Transition of MAXI J0637-430 to the Hard State Seen with NICER

ATel #13427; R. Remillard, D. Pasham (MIT), K. Gendreau, Z. Arzoumanian (NASA/GSFC), J. Homan (Eureka Scientific & SRON), D. Altamirano (University of Southampton), J. Steiner (SAO)
on 30 Jan 2020; 05:13 UT
Credential Certification: Ron Remillard (rr@space.mit.edu)

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MAXI J0637-430 has been classified as an X-ray binary system on the basis of double-peaked optical emission lines at H and HeII (ATel #13257) and the high value (~1000) of the X-ray to optical flux ratio (ATel #13278). Deconvolution of the X-ray spectrum from NuSTAR (ATel #13270) suggested that the source is likely a black hole binary that rather quickly settled into a thermal-dominant soft X-ray state. NICER has been observing MAXI J0637-430 every 1-2 days, on average, beginning the day after its discovery with MAXI on 2019 November 2 (ATel #13256).  From November 3 until 2020 January 10, the spectrum was very soft, and the flux variability was atypically low, with power density spectra showing integrated continuum power < 1%.  The NICER 0.4-12 keV count rate peaked on November 6, at about half the flux of the Crab Nebula, and a smooth decline was seen at a rate slightly slower than an exponential curve with e-folding time of 21 days.  The NICER hard color (HC), here defined as the ratio of 4-12 keV source counts relative to 2-4 keV, remained constant at roughly HC ~ 0.05. Then, on January 14 NICER measured a significantly lower count rate (factor of 3 from January 10) and a harder spectrum, with HC in the range 0.2-0.3. This harder spectral shape, with gradually decreasing flux, has continued through 10 additional intervals ending on January 25. During this time, the light curves in 1 s bins show significant source flickering, and the power density spectra show a power continuum (no features) with integrated rms (0.1 to 10 Hz) near 10%. We conclude that MAXI J0637-430 has entered the hard state, and we strongly encourage additional observations at radio and optical wavelengths.