TITLE:   GCN CIRCULAR
NUMBER:  44005
SUBJECT: GRB 260310A/AT2026fgk: radio detection at 15GHz
DATE:    26/03/14 15:23:56 GMT
FROM:    Lauren Rhodes at McGill <lauren.rhodes@mcgill.ca>

Lauren Rhodes (McGill), Andrew Hughes, Rob Fender (Oxford), Joe Bright (Oxford/Breakthrough Listen), Dave Green, Dave Titterington (Cambridge) report:

We observed the position of GRB 260310A/AT 2026fgk (Fermi GBM Team, GCN 43951; Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65; O’Neill et al. 2026, TNS Discovery Report 294132) with the Arcminute Microkelvin Imager Large-Array (AMI-LA) at 15.2 GHz beginning at 2026 March 13 23:20:06 for a total of 8 hours. The flux standard 3c286 was used to calibrate the bandpass response and flux scale of the AMI-LA and J1435+7605 was used as an interleaved complex gain calibrator.

We detect an unresolved source with coordinates consistent with those of GRB 260310A/AT 2026fgk. We measure a peak flux density of 9.8 +/- 0.5 mJy/beam. Given the high flux density of the source, we cross checked with the VLASS survey and find no preexisting radio emission at the afterglow coordinates down to a 3sigma upper limit of around 0.5mJy/beam.

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