[vsnet-grb-info 22098] LIGO/Virgo S190425z: ATLAS observations of the S190425z skymap

[GCN Circulars]

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  <s.smartt at qub.ac.uk>

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.