XB-NEWS detects a new optical rise during the current outburst of MAXI J0637-430

ATel #13454; Maria Cristina Baglio, D. M. Bramich, David M. Russell, Payaswini Saikia, Saarah F. Pirbhoy (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 6 Feb 2020; 11:49 UT
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Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The black hole candidate MAXI J0637-430 was discovered by the MAXI X-ray telescope in 2019 November (ATel #13256). The optical counterpart of the system was first detected by Swift/UVOT (ATel ##13257) and then confirmed by other ground-based observatories (ATel #13260, #13276, #13278, #13291). On January 14 (MJD 58862), a significantly lower X-ray count rate and a harder spectrum was reported by NICER (ATel #13427). The harder spectral shape, together with a decreasing flux, has continued until January 25 (MJD 58873), when the source transitioned back to the hard state.

We have been monitoring the source outburst since its start with Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes. For the analysis of the LCO data, we are making use of a new real-time data analysis pipeline, the ''X-ray Binary New Early Warning System (XB-NEWS; see Russell et al. 2019 and Pirbhoy et al. 2020, ATel #13451 for details).

We started our monitoring on 5 Nov 2019 (MJD 58792), soon after the first detection of the outburst with MAXI, and the outburst was at its peak, with the following calibrated magnitudes: i'=16.75+-0.01, g'=16.23+-0.01, r'=16.59+-0.01, z'=16.92+-0.03. After that, the outburst started its decay, reaching a minimum flux at around MJD 58871 (January 23; g'=17.54+-0.01, i'=17.96+-0.02, r'=17.80+-0.01, z'=18.14+-0.03). Soon after this, the source started a slow re-brightening, reaching in our last observations (4 Feb 2020, MJD 58883) the following magnitudes: g'=17.32+-0.01, i'=17.68+-0.02, r'=17.57+-0.01, z'=17.71+-0.05. Interestingly, the start of this optical rise happened at approximately the same time as the transition to the hard state reported in ATel #13427 (i.e. MJD 58873). The observed brightening is higher in z' (0.43+-0.06) than in g' (0.22+-0.01), suggesting the recovery of the jet after the transition to the hard state observed with NICER.
The brightening has not reached its peak yet. The optical monitoring of the source with LCO is still ongoing. Further multi-wavelength observations are encouraged in order to confirm the re-brightening and to determine its nature.

The LCO observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. We aim to announce any new outbursts of these sources detected by XB-NEWS in near real-time at the onset of the outburst, and any other light curve morphological changes worth reporting. This work makes use of observations from the Las Cumbres Observatory (LCO). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical LCO light curves of MAXI J0637-430