AMI-LA 15.5 GHz observations of radio flaring from the black hole candidate MAXI J1820+070 in transition

ATel #11827; Joe Bright, Sara Motta, Rob Fender (University of Oxford), Yvette Perrott, David Titterington (MRAO, Cambridge)
on 8 Jul 2018; 21:59 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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We report further (Atel #11420) observations of the black hole XRB candidate MAXI J1820+070 with the Arcminute Microkelvin Imager Large Array (AMI-LA). Observations were initiated after reports (ATels #11820, #11823) that the source was rapidly undergoing a state transition. Data were taken at a central frequency of 15.5 GHz, with J1824+1044 and 3C 286 used as the phase and absolute flux calibrator respectively. Date were reduced using the custom software pipeline REDUCE_DC (e.g. Davies et al. 2009, Perrott et al. 2015).

We observed the source intermittently between 18:31:41.4 and 03:05:32.5 (UTC) on 05-Jul-2018/06-Jul-2018, 19:51:31.3 and 04:52:18.9 (UTC) on 06-Jul-2018/07-Jul-2018 and 19:29:38.9 and 04:48:22.9 on 07-Jul-2018/08-Jul-2018. As variability is expected in the radio emission during a state transition we calculate a channel and baseline averaged UV-amplitude coefficient directly from the UV data in 10 minute time intervals (note that there are no other sources in the field). The flux density on the night of 05-July-2018 showed a significant drop from our previous measurement on 02-Jul-2018, falling from ~40 mJy to ~10 mJy. The source then continued to decline over the course of this observation, ending at ~6 mJy. The following night (06-July-2018) we observe a radio flare, with the flux density at the start of the night at ~15 mJy, rising to ~40 mJy over a ~3.5 hour period, and then decaying back to ~15 mJy over ~4.5 hours, at which point the source was no longer observable. On the final night of our observations (07-July-2018) we measure a flux density of ~1.5 mJy and see no major variation over the course of the observation.

These observations, showing significantly quenched radio emission and radio flaring, are consistent with the reports of a state transition by the NICER team.

We plan on continued monitoring of this source and would like to thank the Mullard Radio Astronomy Observatory staff for carrying out these observations.