TITLE: GCN CIRCULAR NUMBER: 37590 SUBJECT: LIGO/Virgo S240919bn: GOTO optical upper limits DATE: 24/09/23 16:54:48 GMT FROM: soheb.mandhai@manchester.ac.uk

S. Mandhai, A. Kumar, D. O'Neill, B. P. Gompertz, G. Ramsay, R. Starling, K. Ackley; M. J. Dyer; J. Lyman; K. Ulaczyk; F. Jimenez-Ibarra; D. Steeghs; D. K. Galloway; V. Dhillon; P. O'Brien; K. Noysena; R. Kotak; R. P. Breton; L. K. Nuttall; E. Palle and D. Pollacco report on behalf of the GOTO collaboration:

The Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022; Dyer et al 2024) serendipitously covered a part of the localisation region of the GW trigger S240919bn (LIGO Scientific Collaboration, GCN 37552, GCN 37559) in survey mode on 2024-09-20 from 02:03:57 UT. The observations covered the location of 6 out of the 10 X-ray sources detected by the Swift XRT (Page et al. GCN 37589). Each observation consisted of 4x45s exposures in the GOTO L-band (400-700 nm).

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations of the same pointings.

No new candidate optical counterparts are identified in the difference images at the locations of the below-mentioned X-ray sources detected by the Swift-XRT (Page et al. GCN 37589). The upper limits are as follows:

Event Date UT t-t0 (hours) 3-sigma UL (mag) ========================================================================================

Rank 3 sources ------------------- S240919bn_X10 2024-09-20 02:06:52 19.85 >19.26 S240919bn_X11 2024-09-20 02:03:57 19.80 >19.23

Rank 4 sources ------------------- S240919bn_X5 2024-09-20 02:03:57 19.80 >19.24 S240919bn_X6 2024-09-20 02:03:57 19.80 >19.24 S240919bn_X7 2024-09-20 02:06:52 19.85 >19.20 S240919bn_X2 2024-09-20 02:06:52 19.85 A source in the difference images but pre-gw event detections. Not related to S240919bn.

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