AstroSat and MAXI view of the black hole binary 4U 1630−472 during 2016 and 2018 outbursts

Abstract

ABSTRACT We present an in-depth spectral and timing analysis of the black hole binary 4U 1630−472 during 2016 and 2018 outbursts as observed by AstroSat and MAXI. The extensive coverage of the outbursts with MAXI is used to obtain the hardness intensity diagram (HID). The source follows a ‘c’-shaped profile in agreement with earlier findings. Based on the HIDs of previous outbursts, we attempt to track the evolution of the source during a ‘super’-outburst and ‘mini’-outbursts. We model the broad-band energy spectra (0.7–20.0 keV) of AstroSat observations of both outbursts using phenomenological and physical models. No Keplerian disc signature is observed at the beginning of 2016 outburst. However, the disc appears within a few hours after which it remains prominent with temperature (Tin) ∼ 1.3 keV and increase in photon index (Γ) from 1.8 to 2.0, whereas the source was at a disc dominant state throughout the AstroSat campaign of 2018 outburst. Based on the HIDs and spectral properties, we classify the outbursts into three different states – the ‘canonical’ hard and soft states along with an intermediate state. Evolution of rms along different states is seen although no quasi-periodic oscillations are detected. We fit the observed spectra using a dynamical accretion model and estimate the accretion parameters. Mass of the black hole is estimated using inner disc radius, bolometric luminosity, and two-component flow model to be 3–9 M⊙. Finally, we discuss the possible implications of our findings.