TITLE:   GCN CIRCULAR
NUMBER:  24107
SUBJECT: LIGO/Virgo S190408an: ATLAS observations of the S190412m skymap 
DATE:    19/04/12 14:57:03 GMT
FROM:    Stephen Smartt at Queen's U/Belfast  <s.smartt@qub.ac.uk>

H. Weiland (IfA, Univ. of Hawaii), K. W. Smith, D. R. Young, (Queen's 
University Belfast), L. Denneau, H. Flewelling, A. Heinze, J. Tonry 
(IfA), S. Smartt,  O. McBrien, J. Gillanders,nS. 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 BBH event S190412m 
(The LIGO Scientific Collaboration and the Virgo Collaboration, GCN 
24098) 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/c 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 covered 137 square degrees of the bayestar map 90% credible 
region and covered a sky region totalling 91% of the event’s 
full localisation likelihood. Data acquisition began at 58585.300570
or 2019-04-12 07:12:49 (UTC), 41 mins after the PRELIMINARY notice 
and 102 mins after the GW merger event. All data acquisition finished
approximately 4hrs later. 

We found no new transients to magnitudes of o < 19.1 (the median of
the 5 sigma limits of the individual 30 sec images) between 
~40 to ~240 minutes after the BBH merger. 

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.