MAXI J1820+070 drops to its pre-outburst quiescent level after 5 years of activity

ATel #16192; M. Cristina Baglio, David M. Russell, Kevin Alabarta, Payaswini Saikia, D. M. Bramich (NYU Abu Dhabi), Jeroen Homan (Eureka Scientific), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 16 Aug 2023; 06:47 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The black hole X-ray binary MAXI J1820+070 was discovered in 2018 thanks to the detection of a bright outburst at optical (with All-Sky Automated Survey for Supernovae, ASAS-SN) and X-ray frequencies (with MAXI/GSC; ATel #11399). Since the discovery outburst, the source underwent three re-brightenings, that were extensively monitored at all frequencies (see e.g. ATel #12573, #12569, #12577). The last report of optical activity was in April 2021, thanks to the monitoring performed with the Las Cumbres Observatory (LCO) network of telescopes and the Al Sadeem Observatory (ATe l#14582), which, however, only showed low-level activity, also observed at UV and X-ray frequencies, and also reported in the radio (Atel#15277).

We have kept on monitoring the system regularly with the 1m and 2m optical telescopes of the LCO network, as part of an ongoing monitoring program of ~50 low mass X-ray binaries (LMXBs; Lewis et al. 2008). All images are analyzed with the real-time data analysis pipeline "X-ray Binary New Early Warning System"(XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details).

After the low-level activity observed in 2021, the source maintained an approximately constant level of flux, with average magnitudes of i'=17.13+/-0,01, g'=17.76+/-0.01. However, the magnitudes were still significantly brighter than the ones reported in the PanStarrs catalog for a faint object at a position that is coincident with that of the source (i'= 18.039 +/- 0.101, g'=19.381 +/- 0.073; ATel #11418), suggesting that quiescence was still to be reached.

On MJD 60106 (11 June 2023), we observed a significant decrease in the optical flux, with magnitudes z_s=17.97+/-0.01, i'=18.22+/-0.01, r'=18.58+/-0.01, g'=19.31+/-0.02 (previous observations performed on MJD 60098 have magnitudes z_s=17.12+/-0.01, i'=17.34+/-0.01, r'=17.55+/-0.01, g'=18.08+/-0.01).
The flux decrease is confirmed by observations performed in the later weeks. The faintest flux detected corresponds to mags z_s=18.11+/-0.01, i'=18.42+/-0.01, r'=18.81+/-0.01, g'=19.64+/-0.03 on MJD 60161 (5 August 2023), which are slightly fainter than the magnitudes reported in the PanStarrs catalog, suggesting that the source has finally reached quiescence after 5 years of continuous activity.

We will keep on monitoring MAXI J1820+070 with LCO regularly during quiescence.
Additionally, we have an upcoming Swift DDT observation to investigate if the X-ray luminosity has decreased as well.

Light curve of MAXI J1820+070