GCN Circular 20931
Subject
LIGO/Virgo G275697: AMI Large Array Search for a Radio Afterglow
Date
2017-03-22T17:00:26Z (8 years ago)
From
Kunal Mooley at U of Oxford <kunal.mooley@physics.ox.ac.uk>
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.