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 soucres. Although they are systematically softer in X-rays than the general Galactic center source population of accretion powered CVs, their X-ray colors indicate a hard component consistent with emission from thermal plasma with kT>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

Massive Stellar X-ray Sources in the Galactic Center
Jon C. Mauerhan
Department of Physics and Astronomy, University of California, Los Angeles, CA 90095;
mauerhan@astro.ucla.edu
Michael P. Muno
Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91104;
mmuno@srl.caltech.edu
Franz E. Bauer
Columbia Astrophysical Laboratory, New York, NY 10027
Mark R. Morris
Department of Physics and Astronomy, University of California, Los Angeles, CA 90095
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 Galactic nuclear bulge. K-band spectroscopy
has revealed 13 new Wolf-Rayet and O-supergiant counterparts to Chandra soucres. Although they
are systematically softer in X-rays than the general Galactic center source population of accretion
powered CVs, their X-ray colors indicate a hard component consistent with emission from thermal
plasma with kT>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 has been observed by the Chandra X-ray Observatory
on numerous occasions over the last decade, producing a catalog of X-ray points greater in
number than anywhere else in the Milky Way by an order of magnitude. Although dominated
by cataclysmic variables (CVs) with faint infrared (IR) counterparts (Ks > 18 mag; Laycock
et al. 2005), IR spectroscopy of bright counterparts to the X-ray population has revealed
a growing number of massive stellar X-ray sources near the Galactic center (GC), outside
of the well known Arches, Quintuplet and Sgr A∗ star clusters (Muno et al. 2006, Mikles
et al. 2006, Mauerhan et al. 2007, Hyodo et al. 2007). The X-ray emission from these
sources has been interpreted as being due to Wolf-Rayet (WR) and O stars in colliding-wind
binaries (CWBs), wind-accreting neutron stars or black holes in high-mass X-ray binaries
(HMXBs), or extraordinary single WR/O stars. The identification of objects such as these
has important implications for the mode and recent history of massive star formation in
the GC, the tidal disruption of GC stellar clusters, the evolution of massive binaries, and
the role of magnetic fields in controlling accretion at low M˙ and channeling the supersonic
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winds from O stars. Here we present the most recent discoveries of WR/O counterparts to
Chandra X-ray sources in the GC.
Figure 1. K-band spectra of 8 out of the 13 WR/O stars identified as counterparts to
Chandra sources in the Galactic center (obtained at the IRTF, UKIRT, and Keck). The
counterparts span the range of post-main-sequence evolution for massive stars, including
O-supergiants, and Wolf-Rayet stars of subtype WNE, WNL, and WC.
2. Observations and Results
A catalog of over 10,000 X-ray point sources has been assembled from Chandra observations
of the inner 2◦ × 0.8◦ of the Galaxy, incorporating all ACIS-I observations of the region
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as of 2007 September (Muno et al. in prep). 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 Sgr A∗, where there is 1 Msec
of exposure. We cross-correlated the Chandra GC point-source catalog with NIR 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
Galactic center, we restict our sample to sources having soft X-ray colors indicative of an
interstellar absorbing column of hydrogen with NH > 4 × 1022 cm−2. The IR sample was
restricted to sources having colors with J −H > 2.3 mag and H −Ks > 1.1 mag, consistent
with our adopted cutoff value of NH . Further restricting our IR sample to sources with
Ks < 12, we produced a list of 125 X-ray/IR matches of which 39±8 should be genuine, as
determined from Monte Carlo simulation.
To determine the nature of these counterparts we obtained K-band spectra of 39 stars,
of which 13 were discovered to have WR/O counterparts, in excellent agreement with our
predicted success rate. This work brings the total number of known X-ray-emitting WR/O
stars in the Galactic center to 18, outside the Arches and Quintuplet clusters. For the
stars of this work, spectral-types were determined by comparison with spectra of previously
identified Galactic WR/O stars in the literature (Hanson et al. 1996, Figer et al. 1996).
Figure 1 exhibits K-band spectra of 8 of the 13 counterparts discovered, spanning the range
of massive star post-main-sequence evolution, from O-supergiants to nitrogen-type WR stars
(WN) to carbon-type WR stars (WC). Broad emission lines of H at 2.16 µm, and HeI and
NIII near 2.12 µm dominate the spectra of transitional O supergiants, while HeII emission
near 2.19 µm emerges in the helium-burning WR stars. The emission lines of the WC9
stars of our sample are extremely diluted, most likely due to continuum emission from dust.
The WN6 and WN8 have very broad lines of H, HeI, HeII, and NIII/CIII, indicative of a
supersonic wind with v ≈2000-3000 km s−1. The flat-topped shape of the lines indicates
that the wind is extended and optically-thin.
2.1. X-ray Properties
Figure 2 shows a plot of hard X-ray color vs. 2-8 keV photon flux. Hard color 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 vertical lines locate the hard color values expected for thermal plasmas
of various temperatures, absorbed by NH = 6 × 1022 cm−2. The WR/O counterparts are
systematically softer in X-rays than the general Galactic center field population presumed
to be accretion powered CVs. However, their X-ray colors do indicate the presence of a hard
component with kT ≈ 2− 6 keV.
3. Origin of the 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. Given the high binary
fraction observed among massive stars (Mason et al. 1998), we regard the CWB hypothesis
as the most likely. However, we cannot yet rule out the possibility that our sample also
contains wind-accreting neutron stars and black holes in HMXBs. However, if there are
HMXBs in this sample, their low X-ray luminosities (Lx ≈ 1031−33 erg s−1) compared with
known HMXBs would imply a relatively low accretion rate. It has been suggested that the
propeller effect produced by a neutron star magnetosphere could prevent the accretion of a
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Mauerhan; Massive Stellar X-ray Sources in the Galactic Center 4           Origin of hard X-ray emission
CIII
H
H
rays from confirmed WR/O stars consistent with thermal plasma
• Counterparts softer in X-rays relative to field CVs
• But harder in X-rays than single massive stars (kT>2keV)
• Consistent with colliding-wind binary interpretation
• Single-star scenario: oblique magnetic rotators
 WR/O
 field
Figure 2. Hard X-ray color vs. flux for the WR/O counterparts to handra sources
in the Galactic center. In general, the WR/O stars are softer in X-rays than the field
population of CVs, but nonetheless exhibit evidence for hard X-ray emission from a
kT > 2 keV plasma.
wind at low M˙ , which would make quiescent HMXBs in the Galactic center undetectable
in X-rays (Liu & Li 2006). Single star scenarios also remain possible. 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 Orionis C (Wade et al. 2006).
In conclusion, if these WR/O sources are members of CWBs, higher signal-to-noise
spectra at medium to high resolution could reveal the spectral signature of a lower luminosity
OB companion. Furthermore, multi-epoch spectroscoptic monitoring for velocity variations
could also confirm the presence of companions and constrain their masses. These future
data will discriminate the various models for hard X-ray emission and elucidate the nature
of these objects.
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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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