We present results of a spectroscopic survey of bright near-infrared

counterparts to X-ray point sources from a deep Chandra survey of the Galactic nuclear bulge. K-band spectroscopy has revealed 13 new Wolf-Rayet and

O-supergiant counterparts to Chandra sources in the Galactic center (GC). Although they are systematically softer in X-rays than the general GC source population of accretion powered cataclysmic variables (CVs), their X-ray colors indicate a hard component consistent with emission from plasmas with E > 2 keV. Such hard X-ray emission is not ubiquitous among single Wolf-Rayet and O stars,

but is common among Wolf-Rayet+OB binaries with colliding supersonic winds.

Although we regard colliding-wind binary hypothesis as the most likely scenario,

it remains possible that several of these objects are wind-accreting neutron stars

or black holes in supergiant high-mass X-ray binaries, or extraordinary single

stars emitting hard X-rays

Bauer, Franz E.

Mauerhan, Jon C.

Morris, Mark R.

Muno, Michael P.

Caltech Authors - Main

HOT AND COOL: BRIDGING GAPS IN MASSIVE-STAR EVOLUTION
ASP Conference Series, Vol. 425, c© 2010
C. Leitherer, P. D. Bennett, P. W. Morris, and J. Th. van Loon, eds.
Massive Stellar X-ray Sources in the Galactic Center
Jon C. Mauerhan
Infrared Processing and Analysis Center, California Institute of
Technology, Pasadena, CA 91125, USA; mauerhan@ipac.caltech.edu
Michael P. Muno
Department of Physics and Astronomy, University of California, Los
Angeles, CA 90095-1547, USA
Mark R. Morris
Space Radiation Laboratory, California Institute of Technology,
Pasadena, CA 91125, USA
Franz E. Bauer
Columbia Astrophysics Laboratory, Columbia University, Pupin
Laboratories, 550 West 120th Street, New York, NY 10027, USA
Abstract. We present results of a spectroscopic survey of bright near-infrared
counterparts to X-ray point sources from a deep Chandra survey of the Galac-
tic nuclear bulge. K-band spectroscopy has revealed 13 new Wolf-Rayet and
O-supergiant counterparts to Chandra sources in the Galactic center (GC). Al-
though they are systematically softer in X-rays than the general GC source pop-
ulation of accretion powered cataclysmic variables (CVs), their X-ray colors in-
dicate a hard component consistent with emission from plasmas with E > 2 keV.
Such hard X-ray emission is not ubiquitous among single Wolf-Rayet and O stars,
but is common among Wolf-Rayet+OB binaries with colliding supersonic winds.
Although we regard colliding-wind binary hypothesis as the most likely scenario,
it remains possible that several of these objects are wind-accreting neutron stars
or black holes in supergiant high-mass X-ray binaries, or extraordinary single
stars emitting hard X-rays.
1. Introduction
The nuclear bulge of the Galaxy contains the greatest concentration of X-ray
point sources in the sky (Muno et al. 2009). Although the population is domi-
nated by cataclysmic variables (CVs), there are likely to be ∼100 sources that
are the products of massive-star evolution, including high-mass X-ray binaries
(HMXBs) and colliding-wind binaries (Mauerhan et al. 2009). IR spectroscopy
of bright counterparts to this population are revealing a growing number of
massive stellar X-ray sources, outside of the well known Arches, Quintuplet and
SgrA? star clusters (Muno et al. 2006; Mikles et al. 2006; Mauerhan et al. 2007;
Hyodo et al. 2008). The identification of such objects has important implica-
tions for the mode and recent history of massive star formation in the GC, the
dynamical evolution of GC stellar clusters, and the evolution of massive binaries.
221
222 Mauerhan et al.
Here we present the most recent discoveries of WR/O counterparts to Chandra
X-ray sources in the GC.
2. Sample
A catalog of 9017 X-ray point sources has been assembled from Chandra observa-
tions of the inner 2◦×0.◦8 of the Galaxy, incorporating all ACIS-I observations of
the region as of 2007 September (Muno et al. 2009). Assuming a distance of 8 kpc
to the GC, the catalog is complete down to LX > 10
32 erg s−1 over one square
degree and an order of magnitude more sensitive in the few arcminutes around
SgrA?, where there is 1Msec of exposure. We cross-correlated the Chandra
GC point-source catalog with near-IR catalogs produced by the Simultaneous
3-color Infrared Imager for Unbiased Surveys (SIRIUS) on the 1.4-meter Infrared
Science Facility (Nishiyama et al. 2006), and the Two-Micron All-Sky Survey
(2MASS; Cutri et al. 2003). In order to select Chandra sources at or beyond the
GC, we restricted our sample to sources having soft X-ray colors indicative of
an interstellar absorbing column of hydrogen with NH > 4× 10
22 cm−2. The IR
sample was further restricted to sources having colors with H −Ks > 0.9mag,
consistent with our adopted cut-off value of NH. Further restricting our IR sam-
ple to sources with Ks < 12mag, we produced a list of 261 X-ray/IR matches
of which 61± 30 (2σ) should be genuine, as determined from simulations which
yielded the expected number of random matches between the X-ray and infrared
catalogs (see Mauerhan et al. 2009).
3. Observations
Figure 1. Example K-band spectra of hydrogen-rich (WN7–8h; left panel)
and hydrogen-poor (WN5–7; right panel) WR X-ray sources. Broad emission
lines of He i and He ii dominate the spectra.
X-ray Sources in the Galactic Center 223
Figure 2. Photon flux vs. hard X-ray color (HR2) comparing massive-star
counterparts (large symbols) to the field population of CVs (small points).
Sources associated with the Arches (filled stars with circles) and Quintuplet
(filled stars) clusters are also included for comparison (those from Law &
Yusef-Zadeh 2004). Model tracks of thermal and non-thermal plasma ab-
sorbed by a hydrogen column of NH = 6.0 × 10
22 cm−2 are plotted for com-
parison. The figure indicates that the majority of massive X-ray emitting
stars have X-ray luminosities of LX ≈ 10
32–1034 erg s−1 (0.2–8.0 keV).
NIR Spectra: To determine the nature of these counterparts we have
been performing K-band spectroscopy of candidates at several observational
facilities, including the IRTF, AAT, UKIRT, and Keck. So far we have identified
17 WR/O counterparts to the Chandra hard X-ray sources. The spectral-types
span a wide range of massive-star evolutionary stages, including early to late
O supergiants, Ofpe/WN9, WNh, WNE and WC stars (Mauerhan et al., in
prep.). Figure 1 exhibits example K-band spectra of 6 of the 13 counterparts
discovered. These particular spectra were obtained with UKIRT/UIST and the
AAT/IRIS2, and are of moderate resolution (R ≈ 2000). The example spectra
include hydrogen-rich WN7–8h stars and hydrogen-deficient WN5–7 stars, all
of which exhibit broad emission lines of H i, He ii and N iii. The WN5–7 stars
have particularly broad flat-topped emission lines, indicative of very extended
supersonic winds with v ≈ 2000–3000 km s−1. The spectra of the additional
discoveries, including O supergiants and WC stars, will be presented in a
forthcoming paper (Mauerhan et al., in prep.).
X-ray Photometry : Figure 2 shows a plot of hard X-ray color (HR2)
vs. 2–8 keV photon flux for all massive stellar X-ray sources known in the GC,
224 Mauerhan et al.
including sources from the Arches and Quintuplet clusters. HR2 is defined as
(h− s)/(h+ s), where h and s are the fluxes in the 3.3–4.7 and 4.7–8.0 Chandra
bands, respectively. The massive stars are systematically brighter and slightly
softer in X-rays than the field population dominated by CVs, and in most cases
have plasma energies above 2 keV. The data are consistent with both thermal
and non-thermal emission mechanisms, and indicate that the majority of massive
X-ray–emitting stars have X-ray luminosities of LX ≈ 10
32–1034 erg s−1 (0.2–
8.0 keV). Sources in the Arches cluster have relatively high X-ray luminosities.
4. The Nature of Massive Stellar X-ray Sources in the Galactic
Center
Hard X-ray Emission: The hard X-ray emission (kT > 2 keV) observed for
most of the stars in our sample is not ubiquitous among single WR/O stars.
However, X-rays from WR+OB binaries with colliding, supersonic stellar winds
typically exhibit a hard component. We regard this scenario as the most likely
explanation, given the high binary fraction of massive stars (Mason et al. 1998),
and the inevitable collision of supersonic winds involving systems containing WR
stars and O supergiants. Since the X-ray luminosity of a wind collision zone is
proportional to 1/D (Usov 1992), where D is the binary separation, Fig. 2
might be an indication that the sources of the Arches cluster are colliding-wind
binaries with relatively small separations; this would not be surprising since
dense stellar cluster environments, where gravitational encounters are frequent,
will eventually lead to a hardening of massive close binaries.
However, we cannot yet rule out the possibility that our sample also contains
wind-accreting neutron stars and black holes in HMXBs, in which case the X-
ray emission may be non-thermal. If there are HMXBs in this sample, their
relatively low X-ray luminosities (LX ≈ 10
31–1033 erg s−1) compared with most
known HMXBs (Liu et al. 2006) would imply a low accretion rate. If confirmed,
such HMXBs would help determine the luminosity function of HMXBs at the
low end, which would have implications for models of magnetic resistance to
low-rate accretion of a stellar wind (Liu & Li 2006). Single-star scenarios should
also be considered. O stars with strong magnetic fields can channel supersonic
winds from opposite stellar hemispheres into a self-collision zone near the stellar
equator. Such a model has been developed to explain the hard X-ray emission
from the late-O giant θ1 OrionisC (e.g., see Wade et al. 2006) and was also
considered in the case of the oxygen-rich WR star WR142 (Oskinova et al. 2009),
although the X-ray luminosity in the latter case is over an order of magnitude
fainter than the faintest star in our sample.
Further observations are required to discriminate between the various
scenarios discussed above. Particularly, radial-velocity measurements over
several epochs could reveal the presence of companions through their Doppler
motions, while allowing for a constraint on component masses. We will be
performing such experiments in the near future.
Origins: Figure 3 illustrates the spatial distribution of the massive stellar X-ray
sources in the GC. Given their distribution, the origin and formation mode of
these stars is unclear, as all but several do not appear to be associated with
X-ray Sources in the Galactic Center 225
known stellar clusters in the GC region. It is possible that these objects are
members of stellar clusters that had lower initial masses and densities than the
Arches and Quintuplet, and so have been tidally disintegrated to the point where
they are not detectable as a stellar density enhancement above the crowded
background field (e.g., see Portegies Zwart et al. 2002). It is also possible that
dynamical interaction within clusters may be responsible for the ejection of mas-
sive binaries, in which case a few members from our sample may have actually
originated in the Arches and Quintuplet, even though they are now separated
from the clusters by ∼10 pc. Alternatively, these stars may be evidence for a
mode of isolated massive star formation occurring in the GC region, in tandem
with the formation of dense stellar clusters such as the Arches and Quintuplet.
This may be the case for the six sources concentrated near the SgrB H ii region.
The continued discovery of massive stellar X-ray sources in the GC will give a
more complete picture of their overall spatial distribution, while follow-up obser-
vations aimed at surveying the stellar content of their immediate surroundings
and constraining their kinematics will help elucidate the origins of these massive
hard-X-ray–emitting stars.
SgrB
10’ (~23 pc)
SgrA West
Arches                
Quintuplet            
Figure 3. Spitzer λ8 µm image of the Galactic center region, illustrating
the spatial distribution of massive stellar X-ray sources not within the Arches
or Quintuplet clusters. The positions are approximate.
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Massive Stellar X-ray Sources in the Galactic Center

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Massive Stellar X-ray Sources in the Galactic Center

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