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

We study the spectral and temporal properties of MAXI J0637-430 during its 2019-2020 outburst using Neutron Star Interior Composition Explorer (NICER), AstroSat , and 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 NICER and Swift-XRT, and 0.5-25 keV with AstroSat. The spectra can be modelled using a multicolour disc emission (DISKBB) convolved with a thermal Comptonization component (thcomp). The disc temperature decreases from 0.6 to 0.1 keV during the decay with a corresponding decrease in photon index (Γ) from 4.6 to 1.8. The fraction of Compton-scattered photons (f_cov) remains <0.3 during the decay upto 2020 mid-January and gradually increases to 1 as the source reaches hard state. Power density spectra generated in the 0.01-100 Hz range display no quasi-periodic oscillations, although band-limited noise is seen towards the end of 2020 January. During AstroSat observations, Γ lies in the range 2.3-2.6 and rms increases from 11 to 20 per cent, suggesting that the source was in an intermediate state till 2019 November 21. Spectral fitting with the relativistic disc model (kerrbb), in conjunction with the soft-hard transition luminosity, favour a black hole with mass $3\!-\!19\, \mathrm{ M}_{\odot }$ with retrograde spin at a distance <15 kpc. Finally, we discuss the possible implications of our findings.