TITLE: GCN CIRCULAR NUMBER: 40634 SUBJECT: GRB 250605A: LT optical observations DATE: 25/06/05 22:51:33 GMT FROM: Ben Gompertz at U of Birmingham b.gompertz@bham.ac.uk
B. P. Gompertz (U. Birmingham), Dimple (U. Birmingham), A. Martin-Carrillo (UCD), G. Corcoran (UCD) and D. B. Malesani (DAWN/NBI and Radboud), report on behalf of a larger collaboration:
We initiated follow-up observations of GRB 250605A (Gupta et al., GCN 40633) with the IO:O camera on the 2m robotic Liverpool Telescope (LT). Observations began at 20:48:52.89 UT, ~2 hours after the Swift/BAT trigger, and consisted of 10x60 s exposures in the SDSS i filter. Observations were taken close to twilight, at high airmass, and the target was 28 deg from the moon at 77% illumination.
Within the Swift/XRT error box, we detect a possible optical counterpart, not present in archival imaging, with an AB magnitude of i ~ 21.1 at RA = 11:07:49.7, Dec = -19:47:15.4. However, this detection is only significant at the 2-sigma level and further, deeper observations are required to confirm its reality. Formally, we measure a 3-sigma upper limit of i > 20.6 within the XRT localisation region.
We also note the presence of a faint source visible in the Legacy Survey archival imaging with an AB magnitude of i = 23.54. The source position is consistent with the possible optical counterpart, and may be the host galaxy of GRB 250605A.
Magnitudes are calibrated against nearby Pan-STARRS stars and not corrected for galactic extinction.
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