Optical Observations of MAXI J0637-430

ATel #13278; F. J. Hambsch, G. Myers, L. A.G. Monard, J. Patterson (Center for Backyard Astrophysics)
on 7 Nov 2019; 14:15 UT
Credential Certification: Jules Halpern (jules@astro.columbia.edu)

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During 14 hours over 5 observing sessions spanning 3 nights (2019 November 3-5), we obtained time-series photometry of the optical counterpart of the new X-ray transient MAXI J0637-430 (discovered on 2019 November 2, Negoro et al. ATel #13256), with telescopes in Chile, Australia, and South Africa. The measurements were obtained through V and "clear" (4000-7000 A) bands. The star showed irregular variations of ~0.1 mag on a timescale of a few minutes, averaging V = 16.3 on all nights and at each longitude. The measured astrometric position was RA 06:36:23.69, Decl -42:52:04.0 (J2000.0), with an estimated error of 0.4 arcsec. The spread in observatory longitudes limits variations on several-hour to several-day timescales to <0.1 magnitudes. We note for comparison that the large 17-hour "superhump" (quasi-orbital, modulated by disk precession) signal of the 2018 X-ray nova MAXI J1820+070 = ASASSN-18ey did not appear in visual light until day 75 of the outburst.

From the NuSTAR observation reported by Tomsick et al. (ATel #13270), the F_X(2-10 keV)/F_V(5000-6000 A) ratio is ~1000, typical of accreting black holes and neutron stars in their bright states, and compared to ~1 for disk-accreting white dwarfs (dwarf novae).