TITLE: GCN CIRCULAR NUMBER: 20931 SUBJECT: LIGO/Virgo G275697: AMI Large Array Search for a Radio Afterglow DATE: 17/03/22 17:00:26 GMT FROM: Kunal Mooley at U of Oxford K. P. Mooley (Hintze Fellow, Oxford) reports on behalf of a larger collaboration We observed 52 galaxies from the GLADE catalog (weighted by the B-band luminosities) and 8 galaxies from the CLU catalog (stellar mass-weighted from Cook et al., LVC GCN 20800) within the BAYESTAR error region for the neutron star merger candidate G275697 (Siellez et al. LVC GCN 20763). A plot of the galaxy selection function and the list of targeted galaxies can be found here: https://www.dropbox.com/s/mho6k0cm01jaqqm/G275697_AMILA_LVC.pdf?dl=0. The observations, designed to detect a possible orphan afterglow, were carried out with the AMI Large Array (AMI-LA) at 15 GHz over two epochs within 15 days post-trigger. The median 4sigma detection threshold in each epoch is 200 uJy at the pointing center (400 uJy at the half power radius of the beam, 3 arcmin). We manually inspected all the sources detected above 4sigma within approximately 4 arcmin (= 230 kpc separation at 200 Mpc) of the AMI-LA beam, amounting to a total search area of 0.8 sq. deg and a containment volume probability of 1.3%. We have devised a ranking scheme (rank 1-4; with 1 being the most likely to be associated with the GW event and 4 being the least likely) for transient candidates similar to that of Swift. A detailed description of the ranking scheme can be found in the URL mentioned above. Among the ~70 unique point sources detected in the AMI-LA observations, we found two candidates which warrant further follow up. The candidates are detailed below. All quoted upper limits are 3sigma. We note that the observations for a third epoch with the AMI-LA are ongoing, and further information on these and other candidates will be reported in subsequent GCNs. Rank 2: ====== 1) AMILA J220556+503620 March 02.38 (UT): <312 uJy March 09.36 (UT): 620+/-75 uJy Comments: All sources in the field are ~10% smaller than the synthesized beam. Nearby source is stable. Rank 3: ====== 1) AMILA J213542+394253 March 03.59 (UT): 1257+/-78 uJy March 10.57 (UT): 1621+/-83 uJy Comments: Variable (>3sigma). No nearby source for flux density comparison. We thank the AMI staff for scheduling these observations.