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X-ray properties of the microquasar GRS 1915+105 during a variability class transition

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Abstract. We present a detailed X-ray study of the microquasar GRS 1915+105 during a variability class transition observed in 2000 June with the Pointed Proportional Counters (PPCs) of the Indian X-ray Astronomy Experiment (IXAE). We supplement this observation with data from the RXTE archives. The source made a transition from a steady low—hard state to a regular oscillatory behaviour in the light curve known as bursts or class ρ (variability classes from Belloni et al.) between 2000 May 11 and 17 and reverted back to the low—hard state on 2000 June 27. A gradual change in the burst recurrence time from about 75 s to about 40 s was observed, which then increased to about 120 s during the ∼40 d of class ρ. The regular bursts disappeared from the X-ray light curves and the class transition was observed to occur within 1.5 h on 2000 June 27 with the PPCs. A correlation is found between the observed quasi-periodic oscillation frequency at 5-8 Hz in the quiescent phase and the average X-ray intensity of the source during the class ρ. We notice a strong similarity between the properties of the source during the class ρ and those during the oscillatory phase of the observations of class α, which contains a long stretch (∼1000 s) of steady low—hard state in the light curve along with the regular periodic bursts. From the timing and spectral analysis, it is found that the observed properties of the source over tens of days during the class ρ are identical to those over a time-scale of a few hundreds of seconds in the class α. Examining the light curves from the beginning of the RXTE/PCA and RXTE/ASM observations, it is found that the change of state from radio-quiet low—hard state to high state occurs through the X-ray classes ρ and α, which appear together during the state transition. It is further inferred that the source switches from low—hard state to the class ρ through the intermediate class α.

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