TITLE:   GCN CIRCULAR
NUMBER:  36308
SUBJECT: LIGO/Virgo/KAGRA S240422ed: GOTO counterpart search update and AT2024hfx, AT2024hgl observations
DATE:    24/04/25 18:03:51 GMT
FROM:    Amit Kumar at University of Warwick, UK <amitkundu515@gmail.com>

D. Steeghs, K. Ackley, M. J. Dyer, J. Lyman, D. O'Neill, K. Ulaczyk; F. Jimenez-Ibarra, A. Kumar, B. Godson, T. Killestein, B. Gompertz, M. Kennedy, S. Belkin, J. Rana, D. 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 additional optical observations and candidate vetting with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022) in response to the LIGO/Virgo/KAGRA candidate NSBH event S240422ed (The LIGO-Virgo-KAGRA Collaboration, GCN 36236). 

Since our initial update (Ackley et al. GCN 36257), additional passes have been made across the skymap using deeper 12x60s exposure sets. In total, 82% of the total probability region has been searched and vetted. We did not identify any additional sources of interest that can be associated with an EM counterpart to S240422ed.

AT2024hfx / GOTO24aph (https://www.wis-tns.org/object/2024hfx)  was detected in multiple passes, with the first GOTO detection at L = 18.4 just 5 hrs after the i = 20.1 Pan-STARRS detection reported in Smith et al. GCN 36303. This is in line with the detections reported in Groot et al. GCN 36304. The reported detections, archival ZTF detection and presence of an underlying blue source is more in line with a CV/dwarf nova event.

Observation details for GOTO24aph are as follows:

Date Obs                                  Tel               Exp (s)    Mag            Err         Lim Mag    Filter
2024-04-24T11:52:37	     GOTO-S     600         18.366	0.128     18.9            L
2024-04-24T21:02:34	     GOTO-N     720         18.393	0.075 	 19.6           L
2024-04-24T21:33:05	     GOTO-N     720         18.551	0.059     19.8            L

AT2024hgl (Groot et al., GCN 36307) ; no source is detected at this position with 5-sigma forced photometry upper limits L>19.7 for visits on both April 23 and April 25:  

2024-04-23T09:05:41	    GOTO-S   >19.7
2024-04-25T10:25:48	    GOTO-S   >19.7

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).