Spectro-temporal analysis of the black hole X-ray binary 4U 1630-472 using NICER

Abstract

Here we present the timing analysis of the NICER-XTI observations of black hole X-ray binary (XRB) 4U 1630-472 during its 2018, 2020, and 2021 outbursts. The excellent time resolution of NICER-XTI (100 ns) allowed us to study the variations in the X-ray flux for this source with great precision. 4U 1630-472 is a Galactic black hole (M∼10M⊙) with a binary companion which is a Be-type star and it is situated roughly 11 kpc away. We did not observe a typical Q-shape or C-shape evolution in the Hardness Intensity Diagram (HID) of the three outbursts, however, the source still showed significant spectral evolution. The transient black hole X-ray binary shows Low-frequency Quasi-Periodic Oscillations (QPOs) in the range 0.1-4 Hz. The central frequency of the QPO varies from outburst to outburst, and the QPOs in the 2021 outburst even show the expected evolution of the central frequency with the count rate during the initial rising phase. Our study aims to probe the emission variability behavior of the black hole XRBs, its evolution with accretion rate, and the connection with the various radiative components.