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GCN Circular 20791

LIGO/Virgo G275697: iPTF Optical Transient Candidates
2017-03-02T07:30:39Z (7 years ago)
Mansi M. Kasliwal at Caltech <>
M. M. Kasliwal (Caltech), S. M. Adams (Caltech), C. Cannella (Caltech), R.
Lunnan (Caltech), R. Ferretti (OKC), T. Kupfer (Caltech), L. P. Singer
(NASA/GSFC), S. B. Cenko (NASA/GSFC), R. Walters (Caltech), T. Barlow
(Caltech), J. Rana (IUCAA), V. Bhalerao (IIT-B), A. A. Miller
(Northwestern/Adler), Y. Cao (UW), R. Laher (IPAC), F. Masci (IPAC)

report on behalf of the iPTF (intermediate Palomar Transient Factory) and
GROWTH (Global Relay of Observatories Watching Transients Happen)

We performed tiled observations of LIGO/Virgo G275404 (LVC, GCN 20738) and
LIGO/Virgo G275697 (LVC, GCN 20763) using the Palomar 48-inch Oschin
telescope (P48) on the night of 2017-03-01 UTC (delay due to inclement
weather). We imaged 84 fields spanning 633 square degrees, with a 13%
chance of containing the true location of G275404. Of these, 47 fields (361
square degrees) were imaged twice, containing 6% probability of containing
G275404, and searched for transient candidates.

During preliminary sifting through candidate variable sources using image
subtraction by our IPAC (Masci et al. 2016) and NERSC (Cao et al. 2016)
pipelines, a total of 140 candidates were saved in the fields imaged.
Applying standard iPTF vetting procedures and removing transients with a
history of previous variability, we flagged 2 optical transient candidates
in the 90% contour of G275697, listed below, for further follow-up.

Name        RA (J2000)      Dec (J2000)     UTC          R-mag
iPTF17bou   148.739102      -7.943791       09:07        18.72
fading (0.3 mag intranight); nuclear/stellar?
iPTF17bpw   147.851104      -9.136385       09:03        19.16
pstar=0.013; nuclear

Positions are stated in the ICRS. Discovery times are noted in UTC hh:mm on
2017-03-01. Magnitudes are based on image subtraction and are in the Mould
R filter, calibrated with respect to point sources in SDSS as described in
Ofek et al. 2012.

We caution that many candidates are outside the SDSS footprint and lack a
secure star/galaxy classification for the underlying source. We flag these
as "nuclear/stellar?". Where available, we provide machine-learning
probability scores on whether the underlying source is a galaxy/star (0/1)
(Miller et al. 2016).

We are grateful to the Palomar crew (especially John Henning, Jeff
Zolkower, Carolyn Heffner, Jamey Eriksen, Nick Ganciu) for their hard work
in reviving a faulty declination encoder essential to collecting this
dataset during the last two days of iPTF survey operations.
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