GCN Circular 35715

S. Belkin, M. Kennedy, B. P. Gompertz, K. Ackley, R. Starling, T. Killestein, 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 240209A (Fermi GBM team, GCN 35712
Not found
 
 
). Targeted observations were performed by GOTO-North between 05:27:38 on 2024-02-11 (starting 47 hrs after trigger) and 03:58:21 on 2024-02-12. 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 1 candidate optical counterpart (GOTO24pw/AT2024cew
Not found
 
 
) within the GBM 90% localisation region. 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). The most recent non-detection was taken by ATLAS on 2024-02-01.

Name | RA(J2000) | Dec(J2000) | Filter | t - trig(days) | Mag(AB)
GOTO24pw | 17:01:05.29 | +68:43:46.31 | L | 1.96 | 19.64 +/- 0.14

The source is seen to decay as a power-law with an index of t^-1.80+/-0.16 across 3 epochs of observations. The most recent detection magnitude was L = 20.45 +/- 0.19 at 2.90 days after trigger.

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

We note the presence of the GLADE+ (Dalya et al. 2022) catalogued galaxy HyperLEDA 2722973 (z=0.1629+/-0.0150) approximately 7.3" away. At this redshift the sky-projected offset of the transient from this galaxy is 20.58 kpc. This transient is spatially coincident with the cluster ZwCl 8111.

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).