XB-NEWS detects a new outburst from the black hole X-ray binary MAXI J1820+070

ATel #14492; M. C. Baglio, D. M. Russell, P. Saikia, D. M. Bramich (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 29 Mar 2021; 17:58 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The black hole X-ray binary MAXI J1820+070 was detected on 2018 March 11 by the MAXI/GSC nova alert system (ATel #11399), and later identified as the optical transient ASASSN-18ey, that was discovered a few days earlier, on 2018 March 6 (ATel #11400). A quiescent optical counterpart was also found in the Pan-STARRS catalogue (g'=19.38 +/- 0.07; i'=18.04 +/- 0.13).

The activity of the source lasted for more than 2 years, comprising four outbursts (e.g. ATel #11399, #11418, #11439, #11533, #11820, #11827, #12057, #12128, #12534, l#12567, #12596, #13014, #13025, #13041, #13502, #13530). The outbursts were monitored with the 1m and 2m optical telescopes of the Las Cumbres Observatory (LCO) network (see e.g. ATel #11418, #12128, #12534), as part of an ongoing monitoring of ~50 low mass X-ray binaries (LMXBs; Lewis et al. 2008). All images are analyzed thanks to a real-time data analysis pipeline, the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details).

Our monitoring shows that MAXI J1820+070 went back to quiescence after the last outburst around ~ 2020 June 29 (MJD 59029). However, the XB-NEWS g' and i' magnitudes were still ~0.6 and ~0.3 mags (respectively) brighter than the known quiescent magnitude of the source.

MAXI J1820+070 has been in quiescence since then, with an average magnitude of i'=17.65 +/- 0.16, R=17.83 +/- 0.17, V=18.51+/- 0.24, which is slightly fainter than the level reached after the previous outbursts.
On 2021 March 28 (MJD 59301) XB-NEWS detected an increase in the optical flux, with the following magnitudes: g'=17.77+/- 0.11, r'=17.32+/- 0.08, i'=17.12+/- 0.09, V=17.72 +/- 0.01, R=17.13 +/- 0.01. In i'-, R- and V-bands, the filters for which the monitoring was regularly performed during the last 9 months, the magnitude recently changed as follows:
i'= 17.92 +/- 0.02 , R= 18.13 +/- 0.02, V=18.82 +/- 0.04 on MJD 59280 (2021 March 7)
i'= 17.55 +/- 0.01, R= 17.56 +/- 0.01, V= 18.18 +/- 0.01 on MJD 59293 (2021 March 20)
i'= 17.26 +/- 0.01, R=17.13 +/-0.01, V=17.72 +/- 0.01 on MJD 59300 (2021 March 27)
i'=17.12 +/- 0.09 on MJD 59301 (2021 March 28).

Considering these values, it is likely that the optical flux started to rise between MJD 59280 (2021 March 7) and 59293 (2021 March 20).
The increase in flux is more evident at shorter wavelengths (we measure a magnitude difference of 1.02+/-0.11 mag, 0.65+/-0.08 mag, 0.66+/-0.09 mag in g'-, r'-, i'-band, respectively). This suggests there is a significant increasing contribution of the accretion disc, which could be an indicator of a new outburst.

No renewed activity has been observed in the X-rays so far.

Multi-wavelength observations are encouraged in the next few days/weeks to confirm the nature of this renewed activity, and to measure the possible delay between optical and X-rays.

The LCO observations are part of an on-going monitoring campaign of ~ 50 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Light curve of MAXI J1820+070