[vsnet-grb-info 13650] Fermi394416326: iPTF detection of a possible optical afterglow

[GCN Circulars]

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  <lsinger at caltech.edu>

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.