The complex behaviour of the microquasar GRS 1915+105 in the ρ class observed with BeppoSAX. III. The hard X-ray delay and limit cycle mapping

Context. The microquasar GRS1915+105 was observed by BeppoSAX in October 2000 for about ten days while the source was in ρ mode, which is characterized by a quasi-regular type I bursting activity. Aims: This paper presents a systematic analysis of the delay of the hard and soft X-ray emission at the burst peaks. The lag, also apparent from the comparison of the [1.7-3.4] keV light curves with those in the [6.8-10.2] keV range, is evaluated and studied as a function of time, spectral parameters, and flux. Methods: We apply the limit cycle mapping technique, using as independent variables the count rate and the mean photon rate. The results using this technique were also cross-checked using a more standard approach with the cross-correlation methods. Data are organized in runs, each relative to a continuous observation interval. Results: The detected hard-soft delay changes in the course of the pointing from ~3 s to ~10 s and presents a clear correlation with the baseline count rate.