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GCN Circular 21553

Subject
LIGO/Virgo G298048: Pan-STARRS detections and izy photometry of the possible optical/NIR counterpart in NGC4993
Date
2017-08-18T08:37:20Z (7 years ago)
From
S. J. Smartt at Queens U Belfast <s.smartt@qub.ac.uk>
K. C. Chambers (IfA), M. E. Huber (IfA), S. J. Smartt, (QUB),
K. W. Smith (QUB), D. R. Young, M. Coughlin (Harvard), T.-W. Chen
(MPE), L. Denneau, H. Flewelling, A. Heinze, E. Kankare (QUB),
T. Lowe, E. A. Magnier (IfA), A. Rest (STScI), B. Stalder (IfA),
A. S. B. Schultz, C. W. Stubbs (Harvard) J. Tonry, C. Waters,
R. J. Wainscoat, H. Weiland, M. Willman (IfA), D. E. Wright (QUB)

We report Pan-STARRS imaging observations of the the skymap of
G298048. A set of six images were taken, in twilight and at low
airmass. We had planned to tile the northern part of the skymap, but
after the discovery of the transient SSS17a/DLT17ck in GCN 21529
(Coulter et al) and 21531 (Valenti et al.) we immediately switched to
cover the NGC4993 in 2x30s in each of Pan-STARRS i, z, y, ensuring the
postion of the transient object was on a good, clean CCD cell.

Difference images were produced by subtracting the Pan-STARRS1 3Pi
reference image from these separate 30s exposures (Chambers et
al. arXiv:1612.05560, and available at http://panstarrs.stsci.edu).

We recover the transient as Pan-STARRS object PS17gl at coordinates 

RA = 13:09:48.08  DEC = -23:22:53.2 
RA = 197.45033    DEC = -23.38144 

Image log and photometry in the Pan-STARRS system (Tonry et al. 2012
Magnier et al. arXiv:1612.05242)  : 


Date (UT)             RA            DEC            Exp  Filt Airmass
2017-08-18T05:33:01  198.347081   -23.075627 2000  30.0 y    2.83   17.28 0.13
2017-08-18T05:33:46  198.347076   -23.075658 2000  30.0 z    2.84   17.31 0.09
2017-08-18T05:34:31  198.347062   -23.075632 2000  30.0 i    2.86   17.23 0.10
2017-08-18T05:35:45  198.347073   -23.075633 2000  30.0 y    2.89   17.47 0.16
2017-08-18T05:36:30  198.347070   -23.075638 2000  30.0 z    2.91   17.21 0.08
2017-08-18T05:37:14  198.347060   -23.075617 2000  30.0 i    2.93   17.26 0.08

There is no evidence, within the uncertainties for fading between the
exposures. If the Swope 1m photometry point of i=16 (Coulter et al.)
is correct then this would imply substantial fading. But this needs
checked, since DLT17ck was reported at R=17.3, is more consistent with 
the colours we measure :

i - z = -0.02 +/- 0.02
z - y = -0.26 +/- 0.18 

This is consistent with the DECam photometry by Nicholl et
al. (GCN21541) and that within the uncertainties, the colours are
consistent with 0, and being flat in Fnu as reported by the J-band
detection of Tanvir & Levan (GCN21544). Further observations are
planned with the same filter setup as long as the target is
observable, or firmly ruled out as a counterpart to G298048.
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