New analysis of the ρ-class bursts, known as the “heartbeat” of GRS 1915+105: Pulse profile and spectral properties

¹ INAF, IASF Palermo, via U. La Malfa 153, 90146 Palermo, Italy
e-mail: teresa.mineo@inaf.it
² INFN, Sezione Roma 1, Piazzale A. Moro 2, 00185 Roma, Italy
³ INAF, IAPS, via del Fosso del Cavaliere 100, 00113 Roma, Italy

Received: 28 December 2020
Accepted: 30 March 2021

Abstract

Context. We present the results of a new analysis of three long Rossi-XTE observations of the microquasar GRS 1915+105 in the ρ class, performed in 1997, 1999, and 2000, and characterized by different peak profiles. The first data set, labeled G-1, is dominated by a single peak, while in the third observation (G-3), all bursts show a clearly detectable couple of peaks. The second observation (G-2) shows an intermediate structure with a single peak and an emerging shoulder on the decay side.

Aims. We devised a new procedure to obtain mean burst profiles in every energy channel independently of the recurrence time intervals of the bursts, variable from 45 s to 53 s in the considered observations, with the aim of investigating the different features of peaks and the eventual spectral variations.

Methods. All the bursts were aligned at a common time bin on the decaying portion of the bursts that is stable in simultaneous light curves at different energies. An averaging algorithm was then applied without modifying the statistical properties or scaling the burst lengths. We analyzed the peak amplitude ratios and the dependence of their delays on energy. The spectral distributions were evaluated for the various components: a stable multi-temperature disk plus a power law Comptonization component was used for the baseline emission and temperature differences of peak components were evaluated with the inclusion of an additional blackbody.

Results. In addition to the well-observed double peak (P1 and P2) pattern, we detected a third small peak (P3) in the structured G-3 light curve. This peak, differently from the other two, exhibits a fast rising and a slower exponential decay, with a e-folding time constant of 1.32 s. The blackbody temperatures of P2 and P3 are higher than P1 and the power law spectrum of P3 is the flattest one.

Conclusions. The time and spectral behavior of P3 is interpreted as a signature of a relatively hot plasma outflow from the disk into the corona and its duration is consistent with the crossing timescale of the particles through the corona where electrons radiate.