[vsnet-grb-info 15205] Fermi 425667201: optical counterpart search

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  16497
SUBJECT: Fermi 425667201: optical counterpart search
DATE:    14/06/30 17:45:25 GMT
FROM:    Leo Singer at CIT/PTF  <lsinger at caltech.edu>

L. P. Singer (Caltech), M. M. Kasliwal (Carnegie Observatories/Princeton),
V. B. Bhalerao (IUCAA), S. B. Cenko (NASA/GSFC), Y. Cao (Caltech),
G. Duggan (Caltech), D. A. Perley (Caltech), and J. Johansson
(Stockholm University) report on behalf of the intermediate Palomar
Transient Factory (iPTF) collaboration:

Fermi GBM reported trigger 425667201 at 2014-06-28 16:53:18.98 UT.
Starting 2014-06-29 09:17:13 UT (JD 2456837.88696, 16.4 hours after
the trigger), we began our search for optical counterparts using the
Palomar 48-inch Oschin telescope (P48). Based on the final Fermi GBM
localization, we selected 10 fields covering an area of 76 deg^2,
with a 20% chance of containing the true location of the source.

Sifting through candidate transient sources using image subtraction
and standard intermediate Palomar Transient Factory vetting
procedures, we detected several optical transients, of which we list
five below.

iPTF14deq, at r = 19.74 +/- 0.05 mag, at the coordinates:
 RA(J2000)  =  23h 49m 29.57s (357.373189 deg)
 Dec(J2000) = +30d 47' 50.4"  (+30.797328 deg)

iPTF14det, at r = 17.62 +/- 0.01 mag, at the coordinates:
 RA(J2000)  =  23h 29m 53.05s (352.471034 deg)
 Dec(J2000) = +29d 12' 12.3"  (+29.203413 deg)

iPTF14dev, at r = 19.13 +/- 0.03, at the coordinates:
 RA(J2000)  =  23h 44m 29.96s (356.124852 deg)
 Dec(J2000) = +27d 04' 26.2"  (+27.073938 deg)

iPTF14dfc, at r = 20.58 +/- 0.09 mag, at the coordinates:
 RA(J2000)  =  23h 39m 53.79s (354.974131 deg)
 Dec(J2000) = +35d 23' 43.3"  (+35.395371 deg)

iPTF14dfd, at r = 20.24 +/- 0.07 and rising, at the coordinates:
 RA(J2000)  =  23h 55m 11.06s (358.796090 deg)
 Dec(J2000) = +27d 05' 21.2"  (+27.089213 deg)

The first four (iPTF14deq, iPTF14det, iPTF14dev, iPTF14dfc) showed no
significant optical evolution over the course of an hour, nonetheless
consistent (within the uncertainty in our photometry) with a fading
of ~0.08 mag per hour for a typical optical afterglow (assuming a
decay index alpha ~ -1). The last, iPTF14dfd, brightened by
0.65 +/- 0.01 mag over the course of half an hour.

The first four had plausible nearby host galaxies in SDSS, but none
had known redshifts. However, the candidates listed above had host
galaxies with photometric redshifts that were compatible with the
absolute magnitudes of typical long GRB afterglows, M_B >~ -20 (see
Kann et al. 2011, http://dx.doi.org/10.1088/0004-637X/734/2/96).

We obtained spectra of these five candidates with the Low Resolution
Imaging Spectrometer (LRIS) mounted on the 10-m Keck I telescope. None
of the spectra are consistent with an optical afterglow.

We interpret iPTF14deq as an SN II at z=0.083 due to a blue continuum
and weak Ha emission lines with a low-velocity P-cygni profile.

We classify iPTF14det as an SN II-p at z=0.0197 due to broad Ha
emission with a pronounced P-cygni profile.

iPTF14dev is a type Ia supernova at a redshift of z=0.0655 with a
spectrum consistent with a phase of about 6 days before maximum light.

iPTF14dfc is a type I (probably, type Ia or Ic) supernova. Based on fitting the
SN features with SNID (Stephane & Tonry 2007, http://dx.doi.org/10.1086/520494),
we tentatively suggest a redshift of z=0.25.

In iPTF14dfd, we find double-peaked Ha emission lines and He II 4686 at a
redshift of z=0, superimposed on a blue continuum. Combined with its rapid
brightening and lack of an associated galaxy, it is likely a CV outburst.

The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi425667201.pdf
shows the locations of the optical transients and the 10 P48 fields
in relation to the Fermi GBM 1- and 2-sigma statistical+systematic
contours.

We thank the Fermi-GBM team for promptly sharing their detailed
localizations with us.