Speaker
Description
Following on two ISSI-BJ workshops organized by Tomaso Belloni and Dipankar Bhattacharya in 2017 & 2018, with the title “Understanding multi-wavelength rapid variability: accretion and jet ejection in compact objects”, we developed AstroSat anticipated ToO proposals to observe a state transition in a known black hole X-ray binary (BHXB) transient. Three separate, but identical, proposals were submitted, each of them choosing 5 different system, for either Indian, Canadian or International time. The plan was to trigger the ToO observations when any one of the 15 went into an outburst and reached a flux level of >0.4 Crab in both the hard (>0.4 keV) and soft (<0.4 keV) bands. During the 2018 workshop we submitted the three proposals for the A05 call for a total stare time of 864 ksec, with LAXPC as the primary instrument. The same proposals were submitted for subsequent annual calls, but it was not until March 2023 (A12) that one of the targets reached the required threshold to trigger, namely 4U 1630−47. Here we present a spectral and temporal analysis of both the AstroSat and contemporaneous NICER observations of the outburst. We observed a continuous flux evolution of the source, allowing us to probe variability across different flux states. We employ a simultaneous modeling of the power density spectra (PDS), the real and imaginary parts of the cross-spectrum (CS) to identify variability features. We found strong Type-C QPOs (∼3.6–6.1 Hz) with high fractional rms amplitude. Subsequently, a quasi-regular modulation (QRM) at 30 mHz emerges, likely linked to radiation-pressure instabilities in the inner disk. At higher flux, two simultaneous QPOs are detected at 4.6 Hz and 13 Hz. The ∼13 Hz QPO shows a strong correlation with disk flux, suggesting a close connection to the inner accretion flow, while the lower frequency component (∼4.6 Hz) likely arises from a distinct mechanism. As the outburst progresses, ∼4.6 Hz disappeared and a ∼ 12.2 Hz QPO appeared in place of ∼ 13 Hz QPO in the PDS. We also observed a bimodal distribution in the rms and phase lags energy spectra of these components. These results indicate the coexistence of multiple variability mechanisms in the system.