TITLE: GCN CIRCULAR NUMBER: 42682 SUBJECT: LIGO/Virgo/KAGRA S251112cm: ATLAS targeted observations of the skymap DATE: 25/11/14 15:59:20 GMT FROM: Adam Smith at Queen's University Belfast <asmith67@qub.ac.uk> A. Smith, M. Nicholl, (QUB), J. Gillanders, S. J. Smart, K. W. Smith, (Oxford), D. R. Young, C. R. Angus, M. D. Fulton, T. Moore, A. Aamer, S. Sim, J. Weston, X. Sheng, D. Magill, P. Ramsden (QUB), S. Srivastav, H. Stevance, F. Stoppa, A. Cooper, J. Tweddle, L. Eastman (Oxford), L. Shingles (GSI/QUB), L. Rhodes (TSI/McGill), L. Denneau, J. Tonry, H. Weiland, R. Siverd (IfA, University of Hawaii), N. Erasmus, W. Koorts (South African Astronomical Observatory), A. Rest (STScI), T.-W. Chen (NCU), C. Stubbs (Harvard), J. Sommer (LMU/QUB), B. Schmidt (ANU): We report observations of the Bilby.fits skymap of the compact binary merger candidate S251112cm (The LIGO-Virgo-Kagra Collaboration, GCN 42650) with the ATLAS telescope system (Tonry et al., 2018, PASP, 13, 164505). ATLAS is a quintuple 0.5m optical telescope survey system (Hawaii x 2, South Africa, Chile, Tenerife) employing three filters, cyan, orange, and ‘clear’. In our primary NASA mission for Near-Earth Object discovery, we cover the entire visible night sky every 24hrs to AB magnitude depths m ~ 19.5, weather and Moon permitting. During focused science operations, we covered part of the accessible skymap of S251112cm. A sequence of quads (4 x 120-sec images) was taken at each pointing position. The images were processed with the ATLAS pipeline, and reference images were subtracted from each one. Transient candidates were identified and run through our standard filtering procedures (Smith et al., 2020, PASP, 132, 1). We covered 665.14 square degrees of the bilby.fits skymap 90% area, and covered a sky region totalling 45.08% of the event's full localisation likelihood, South of declination = 11 degrees. Data acquisition began at MJD 60991.811354 or 2025-11-12 19:28:20 (UTC), 3.5hrs after the LVC Preliminary/Initial notice and 4.2hrs after the merger event at 2025-11-12 15:18:45.362 (UTC). Observations lasted between ~4.2hrs to 41.3hrs after the GW detection. The depths of our images typically reached m_o < 19.5 AB mag. We found 26 transient sources, including 5 previously unreported transients. However, all had been previously detected by ATLAS before the merger event, or reported to the IAU Transient name server (https://www.wis-tns.org) and/or are associated with host galaxies with photometric redshifts beyond z > 0.05. We are reporting all discoveries to the TNS, where they can be tracked, classified, searched, and commented upon. We encourage further information to be reported on the TNS object pages. We checked the ATLAS forced photometry for the 3 GOTO objects reported by Ackley et al. (GCN 42658), and confirmed that all three have flux detected before the S251112cm GW event time. This flux on the individual 30 sec images are each below 5-sigma significance but the combined detections over multiple nights indicate the source was detected. These detections can be checked on ATLAS forced photometry server (Shingles et al. AstroNote 2021-7). The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for Near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the University of Oxford, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile. View this GCN Circular online at https://gcn.nasa.gov/circulars/42682. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...