Study of Timing Evolution from Nonvariable to Structured Large-amplitude Variability Transition in GRS 1915 + 105 Using AstroSat

In this work, we present a ∼90 ks continuous monitoring of the Galactic microquasar GRS 1915 + 105 with AstroSat when the source undergoes a major transition from a nonvariable, χ class (similar to radio-quiet χ class) to a structured, large-amplitude, periodic heartbeat state (similar to ρ class). We show that such a transition takes place via an intermediate state when the large-amplitude, irregular variability of the order of hundreds of seconds in the soft X-ray band turned into 100-150 s regular, structured, nearly periodic flares. The properties of strong low-frequency (LF) quasi-periodic oscillation (QPO) in the frequency range 3-5 Hz also evolve marginally during these variability transitions. We also study time-lag and rms spectra at the QPO and harmonic component and the dynamic power spectra. We note a few important differences between the heartbeat state and the ρ class. Interestingly, the time-averaged LF QPO properties in the hard X-ray band are relatively stable in three states when compared to the significant evolution observed in the slow variability properties at millihertz frequencies. Such relative stability of LF QPOs implies that the inner disk-corona coupled accretion flow, which determines the LF QPO properties, may be uninterrupted by the launch of long, large-amplitude flares.