extremely variable, unlike any other known X-ray source (see Remillard & Morgan 1998). GRS 1915+105 was observed extensively with the Proportional Counter Array (PCA) on board RXTE: from these data, the variability of the source appears even more extreme (Greiner, Morgan & Remillard 1996). Quasi-periodic oscillations with frequencies ranging from 0.001 to 67 Hz have been observed (Morgan, Remillard & Greiner 1997, Chen, Swank & Taam 1997). A subset of observations, showing a remarkably regular behavior, has been presented by Taam, Chen & Swank (1997) and Vilhu & Nevalainen (1998), with different interpretations. Other observations did not show spectacular variability (see Trudolyubov, Churazov & Gilfanov 1999). The source is suspected to host a black hole because of its similarity with the other Galactic superluminal source GRO J1655-40 (Zhang et al. 1994), for which a dynamical estimate of the mass is available (Bailyn et al. 1995) and because of its very high X-ray luminosity.

Belloni et al. (1997a,b) showed, from the analysis of the energy spectra of selected observations, that the large variability of the X-ray flux of GRS 1915+105 can be interpreted as the appearing/disappearing of a detectable inner region of the accretion disk, caused by the onset of thermal-viscous 