We present X-ray, infrared, optical and radio observations of four previously unidentified Galactic plane X-ray sources, AX J163252-4746, AX J184738-0156, AX J144701-5919 and AX J144547-5931. Detection of each source with the Chandra X-ray Observatory has provided sub-arcsecond localizations, which we use to identify bright infrared counterparts to all four objects. Infrared and optical spectroscopy of these counterparts demonstrate that all four X-ray sources are extremely massive stars, with spectral classifications Ofpe/WN9 (AX J163252-4746), WN7 (AX J184738-0156 = WR121a), WN7-8h (AX J144701-5919) and OIf+ (AX J144547-5931). AX J163252-4746 and AX J184738-0156 are both luminous, hard, X-ray emitters with strong Fe XXV emission lines in their X-ray spectra at ~6.7 keV. The multi-wavelength properties of AX J163252-4746 and AX J184738-0156 are not consistent with isolated massive stars or accretion onto a compact companion; we conclude that their X-ray emission is most likely generated in a colliding-wind binary system. For both AX J144701-5919 and AX J144547-5931, the X-ray emission is an order of magnitude less luminous and with a softer spectrum. These properties are consistent with a colliding-wind binary interpretation for these two sources also, but other mechanisms for the generation of X-rays cannot be excluded. There are many other as yet unidentified X-ray sources in the Galactic plane, with X-ray properties similar to those seen for AX J163252-4746, AX J184738-0156, AX J144701-5919 and AX J144547-5931. This may indicate a substantial population of X-ray-emitting massive stars and colliding-wind binaries in the Milky Way.
Wolf-Rayet (WR) stars and their O-type supergiant progenitors (Of) evolve from the most massive stars in our Galaxy, with initial masses 25M ⊙ . These evolved stars, particularly WR, have extremely strong stellar winds, experiencing high mass-loss rates of Ṁ ∼ 10 -5 M ⊙ yr -1 and, in some cases, have luminosities > 10 6 L ⊙ (Crowther 2008). Their short lifetimes make them very rare; < 400 WR stars are known in our Galaxy (van der Hucht 2006;Martins et al. 2008;Shara et al. 2009;Mauerhan et al. 2009Mauerhan et al. , 2010)), and are usually only found in the Galactic plane.
Massive stars have historically been discovered through optical and infrared observations and are classified based on their spectral characteristics in these wavebands. However, X-ray observations are now becoming a newly recognized technique for discovering massive stars and are also a powerful tool in assessing their physical environments (e.g. Mauerhan et al. 2010), allowing us to determine if they are isolated or in a high-mass X-ray binary (HMXB) or colliding-wind binary (CWB) system. By discovering more of these massive stars and understanding their emission mechanisms, we can determine how mass-loss drives the different stages of stellar evolution.
The most accepted model for X-ray generation in a single hot star is the instability-driven wind-shock picture, which attributes the production of soft, thermal X-ray emission, with temperatures of kT < 1 keV, to shocks distributed throughout the wind (Lucy & White 1980;Lucy 1982). More exotic models of X-ray generation in isolated massive stars include the magnetically channeled wind-shock mechanism, which was first explored in detail by Babel & Montmerle (1997a,b). In this case a radiatively driven stellar wind is magnetically channeled from the two hemispheres of the star. These streams collide at the magnetic equator and the rapid deceleration causes shock heating resulting in high levels of hard thermal X-ray emission. There is also the possibility of isolated OB supergiant stars producing intrinsic hard non-thermal X-rays through inverse Compton scattering (Chen & White 1991).
Alternatively, the X-ray emission may not be completely intrinsic to the massive star but created through a binary interaction. X-rays can be generated in high-mass X-ray binaries (HMXBs) through gravitational accretion onto a compact object such as a neutron star (NS) or black hole (BH). The two main classes of HMXBs are the Be X-ray binary systems (BeX) and the supergiant X-ray binaries (SGXB). BeXs are accretion fed by a disk around a Be star and are often transient X-ray sources whereas SGXBs are wind-fed, and persistent, X-ray sources (McClintock & Remillard 2006).
Colliding-wind binary (CWB) systems are another class of massive stellar binaries marked by extreme wind loss and highenergy emission. Originally predicted by Prilutskii & Usov (1976) and Cherepashchuk (1976), the supersonic winds from the two massive stars in a binary produce shock-heated gas (Stevens et al. 1992), resulting in hard, thermal X-ray emission (Pittard & Parkin 2010) and possibly γ-rays, likely produced by inverse Compton scattering (e.g. Benaglia & Romero 2003;Pittard & Dougherty 2006;Reimer et al. 2006;De Becker 2007). The detection of this high-energy emission allows us to probe the nature of these shocks and provides a laboratory for investigating particle acceleration in a very different density regime to that in supernova remnants.
In this Paper we discuss our classification of four previously unidentified Galactic X-ray sources; AX J163252-4746, AX J184738-0156, AX J144701-5919 and AX J144547-5931. These sources have been observed with the Chandra X-ray Observatory as part of the “ChIcAGO” (Chasing the Identification of ASCA Galactic Objects) project (Anderson et al. in prep.), a survey designed to localize and classify the unidentified X-ray sources discovered during the ASCA Galactic Plane Survey (AGPS; Sugizaki et al. 2001). ASCA (the Advanced Satellite for Cosmology and Astrophysics) surveyed the inner region of the Galactic plane, detecting 163 X-ray sources with fluxes between 10 -11 and 10 -13 erg cm -2 s -1 in the 0.7 -10.0 keV energy range. Due to the ASCA X-ray telescope’s ∼ 3 ′ spatial resolution, only a third of these X-ray sources have been properly characterized and little is known about the remaining unidentified objects. This unidentified population should contain at least some rare classes of X-ray objects, as modeling by Hands et al. (2004) and Grindlay et al. (2005) has demonstrated that the Galactic populations of cataclysmic variables, bright X-ray binaries, and background Active Galactic Nuclei cannot account for the entire observed flux distribution of X-ray sources detected in the AGPS. In ChIcAGO, we are combining the sub-arcsecond localization capabilities of Chandra with a detailed multi-wavelength follow-up program, with the goal of classifying the > 100 unidentified sources in the AGPS.
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