Revealing the nature of the transient source MAXI J0637-430 through spectro-temporal analysis

Abstract

We study the spectral and temporal properties of MAXI J0637-430 during its 2019-2020 outburst using \textit{NICER}, \textit{AstroSat} and \textit{Swift-XRT} data. The source was in a disc dominant state within a day of its detection and traces out a `c' shaped profile in the HID, similar to the `mini'-outbursts of the recurrent BHB 4U 1630-472. Energy spectrum is obtained in the $0.5-10$ keV band with \textit{NICER} and \textit{Swift-XRT}, and $0.5-25$ keV with \textit{AstroSat}. The spectra can be modelled using a multicolour disc emission (\textit{diskbb}) convolved with a thermal Comptonisation component (\textit{thcomp}). The disc temperature decreases from 0.6 keV to 0.1 keV during the decay with a corresponding decrease in photon index ($\Gamma$) from 4.6 to 1.8. The fraction of Compton scattered photons ($f_{cov}$) remains $<$ 0.3 during the decay upto mid-January 2020 and gradually increases to 1 as the source reaches hard state. Power Density Spectra (PDS) generated in the 0.01-100 Hz range display no Quasi-periodic Oscillations (QPOs) although band-limited noise (BLN) is seen towards the end of January 2020. During \textit{AstroSat} observations, $\Gamma$ lies in the range $2.3-2.6$ and rms increases from 11 to 20\%, suggesting that the source was in an intermediate state till 21 November 2019. Spectral fitting with the relativistic disc model (\textit{kerrbb}), in conjunction with the soft-hard transition luminosity, favour a black hole with mass $3-19$ $M_{\odot}$ with retrograde spin at a distance $<15$ kpc. Finally, we discuss the possible implications of our findings.