TITLE: GCN CIRCULAR NUMBER: 42666 SUBJECT: LIGO/Virgo/KAGRA S251112cm: ATLAS pre-detections of AT2025adgp and AT2025adgq DATE: 25/11/13 12:12:23 GMT FROM: James Gillanders at University of Oxford <jhgillanders.astro@gmail.com> J. H. Gillanders (Oxford), S. J. Smartt, K. W. Smith (Oxford/QUB), S. Srivastav, H. Stevance, F. Stoppa, A. Cooper (Oxford), M. Nicholl, D. R. Young, C. R. Angus, M. D. Fulton, T. Moore, A. Aamer, M. McCollum, S. Sim, J. Weston, X. Sheng, D. Magill, P. Ramsden (QUB), 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 are currently observing the skymap of the LIGO/Virgo/KAGRA GW event S251112cm (LVK Collaboration, GCN 42650) with ATLAS (Tonry et al., 2018, PASP, 13, 164505; Licandro et al., 2023, arXiv:2302.07954). ATLAS is a quintuple 0.5-m optical telescope survey system (Hawaii x2, 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 24 hours to a magnitude depth of m ~ 19.5, weather and Moon permitting. We will report our candidate discoveries in an upcoming GCN. Groot et al. (GCN 42663) report on BlackGEM/MeerLICHT observations of the GW skymap of S251112cm (GCN 42650), and they present two candidate counterparts: AT2025adgp and AT2025adgq. Here, we note that both AT2025adgp and AT2025adgq have historical detections in ATLAS, which precede the GW detection. The public ATLAS data (for these and any other transient) are accessible through the ATLAS Forced Photometry Server (Shingles et al. AstroNote #2021-7). More specifically: AT2025adgp was first detected on MJD 60991.136 (~0.5 days pre-GW) in the c-band with magnitude, m_c = 19.22 +/- 0.25 (AB) mag. AT2025adgq was first detected on MJD 60988.111 (~3.5 days pre-GW) in the o-band with magnitude, m_o = 19.03 +/- 0.18 (AB) mag. Given these pre-detections, we expect that both AT2025adgp and AT2025adgq are young, rising supernovae, unrelated to the GW signal, S251112cm. However, the exact nature of the optical counterpart to a sub-solar-mass compact object merger event is uncertain, and so we cannot rule out that these transient phenomena are related in some way. Further monitoring of these candidate events will occur with ATLAS. 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, University of Oxford, the Queen's University Belfast, 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/42666. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...