GCN Circular 20801

R. Lunnan (Caltech), S. Papadogiannakis (OKC), N. Blagorodnova (Caltech),
A. A. Miller (Northwestern/Adler), L. P. Singer (NASA/GSFC), S. M. Adams
(Caltech), C. Cannella (Caltech), S. B. Cenko (NASA/GSFC), R. Walters
(Caltech), T. Barlow (Caltech), J. Rana (IUCAA), V. Bhalerao (IIT-B),  Y.
Cao (UW), R. Laher (IPAC), F. Masci (IPAC) and M.M. Kasliwal (Caltech)

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

We have continued tiled observations of LIGO/Virgo G275404 (LVC, GCN
20738
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 ) and LIGO/Virgo G275697 (LVC, GCN 20763
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 ) using the Palomar 48-inch
Oschin telescope (P48). Including our first night of observations on
2017-03-01 UTC (GCN 20791
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 ) and a second night on 2017-03-02, we have
observed a total of 256 fields spanning 1747 square degrees. We estimate a
20% chance that these fields contain the true location of G275404 and a
25% chance that they containing the true location of G275697.

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 115 candidates were saved in the fields imaged.
Applying standard iPTF vetting procedures and removing transients with a
history of previous variability, we flagged 9 additional optical transient
candidates in the 90% localization contour of G275404, listed below, for
further follow-up.


iPTF17buf   98.583906       76.596292       05:24  19.59
pstar=0.962; nuclear/stellar?
iPTF17bug   97.67324        77.159053       05:24  18.6    0.0166  specz;
nuclear
iPTF17bur   295.1181        58.707877       11:38  19.14
pstar=0.016; nuclear/stellar?
iPTF17buu   114.100254      74.505262       05:33  19.92           Hostless
in iPTF reference but faint counterpart seen in PanSTARRS.
iPTF17buw   127.499818      73.935927       06:08  20.23
pstar=0.948; nuclear/stellar?
iPTF17bvo   133.140294      86.061384       06:11  19.55           nuclear;
pstar=0.042
iPTF17bvt   115.702547      74.553167       05:33  19.57
nuclear/stellar?; looks like three-star blend in PS1
iPTF17bwl   150.611516      8.822758        08:23  18.77   0.114   specz;
also detected 2017-03-01; nuclear; SDSS QSO
iPTF17bxk   149.643558      6.963137        08:23  19.99   0.077   specz;
offset 13.5" from the center of 2MASX J09583513+0657389

Two known transients detected by Pan-STARRS on 2017-02-16 also fall in this
localization region and were recovered with iPTF : PS17bjk (iPTF17bez) and
PS17bjd (iPTF17bfa).

Positions are stated in the ICRS. Discovery times are noted in UTC hh:mm on
2017-03-02. Magnitudes are based on image subtraction and 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 encourage spectroscopic classification of these candidates, and
highlight iPTF17bug as a local candidate (d < 200 Mpc).

We are grateful to the Palomar crew (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.