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GCN Circular 33439

Subject
GRB 230307A: Possible optical afterglow
Date
2023-03-09T15:14:27Z (a year ago)
From
Andrew Levan at U.of Leicester <a.levan@astro.ru.nl>
A.J. Levan (Radboud), B. Gompertz (U. Birmingham), K. Ackley (U. Warwick), M. Kennedy (University College Cork), V. Dhillon (U. Sheffield) report on behalf of a larger collaboration:

"We observed the IPN localisation (Kozyrev et al. GCN 33425)  of GRB 230307A (GBM team GCN 33405, Xiong et al. GCN 33406) with ULTRACAM, mounted on the NTT at La Silla. A total of 200s of exposure (10 x 20s) were obtain simultaneously in the u, g and r filters starting at 01:58:33 UTC approximately 34.3 hours post burst.  Observations covered the entirety of the refined IPN localisation. A comparison of the observation with DSS and legacy survey image reveals a weak new optical source at a location of 

RA(J2000): 04:03:25.83
DEC(J2000): -75:22:42.7
 (+/- 0.5���)

The source has an r-band magnitude of approximately r=20.5 at the time of these observations. We note that the source is co-incident (1.9��� offset) with source 2 identified by the Swift-XRT (Evans, GCN 33419). Combined with the absence of the source in pre-imaging we suggest that this is the afterglow of GRB 230307A. 

We note that the source has a modest offset (3���)  from a very red source identified in the legacy survey and WISE. It is possible that this is host galaxy of the GRB. Spectroscopic observations to obtain the redshift of both afterglow and nearby source are encouraged. "
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