TITLE: GCN CIRCULAR NUMBER: 36715 SUBJECT: GRB 240619A: GOTO candidate optical afterglow DATE: 24/06/20 21:07:53 GMT FROM: Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk> B. P. Gompertz, K. Ackley, S. Belkin, T. Killestein, A. J. Levan, B. Godson, R. Starling, M. J. Dyer, J. Lyman, K. Ulaczyk, F. Jimenez-Ibarra, A. Kumar, 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. Pall'e 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 GRB 240619A (Fermi GBM Team, GCN 36694). Targeted observations were performed by GOTO-South at 08:21:34 UT on 2024-06-19 (3x90s exposures, 4.7 hours after trigger), and by GOTO-North at 21:40:50 UT on 2024-06-19 (4x90s exposures, 18.0 hours after trigger). Observations were taken 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. A new optical source AT 2024lwv<https://www.wis-tns.org/object/2024lwv> (GOTO24cvn) is identified within the GBM 90% localisation region at RA = 10:49:34.70, Dec = +17:16:58.07 with an initial magnitude of L = 17.17 +/- 0.17 mags 4.7 hours after trigger, fading to L = 18.38 +/- 0.09 mags at 18.0 hours after trigger. The source is also present in the data available on the ATLAS forced photometry server (Shingles et al. 2021) with an initial magnitude of o = 16.2 +/- 0.04, 2.3 hours after the GRB trigger. Observations are consistent with a power-law decay of approximately t^-0.8. The transient was not detected in the most recent pre-trigger GOTO observation, taken at 21:54:02 UT on 2024-06-17 (~1.25 days prior to the GRB) to a 5-sigma limiting magnitude of L > 19.2 mags. The transient is spatially coincident (0.1” offset) with the catalogued galaxy PSO J162.3946+17.2828 with a photometric redshift of 0.63 +/- 0.19 in the PS1-STRM catalogue (Beck et al. 2020). Due to the galaxy association, rapid decay, and lack of detection in pre-GRB imaging, we propose this source as the optical afterglow of GRB 240619A. Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction. Observations are ongoing. 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/36715. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...