AMI-LA observations of the candidate gamma-ray burst GRB221009A/Swift J1913.1+1946

ATel #15653; Joe S. Bright, Lauren Rhodes, Rob P. Fender (University of Oxford), David A. Green (University of Cambridge)
on 10 Oct 2022; 12:06 UT
Credential Certification: Joe Bright (joe.bright@physics.ox.ac.uk)

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We observed the field of the candidate gamma-ray burst GRB221009A (Swift J1913.1+1946; ATel #15650, ATel #15651) with the Arcminute Microkelvin Imager Large-Array (AMI-LA; Zwart+2008, Hickish+2018) at 15.5 GHz beginning at UT 16:25:25.5 on 09-Oct-2022 (approximately 2 hours 15 minutes after the initial BAT trigger reported in ATel #15650) for a total of 4 hours. The flux standard 3c286 was used to calibrate the bandpass response and flux scale of the AMI-LA and J1925+2106 was used as an interleaved complex gain calibrator.

We detect a bright unresolved source at a position consistent with the one reported in ATel #15650 at a (preliminary) flux density of 39 +/- 2 mJy (including both a statistical uncertainty and a 5% absolute flux scale uncertainty). There is no significant emission at this position in either the VLA Sky Survey (VLASS; Lacy+2020) or the NRAO VLA Sky Survey (NVSS; Condon+1998), and so we identify this new source as the radio counterpart to GRB221009A/Swift J1913.1+1946. Further observations are planned.

We thank the staff at the Mullard Radio Astronomy Observatory for carrying out these observations and operating the AMI-LA.