TITLE:   GCN CIRCULAR
NUMBER:  20503
SUBJECT: LIGO/Virgo G270580:  ATLAS coverage and 4 optical transients
DATE:    17/01/22 22:58:06 GMT
FROM:    S. J. Smartt at Queens U Belfast  <s.smartt@qub.ac.uk>

K. W. Smith (QUB), J. Tonry, L. Denneau, A. Heinze, B. Stalder,
H. Weiland (IfA), C. W. Stubbs (Harvard), D. R. Young
(QUB), S. J. Smartt, (QUB), A. Rest (STScI), K. C. Chambers (IfA),
T.-W. Chen (MPE), M. Coughlin (Harvard), M. E. Huber (IfA),
D. E. Wright (QUB), H. Flewelling, E. A. Magnier, A. S. B. Schultz,
C. Waters, R. J. Wainscoat (IfA)

We report the following observations of the skymap for LIGO/Virgo
G270580 (discovered MJD = 57773.5215; LIGO Scientific Collaboration
and Virgo, GCN 20486) with the ATLAS telescope system. ATLAS
is a twin 0.5m telescope system on Haleakala and Mauna Loa (see Tonry
et al. ATel 8680 and Tonry 2011, PASP, 123, 58). The first unit is
operational on Haleakala and is robotically surveying the sky in two
filters cyan and orange (denoted c and o, all mags are in AB
system). More information is on http://www.fallingstar.com

ATLAS covered 1294 sq. deg. of the ~non-zero probability regions on
the nights MJD=577773,57774. We estimate that the coverage equates to
39.9% total all-sky probability of containing the source (from the
LALInference map; Veitch et al., PRD 91, 042003).  A map of the ATLAS
footprints will be placed on GraceDB.  The probability region was
covered in normal ATLAS survey mode, with typically 5 exposure per
footprint.

We detect the following transients. None are convincing counterparts
to the GW source for various reasons, but should be spectrally
classified. All four are in the Liverpool Telescope queue. 

Name       | RA (J2000)  | Dec (J2000) | Disc. MJD | Disc. Date  | Disc Mag | Notes
ATLAS17ajr | 12:28:58.69 | +31:39:37.7 | 57773.63  | 20170120.63 |  17.73 o | 1. AT2017mq
ATLAS17akl | 14:16:31.06 | +39:35:10.7 | 57766.67  | 20170113.67 |  17.72 o | 2. AT2017mf
ATLAS17akq | 13:16:05.81 | +36:04:19.9 | 57774.56  | 20170121.56 |  18.17 o | 3. 
ATLAS17ajv | 10:26:42.36 | +36:40:49.9 | 57774.49  | 20170121.49 |  16.32 o | 4. PTSS-17dfc


1. ATLAS17ajr = AT2017mq.  Host galaxy is z=0.03848. Close to, but
outside both the LIB cWB maps, hence released on the IAU Transient
name server. Some 3-sigma detections on forced photometry on
MJD=57761,

2. ATLAS17akl : 5 arcseconds from SN2006cb (and not related to that
SN). Amateur discovery on TNS (as AT2017mf). This is not within LIB,
but is within cWB map. However ATLAS detection on 57766.67, shows it
to have occurred 7 days before the GW detection.

3. ATLAS17akq : transient coincident with core of SDSS r=20.25 galaxy
(photo-z z=0.12).  This is within the LIB map. But there is some
evidence of 3-4 sigma detections 8-12 days before this discovery.

4. ATLAS17ajv : we independently discovered this object which is
PTSS-17dfc (Zhao et al. GCN 20497).  No detection (o < 18.3) on
MJD=57766. This is outside the LIB map, but within the cWB 90%
contours.  Note we do not register objects publicly on the TNS if they
fall within the LIGO maps (and timeframe), hence we did not register
this one.