Optical/X-ray Flux Decoupling in MAXI J1820+070

ATel #11574; Amanda Townsend, Sarik Jeram, Stephen Eikenberry, Yigit Dallilar, Alan Garner, Amy Gottlieb, Connor McClellan, Gustavo Perez Sanchez, Rhiannon Robshaw, David Rosenbaum, Alma Spahic (University of Florida)
on 25 Apr 2018; 21:50 UT
Credential Certification: Stephen Eikenberry (eiken@astro.ufl.edu)

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We report optical monitoring of the black hole candidate X-ray binary MAXI J1820+070 (e.g. ATEL #11399, #11418, #11420) using the University of Florida Rosemary Hill Observatory 14-inch telescope. We used a DMK CCD in white light (roughly corresponding to Gaia G-band) to obtain 17 epochs of data with 1.1Hz cadence for several hours per epoch, beginning on 28 March 2018 UT (MJD 58205). In addition to fast variability signatures (see also ATEL #11421, #11423, #11426, #11432, #11437, #11451, #11510), we have analyzed a subset of the data to study the long-timescale optical behavior in the outburst. We analyzed aperture photometry of 10-minute averages of the 1.1Hz data over several nights. We find that the optical flux initially tracked the fading of the X-rays, as seen on long timescales by the Swift Burst Alert Telescope (BAT), fading slowly from a magnitude of G=11.7 mag on MJD 58205. However, the optical flux appeared to decouple from the X-ray lightcurve over the time period MJD 58221 to 58227 (see figure attached). During this time period, the Swift BAT 15-50 keV X-ray flux decreased smoothly by about 10%, while the optical flux dropped sharply by about 0.35 mag (35%) in G, followed by a sharp increase in flux/variability. This apparent decoupling does not seem to match simple X-ray reprocessing models for producing the dominant optical flux during the outburst. However, the reported fast optical/IR variability (see references above) seems to indicate that the optical flux may arise in the inner regions of a jet, as seen in other sources such as V404 Cyg (Gandhi et al., 2016; Gandhi et al., 2017; Dallilar et al., 2017). The jet scenario would also be consistent with the decoupling of the X-ray/optical fluxes we observe. If this is the case, the relatively fast fading and instability in the optical flux may indicate that the jet in MAXI J1820+070 is weakening and/or becoming unstable, while the X-ray flux continues to slowly and smoothly fade on long timescales.

ATEL #11539 report fading of the radio flux (which may also have a jet origin), which seems to roughly correlate with the Swift/BAT X-ray fading. However, in systems such as V404 Cyg in its 2015 outburst, the radio flux was seen to probe a much larger, steadier region of the jet as compared to the optical/IR flux arising from the inner jet (i.e. Dallilar et al., 2017).

The authors thank T. Maccarone for pointing out to us how very bright this source had become, and the University of Florida for its support of the Rosemary Hill Observatory.

Swift/BAT data on MAXI J1820+070 were taken from the Swift/BAT transient monitor (Krimm et al., 2013).

RHO + Swift Optical/X-ray Lightcurve of MAXI J1820+070