Comparing the ρ and χ class spectra of the microquasar GRS 1915+105 observed with BeppoSAX

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
e-mail: mineo@ifc.inaf.it
² INAF, Istituto di Astrofisica e Planetologia Spaziali, via del Fosso del Cavaliere 100, 00113 Roma, Italy
³ INFN, Sezione Roma 1, Piazzale A. Moro 2, 00185 Roma, Italy

Received: 6 September 2016
Accepted: 17 November 2016

Abstract

Context. BeppoSAX observed GRS 1915+105 during two variability classes at the same 2–10 keV flux level. The ρ class is characterized by quasi-periodic flares recurring on a time-scale of 1 to 2 min, namely heartbeat, while the χ class is characterized by no strong temporal variability.

Aims. The aim of this work is to coherently analyze the source spectrum in these two classes and to gain insight into the source conditions that inset the heartbeats.

Methods. A single χ spectrum was accumulated, while ρ data were split in runs where five phase-resolved spectra were selected. In addition to the multicolor disc black body, the fitting model includes a hybrid Comptonization plus a Compton reflection component.

Results. Our results show that the emission in the ρ class is dominated by the multi-temperature disk, while in the χ class the Comptonised component is dominant. The disk temperature varies between a maximum of 1.95 ± 0.11 keV reached at the peak and a minimum of 0.95^+0.08_-0.20 keV in the χ class. In both classes we detect a significant contribution from a non-thermal electron population to the total Comptonized emission. A broadened iron emission line is detected in the χ spectrum. We interpret the line shape as being due to reflection from an accretion disk extremely close to the black-hole (R_in = 4.1^+1.5_-0.9 gravitational radii), with an equivalent width of 200 ± 20 eV. Concomitantly, upper limits of ~150 eV can be derived from the ρ spectra.

Conclusions. In the framework of coupled disc-corona models, these results point out that the source emission is strongly affected by the fractions of accretion energy distributed between the disk, the corona, and possibly the wind, with no indication on the conditions that inset the heartbeats.