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GCN Circular 21262

Subject
LIGO/Virgo G288732: Pan-STARRS coverage of the Fermi/LAT candidate position
Date
2017-06-22T16:11:05Z (7 years ago)
From
S. J. Smartt at Queens U Belfast <s.smartt@qub.ac.uk>
K. W. Smith, K. C. Chambers (IfA), M. E. Huber (IfA), S. J. Smartt,
T.-W. Chen (MPE), M. Coughlin (Harvard), D. E. Wright, D. R. Young,
E. Kankare (QUB), H. Flewelling, T. Lowe, E. A. Magnier,
A. S. B. Schultz, C. Waters, R. J. Wainscoat, M. Willman
(IfA),J. Tonry, L. Denneau, A. Heinze, B. Stalder, H. Weiland (IfA),
C. W. Stubbs (Harvard), A. Rest (STScI),


GCN 21227 (Omedi et al.) reported detection of a weak gamma-ray
candidate with Fermi/lAT at position R.A.,Dec.=128.11, 43.39, (J2000)
with a localization error of 0.24 degree, which is within the skymap
of the LIGO GW Binary Merger Candidate G288732 (GCN 21221, Becsy et
al.), discovered at 2017-06-08 02:01:16.492 UTC (57912.08421866)

We report that we observed the Fermi/LAT postion with the Pan-STARRS1
telescope (Chambers et al. arXiv:1612.05560). We began taking data at
2017-06-09T 06:03:40 UT, centered on the Fermi position with a set of
8 dithered exposures in i and z band filters. The PS1 camera 
encloses a circle of 2.9 degree diamater, and therefore enclosed the 
whole of the Fermi/LAT error box. 

Difference images were produced by subtracting the Pan-STARRS1 3Pi
reference image from these separate exposures and a nightly combined
stack of the dithers (3Pi data described in Chambers et
al. arXiv:1612.05560, and available at http://panstarrs.stsci.edu).
Using techniques discussed in Smartt et al. (2016, MNRAS, 462, 4094),
we located and vetted transients with quality filters and a machine
learning algorithm on the difference images.

No fast evolving transients (with variable lightcurves) were found
during the first night of data (MJD ~ 57913.25 to 57913.30) in the
individual 240s exposures, with limits of z > 18.5 +/- 0.5 (given the
airmass range of 1.9 - 2.8 and poor image quality).

Three transients were found in the stacked data covering several 
nights between 57913 and 57924. However all three are outside the
error radius of 0.24 degrees from Fermi/LAT. In summary, no possible
optical counterpart to the Fermi/LAT source is detected to 
i,z ~ 18.5 (within 24hrs) and i,z ~ 20.5 (daily stacked limits up to 
5 days after). 

The detected sources are below (where AngSep = angular separation
from the Fermi/LAT position in degrees)  

Name      RA (J2000)   Dec (J2000)  Disc. MJD  Disc Mag    AngSep 
PS17diu   08 35 24.13  +44 00 29.5  57913.26    18.82 (z)  0.82
PS17dit   08 27 39.44  +42 36 36.2  57916.28    20.56 (i)  0.78
PS17djl   08 30 31.25  +43 55 04.2  57916.29    20.45 (i)  0.63

PS17diu is associated with SDSS J083524.25+440029.3, an r=19.36 mag
galaxy with a host photoZ=0.083 (+/- 0.038) implying a transient M_i =
-19.36. The lightcurve is relatively flat over 10 days. This is likely
an old SN, and this is likely an old SN around peak.

PS17dit is likely associated with 2MASX J08273960+4236311; a 18.50 mag
galaxy with z=0.151, implying a transient at M = -18.8. The lightcurve
is flat for 4 days, and this is likely an old SN around peak.

PS17djl is likely associated with SDSS J083031.05+435504.3; an r=19.81
mag galaxy with A host photoZ=0.217 (+/- 0.051) implying a transient
at M = -19.7. Again, the lightcurve is flat for 4 days, also
suggesting this is likely an old SN around peak.
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