TITLE:   GCN CIRCULAR
NUMBER:  20790
SUBJECT: LIGO/Virgo G275404: iPTF Optical Transient Candidates
DATE:    17/03/02 07:29:15 GMT
FROM:    Mansi M. Kasliwal at Caltech  <mansi@astro.caltech.edu>

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)
collaborations:

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 17%
chance of containing the true location of G275404. Of these, 47 fields (361
square degrees) were imaged twice, containing 11% of the 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 27 optical transient candidates
in the 90% localization contour of G275404, listed below, for further
follow-up.

Name         RA              Dec            UTC          R-mag      z
Notes
iPTF17bpa   150.966591      1.095135        09:07        18.72   0.0456
specz; off center from host
iPTF17bpb   154.902074      -2.992951       09:25        19.56   0.0511
photz; pstar=0.016; nuclear?
iPTF17bpc   155.671912      -3.647882       09:25        19.31
pstar=0.001; nuclear?
iPTF17bpd   155.181672      -4.163867       09:25        19.28
pstar=0.804; nuclear/stellar?
iPTF17bpk   157.626592      -11.121047      10:20        19.63
nuclear/stellar?
iPTF17bpm   161.324311      -14.376753      10:04        19.26
pstar=0.229; nuclear/stellar?
iPTF17bpn   161.238427      -14.017486      10:04        19.56
pstar=0.46; nuclear/stellar?
iPTF17bpo   158.494163      -12.65084       10:02        19.03
pstar=0.96; nuclear/stellar?
iPTF17bps   161.952768      -15.243493      10:04        19.76
hostless (very slow mover?)
iPTF17bpt   157.657157      -7.372908       09:29        19.49
fading (0.5 mag intra-night); pstar=0.89; nuclear/stellar?
iPTF17bpu   157.559861      -6.799173       09:29        19.81
pstar=0.612; nuclear/stellar?
iPTF17bpy   156.687516      -7.684226       09:29        19.98
pstar=0.933; nuclear/stellar?
iPTF17bqc   158.470076      -11.493302      10:02        19.62
pstar=0.717; nuclear/stellar?
iPTF17bqh   156.749276      -10.358467      10:00        19.49
rising (0.4 mag intra-night); nuclear/stellar?
iPTF17bqi   156.87908       -7.04181        09:29        19.41
pstar=0.962; nuclear/stellar?
iPTF17bqk   162.188261      -15.49746       10:04        19.76
nuclear/stellar?
iPTF17bqz   157.651689      -7.696641       09:29        19.83
pstar=0.885; nuclear/stellar?
iPTF17bro   288.344596      58.81211        10:59        19.82
rising (0.5 mag intra-night), nuclear?
iPTF17brp   291.671757      79.220007       11:47        19.42   0.076
photz; pstar=0.056; off center from host galaxy
iPTF17brr   289.432632      79.995864       11:47        19.88
pstar=0.898; nuclear/stellar?
iPTF17bru   294.594305      63.706556       11:36        19.95          off
center from host galaxy
iPTF17bsd   156.642225      -10.970993      10:00        19.68
rising (0.3 mag intra-night); nuclear
iPTF17bsi   297.077103      78.352918       12:01        19.08   0.014
photz; off center from host galaxy
iPTF17bst   156.998904      -7.216924       09:29        19.87
pstar=0.973; nuclear/stellar?
iPTF17btb   302.938179      72.930472       12:03        18.56
nuclear/stellar?
iPTF17btc   309.740925      72.580678       12:03        18.97
nuclear/stellar?
iPTF17bth   307.33055       68.241724       12:05        19.31
nuclear/stellar?

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 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. In
particular, we highlight iPTF17bpt, iPTF17bqh, iPTF17bsd as fast-evolving
by more than 0.3 mag in the same night; iPTF17bpa and iPTF17bsi as local
transients (d < 200 Mpc), and iPTF17bpa and iPTF17btb as bright transients
(m < 19 mag).

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.