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

I. Timing analysis

¹ Dipartimento di Fisica, Università La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy e-mail: enrico.massaro@uniroma1.it
² Stazione Astronomica di Vallinfreda, via del Tramonto, Vallinfreda (RM), Italy
³ INFN – Sezione di Roma1 (retired), Roma, Italy
⁴ INAF, Istituto di Astrofisica Spaziale e Fisica cosmica di Roma, via del Fosso del Cavaliere 100, 00113 Roma, Italy
⁵ INAF, Istituto di Astrofisica Spaziale e Fisica cosmica di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
⁶ Astronomical Institute, University of Amsterdam, The Netherlands
⁷ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy

Received: 17 July 2009
Accepted: 1 December 2009

Abstract

GRS 1915+105 was observed by BeppoSAX  for about 10 days in October 2000. For about 80% of the time, the source was in the variability class ρ, characterised by a series of recurrent bursts. We describe the results of the timing analysis performed on the MECS (1.6–10 keV) and PDS (15–100 keV) data. The X-ray count rate from GRS 1915+105 showed an increasing trend with different characteristics in the various energy bands: in the bands (1.6–3 keV) and (15–100 keV), it was nearly stable in the first part of the pointing and increased in a rather short time by about 20%, while in the energy range (3–10 keV) the increase had a smoother trend. Fourier and wavelet analyses detect a variation in the recurrence time of the bursts, from 45–50 s to about 75 s, which appear well correlated with the count rate. From the power distribution of peaks in Fourier periodograms and wavelet spectra, we distinguished between the regular and irregular variability modes of the ρ class, which are related to variations in the count rate in the 3–10 keV range. We identified two components in the burst structure: the slow leading trail, and the pulse, superimposed on a rather stable level. Pulses are generally structured in a series of peaks and their number is related to the regularity modes: the mean number of peaks is lower than 2 in the regular mode and increases up to values higher than 3 in the irregular mode. We found that the change in the recurrence time of the regular mode is caused by the slow leading trails, while the duration of the pulse phase remains far more stable. The evolution in the mean count rates shows that the time behaviour of both the leading trail and the baseline level are very similar to those observed in the 1.6–3 and 15–100 keV ranges, while that of the pulse follows the peak number. These differences in the time behaviour and count rates at different energies indicate that the process responsible for the pulses must produce the strongest emission between 3 and 10 keV, while that associated with both the leading trail and the baseline dominates at lower and higher energies.