MAXI 1820+070 has completed the decline from the recent optical re-brightening following the soft to hard transition

ATel #12157; U. Munari and L. Zampieri (INAF Padova), P. Ochner (Univ. Padova and INAF Padova), and F. Manzini (ANS Collaboration)
on 2 Nov 2018; 15:22 UT
Credential Certification: Luca Zampieri (luca.zampieri@oapd.inaf.it)

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We are continuing to regularly collect spectra and UBVRI photometry of MAXI 1820+070 with the Asiago 1.22m, 1.82m and Schmidt 67/92 telescopes, and ANS Collaboration telescope ID 2300, in coordination with optical timing observations carried out by the AQUEYE+IQUEYE Collaboration (ATel #11723, #11824, #11936). MAXI 1820+070 (= ASASSN-18ey) has been proposed to be a black hole X-ray binary by Baglio et al. (ATel #11418), following the discovery of the source by Kawamuro et al. (ATel #11399) on 2018 March 11, and is being intensively monitored worldwide over the whole range of the electromagnetic spectrum (e.g. ATel #12061, #11887, #11855, #11833, #11831, #11578, #11490). After the hard to soft transition in the X-ray band reported by Homan et al. (ATel #11820) on July 5, a reverse transition from the soft to hard state was observed starting at around September 22 (Negoro et al., ATel #12057). With a certain delay after this transition, also the optical emission of MAXI 1820+070 started to change. Baglio et al. (ATel #12128) reported a re-brightening at optical wavelengths taking place after October 5, delayed by almost two weeks from the increase in the hard X-rays and the subsequent radio detection (indicating the likely reactivation of the jet; Bright et al., ATel #12061). Here we confirm the results of Baglio et al. and provide further information on the evolution of the optical emission during October 2018. Averaging over the short time-scale periodicities displayed by the object, the mean brightness and colors of MAXI 1820+070 in our data are given in the table reported below. Following the soft to hard transition of September 22, the brightness decline became faster and the source reached a sharp minimum centered on Oct 3.8 UT, while keeping the same blue colors it had before. The recovery from this sharp minimum was accompanied by the colors becoming significantly redder, with a new maximum being reached around Oct 12.0 UT. MAXI 1820+070 then entered a smooth decline phase characterized by stable colors, redder than those in the pre-brightening phase. In our last observing run for Oct 30.8 UT, effectively marking the conclusion of the October re-brightening event, we measured a magnitude B=15.02 and colors B-V=+031, V-R=+030. The main lasting effect of the October evolutions is then a change in the optical colors from a median value of B-V=+0.16 and V-R=+0.18 before re-brightening, to B-V=+0.30 and V-R=+0.30 during and after it.