XMM-Newton Observations Reveal Very High X-ray Luminosity from the Carbon-rich Wolf-Rayet Star WR 48a

TL;DR

XMM-Newton observations of WR 48a reveal it as the most X-ray luminous Wolf-Rayet star in the Galaxy, with thermal emission likely from colliding winds or accretion onto a compact object, but its exact nature remains uncertain.

Contribution

First X-ray detection of WR 48a showing high luminosity, suggesting possible colliding winds or accretion scenarios in a binary or triple system.

Findings

X-ray luminosity ~ 10^35 erg/s, the highest for Wolf-Rayet stars.

Thermal emission with dominant hot component at ~3 keV.

No significant short-term X-ray variability detected.

Abstract

We present XMM-Newton observations of the dusty Wolf-Rayet star WR 48a. This is the first detection of this object in X-rays. The XMM-Newton EPIC spectra are heavily absorbed and the presence of numerous strong emission lines indicates a thermal origin of the WR 48a X-ray emission, with dominant temperature components at kT_cool approx. 1 keV and kT_hot approx. 3~keV, the hotter component dominating the observed flux. No significant X-ray variability was detected on time scales < 1 day. Although the distance to WR 48a is uncertain, if it is physically associated with the open clusters Danks 1 and 2 at d ~ 4 kpc, then the resultant X-ray luminosity L_X ~ 10^(35) ergs/s makes it the most X-ray luminous Wolf-Rayet star in the Galaxy detected so far, after the black-hole candidate Cyg X-3. We assume the following scenarios as the most likely explanation for the X-ray properties of WR 48a: (1) colliding stellar winds in a wide WR+O binary system, or in a hierarchical triple system with non-degenerate stellar components; (2) accretion shocks from the WR 48a wind onto a close companion (possibly a neutron star). More specific information about WR48a and its wind properties will be needed to distinguish between the above possibilities.