TITLE:   GCN CIRCULAR
NUMBER:  20713
SUBJECT: LIGO/Virgo G274296:  ATLAS confirmation of the bright MASTER transient and earlier explosion date
DATE:    17/02/21 15:14:53 GMT
FROM:    S. J. Smartt at Queens U Belfast  <s.smartt@qub.ac.uk>

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

Lipunov et al. (GCN 20712) reported the discovery of a very bright,
mag=15.7 transient in the sky localisation map of G274296 (GCN20689)
at the position : 

MASTER OT J105519.53+365834.1 RA(2000)=10h 55m 19.53s +36d 58m 34.1s  

at 2017-02-18 18:18:18 UT (MJD=57802.76271) 

and close to (within 1.7”) of the centre of an edge-on spiral galaxy
at z = 0.043224 (KUG 1052+372; PGC 032819)

With the ATLAS telescope system (see Tonry et al. GCN 20382 and
www.fallingstar.com), we confirm the reality of this bright transient,
however it exploded well before the GW detection G274296.  The MJD of the
GW source was 57801.2541094 as announced in GCN20689.

In ATLAS routine survey mode we detect the transient (internally called
ATLAS17bbp) on two images on the night of MJD=57794 at mag = 18.23
(orange filter) and again on 5 images on each of 57798 and 57802, the
latest with m=17.40 +/- 0.07 (the orange bandpass is approximately
equivalent to o ~ 0.56r+0.44i).

The difference in the magnitudes between ATLAS and MASTER is likely
due to ATLAS performing difference imaging photometry and the transient being
located close to the galaxy core.

The ATLAS17bbp coordinates are RA=163.83115 DEC=+36.97640
(10:55:19.47 +36:58:35.0)

While this is certainly real, and within the LIGO localisation banana,
it has an explosion date at least 7 days before G274296.  We measure a
1.2" difference in the astrometric coordinates of PGC 032819 and
ATLAS17bbp, but further analysis is required to determine if it
actually is coincident with the galaxy nucleus.

The source fell outside the northern egde of the Pan-STARRS1 coverage
(Chambers et al. GCN 20699).