Fading of MAXI J0637-430 towards quiescence

ATel #13779; Angad Johar, David M. Russell, Maria Cristina Baglio, D. M. Bramich, Payaswini Saikia (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 4 Jun 2020; 11:33 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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The X-ray transient source MAXI J0637-430 was first discovered by the MAXI/GSC nova alert system on 2nd November 2019 (ATel #13256). It was classified as an X-ray binary system, with the optical counterpart first detected by Swift/UVOT (ATel #13257), and later confirmed by ground-based observatories (ATel #13260, #13276, #13278, #13291). Optical spectroscopy performed by the Goodman Spectrograph on the SOAR telescope found strong evidence that the source was a low mass X-ray binary in outburst (ATel #13260). Spectral and timing properties assessed from data of the NuSTAR X-ray telescope suggested that the source is a candidate black hole transient that entered the soft state at an unusually quick rate (ATel #13270).

Here we report on observations of an optical fading of the source over the last 3 months, towards quiescence. We have been monitoring MAXI J0637-430 in optical bands with the Las Cumbres Observatory (LCO) 1- and 2-m (Faulkes) telescopes (in g', r', i', z' bands). The analysis of the LCO data are performed with 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 near its peak. After that, the outburst started its decay, reaching a minimum flux around 23 Jan 2020 (MJD 58871). Soon after this, the source started a slow re-brightening, peaking around 4 Feb 2020 (MJD 58883; ATel #13454).

From 5 Feb 2020 (MJD 58884) to 6 Mar 2020 (MJD 58914), the magnitude showed slight variability with a steady rate of fading, from magnitudes of g'=17.34+-0.01, i'=17.74+-0.01 on 5 Feb 2020 to g'=17.60+-0.01, i'=17.94+-0.04 on 6 Mar 2020. More sparse observations after this show a sharper rate of decline, fading to g'=18.24+-0.01, i'=18.36+-0.01 by 25 Apr 2020 (MJD 58964). The next observation of the source occurred after a gap of 9 days, with the brightness of the source dropping by ~3 mag to g'=20.97+-0.18, i'=20.43+-0.18 on 4 May 2020 (MJD 58973). The source then re-flared, reaching g'=19.61+-0.04, i'=19.74+-0.105 on 8 May 2020 (MJD 58977), then declined again to g'=21.09+-0.14, i'=20.72+-0.10 on 16 May 2020 (MJD 58985). Since then, observations have resulted in non-detections, with conservative upper limits of i' >> 19.2 on 30 May 2020 (MJD 58999), and z' >> 18.3 on 31 May 2020 (MJD 59000).

The magnitudes g'=21.1, i'=20.7 are the faintest yet observed from the source since its discovery. The quiescent magnitude of MAXI J0637-430 is unknown, so it is uncertain if the source has reached the quiescent level. Further, multi-wavelength observations are encouraged, especially deep X-ray observations. The source will be Sun-constrained for ground-based optical telescopes for the next few months.

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. 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