GCN Circular 39211

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

We report observations of the Bilby.fits skymap of the NSBH merger event S250206dm (The LIGO-Virgo-Kagra Collaboration, GCN 39175
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 ) by the ATLAS survey (Tonry et al., 2018, PASP, 13, 164505). ATLAS is a quadruple 0.5m optical telescope survey system (Hawaii, South Africa, Chile) employing two filters, cyan and orange. In our primary NASA mission for Near-Earth Object discovery, we cover the entire visible night sky every 24 hrs to magnitude depths m ~ 19.5, weather and Moon permitting.

We targeted the accessible skymap of S250206dm with a sequence of quads (4 x 110 s images) obtained at each pointing position. Data acquisition began at MJD 60713.098 or 2024-04-22 02:21:07 (UTC), 0.8 hrs after the LVC initial alert and 5.0 hrs after the merger event (which was 60712.89). 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). We covered 389 square degrees of the bilby.fits skymap 90% area, and covered a sky region totalling 35% of the event's full localisation likelihood. 

Observations lasted between ~5 hrs to 10.5 hrs after the NSBH/BNS merger. We found no plausible new transient sources that had not been previously detected by ATLAS before the merger event or reported to the IAU Transient name server. The 5-sigma depths of our images were typically o < 19.5 +/- 0.6 AB mag. We are reporting all discoveries to the TNS, where they can be tracked, classified, searched, and commented upon. 

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.