I. INTRODUCTION

GRS 1915+105 is a superluminal jet source, well-known since its discovery by the WATCH experiment on GRANAT (Castro-Tirado et al. 1992). The source distance is about 10 kpc, and its inclination 70 degrees. It shows properties very similar to those of the other famous microquasar GRO J1655-40, for which accurate dynamical measurements yielded M ~ 7 solar masses (Orosz & Bailyn 1997; Shahbaz T. et al. 1999). Both sources are thus believed to host a black hole (BH). The value of the BH spin can been estimated via QPO frequencies with general-relativistic (GR) effects taken into account (Cui W. et al. 1998), and by discussing the source X-ray brightness in its soft state (Zhang et al. 1997).

GRS 1915+105 exhibits a strong, broad (EW > 200 eV) and asymmetric emission line around 6.4 keV in the spectrum observed by BeppoSAX on April 19, 1998. This is the only case, to our knowledge, in which the iron K alpha fluorescent line is so prominent in this source. Out of so many satellite data available for this source, no other observation shows such an intense Fe K alpha emission, and the EW value is normally confined between 20 and 40 eV. The most natural interpretation for such a broad line is emission from a relativistic accretion disc (Fabian et al., 1989; see Tanaka et al., 1995, for the famous case of MCG-6-30-15). Thus, for the first time we can employ the K alpha line diagnostics to test the BH assumption in GRS 1915+105.