A NuSTAR Observation of MAXI J0637-430: A New X-ray Transient and Likely Black Hole X-ray Binary

ATel #13270; John A. Tomsick (UCB/SSL), Javier Garcia (Caltech), Andy Fabian (Cambridge), Dom Walton (Cambridge), Jiachen Jiang (Tsinghua), Felix Fuerst (ESA), Douglas Buisson (Southampton), Aarran Shaw (UNR), Jeremy Hare (GSFC), Matteo Bachetti (INAF), Riley Connors (Caltech), Posahk Gandhi (Southampton), Yanjun Xu (Caltech)
on 6 Nov 2019; 19:26 UT
Credential Certification: John A. Tomsick (jtomsick@ssl.berkeley.edu)

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The first detection of X-ray emission from MAXI J0637-430 was on 2019 Nov. 2 (Negoro et al., ATEL 13256). It brightened over the next day and was localized by Swift, being detected by XRT and UVOT (Kennea et al., ATEL 13257). Strader et al. (ATEL 13260) obtained an optical spectrum, which exhibited hydrogen and helium emission lines with properties typical of a low-mass X-ray binary.

An observation of the source with NuSTAR started on Nov. 5 at 14.4h UT, and we analyzed the first 11 ks of data from the observation, using both NuSTAR instruments (FPMA and FPMB). The continuum of the 3-79 keV energy spectrum is dominated by a thermal disk-blackbody component plus a power-law, which is similar to what was seen by Swift/XRT (ATEL 13257). However, with NuSTAR's broader bandpass and larger number of counts, the quality of the spectrum is higher, and this model gives a reduced chi2 of 1.35 (455 degrees of freedom). There are positive residuals in the FeK region and above 20 keV that may be due to presence of disk reflection. Although adding a reflection component causes the reduced chi2 to decrease to 1.09 and flattens the residuals, it is not entirely clear that reflection is a unique interpretation, and other models will also be tested in the future.

Based on a model with a disk-blackbody, a power-law, and a reflection component, the inner disk temperature is 0.628+/-0.004 keV, and the power-law photon index is 2.40+/-0.04 (90% confidence errors). The 1-100 keV flux is 4.8e-9 erg/cm2/s, and the 2-10 keV flux is 1.9e-9 erg/cm2/s, corresponding to a flux of ~95 mCrab.

Although we have not yet performed a detailed timing analysis, the count rate does not show strong variability over the first 11 ks (4 NuSTAR orbits) of the observation. The spectral and timing properties are consistent with an accreting black hole in the soft state. While we do conclude that this source is likely a black hole transient, it is unusual for such sources to enter the soft state so quickly. More often, such systems are in the hard state for at least the first several days of their outbursts.

We thank the NuSTAR operations teams (the SOC at Caltech and the MOC at UC Berkeley's Space Sciences Laboratory) for support with the execution of these observations.