[vsnet-grb-info 17945] IceCube-160427A: iPTF Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  19392
SUBJECT: IceCube-160427A: iPTF Observations
DATE:    16/05/06 03:05:02 GMT
FROM:    Leo Singer at GSFC/iPTF  <leo.p.singer at nasa.gov>

L. P. Singer (NASA/GSFC), M. M. Kasliwal (Caltech), S. B. Cenko (NASA/GSFC),
E. O. Ofek (Weizmann), N. Strotjohann (DESY), A. Franckowiak (DESY),
M. Kowalski (DESY), and G. Tesic (PSU) report on behalf of the
Intermediate Palomar Transient Factory (iPTF) collaboration:

Following the detection and updated localization of the cosmic neutrino
candidate IceCube-160427A (GCN 19363), we began searching for optical
counterparts using the Palomar 48-inch Oschin telescope (P48) on
2016-05-04 at 05:56 UT, 7.0 days after the trigger. The delay was due to
prolonged adverse weather conditions at Palomar.

We imaged 2 fields covering an area of 16 deg^2 to a limiting magnitude of
R=20.6--21, enclosing the entire final IceCube error circle. We estimate a
nearly 100% prior probability that these fields contain the true location
of the source. Sifting through candidate variable sources using image
subtraction and standard iPTF vetting procedures, we detected the
following optical transient candidates:

   name       RA        Dec     sep  mag   t   
-----------------------------------------------
iPTF16apj 240.638236  +7.247857 2.1 20.07 7.00
iPTF16apk 240.284222  +7.124179 2.2 20.58 7.00
iPTF16apl 241.437244  +9.961542 1.1 20.27 7.00

All three transients are positionally consistent with the nuclei of their
respective host galaxies. All three have a prior history of variability in
iPTF dating to March 2016.

We checked for excess emission at the positions of the SN candidates
reported by Pan-STARRS (GCN 19381). PS16cgv is at the edge of an iPTF
chip, so reliable image subtraction is not possible. None of the other
candidates had significant detectable excesses in our difference images,
not surprising given the shallower depth of our observations relative to
Pan-STARRS.

Positions are in J2000. Separations are in degrees and relative to the
best-estimate IceCube position (240.57 +9.34). Magnitudes 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, http://dx.doi.org/10.1086/664065).
Times are in days since the IceCube trigger.