TITLE:   GCN CIRCULAR
NUMBER:  24653
SUBJECT: LIGO/Virgo S190521r: No candidate counterparts from ATLAS observations of the skymap
DATE:    19/05/22 13:56:40 GMT
FROM:    Shubham Srivastav at QUB  <S.Srivastav@qub.ac.uk>

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S. Srivastav, P. Clark, K. W. Smith,  D. R. Young, S. J. Smartt (QUB)
L. Denneau, H. Flewelling, A. Heinze, J. Tonry, H. Weiland (IfA, Univ. Hawaii),
O. McBrien, J. Gillanders, D. O’Neil, S. Sim (QUB) A. Rest (STScI), B. Stalder (LSST), C. Stubbs (Harvard)


We report observations of the BAYESTAR skymap of the BBH event S190521r (The LIGO Scientific Collaboration and the Virgo Collaboration, GCN 24632) 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 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 488 square degrees of the bayestar map 90% credible region and covered a sky region totalling 96% of the event’s full localisation likelihood. Data acquisition began at MJD 58624.333909 or 2019-05-21 08:00:49.7 (UTC), ~11 mins after the PRELIMINARY notice and ~17 mins after the GW merger event. All data acquisition finished approximately 5 hours later.

We found no new transients to magnitudes of o < 18.7 (the median of the 5 sigma limits of the individual 30 sec images) between ~20 to ~320 minutes after the BBH merger.

We also report previous detection of the CNEOST transient (Li et al., GCN 24647) by ATLAS, designated as ATLAS19kvc. ATLAS19kvc (or AT 2019fsk) was first detected on 2019-05-15.43 UT (MJD 58618.43) at a magnitude of 
m_o = 17.33 +/- 0.06, with subsequent detections on 2019-05-17 and 2019-05-21. ATLAS19kvc is therefore
unrelated to the gravitational wave event S190521r.

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.