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Optical observations of MAXI J1820+070 confirm the rebrightening

ATel #12596; Maria Cristina Baglio, Dave M. Russell (NYU Abu Dhabi), Thabet Al Qaissieh, Alejandro Palado, Aldrin Gabuya (Al Sadeem Observatory, Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 19 Mar 2019; 17:00 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

Subjects: Optical, Binary, Black Hole, Transient

Referred to by ATel #: 12608, 12747, 12988, 13066, 14492

The black hole candidate X-ray binary MAXI J1820+070 (ASASSN-18ey) was first detected in outburst in March 2018 (ATel#11399). The outburst lasted for approximately eight months, before starting its decline towards quiescence (ATel #12157). However, according to optical observations, the quiescent level has probably never been reached by the source since last year's outburst (ATel#12534).
Very recently (between 2019-03-09 and 11), a rebrightening of the target has been detected at optical frequencies, the V-band magnitude increasing from a faint value (V=17.2) up to V=15.4 (ATel#12567). The Swift satellite showed a rebrightening also in the X-rays on 2019-03-12, together with a 1.1 magnitude increase in brightness at UV frequencies (UM2 filter, ATel#12573). Finally, joint radio (AMI-LA) and X-rays (Swift) observations performed on 2019-03-13 and 2019-03-14 showed a strong brightening (more than one order magnitude) in both X-rays and radio over 1 day (ATel#12577).

We have been monitoring the source during the outburst since its start using 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; for details see ATel#12128). In our most recent observations, we observe the target brightening from R=17.2 on March 04 to R = 15.6 on March 9th, which allows us to measure the time in which the rebrightening started between March 4th and March 9th, in agreement with the results reported in ATel#12567.

Here follow our most recent optical magnitudes, obtained with LCO and the Al Sadeem observatory telescope:

2019-03-15: R=14.25+/-0.01
2019-03-18: g'=14.05+/-0.01, r'=13.88+/-0.02, i'=13.68+/-0.02, y=13.48+/-0.03,

which show that the rebrightnening is still ongoing. The source is still 2 magnitudes fainter in the g' band than its maximum brightness during the outburst, which it reached on MJD 58207 (30th March 2018). We measure a rise rate of 0.23 mags/day in the R-band, according to our Al Sadeem telescope light curve. The last observations also show a fairly varying light curve on minutes time-scales in the R-band, with a fractional rms of a few per cent.
The optical monitoring will continue in order to determine the nature of the rebrightening. Further multi-wavelength observations are therefore encouraged.

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).

MAXI J1820+070 light curves