TITLE: GCN CIRCULAR NUMBER: 19760 SUBJECT: IceCube-160731A: iPTF P48 Observations DATE: 16/08/04 00:11:38 GMT FROM: Leo Singer at GSFC/iPTF L. P. Singer (NASA/GSFC), M. M. Kasliwal (Caltech), Varun Bhalerao (IUCAA), Anna Ho (Caltech), Tom Barlow (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-160731A (http://gcn.gsfc.nasa.gov/notices_amon/6888376_128290.amon), we began searching for optical counterparts using the Palomar 48-inch Oschin telescope (P48) on 2016-07-31 at 05:22 UT, 3.5 hours after the trigger. We imaged one field covering an area of 7.1 deg^2 to a limiting magnitude of R=19.1--19.7. Unfortunately, the EHE localization coincides with an inoperable CCD. Sifting through candidate variable sources using image subtraction and standard iPTF vetting procedures, we do not detect any optical transients close to the IceCube error circle. Two optical transient candidates are found at 1.1 and 2.0 deg from the neutrino candidate position: name RA Dec sep mag t ----------------------------------------------- iPTF16elf 213.555124 -0.894361 1.1 17.25 3.5 iPTF16elg 212.640918 -0.988094 2.0 18.71 3.75 The two transients are positionally consistent with the nuclei of their respective host galaxies, CGCG 018-088 at z=0.038 and SDSS J141033.84-005918.0 at z=0.054. Positions are in J2000. Separations are in degrees and relative to the best-estimate IceCube position (214.5440d -0.3347d). 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 hours since the IceCube trigger. The diagram http://asd.gsfc.nasa.gov/Leo.Singer/iptf/IceCube-160731.pdf shows the P48 field in relation to the IceCube 1-sigma error circles.