Modeling the low-state spectrum of the X-ray nova XTE J1118 + 480
Abstract
Based on recent multiwavelength observations of the new X-ray nova XTE J1118 + 480, we can place strong constraints on the geometry of the accretion flow in which a low/hard-state spectrum, characteristic of an accreting black hole binary, is produced. We argue that the absence of any soft blackbody-like component in the X-ray band implies the existence of an extended hot optically thin region, with the optically thick cool disk truncated at some radius Rtr ≳ 55RSchw. We show that such a model can indeed reproduce the main features of the observed spectrum: the relatively high optical to X-ray ratio, the sharp downturn in the far-UV band, and the hard X-ray spectrum. The absence of the disk blackbody component also underscores the requirement that the seed photons for thermal Comptonization be produced locally in the hot flow, e.g., via synchrotron radiation. We attribute the observed spectral break at ∼ 2 keV to absorption in a warm, partially ionized gas.