TITLE: GCN CIRCULAR NUMBER: 14967 SUBJECT: Fermi394416326: iPTF detection of a possible optical afterglow DATE: 13/07/03 08:24:18 GMT FROM: Leo Singer at CIT/PTF L. P. Singer (Caltech), S. B. Cenko (NASA/GSFC), M. M. Kasliwal (Carnegie Observatories), D. A. Brown (Syracuse), O. Yaron (Weizmann Institute of Science), E. Bellm (Caltech), S. Caudill (Milwaukee), S. Tinyanont (Harvey Mudd), D. Khatami (Pomona), and A. J. Weinstein (Caltech) report on behalf of the intermediate Palomar Transient Factory (iPTF) collaboration: We have imaged 72 deg^2 of the vicinity of the final localization of the Fermi-GBM trigger 394416326 with the Palomar 48 inch Oschin telescope (P48). Images were obtained in the Mould R filter in 2 visits to each of 10 fields. Within the GBM error circle, we detect a bright point source at the position: RA(J2000) = 14h 29m 14.78s DEC(J2000) = +15d 46' 26.4" which is 3.8 degrees away from the center of the final GBM localization (68% confidence radius of 3.99 degrees). At 04:17 UT on 2013 July 2 (4.2 hours after the Fermi-GBM trigger), we measure a magnitude of R = 17.4 for the source, dubbed iPTF13bxl. Nothing was detected at this location in previous P48 images of the field taken on 2011 February 5 to a limiting magnitude of R > 21.1. Automatic follow-up of iPTF13bxl was obtained with the robotic Palomar 60 inch telescope beginning at 4:10 UT on 2013 July 3 (28.1 hours after the burst trigger). At this time we measure a magnitude of r' = 18.7. Assuming the source decays as a single power-law from the time of our P48 to discovery to this time, we infer a decay index of 0.54. We observed iPTF13bxl with the Double Beam Spectrograph on the Palomar 200-inch (P200) on 2013-07-03 04:24:04, 28.3 hours after the burst. The spectrum has a largely featureless blue continuum with no strong, narrow features in emission or absorption between 3800A and 9000A. We triggered target-of-opportunity observations of iPTF13bxl with the Swift satellite, beginning at 00:50 UT on 2013 July 3 (1.03 d after the Fermi-GBM trigger). A total exposure time of 1.4 ks was obtained with the on-board X-Ray Telescope (XRT). A bright source is detected at the location of iPTF13bxl in the XRT. We measure a preliminary count rate of 0.3 ct s^-1 at this time. Assuming a power-law spectrum with a photon index of 2, this corresponds to a 0.3-10.0 keV X-ray flux of ~ 10^-11 erg cm^-2 s^-1. We note that close to iPTF13bxl are two SDSS sources: SDSS J142914.75+154626.0, at a separation of 0.6", a faint source classified as a star with r = 23.01, and SDSS J142914.57+154619.3, at a separation of 7.6", a bright galaxy with a photometric redshift of 0.09 +/- 0.02. Without a secure spectroscopic redshift, we cannot definitively associate this source with the Fermi-GBM GRB. The bright X-ray emission and relatively steep decay in the optical seem to rule out an (unrelated) supernova, but other potential interlopers remain. Follow-up observations are ongoing and encouraged to help identify the nature of this source.