MAXI J1820+070 is close to quiescence

ATel #12534; David M. Russell, Maria Cristina Baglio (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 27 Feb 2019; 05:31 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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MAXI J1820+070 (ASASSN-18ey) is a black hole candidate X-ray binary that had its first detected outburst in 2018 (e.g. ATel #11399, #11418, #11423). We have been monitoring the optical counterpart (ATel #11418, #11533, #12128) with the Las Cumbres Observatory (LCO) 1- and 2-m (Faulkes) telescopes (g', r', i', y bands), and with the Meade LX850 16-inch (41-cm) telescope of the Al Sadeem Observatory (UAE). In September 2018 the source was fading and performed a transition from the soft state to the hard state (ATel #12057, #12061, #12064, #12068). Since an optical brightening of the source over the soft to hard transition (ATel #12128), the source continued to fade (ATel #12157). We observed a fade of more than two magnitudes, from g' = 14.2, i' = 13.9 on 2018 Oct 14 (MJD 58405) to g' = 16.4, i' = 16.1 (errors are <0.1 mag) on 2018 Nov 26 (MJD 58448), before becoming Sun constrained.

Since 2019 Feb 5 (MJD 58519) we have continued our LCO optical monitoring of MAXI J1820+070. It is fainter than it was in November, with mean magnitudes in February (most recently on 2019 Feb 26; MJD 58540) of g' ~ 18.3, i' ~ 17.4 (errors on individual magnitudes are 0.05 - 0.2 mag depending on conditions). This is approximately 0.6 - 1.1 magnitudes above the pre-outburst quiescent level of g' = 19.4, i' = 18.0 as measured by PanStarrs. There is no clear increasing/decreasing trend over the last 3 weeks. We detect strong variability, with the i'-band magnitude spanning a range between i' = 16.64 +- 0.13 and 18.32 +- 0.16.

These observations imply that MAXI J1820+070 is probably ending its outburst, which lasted almost a year. There is evidence for low level accretion activity, and the source is most likely in the hard state at a very low luminosity. Multi-wavelength observations during decay to quiescence are rare but useful (e.g. Plotkin et al. 2017, ApJ, 834, 104) and so are encouraged, as well as observations in quiescence.

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.

MAXI J1820+070 light curves