TITLE: GCN CIRCULAR NUMBER: 18106 SUBJECT: Fermi GBM GRB Trigger 458852670: iPTF P48 Observations DATE: 15/08/02 01:22:55 GMT FROM: S. Bradley Cenko at NASA/GSFC S. B. Cenko, L. P. Singer (NASA/GSFC), M. M. Kasliwal (Carnegie), and V. Bhalerao (IUCAA) report on behalf of a larger collaboration: We conducted follow-up observations of Fermi trigger 458852670 (t0 = 2015-07-17 19:04:26.72) with the Palomar 48-inch Oschin telescope (P48). Observations began at 4:15 UT on 2015-07-18, ~ 9.2 hours after the GBM trigger time. We imaged 20 fields covering an area of 145 deg^2 in and around the 1-sigma statistical+systematic uncertainty region of the final Fermi GBM localization. We estimate an 11% a priori 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: iPTF15bts, at the coordinates: RA(J2000) = 15h 23m 52.94s (230.970585 deg) Dec(J2000) = +31d 48' 39.3" (+31.810910 deg) The source is located near the resolved galaxy SDSS J152352.73+314839.8, with a spectroscopic redshift of 0.074. The following P48 photometry indicates no evidence for significant intra-night variability: +9.28 hr: R = 19.96 +/- 0.07 +11.50 hr: R = 20.14 +/- 0.10 iPTF15btq, at the coordinates: RA(J2000) = 16h 10m 59.75s (242.748965 deg) Dec(J2000) = +29d 58' 02.9" (+29.967474 deg) The source is located near the resolved source SDSS J161059.19+295804.3. The following P48 photometry indicates no evidence for significant intra-night variability: +9.64 hr: R = 20.31 +/- 0.09 +11.86 hr: R = 20.24 +/- 0.07 A low resolution spectrum obtained with the Double Spectrograph (DBSP) on the Palomar 200-inch Hale Telescope at 6:27 UT on 2015-07-23 suggest that the source is a Type II SN (based on a blue continuum and broad Balmer absorption features) at a redshift of z = 0.094 (from SN features). iPTF15btp, at the coordinates: RA(J2000) = 15h 55m 43.81s (238.932550 deg) Dec(J2000) = +34d 05' 58.1" (+34.099463 deg) The source is located near the resolved source SDSS J155543.32+340557.1. The following P48 photometry indicates some evidence for intra-night variability: +9.50 hr: R = 20.88 +/- 0.17 +11.72 hr: R = 20.33 +/- 0.10 A low resolution spectrum obtained with DBSP on the Palomar 200-inch Hale Telescope at 5:10 UT on 2015-07-23 suggest that the source is a Type II SN (based broad Balmer and Ca absorption features) at a redshift of z = 0.047 (from SN features). iPTF15btm, at the coordinates: RA(J2000) = 16h 12m 45.71s (243.190463 deg) Dec(J2000) = +25d 40' 39.2" (+25.677557 deg) The source is located near the resolved source SDSS J161245.43+254037.0. The following P48 photometry indicates no evidence for significant intra-night variability: +9.69 hr: R = 20.96 +/- 0.15 +11.92 hr: R = 20.90 +/- 0.18 iPTF15bsp, at the coordinates: RA(J2000) = 15h 54m 21.59s (238.589958 deg) Dec(J2000) = +32d 55' 03.8" (+32.917731 deg) The source is located near the resolved galaxy SDSS J155421.06+325508.3, with a spectroscopic redshift of z = 0.061. The following P48 photometry indicates no evidence for significant intra-night variability: +9.36 hr: R = 18.33 +/- 0.03 +11.58 hr: R = 18.26 +/- 0.04 iPTF15bsn, at the coordinates: RA(J2000) = 15h 22m 31.95s (230.633117 deg) Dec(J2000) = +30d 01' 20.3" (+30.022302 deg) The source is located near the resolved source SDSS J152232.07+300104.5, with a spectroscopic redshift of z = 0.066. The following P48 photometry indicates some evidence for intra-night variability: +9.81 hr: R = 18.91 +/- 0.03 +11.47 hr: R = 18.90 +/- 0.03 A low resolution spectrum obtained with FLOYDS on the Faulkes Telescope North on 2015-07-18 suggest that the source is a Type Ia SN. iPTF15bsk, at the coordinates: RA(J2000) = 15h 35m 12.81s (233.803394 deg) Dec(J2000) = +25d 08' 29.3" (+25.141482 deg) The source is located near the resolved galaxy SDSS J153512.98+250827.8, with a spectroscopic redshift of z = 0.050. The following P48 photometry indicates no evidence for significant intra-night variability: +9.19 hr: R = 19.46 +/- 0.04 +10.86 hr: R = 19.46 +/- 0.04 iPTF15bsh, at the coordinates: RA(J2000) = 15h 21m 22.85s (230.345221 deg) Dec(J2000) = +22d 29' 59.0" (+22.499726 deg) The source is located in the nucleus of the resolved source SDSS J152122.84+222958.9 (photo-z = 0.12 +/- 0.02). The following P48 photometry indicates no evidence for significant intra-night variability: +9.15 hr: R = 19.39 +/- 0.06 +11.39 hr: R = 19.39 +/- 0.05 Times are relative to the GBM trigger. 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). We do not consider any of these sources to be likely candidates for the optical afterglow of Fermi trigger 458852670. The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi458852670.pdf shows the locations of our candidates and the P48 fields in relation to the Fermi GBM 1- and 2-sigma statistical+systematic contours.