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GCN Circular 32222

Subject
GRB 220611A optical counterpart
Date
2022-06-18T19:10:23Z (2 years ago)
From
Giovanna Pugliese at API/UvA <pugliese@astroduo.org>
D. B. Malesani (Radboud Univ. and DAWN/NBI), G. Pugliese (Amsterdam Univ. and Leiden Observatory), P. Schady (Univ. Bath), A. J. Levan (Radboud Univ.), B. P. Gompertz (Birmingham), report on behalf of the Stargate collaboration:

We observed the candidate counterpart (O'Connor et al., GCN 32203; Rastinejad et al., GCN 32208) of GRB 220611A (Cenko et al., GCN 32191) using  the ESO VLT UT1 (Antu) equipped with the FORS2 imager. Observations were taken in twilight using the I filter, at an average airmass of 2.6, and with a delivered seeing of 1.9". The mean time of the observation (twenty 30-s images) was Jun 18.42 UT (6.67 days after the GRB).

The object noted by O'Connor et al. (GCN 32203) is detected in the stack of our data. Calibrating against three stars from the SkyMapper survey, we measure a preliminary magnitude of i = 23.0 +- 0.2 (AB).

Compared to the archival brightness r = 24.5 from the Legacy Survey (Rastinejad et al., GCN 32208), our measurement favors some transient contribution, consolidating the association of this object to GRB 220611A. Alternatively, the source could be very red, which would however be in contrast with the non-detection in the Legacy Survey z-band image.

We note that the observed magnitude is fainter by ~4.5 mag than what SN 1998bw would look like at z = 0.049, likely ruling out a traditional collapsar GRB in MCG-06-10-007. GRB 220611A might be a background event (consistent with the detection of a potential host galaxy in the Legacy Survey) or, speculatively, originate from a merger event, which would be more consistent with the offset and spectral type of MCG-06-10-007.

We acknowledge support with this difficult observation from the VLT staff at Paranal, in particular Cedric Ledoux and Joseph Anderson. 

[Editor's Note: GRB Name corrected from GRB 221106A to GRB 220611A.]
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