Simultaneous Radio and X‐Ray Observations of the Wolf‐Rayet Star WR 147

Abstract

We present results of simultaneous ASCA/Very Large Array (VLA) observations of the Wolf-Rayet star WR 147 (AS 431). This WN 8 star is an optical double and may be a WR+OB colliding wind binary system. The new observations place tight constraints on the origin of its X-ray and radio emission. The X-ray emission is due to a multitemperature, optically thin thermal plasma, with the dominant contribution coming from plasma at kT ≈ 1 keV. The absorption column density derived from the X-ray spectrum is NH = 2 × 1022 cm-2, which agrees well with estimates based on the visual extinction but is too large to explain by wind absorption alone. The X-ray temperature structure is consistent with colliding wind shock emission, but the unabsorbed X-ray luminosity Lx = 1032.55 ergs s-1 (0.5-10 keV) is several times smaller than that predicted from colliding wind shock models. The VLA data provide the most complete picture ever obtained of the radio spectral energy distribution of a WR star and consist of near-simultaneous observations at five different wavelengths (1.3, 2, 3.6, 6, and 21 cm). The radio emission consists of a thermal free-free component from the WR wind and a nonthermal component. If the nonthermal emission is due to relativistic particles accelerated by the Fermi mechanism in wind shocks, then the flux is expected to decline at high frequencies according to Sν ∝ ν-0.5. However, the observed falloff is much steeper and cannot be reproduced by a simple power law or by synchrotron models that assume power-law electron energy distributions. A surprising result is that the nonthermal emission can be accurately modeled as synchrotron radiation from relativistic electrons that are nearly monoergetic.