GCN Circular 16425

M. M. Kasliwal (Carnegie Observatories/Princeton), S. B. Cenko
(NASA/GSFC), and L. P. Singer (Caltech) report on behalf of the
intermediate Palomar Transient Factory (iPTF) collaboration:

Fermi GBM reported trigger 424934131 at 2014-06-20 05:15:28.02 UT.
Starting 2014-06-20 05:25:25 UT (10 min after trigger),
we began our search for optical counterparts using the Palomar 48-inch
Oschin telescope (P48). Based on the the ground and final Fermi GBM
localizations, we observed 20 fields covering 147 deg^2, with an
estimated 59% chance of containing the true location of the event.

Sifting through candidate transient sources using image subtraction
and standard intermediate Palomar Transient Factory vetting procedures,
we detected several optical transients. The fastest fading transient was:

iPTF14cva, at the coordinates:
  RA(J2000)  =  18h 47m 29.01s (281.870873 deg)
  Dec(J2000) = +49d 43' 51.7"  (+49.731037 deg)

iPTF14cva faded from R=17.6 mag to R=18.8 mag between 05:30 UT to 06:42 UT.

We observed iPTF14cva with the Gemini Multi-Object Spectrograph
mounted on the 8m Gemini North telescope beginning at 14:00 UT on
2014 June 20 (~ 8.8 hr after the GBM trigger). Two 900 s spectra were
obtained, covering the wavelength range from 4000-9300 A.
Super-imposed on a relatively flat continuum, we detect a number of
strong absorption features, including Mg I, Mg II, Fe II, Al II, Si
II, and Si II*, at a common redshift of z = 2.04. The detection of
the fine structure Si II* feature, together with the lack of Ly-alpha
absorption in the spectrum, imply that this is the redshift of the
Fermi GBM GRB. In addition, we detect strong absorption features (Mg
II, Fe II) from an intervening system at a redshift of z = 0.88.

The diagram
http://www.its.caltech.edu/~lsinger/iptf/Fermi424934131.pdf shows the
locations of the afterglow and the 20 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.