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GCN Circular 20398

Subject
LIGO/Virgo G268556: iPTF Optical Transient Candidates
Date
2017-01-08T02:57:57Z (7 years ago)
From
Mansi M. Kasliwal at Caltech <mansi@astro.caltech.edu>
M. M. Kasliwal (Caltech), L. P. Singer (NASA/GSFC), E. Karamehmetoglu
(OKC),  S. B. Cenko (NASA/GSFC), R. Quimby (SDSU), D. Cook (Caltech), T.
Barlow (Caltech), V. Bhalerao (IUCAA), J. Rana (IUCAA), A. Goobar (OKC), J.
Sollerman (OKC), R. Amanullah (OKC), Y. Cao (UW Seattle), A. A. Miller
(Northwestern/Adler)

report on behalf of the iPTF and GROWTH collaborations:

We have performed tiled observations of LIGO/Virgo G268556 (LVC, GCN 20364)
using the Palomar 48-inch Oschin telescope (P48). Due to weather, our
observations only began on 2017-01-07 UTC. The northern probability island
from the updated LALInference localization (LVC, GCN 20385) was observable
from Palomar for most of the night. Starting at 13:03 UT (2.88 d after the
LIGO trigger), we imaged 85 of the fields two or more times, spanning 622
square degrees. We estimate a 32% prior probability that these fields
contain the true location of the source.

During preliminary sifting through candidate variable sources using image
subtraction by both our IPAC pipeline (Masci et al. 2016) and NERSC
pipeline (Cao et al. 2016), and applying standard iPTF vetting procedures,
we flagged the following optical transient candidates for further follow-up:

  name       RA        Dec      time  mag    z     notes
-------- ---------- ---------- ----- ----- ------ ---------
iPTF17bo 132.510142 +46.663565 06:13 18.57 0.038   =ATLAS16ecj (GCN 20377)
iPTF17bq 132.192751 +44.006807 06:11 19.11 0.054   spec-z
iPTF17bs 128.827313 +44.022911 06:11 20.35 0.049   photo-z
iPTF17bt 130.174781 +39.306960 06:08 20.05 0.1166  spec-z
iPTF17bu 124.471393 +40.726652 06:06 20.69 0.0608  spec-z
iPTF17bv 124.186115 +41.266753 06:06 19.23 0.039   spec-z
iPTF17cc 125.336819 +35.420771 06:00 20.06 0.064   spec-z
iPTF17ce 122.265555 +24.773640 05:55 20.19 0.350   photo-z
iPTF17cf 122.967123 +25.422642 05:55 19.06 0.199   photo-z
iPTF17ch 123.698029 +27.836746 05:53 20.22 0.118   photo-z
iPTF17ck 115.465769 +29.353904 07:08 19.18 0.169   photo-z
iPTF17cp 115.146554 +17.018280 07:05 19.90 0.072   spec-z
iPTF17cs 115.613615 +13.805968 07:03 19.48 0.066   photo-z
iPTF17cv 114.643374 +23.994179 07:01 20.60 0.081   photo-z
iPTF17dk 120.728531 +16.489364 07:21 20.00 0.069   photo-z
iPTF17dn 115.013574 +13.340217 09:11 19.08 0.037   photo-z
iPTF17du 114.886641 +10.661666 07:15 20.05 0.412   photo-z
iPTF17dz 126.106981 +24.997750 09:40 19.33 0.235   photo-z
iPTF17eb 118.169444 +10.174427 09:20 19.78 0.573   photo-z
iPTF17ec 111.440505 +17.716475 09:13 19.50         no redshift available
iPTF17ee 146.552954 +60.363235 10:07 19.15 0.102   possibly variable
iPTF17ef  117.10532 +20.863792 07:06 20.32 0.162   photo-z
iPTF17eh 116.453873  +9.318206 07:15 19.75 0.077   photo-z
iPTF17ei 114.113854 +25.736748 09:29 19.56 0.101   photo-z
iPTF17ej 116.903063 +25.599466 09:29 20.40 0.543   photo-z
iPTF17ep 137.758943 +62.963989 09:38 19.97 0.115   photo-z
iPTF17et  126.46248 +32.611008 07:30 19.46 0.124   spec-z
iPTF17ez 118.434093 +19.300562 07:14 20.19 0.538   photo-z
iPTF17fa 130.091957 +38.723745 06:08 19.94 0.673   photo-z
iPTF17fb 123.253486 +41.307429 06:06 20.38 0.282   photo-z
iPTF17fc 123.845936 +40.118490 06:04 19.78 0.067   photo-z

Some additional transients not listed above fell just outside the 90%
contour localization contour, e.g. iPTF17cw 135.909904 +43.097300.

Positions are stated in the ICRS. Times are in UTC. Magnitudes are based on
image subtraction; they are in the Mould R filter and in the AB system,
calibrated with respect to point sources in SDSS as described in Ofek et
al. 2012.

None of the above transients show prior history of detection in iPTF
archival images. All of the above transients coincide with a galaxy that is
visible in iPTF and/or SDSS images. Archival spectroscopic or photometric
redshifts of the transients' likely host galaxies are given above. Of the
transients, those that are most consistent with the directional distance
estimate from LIGO (e.g., |[galaxy distance] - [GW mean distance]| < 1.5 *
[GW distance std.
dev.]) are iPTF17ce, iPTF17ck, iPTF17dz, iPTF17ef, and iPTF17ei.

We encourage spectroscopic classification of these candidates.
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