur-
thermore scales with ρ01.5, a shallower dependence on density
than implied by a proportionality with Γ (Jord´ an et al., 2004).
The numbers of less luminous sources, mostly cataclysmic
variables – hitherto only observed in globular clusters of our
Milky Way – appear to have a more shallow dependence on
central density N ∝ ρ0γwith γ = 0.6 − 0.7 (Heinke et al.,
2003; Pooley et al., 2003). Heinke et al. (2006) find different
values of Γ for bright and faint cataclysmic variables, with the
boundary near 1032ergs−1. Finally, Kong et al. (2006) show that
the number of sources (2-5 only!) in the low-density globular
cluster NGC288, a mixture of cataclysmic variables and bina-
ries of main-sequence stars, scales with the mass of this cluster.
Pooley & Hut (2006) successfully describe the number N of X-
ray sources in globular clusters with an equation of the form
N = aM + bΓ.
There are many uncertainties in the theoretical description
of the formation of X-ray sources in globular clusters. We men-
tion only the retained fraction of neutron stars; and the details
of the spiral-in process which changes the wide orbit of the pro-
genitor binary into the close orbit of a cataclysmic variable. To
help untangle these uncertainties empirically we have observed
a variety of globular clusters both in X-rays and in the optical.
We have selected these clusters to span a wide range in central
density, core radius, and mass. In this paper we discuss our ob-
servations of two clusters with relatively large core radii and low
central densities: M55 and NGC6366. Some parameters of both
clusters are given in Tables1 and 6.
Both clusters were observed with the ROSAT PSPC, which
detected one source in the core of each cluster (Johnston et al.,
1996). The position of the source in M55 was more accurately
determined with a ROSAT HRI observation Verbunt (2001),
and this source was later optically identified with a dwarf nova
(Kaluzny et al., 2005). XMM detected five sources in the core
of M55, one of them coincident with the ROSAT source (Webb
et al., 2006).
>
2. X-ray observations
M55 was observed for 33.7ks on 2004 May 11, and NGC6366
was observed for 22.0ks on 2002 July 5, both with the Advanced
CCD Imaging Spectrometer (ACIS) on the Chandra X-Ray
Observatory with the telescope aim point on the back-side il-
luminated S3 chip. The data were taken in timed-exposure mode
withthestandardintegrationtimeof3.24sperframeandteleme-
tered to the ground in faint mode.
Data reduction was performed using the CIAO 3.3 software
provided by the Chandra X-ray Center1. The data were repro-
cessed using the CALDB 3.2.2 set of calibration files (gain
maps, quantum efficiency, quantum efficiency uniformity, ef-
fective area) including a new bad pixel list made with the
acis run hotpix tool. The reprocessing was done without in-
cluding the pixel randomization that is added during standard
processing. This omission slightly improves the point spread
function. The data were filtered using the standard ASCA grades
(0, 2, 3, 4, and 6) and excluding both bad pixels and software-
flagged cosmic ray events. Intervals of strong background flaring
were searched for, but none were found. The extraction of counts
and spectra and the generation of response files was accom-
plished with ACIS Extract2(Broos et al. 2002, version 3.107),
which calls many standard CIAO routines.
2.1. Source detection
The CIAO wavelet-based wavdetect tool was employed for
source detection in both the 0.5–6.0 and 0.3–10.0keV bands.
We detected 29 sources on the entire S3 chip in the M55 ob-
servation and 12 in NGC6366 observation. Of these sources,
15 (14) lie within the 2.?89 half-mass radius (2.?83 core radius)
of M55, and 3 (3) lie within the 2.?63 half-mass radius (1.?83
core radius) of NGC6366. We also searched part of the adja-
cent S4 CCD in the M55 observation since part of the half-mass
region fell on this chip, but no sources were detected in this area.
Furthermore, we examined adaptively-smoothed images (made
from the CIAO csmooth tool) for significant point sources. In
the M55 observation, we found two additional possible sources
within the core radius. In the NGC6366 observation, we also
found two additional possible sources within the half-mass ra-
dius, one of which was within the core radius. Fig. 1 shows the
location of the detected sources with respect to the cluster center
and the core and half-mass radius.
All sources are consistent with being point sources, except
for CX1 in NGC6366, which appears to extend to a radius of
about 20??(Fig.2). Visual inspection suggests the superposition
of a point source and extended emission. To assess their spatial
coincidence, we fit two gaussians plus a constant background
to an image of the region. One gaussian approximates the point
spread function, and the other is broader and describes the ex-
tended emission. Our best fit model gives source counts in rough
agreement with the methods described in §2.2 and given in
Table3; according to the model, the point source has 17.4 net
counts, and the extended source has 186.1 net counts. The two
gaussians are offset from each other by 2.??4 ± 1.??1.
We will give two estimates of how many sources are related
to each cluster. For the first estimate, we assume that all sources
outside the half-mass radius are fore- or background sources,
No = 14 for M55 and 9 for NGC6366. Taking the size of the
S3 detector as 8.?4 × 8.?4 = 70.6sq.arcmin, the expected number
of fore- or background sources within the half-mass radius rh,
expressed in arcmin, follows as Ne= Noπrh2/(70.6−πrh2). This
gives Ne= 8.3 for M55 and 4.0 for NGC6366.
1http://asc.harvard.edu
2http://www.astro.psu.edu/xray/docs/TARA/ae users guide.html

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C. G. Bassa et al.: X-ray and optical observations of M55 and NGC6366: evidence for primordial binaries3
Fig.1. 10?× 10?Digitized Sky Survey R-band images of M55 (left) and NGC6366 (right). The positions of the Chandra X-ray
sources are depicted with circles and numbered according to their CX designation (see Table2 and 3). The two large rectangles on
each image denotes the field-of-view of the two FORS2 chips, while the smaller slanted rectangles denote the field-of-view of the
Hubble Space Telescope ACS/WFC chips.
The second estimate may be found from the logN-logS re-
lationships of Giacconi et al. (2001); at our detection limits, this
leads to an expected number of background sources within the
half-mass radius of 6–9 for M55 and 3–4 for NGC6366. These
numbers are wholly compatible with our first estimate. In M55
we detect 17 sources within the core, of which we estimate 8-11
cluster members. The probability to find 14 or more sources for
expected 8.3 is about 4%; so it is likely that at least 4 sources
are members. In the case of NGC6366 we estimate that 1 of the
4 sources within the half-mass radius is a cluster member, but
there is a sizable probability (37%) that all sources are fore- or
background.
We repeat our first estimate for sources with LX > 4 ×
1030ergs−1[0.5–6.0keV], the limit used in the Pooley et al.
(2003) study. Above the corresponding flux limit of 1.2 ×
10−15ergs−1cm−2for M55, we detected 14 sources outside the
half-mass radius of M55, and expect 8.3 sources unrelated to the
cluster within the half-mass radius. 16 sources within the half-
mass radius are observed above the flux limit. Above the flux
limit for NGC6366 of 2.6 × 10−15ergs−1cm−2, we observe 9
sources outside the half-mass radius, which predicts 4.0 unre-
lated sources within the core, compared to the observed number
of 5. The probability to observe 14 or more sources when 8.3 are
expected is about 4%. Thus from the source numbers alone we
have marginal evidence for sources above the luminosity limit
used by Pooley et al. (2003) related to M55; and no evidence for
such sources related to NGC6366.
In Tables2 and 3 we list the properties of the X-ray sources
detected by Chandra in M55 and NGC6366, respectively.
2.2. Count Rates
We extracted source counts in the following bands: 0.5–1.5keV
(Xsoft), 0.5–4.5keV (Xmed), and 1.5–6.0keV (Xhard). The de-
tected count rate was corrected for background, exposure vari-
ations, and foreground photoelectric absorption. We make these
corrections in order to produce a X-ray color-magnitude diagram
(CMD) that can be compared to the X-ray CMDs that have re-
sulted from Chandra observations of other globular clusters. In
addition, however, attention must be paid to differences in detec-
tor responses and, of course, exposure times and distances.
The background count rates in each band were estimated
from source-free regions on the S3 chip outside the half-
mass radii. The density of background counts in each band
for the M55 observation is 0.0050countspixel−1in Xsoft,
0.0103countspixel−1in Xmed, and 0.0072countspixel−1in Xhard.
For the NGC6366 observation, the background densities are
0.0030countspixel−1in Xsoft, 0.0063countspixel−1in Xmed, and
0.0056countspixel−1in Xhard. The background count rates in the
cores may be somewhat higher, but even factors of a few greater
than this estimate have negligible effects on our analysis.
The exposure variations among sources were at the 15%
level or less in both observations. To account for these variations
in exposure, we applied multiplicative corrections based on the
ratiooftheaverageeffectiveareaofthedetectoratthelocationof
a source in each of the three bands to that in the same band of the
source which had the highest average exposure (CX11 in M55
and CX2 in NGC6366). The individual effective area curves for
the sources were made using the CIAO tool mkarf. The aver-
age effective area of the detector at the location of CX11 in M55
in each of the bands was 494cm2(Xsoft), 449cm2(Xmed), and
381cm2(Xhard); for CX2 in NGC6366, the areas were 509cm2
(Xsoft), 447cm2(Xmed), and 373cm2(Xhard).
While the previous corrections were relatively minor (at the
few percent level or less), the correction for photoelectric ab-
sorption is appreciable for NGC6366 (less so for M55). We
investigated the effects of absorption by the column densities
given in Table1 on three characteristic spectra: a 3keV ther-
mal bremsstrahlung, a 0.3keV blackbody plus power law with
photon index of 2, and a power law with a photon index of 2.

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4C. G. Bassa et al.: X-ray and optical observations of M55 and NGC6366: evidence for primordial binaries
Table 2. Chandra X-ray sources detected in our observation of M55. The positions of the X-ray sources have been corrected for the
bore-sight correction of −0.??270 in right ascension and +0.??080 in declination. Positional uncertainties are given in parentheses and
refer to the last quoted digit and are the centroiding uncertainties given by ACIS Extract. They do not include the uncertainties in
the bore-sight correction (0.??044 in right ascension and 0.??045 in declination). The X-ray bands are 0.5–1.5keV (Xsoft), 0.5–4.5keV
(Xmed) and 1.5–6.0keV (Xhard). The first 17 sources are located within the half-mass radius of this cluster and are ordered on 0.5–
6.0keV countrate (Xsoft+ Xhard). The remaining sources are located outside the half-mass radius and are ordered on right ascension.
Some of the X-ray sources are also detected by ROSAT (Johnston et al., 1996; Verbunt, 2001) and XMM (Webb et al., 2006). These
sources are denoted by R and X. CV1 is cataclysmic variable found by Kaluzny et al. (2005).
IDR.A.
(J2000)
Decl.
(J2000)
Counts (Detected/Corrected)
Xsoft
Xmed
60/74.3102/123.5
27/32.844/51.4
14/17.0 33/38.6
4/4.921/24.7
10/11.815/17.5
5/6.012/13.8
10/11.813/14.7
5/6.210/12.1
0/0.08/9.3
3/3.79/10.9
3/3.37/7.3
2/2.45/5.9
0/0.05/6.3
0/0.05/5.4
4/5.45/6.3
5/6.45/5.9
3/3.74/4.6
71/101.4137/170.8
5/6.411/12.8
29/38.592/112.6
4/5.214/15.2
33/44.265/77.3
8/23.811/28.6
207/264.9304/368.1
45/56.248/56.0
0/0.03/3.0
0/0.08/9.4
25/30.037/42.7
9/11.011/13.1
4/5.510/12.3
2/2.212/14.5
fX(0.5–2.5keV)
(ergs−1cm−2)
1.2 × 10−14
5.0 × 10−15
3.4 × 10−15
1.9 × 10−15
1.9 × 10−15
1.3 × 10−15
1.7 × 10−15
1.2 × 10−15
6.1 × 10−16
9.2 × 10−16
8.1 × 10−16
5.8 × 10−16
4.8 × 10−16
4.3 × 10−16
6.4 × 10−16
7.6 × 10−16
5.7 × 10−16
1.8 × 10−14
1.2 × 10−15
9.7 × 10−15
1.5 × 10−15
8.1 × 10−15
3.5 × 10−15
3.9 × 10−14
7.1 × 10−15
6.9 × 10−16
6.4 × 10−16
4.7 × 10−15
1.6 × 10−15
1.0 × 10−15
1.3 × 10−15
fX(2.5–6.0keV)
(ergs−1cm−2)
1.4 × 10−14
5.8 × 10−15
7.9 × 10−15
7.2 × 10−15
2.1 × 10−15
2.3 × 10−15
1.1 × 10−15
1.6 × 10−15
3.6 × 10−15
1.7 × 10−15
6.4 × 10−16
1.2 × 10−15
2.4 × 10−15
1.4 × 10−15
5.8 × 10−16
2.0 × 10−17
6.9 × 10−16
2.0 × 10−14
2.0 × 10−15
2.9 × 10−14
4.7 × 10−15
1.2 × 10−14
1.2 × 10−15
2.6 × 10−14
4.8 × 10−16
3.9 × 10−15
3.1 × 10−15
4.2 × 10−15
7.7 × 10−16
3.0 × 10−15
5.0 × 10−15
Xhard
45/47.9
18/18.5
22/22.8
19/19.8
7/7.5
8/8.2
3/3.1
6/6.5
10/10.2
6/6.5
4/3.6
4/4.2
6/7.0
5/5.1
1/0.8
0/0.0
1/0.8
72/75.8
6/6.0
78/82.9
13/12.0
34/33.9
3/6.5
106/112.5
3/2.0
7/7.1
9/8.8
13/12.6
2/2.2
6/6.0
12/12.1
CX1 (R9/X30/CV1) 19h40m08.s593(3)
CX2 (X42)
CX3 (X12)
CX4 (X13)
CX5
CX6
CX7
CX8
CX9
CX10
CX11
CX12
CX13
CX14
CX15
CX16
CX17
CX18 (R6/X9)
CX19
CX20 (X14)
CX21
CX22 (X19)
CX23
CX24 (R13/X17)
CX25 (X21)
CX26
CX27
CX28 (X20)
CX29 (X45)
CX30
CX31
−30◦58?52.??08(4)
−30◦56?11.??67(4)
−30◦56?47.??09(4)
−30◦56?58.??54(7)
−30◦56?20.??34(8)
−30◦59?01.??01(11)
−30◦59?05.??61(11)
−30◦57?18.??01(8)
−30◦57?39.??85(10)
−30◦56?19.??48(10)
19h39m57.s877(3)
19h40m04.s720(3)
19h40m09.s192(5)
19h40m08.s846(6)
19h39m53.s108(11) −31◦00?14.??99(15)
19h39m51.s190(9)
19h40m03.s279(8)
19h39m54.s920(7)
19h40m04.s932(11) −30◦59?37.??68(15)
19h40m00.s235(8)
19h40m01.s887(8)
19h39m58.s668(12) −30◦59?22.??01(16)
19h40m00.s479(14) −30◦59?38.??08(18)
19h39m59.s793(10) −30◦55?26.??12(13)
19h40m06.s829(10) −30◦57?12.??29(13)
19h40m08.s214(12) −30◦57?15.??47(15)
19h39m42.s130(4)
19h39m44.s842(13) −30◦58?47.??54(17)
19h39m45.s876(4)
19h39m45.s989(19) −31◦01?31.??65(25)
19h39m46.s573(12) −31◦02?26.??47(15)
19h39m47.s994(11) −30◦55?24.??12(14)
19h39m55.s778(4)
19h39m57.s005(15) −31◦03?07.??82(19)
19h40m01.s219(18) −31◦01?02.??60(24)
19h40m03.s284(24) −31◦01?52.??73(31)
19h40m03.s475(14) −31◦02?42.??40(18)
19h40m13.s402(15) −30◦59?45.??22(19)
19h40m16.s842(13) −30◦57?41.??89(17)
19h40m18.s981(20) −31◦00?13.??35(26)
−30◦55?45.??48(5)
−30◦57?40.??03(4)
−31◦02?04.??98(5)
The effects were most prominent in the Xsoftband. Averaging
the results of each spectrum in each band, we use the following
correction factors for NGC6366: 3.72 (Xsoft), 2.47 (Xmed), and
1.17 (Xhard). For M55, the factors are 1.21 (Xsoft), 1.16 (Xmed),
and 1.02 (Xhard). Table2 and 3 list both the observed and fully
corrected counts in each band. The effect of the absorption cor-
rection on the X-ray CMD (Fig. 3) is a uniform shift of the
NGC6366 sources by 0.39units on the left axis and 0.50units
on the bottom axis and a uniform shift of the M55 sources by
0.06units on the left axis and 0.07units on the bottom axis. The
bottom and left axes give the X-ray color and magnitude with-
out this shift (they do, however, include the small corrections for
background subtraction and exposure variations).
2.3. Spectral Fitting
We fit all sources with absorbed power-law spectral models in
Sherpa (Freeman et al., 2001) using Cash (1979) statistics. We
fixed the column density to the value given in Table1, with only
the power law photon index and normalization allowed to vary.
From the best fit spectra, we calculated the unabsorbed fluxes,
given in Tables2, 3.
For the point source NGC6366 CX1a, the fluxes quoted in
Table3 are from a fit of an absorbed power law to the unbinned
spectrum, for which we use Cash statistics, and fix the absorp-
tion to that of NGC6366. This gave a power-law with photon
index 3.5±0.6. For the extended source NGC6366 CX1b we ex-
tracted the spectrum from within a 40??radius centered on CX1a.
We subtracted the spectrum of the point source CX1a, and used
a nearby, source-free, region with 1?radius to estimate the back-
ground. This gave a bremsstrahlung temperature of the extended
component of kT = 2.0+1.3
in Table3.
−0.7keV, and unabsorbed fluxes as listed
3. Optical observations
Optical observations of M55 and NGC6366 were obtained with
the FORS2 instrument at the Unit Telescope 1 (UT1) of the ESO
VLT in April and May 2005. Both globular clusters were ob-
served in three filters, B, R and Hα, with exposure times chosen
to maximize the dynamic range. Table4 provides a condensed

Page 5

C. G. Bassa et al.: X-ray and optical observations of M55 and NGC6366: evidence for primordial binaries5
Table 3. Chandra X-ray sources detected in our observation of NGC6366. The celestial positions of the X-ray sources have been
corrected for the bore-sight correction of +0.??030 in right ascension and −0.??256 in declination. Positional uncertainties are given
in parenthesis and refer to the last quoted digit and are the centroiding uncertainties given by ACIS Extract. They do not include
the uncertainties in the bore-sight correction (0.??091 in right ascension and 0.??082 in declination). The X-ray bands are as defined
in Table2. The first 5 sources are located within the half-mass radius of this cluster and are ordered on 0.5–6.0keV countrate
(Xsoft+ Xhard). The remaining sources are located outside the half-mass radius and are ordered on right ascension. CX1 was also
detected by ROSAT (Johnston et al., 1996; Verbunt, 2001).
IDR.A.
(J2000)
Decl.
(J2000)
Counts (Detected/Corrected)
Xsoft
Xmed
15/32.1
185/280.9
2/7.56/15.0
2/7.75/11.5
0/0.04/10.8
3/12.15/13.4
5/20.115/38.6
2/7.76/15.0
15/54.717/40.9
2/6.78/19.9
1/4.02/5.4
80/207.4
0/0.05/12.7
4/15.16/14.7
29/71.4
fX(0.5–2.5keV)
(ergs−1cm−2)
8.5 × 10−15
4.6 × 10−14
1.4 × 10−15
1.3 × 10−15
5.5 × 10−16
2.7 × 10−15
4.6 × 10−15
1.7 × 10−15
1.5 × 10−14
2.2 × 10−15
1.8 × 10−15
2.9 × 10−14
7.0 × 10−16
5.3 × 10−15
1.0 × 10−14
fX(2.5–6.0keV)
(ergs−1cm−2)
7.0 × 10−16
4.9 × 10−14
2.8 × 10−15
2.4 × 10−15
3.1 × 10−15
3.6 × 10−16
3.7 × 10−15
3.6 × 10−15
1.7 × 10−16
2.2 × 10−15
5.1 × 10−15
1.6 × 10−14
5.2 × 10−15
8.3 × 10−17
5.5 × 10−15
Xhard
5/4.8
111/79.2
5/5.9
3/2.8
4/5.2
2/2.6
10/12.0
6/7.2
2/2.1
6/7.1
2/2.6
45/55.2
6/6.9
2/2.1
13/14.6
CX1a
CX1b (R4) 17h27m42.s78(7)
CX217h27m38.s682(7)
CX317h27m48.s470(19) −05◦02?58.??69(28)
CX417h27m42.s044(12) −05◦03?55.??12(18)
CX517h27m44.s137(12) −05◦04?27.??20(17)
CX617h27m27.s384(8)
CX717h27m28.s726(16) −05◦03?00.??34(24)
CX817h27m29.s883(6)
CX9 17h27m32.s307(7)
CX1017h27m36.s643(17) −05◦02?31.??24(26)
CX1117h27m40.s138(7)
CX1217h27m40.s533(9)
CX1317h27m50.s335(14) −05◦07?10.??27(21)
CX1417h27m52.s830(11) −05◦02?56.??05(16) 16/62.0
17h27m42.s892(7)
−05◦05?05.??28(11) 10/33.2
−05◦05?04.??7(1.1) 87/190.0
−05◦04?47.??63(11)
−05◦04?40.??42(12)
−05◦05?24.??56(8)
−05◦06?56.??56(10)
−05◦01?13.??76(10) 36/141.8
−05◦08?35.??70(14)
Fig.2. (left panel) The adaptively smoothed brightness distribution of NGC6366 CX1 shows a point source and an extended source.
The solid contour contains 90% of the flux in the Chandra point spread function (at the detector location of the source); the dashed
contours indicate the extraction region for the counts of the extended source. (middle panel) Contours from the adaptively smoothed
brightness distribution of CX1 overlaid on the 15minute Hα images. The white diagonal strip is due to the gap between the FORS2
chips, see Fig. 1. There is no evidence for any extended Hα emission. (right panel) The cumulative energy distributions of the point
source (large-stepped distribution) and the extended source. A Kolmogorov Smirnov test gives a probability of 1% that both energy
distributions are the same, and we conclude that the extended source is harder.
log of the observations. FORS2 is a mosaic of two 2k×4k chips
with a pixel scale of 0.??126pix−1. For the majority of the obser-
vations, we used 2 × 2 on-chip binning, providing a pixel scale
of 0.??252pix−1, except when the seeing was below 0.??6, when no
binning was applied.
The images were reduced using the Munich Image Data
Analysis System (MIDAS). All images were bias-subtracted and
flatfieldedusingtwilightflats.Next,wegroupedtheimagesshar-
ing the same combination of chip, filter and exposure time. The
images in each group were aligned using integer pixel offsets
and co-added to remove cosmic rays and increase the signal-to-
noise. Hence, for each cluster we obtained 18 separate stacked
images, 9 for each chip.
Both M55 and NGC6366 are part of a survey of globular
clusters with ACS/WFC on board the Hubble Space Telescope.
M55 was observed for 284s in a V-band filter (F606W) and 384s
in an I-band filter (F814W). For NGC6366 the exposure times
with 570s in both F606W and F814W. Compared to FORS2,
ACS/WFC has a smaller field-of-view (about 3.?4×3.?4) and con-
tains only a few of the X-ray sources. For M55, CX2, CX3, CX8,
CX9, CX11, CX12, CX13 and CX16 are coincident with the
202??× 202??field of view, while for NGC6366 CX1 through
CX5 are coincident.