Iron Line Diagnostics for the GRS 1915+105 Black Hole

Martocchia et al. (2002, Paper I [5]) and Martocchia et al. (2003, Paper II [6]) reported the discovery of intense iron Kα fluorescent emission in two BeppoSAX observations of themicroquasar GRS 1915+105, which took place respectively on April 19-20, 1998, and April 21-22, 2000. During these observations the Fe line is broad and asymmetric, best fitted with a relativistic disc model (see e.g. [2], [8], [4] and Paper I for references on the relativistic line model). These are the only two cases, to our knowledge, in which the iron Kα line is so prominent in this source. The fact that the line is not always such strong in GRS 1915+105 observations should not surprise, since finding the line out of the thermal emission high-energy tail can be awkward in galactic Black Hole (BH) candidates, because of the high disk temperature. Nevertheless, strong, broad, possibly relativistic iron Kα lines have recently been detected in various spectra of galactic BH candidates, e.g. Cyg X-1, XTE J1650500, XTE J1550-564, V4641 Sgr, GRO J165540, XTE J2012+381, GX 339-4, XTE J1748-288, XTE J1908+094. They may help estimating the rotation parameter of the BHs hosted at the center of these sources by means of the iron Kα di-