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

II. Time-resolved spectral analysis

¹ INAF, Istituto di Astrofisica Spaziale e Fisica cosmica di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
e-mail: mineo@iasf-palermo.inaf.it
² Dipartimento di Fisica, Università La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy
³ Dipartimento di Fisica, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
⁴ Stazione Astronomica di Vallinfreda, via del Tramonto, Vallinfreda (RM), Italy
⁵ INFN-Sezione di Roma1 (retired), Roma, Italy
⁶ INAF, Istituto di Astrofisica Spaziale e Fisica cosmica di Roma, via del Fosso del Cavaliere 100, 00113 Roma, Italy
⁷ School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
⁸ ADSC, Data Centre for Astrophysics, Chemin d’Écogia 16, 1290 Versoix, Switzerland
⁹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy

Received: 30 May 2011
Accepted: 17 October 2011

Abstract

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 ρ 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 KT_el, τ, KT_in, 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.