GCN Circular 33830

M. D. Fulton, M. Nicholl, K. W. Smith, S. Srivastav, D. R. Young, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), L. Shingles (GSI/QUB), J. Sommer (LMU/QUB), A. Aamer (UoB/QUB), S. J. Smartt, H. Stevance, L. Rhodes, A. Andersson (Oxford), L. Denneau, J. Tonry, H. Weiland, A. Lawrence, R. Siverd (IfA, University of Hawaii), N. Erasmus, W. Koorts (South African Astronomical Observatory), J. Anderson (ESO), A. Jordan, V. Suc (UAI, Obstech) A. Rest (STScI), T.-W. Chen (TUM), C. Stubbs (Harvard):

We report observations of the bilby.multiorder.fits updated skymap of the NSBH event S230518h (The LIGO-Virgo-Kagra Collaboration, GCN 33816
Not found
 
 
) with the ATLAS telescope system (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 24hrs to mag~19.5, weather permitting. 

During normal science operations, we covered part of the accessible skymap of S230518h. A sequence of quads (4 x 30-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 175 square degrees of the bilby.fits skymap 90% area, and covered a sky region totalling 30% of the event's full localisation likelihood South of declination =-42 degrees. Data acquisition began at MJD 60082.95977 or 2023-05-18 23:02:04 (UTC), 9.4hrs after the LVC Preliminary notice and 10.1hrs after the GW merger event at 2023-05-18 12:59:08 (UTC). 

Observations lasted between ~10hrs to 18hrs after the NSBH merger. We found no new transient sources that had not been previously detected by ATLAS before the GW event or reported to the IAU Transient name server. The depths of our images were shallower than normal due to weather conditions, ranging between m_o < 17.5 to m_o < 18.5.

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 Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.