The peculiar spectral evolution of the new X-ray transient MAXI J0637-430

We studied the transient Galactic black hole candidate MAXI J0637-430 with data from Insight-HXMT, Swift, and XMM-Newton. The broad-band X-ray observations from Insight-HXMT help us constrain the power-law component. MAXI J0637-430 is located at unusually high Galactic latitude; if it belongs to the Galactic thick disc, we suggest a most likely distance less than or similar to 7 kpc. Compared with other black hole transients, MAXI J0637-430 is also unusual for other reasons: a fast transition to the thermal dominant state at the start of the outburst; a low peak temperature and luminosity (we estimate them at approximate to 0.7 keV and less than or similar to 0.1 times Eddington, respectively); a short decline time-scale; a low soft-to-hard transition luminosity (less than or similar to 0.01 times Eddington). We argue that such properties are consistent with a small binary separation, short binary period (P similar to 2 h), and low-mass donor star (M-2 similar to 0.2 M-circle dot). Moreover, spectral modelling shows that a single disc blackbody component is not a good fit to the thermal emission. Soft spectral residuals, and deviations from the standard L-disc proportional to T-in(4) relation, suggest the need for a second thermal component. We propose and discuss various scenarios for such component, in addition to those presented in previous studies of this source. For example, a gap in the accretion disc between a hotter inner ring near the innermost stable orbit, and a cooler outer disc. Another possibility is that the second thermal component is the thermal plasma emission from an ionized outflow.

Automated summary

In this study, we investigated the transient Galactic black hole candidate MAXI J0637-430 using data from Insight-HXMT, Swift, and XMM-Newton. We found that MAXI J0637-430 exhibits unusual characteristics compared to other black hole candidates, including a rapid transition to the thermal dominant state, low peak temperature and luminosity, short decline time-scale, and low soft-to-hard transition luminosity. These properties suggest a small binary separation, short binary period, and low-mass donor star for MAXI J0637-430. Additionally, spectral modelling revealed the need for a second thermal component to explain the thermal emission.