TITLE: GCN CIRCULAR NUMBER: 41103 SUBJECT: GRB 250716A: GOTO tentative optical counterpart candidate DATE: 25/07/16 20:30:19 GMT FROM: Ben Rayson at University of Leicester br155@leicester.ac.uk
B. Rayson, D. O’Neill, B. P. Gompertz, A. Kumar, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration:
We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 250716A (Fermi GBM Team, GCNs 41099, 41102).
Targeted observations were performed beginning at 2025-07-16 17:07:29 UT, (+15.64h post trigger) and continued through to 2025-07-16 19:03:17 UT (+17.57h post trigger). 85 images were taken, across 10 unique pointings, covering 203 sq degrees, within the 90% localisation contour. ~90.0% of the total 2D localisation probability was covered, with an average 5-sigma depth of 19.9 mag.
Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. 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.
A new optical source GOTO25eyp (AT 2025rhc) is identified within the GBM 90% localisation region, lying on the 16% probability contour. The source was initially detected with magnitude L = 18.20 ± 0.04 AB mag at +16.33h post trigger, before fading to L = 18.26 ± 0.04 AB mag at +17.46h post trigger. In total the source was detected in three consecutive epochs. The measured magnitudes are consistent with a decay rate of ~t^(-0.83±0.09). However, we caution that the measured decay is based on a relatively short temporal baseline and is therefore sensitive to scatter in the photometric measurements. The source is coincident with a faint (g=25.15 AB mag) extended source in the legacy survey with a mean photo z=0.98 ± 0.21.
We find no evidence of the source prior to the GRB trigger time in previous GOTO observations or the ATLAS forced photometry server (Shingles et al. 2021). Due to the absence of pre-trigger detections, possible power-law decay, and the presence of a candidate host galaxy, we propose GOTO25eyp / AT 2025rhc as a candidate optical counterpart for GRB 250716A. Follow-up observations are encouraged.
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are 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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