AMI radio detection of the black hole candidate MAXI J1820+070 during the soft to hard transition.

ATel #12061; Joe Bright, Sara Motta, Rob Fender (University of Oxford)
on 25 Sep 2018; 16:10 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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The transient black hole candidate ASASSN-18ey/MAXI J1820+070 (ATel #11820) was discovered by MAXI in March 2019, when it entered the outburst phase (Kawamura et al. ATel #11399, Denisenko et al. ATel. #11400). After an extended hard-state, the source rapidly transitioned to the soft state on MJD 58300 (Homan et al. ATel #11820).

Following the report of a rapid increase in the hard X-ray of MAXI J1820+070, observed by MAXI/GSC and reported by the MAXI team (MAXI alerts and ATel #12057), we triggered the Arcminute Microkelvin Imager Large Array (AMI-LA).

We observed the position of MAXI J1820+070 at 15.5 GHz. Our observation began on 24-Sept-2018/14:16:01.6 and had a total duration of 7 hours. We used J1824+1044 and 3C 286 as the phase and absolute flux calibrator, respectively. Data analysis was carried out as described in Bright et al. ATel #11420. MAXI J1820+070 is clearly detected by AMI-LA, with an integrated flux of 910 +/- 70 uJy (uncertainties including the calibration error). The rms noise of the image was ~30 uJy.

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