TITLE: GCN CIRCULAR NUMBER: 37522 SUBJECT: GRB 240916A: GOTO identification of two new optical sources within the GBM localisation region DATE: 24/09/16 11:16:13 GMT FROM: Ben Gompertz at U of Birmingham b.gompertz@bham.ac.uk

B. P. Gompertz, A. Kumar, D. O’Neill, M. Kennedy, R. Starling, K. Ackley, M. Dyer, F. Jiménez-Ibarra, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, L. Nuttall, E. Palle, D. Pollacco and T. Killestein report on behalf of the GOTO collaboration:

We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022) in response to GRB 240916A, detected by Fermi/GBM and INTEGRAL SPI ACS (Fermi GBM Team, GCN 37518; Pawar et al., GCN 37519).

Targeted observations were performed by GOTO South between 2024-09-16 09:06:48 UT (t0+7.73h) and 2024-09-16 09:23:05 UT (t0+8.00h). In total, 48 images were taken across 6 unique pointings, covering 178.6 square degrees within the GBM 90% localisation contour. This corresponds to ~78.3% coverage of the total 2D localisation probability. Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm). The average 5-sigma limiting magnitude was L > 19.1 mag.

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

We identify 2 new optical sources contained within the GBM 90% probability contour:

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| Internal name | IAU name | ra | dec | L-band mag |

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| GOTO24fzk | AT 2024vlohttps://www.wis-tns.org/object/2024vlo | 234.873760 | -6.94630 | 19.15 +/- 0.19 |

| GOTO24fzn | AT 2024vlphttps://www.wis-tns.org/object/2024vlp | 235.913454 | -7.76478 | 17.80 +/- 0.05 |

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GOTO24fzk/AT2024vlo is consistent with galaxy 6dF J1539293-065643 in the GLADE+ galaxy catalog (Dálya et al., 2021), with a photometric redshift of z = 0.0267+/-0.0339 (121 Mpc).

We find no evidence of these sources 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). However, pre-burst imaging to sufficient depth is not available for the week preceding the GRB.

We caution that due to the target field setting soon after twilight, only a single epoch of observations could be obtained, and we therefore cannot assess the rate of evolution of either candidate. Manual checks for minor planets revealed no matches consistent with the source properties at either position.

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and 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).

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