GCN Circular 36488

A. Kumar, B. P. Gompertz, K. Ackley, J. Lyman, S. Belkin, A. J. Levan, R. Starling, M. J. Dyer, K. Ulaczyk, F. Jimenez-Ibarra, D. O'Neill, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, E. Palle and D. Pollacco report on behalf of the GOTO collaboration:

We report on observations with the Gravitational-wave Optical Transient Observer (GOTO, Steeghs et al. 2022) in response to the Fermi GBM detected GRB 240514B (Fermi GBM team, GCN 36469
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, Roberts et al., GCN 36481
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). Targeted observations were performed by GOTO-South on 2024-05-14 from 18:11:25 to 19:24:33 UT (respectively from 14.128 to 15.347 hours after trigger) distributed over five epochs. Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm).

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using recent survey observations of the same pointings. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogues. Human vetting was carried out in real time on any candidates that passed the above checks.

We identify AT2024ixa
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/GOTO24bph as a possible optical counterpart within the GBM 90% localisation region. The source is coincident with a galaxy in the GLADE+ catalogue (Dalya et al. 2021) with a photometric redshift of z=0.0579+/-0.0349. The source is seen to fade by 8.6 +/- 5.8 mags/day across 5 epochs of observations.

We find no evidence of this source prior to the GRB trigger time in previous GOTO observations, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). We caution that the position has not been covered by GOTO since 2024-01-09 prior to the GBM trigger. Hence, further observations are encouraged to ascertain the nature of the candidate.

Name | RA(J2000) | Dec(J2000) | Epoch(MJD) | dt_trig(hrs) | Filter | Mag(AB)
GOTO24bph | 22:43:13.32 | -00:08:17.37 | 60444.7579 | 14.128 | L | 19.58 +/- 0.18

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and were not corrected for Galactic extinction.

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).