TITLE:   GCN CIRCULAR
NUMBER:  16442
SUBJECT: Fermi 425193729 / iPTF14cyb: Discovery of optical afterglow
DATE:    14/06/23 13:58:46 GMT
FROM:    Varun Bhalerao at IUCAA  <varunb@iucaa.ernet.in>

V. B. Bhalerao (IUCAA), L. P. Singer (Caltech), M. M. Kasliwal
(Carnegie Observatories/Princeton), S. B. Cenko (NASA/GSFC), A. Horesh
(Weizmann Institute) and D. A. Perley (Caltech) report on behalf of
the intermediate Palomar Transient Factory (iPTF) collaboration: 

Fermi GBM reported trigger 425193729 at 2014-06-23 05:22:06.600 UT.
Starting 2014-06-23 05:33:07.776 UT (JD 2456831.73134, 11 min after
trigger), we began our search for optical counterparts using the
Palomar 48-inch Oschin telescope (P48).  Based on the automated Fermi
ground localization, we selected 10 fields covering an area of 74
deg^2.

The final Fermi localization, available 2.6 hours after the trigger,
differed in position by 13.4 deg. Based on the final localization and
an empirical description of the systematic errors of the GBM
localization (Paciesas et al. 2012,
http://dx.doi.org/10.1088/0067-0049/199/1/18), we estimate a 4% chance
that these fields contain 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. The fastest fading
transient was iPTF14cyb, at the coordinates:
  RA(J2000)  =  15h 01m 53.42s  (225.472574 deg)
  Dec(J2000) = +81d 11' 29.1"   (+81.191410 deg)

We note that the position of 1PTF 14cyb is consistent with both the
iniital ground and final localizations.

Measured R magnitudes of iPTF14cyb are
   JD          R
2456831.73527 18.0
2456831.75906 19.3
2456831.78167 19.7

These are consistent with a power-law decay with index -0.93 +- 0.17.

We observed iPTF14cyb with the Gemini Multi-Object Spectrograph
mounted on the 8m Gemini North telescope beginning at 08:10 UT on 2014
June 23 (~ 2.8 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 II, Fe II, Al II, Si II, Al
III, C I, and C IV, at a common redshift of z = 1.92.  The lack of
Ly-alpha absorption in the spectrum suggests 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
= 1.06.

The diagram
http://www.its.caltech.edu/~lsinger/iptf/Fermi425193729.pdf shows the
locations of the afterglow and the 10 P48 fields in relation to the
Fermi GBM 1- and 2-sigma statistical+systematic contours.

We have triggered follow-up observations with Swift, EVLA and CARMA.
We encourage follow-up observations to confirm the nature of the
source. 

We thank GoGo Inflight Wifi for internet at 10,000 feet.