TITLE: GCN CIRCULAR NUMBER: 24197 SUBJECT: LIGO/Virgo S190425z: ATLAS observations of the S190425z skymap DATE: 19/04/25 21:45:52 GMT FROM: Stephen Smartt at Queen's U/Belfast O. McBrien, S. Smartt, K. W. Smith, D. R. Young, (Queen's University Belfast), L. Denneau, H. Flewelling, A. Heinze, J. Tonry, H. Weiland, (IfA, Univ. of Hawaii), J. Gillanders, S. Srivastav, D. O'Neil, P. Clark, S. Sim (QUB), A. Rest (STScI), B. Stalder (LSST), C. Stubbs (Harvard), E. Magnier, A. Schultz, , M. Huber, K. C. Chambers (IfA) We report observations of the BAYESTAR skymap of the BNS event S190425z (The LIGO Scientific Collaboration and the Virgo Collaboration, GCN 24168) 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 o bands, 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 began observing the northern part of the skymap within the first hour of the preliminary notice. ATLAS covered 2652 squ. degrees of the bayestar map 90% credible region and covered a sky region totalling of 37.2% of the event's localisation likelihood. A single image reaches approximately o = 19.5 (5 sigma), We flagged 25 transients but all were either known, or we detected previous flux in our own forced photometry in images before the explosion. These new objects were registered on the TNS. Users are referred there as a reference point. No further convincing counterpart candidates were found above o = 19.5, which were plausibly associated with a galaxy within 100-200 Mpc (i.e. less than 50 kpc separation). Our area did not cover the sky position of the two ZTF candidates (see Kasliwal et al. GCN 24191). In addition we report 5 marginal candidates. These are all orphans (not matched with any known source), but require independent confirmation. They are within the skymap (at least 30% contour). Name | IAU Name | RA (J2000) | Dec (J2000) | Disc. MJD | Disc Mag ATLAS19hxm | AT2019dzv |14:01:45.02 | +46:12:56.1 | 58598.42 | 19.23 o ATLAS19hyx | AT2019ebl |14:32:31.53 | +55:45:00.1 | 58598.44 | 19.28 o ATLAS19hyo | AT2019eao |13:01:18.63 | +52:09:02.1 | 58542.59 | 19.36 o ATLAS19hwn | AT2019ebm |12:59:58.58 | +29:14:30.7 | 58598.40 | 19.42 o ATLAS19hwh | AT2019ebn |13:54:47.42 | +44:46:27.3 | 58318.29 | 19.07 o 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.