TITLE: GCN CIRCULAR NUMBER: 25217 SUBJECT: LIGO/VIRGO S190728q: No candidate counterparts from ATLAS observations DATE: 19/07/29 13:26:10 GMT FROM: Shubham Srivastav at QUB ​S. Srivastav, O. McBrien, K. W. Smith, D. R. Young, M. Dobson, S. J. Smartt, J. Gillanders, P. Clark, D. O'Neil, S. Sim (QUB), L. Denneau, H. Flewelling, A. Heinze, J. Tonry, H. Weiland (IfA, Univ. Hawaii), A. Rest (STScI), B. Stalder (LSST), C. Stubbs (Harvard) We report observations of the skymap for the BBH event S190728q (The LIGO Scientific Collaboration and the Virgo Collaboration, GCN 25187) with the ATLAS telescope system (Tonry et al. 2018, PASP, 13, 164505). ATLAS is a twin 0.5m telescope system on Haleakala and Mauna Loa employing two filters - cyan and orange. While carrying out the primary mission for Near Earth Objects, we can adjust the schedule rapidly to point at LVC gravitational wave skymaps. Sequences of 30 sec images were taken in the ATLAS c band, and at each pointing position a sequence of quads (4 x 30 sec) was taken. The images were processed with the ATLAS pipeline and reference images subtracted from each one. Transient candidates were run through our standard filtering procedures, combined with machine learning algorithms (e.g. Wright et al. 2015, MNRAS, 449, 451). Candidates were spatially cross-matched with known minor planets, and star, galaxy, AGN and multi-wavelength catalogues (as described in Smartt et al. 2016, MNRAS, 462 4094, Stalder et al. 2017, ApJ, 850, 149). We covered 99.04 square degrees of the 90% credible region (GCN 25208; LALInference), totalling 91.7% of the event’s localisation likelihood. Data acquisition began at MJD 58692.36 or 2019-07-28 08:39:33.29 (UTC), ~100 mins after the PRELIMINARY notice and ~114 mins after the GW merger event. We found no new transients to a magnitude of c < 19.8 (the median of the 5 sigma limits of the individual 30 sec images) between 114 to 261 minutes after the BBH merger within the LALInference skymap. This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS 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. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, and the Space Telescope Science Institute.