The complex behaviour of the microquasar GRS 1915+105 in the ρ class observed with BeppoSAX

Context. BeppoSAX observed GRS 1915+105 on October 2000 with a long pointing lasting about ten days. During this observation, the source was mainly in the rho class characterized by bursts with a recurrence time of between 40 and 100 s. Aims. We identify five segments in the burst structure and accumulate the average spectra of these segments during each satellite orbit. We present a detailed spectral analysis aimed at determining variations that occur during the burst and understanding the physical process that produces them. Methods. We compare MECS, HPGSPC, and PDS spectra with several models. Under the assumption that a single model is able to fit all spectra, we find that the combination of a multi-temperature black-body disk and a hybrid corona is able to give a consistent physical explanation of the source behaviour. Results. Our measured variations in KTel, tau, KTin, and R-in appear to be either correlated or anti-correlated with the count rate in the energy range 1.6-10 keV. The strongest variations are detected along the burst segments: almost all parameters exhibit significant variations in the segments that have the highest fluxes (pulse) with the exception of R-in, which varies continuously and reaches a maximum just before the peak. The flux of the multi-temperature disk strongly increases in the pulse and simultaneously the corona contribution is significantly reduced. Conclusions. The disk luminosity increases in the pulse and the R-in - T-in correlation can be most successfully interpreted in term of the slim disk model. In addition, the reduction in the corona luminosity during the bursts might represent the condensation of the corona onto the disk.