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Optical brightening of MAXI J1820+070 over the soft to hard transition observed with LCO and the Al Sadeem Observatory

ATel #12128; Maria Cristina Baglio, Dave Russell (NYU Abu Dhabi), Thabet Al Qaissieh, Alejandro Palado, Aldrin Gabuya (Al Sadeem Observatory, Abu Dhabi), Hinna Shivkumar (Paris-Sorbonne University of Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 18 Oct 2018; 14:22 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

Subjects: Optical, Binary, Black Hole, Transient

Referred to by ATel #: 12157, 12534, 12567, 12596, 12608, 12829, 13014, 14492, 14582

We report on optical observations of the transient black hole binary (BHB) candidate MAXI J1820+070 (ASASSN-18ey), in outburst since March 2018 (ATel #11399, #11400).
Since the outburst started, we have been monitoring the field of the target in the optical using the Las Cumbres Observatory (LCO) 1- and 2-m telescopes (g', r', i', y bands), and with the Meade LX850 16-inch (41-cm) telescope of the Al Sadeem Observatory (UAE), using the 'blue', 'green' and 'red' Baader LRGB CCD-Filters (that approximately correspond to the central wavelengths of the g', V and R filters, respectively). The optical counterpart was always well detected, allowing us to follow the evolution of the outburst at these wavelenghts, and to compare it to the X-rays (see ATel #11418, #11533).

After the detection of the hard-to soft transition in the X-rays (MJD 58300; ATel #11820), the optical fluxes followed a decreasing trend with time, similarly to what is observed in the soft X-rays.

Around September 22nd (MJD 58383), a significant increase in the hard X-rays has been reported by the MAXI team (ATel #12057), suggesting for a transition to the hard-intermediate state (see also ATel #12064). After the transition, MAXI J1820+070 was detected in the radio on MJD 58384 by the Arcminute Microkelvin Imager Large Array (AMI-LA; Atel #12061), with an integrated flux of 910+/-70 uJy. This radio detection is a clear indication that the jet has reactivated in the radio at the passage from the soft to hard-intermediate state.

At the time of the radio detection, the optical fluxes that we obtained from our monitoring kept on decreasing, reaching the minimum flux on MJD 58396 (a delay in the optical flux rise with respect to the radio due to the reactivation of the jet is typically observed in BHBs; see Kalemci et al. 2013, ApJ, 779, 95). We report here the corresponding magnitudes of the target, obtained with the LCO 1-m telescope, calibrated against a group of PanStarrs bright stars in the field:

g'=14.62 +/- 0.01
i'=14.55 +/- 0.05
r'=14.64 +/- 0.02
y=14.57 +/- 0.04

The first signs of the reactivation of the jet at optical frequencies happened to be on October 12th (MJD 58403). Here follow the magnitudes (Al Sadeem Telescope) on that epoch, showing the increase of the flux:

R = 14.08+/- 0.03
g' = 14.16 +/- 0.02

Here also follow the AB magnitudes of MAXI J1820+070 obtained with the LCO two days later, on MJD 58405 (October 14th), that are still consistent with a rise of the optical flux:

g'=14.17 +/- 0.01
i'=13.94 +/- 0.04
r'=14.06 +/- 0.02
y=13.72 +/- 0.03

Besides the increased optical flux, we also notice that the color of the source has changed, with the spectrum becoming more red. This is a strong evidence of the reactivation of the jet also at optical frequencies.

Our last Al Sadeem Observatory observations, moreover, are consistent with a flattening of the light curves, with the following measured magnitudes:

R = 13.86+/- 0.02
g' = 14.19 +/- 0.04

This suggests that the flux is now peaking and may start to decay.

Further multiwavelength observations are strongly encouraged to confirm the peak and decay of the optical flux.