Downscattering Due to Wind Outflows in Compact X‐Ray Sources: Theory and Interpretation

A number of recent lines of evidence point towards the presence of hot,out-flowing plasma from the central regions of compact Galactic andextragalactic X-ray sources. Additionally, it has long been noted that many ofthese sources exhibit an ``excess'' continuum component, above 10 keV, usuallyattributed to Compton Reflection from a static medium. Motivated by thesefacts, as well as by recent observational constraints on the Compton reflectionmodels - specifically apparently discrepant variability timescales for line andcontinuum components in some cases -- we consider possible effects ofout-flowing plasma on the high-energy continuum spectra of accretion poweredcompact objects. We present a general formulation for photon downscatteringdiffusion which includes recoil and Comptonization effects due to divergence ofthe flow. We then develop an analytical theory for the spectral formation insuch systems that allows us to derive formulae for the emergent spectrum.Finally we perform the analytical model fitting on several Galactic X-raybinaries. Objects which have been modeled with high-covering-fraction Comptonreflectors, such as GS1353-64 are included in our analysis. In addition, CygX-3, is which is widely believed to be characterized by dense circumstellarwinds with temperature of order 10^6 K, provides an interesting test case. Datafrom INTEGRAL and RXTE covering the 3-300 keV range are used in our analysis.We further consider the possibility that the widely noted distortion of thepower-law continuum above 10 keV may in some cases be explained by thesespectral softening effects.Comment: 16 pages, 5 figures, accepted for publication in the Astrophysical Journal and scheduled for 1 December 2004, vol 616 issu