TITLE: GCN CIRCULAR NUMBER: 38168 SUBJECT: LIGO/Virgo/KAGRA S241109bn: ATLAS observations and discovery of the potential counterpart AT2024aawt DATE: 24/11/11 17:51:56 GMT FROM: James Gillanders at University of Oxford jhgillanders.astro@gmail.com

J. H. Gillanders (Oxford), K. W. Smith, S. J. Smartt (Oxford/QUB), D. R. Young, M. Nicholl, M. Fulton, M. McCollum, T. Moore, J. Weston, X. Sheng, A. Aamer, C. R. Angus, D. Magill (QUB), P. Ramsden (QUB/Birmingham), L. Shingles (GSI/QUB), S. Srivastav, H. Stevance, A. J. Cooper, F. Stoppa (Oxford), 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. Jordan, V. Suc (UAI, Obstech), A. Rest (STScI), T.-W. Chen (NCU), C. Stubbs (Harvard), J. Sommer (LMU) and B. P. Schmidt (ANU).

We report observations of the skymap of the possible NSBH merger event S241109bn (LVK Collaborations, GCN 38142) with the ATLAS survey (Tonry et al., 2018, PASP, 13, 164505). ATLAS scans the visible night sky every 24-48 hours with a 4x30s tiling pattern, reaching typical limiting magnitudes, m~19.5 (AB mag) in 30 seconds.

Our standard observing strategy tiled 31.7% of the 90% bilby.fits sky localisation area within 24 hours of the GW trigger. This rose to 35.7% within 72 hours. The images were processed with the ATLAS pipeline, and reference images were subtracted. Transient candidates were identified and run through our standard filtering procedures (Smith et al., 2020, PASP, 132, 1). All potential transients that passed our data processing cuts were manually inspected, and cross-matched with catalogued galaxies and stars. We do not find any compelling candidates that lie within the skymap, occurred after the GW trigger time, and are matched with galaxies in the range 0.10 < z < 0.17 (D = 600 +/- 150 Mpc; LVK Collaborations, GCN 38149), to a limit of around m_o~19.5 (AB mag).

We identity one bright transient discovered in images taken 1.57 days after the GW trigger (MJD 60623.50) and registered as AT2024aawt on the IAU Astronomical Transient Name Server (Tonry et al., TNS Astronomical Transient Report No. 232251). We detected the transient at MJD 60625.07 == 2024-11-11 01:40 UTC, at the coordinate location of RA=314.464552727, Dec=-41.64411. Its location is not coincident with any known, catalogued source, although some faint excess is visible in the public DECaLS image of the field. AT2024aawt was discovered with an o-band magnitude, m_o = 17.28 +/- 0.08 (AB mag). Our most recent previous observation at the transient location was on MJD 60624.04 (0.54 days after GW trigger), from which we extracted a 3-sigma limiting magnitude, m_o = 19.50 (AB mag). Forced photometry at the transient location indicates no significant activity for the last 30 days. This is most likely a foreground, Galactic source (such as a dwarf nova in outburst), and thus not related to the GW event, but a spectrum should be taken to confirm.

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; by-products of this search include images and catalogues from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112, 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’s Institute for Astronomy, Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.

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