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GCN Circular 17079

Subject
GRB 141118A: iPTF Optical Observations
Date
2014-11-21T07:37:46Z (10 years ago)
From
Leo Singer at CIT/PTF <lsinger@caltech.edu>
L. P. Singer (Caltech), M. M. Kasliwal (Carnegie Observatories/Princeton),
V. B. Bhalerao (IUCAA), and S. B. Cenko (NASA/GSFC) report on behalf of
the intermediate Palomar Transient Factory (iPTF) collaboration:

We searched for optical counterparts of GRB 141118A (GCN 17073, Hurley
et al.; GCN 17074, Golenetskii et al.) using the Palomar 48-inch Oschin
telescope (P48). At about 18 hours after the burst, we began imaging 20
fields spanning an area of 145 deg2, covering most of the 1-sigma
statistical+systematic region of the final Fermi GBM localization (Fermi
trigger 438020153 / 141118678). Based on the GBM localization, we estimate
a 77% chance that these fields contain the true location of the source.
Coincidentally, we estimate the same containment probability based on the
IPN error box.

Sifting through candidate variable sources using image subtraction and
standard iPTF vetting procedures, we detect the optical transient
iPTF14hvs, at the coordinates:
  RA(J2000)  =  10h 26m 05.29s (156.522055 deg)
  Dec(J2000) = +18d 34' 08.6"  (+18.569044 deg)
This position is just inside the error box. It is also in the outskirts of
the galaxy SDSS J102605.09+183411.6, which has a photoZ of ~0.1. In host-
subtracted P48 photometry, we find marginal evidence that the source faded:
  +18.41 hours:  R = 19.92 +/- 0.14
  +19.06 hours:  R = 20.03 +/- 0.08
Times are relative to the GBM trigger. Magnitudes are in the Mould R filter
and in the AB system, calibrated with respect to point sources in SDSS as
described in Ofek et al. (2012, http://dx.doi.org/10.1086/664065).

We obtained a series of spectra of iPTF14hvs with the Gemini Multi-Object
Spectrograph mounted on the 8 m Gemini North telescope beginning at 15:21
UT on 2014 Nov 11. The spectra cover the wavelength range of 3900-9200 A.
Cross-correlation with a library of supernova spectra using the
``Supernova Identification'' code (SNID; Blondin and Tonry 2007, ApJ 666,
1024) indicates the source is a type Ia supernova several weeks past
maximum light at z ~ 0.11. Spatially offset from the transient location we
identify narrow emission lines of H-alpha, [N II], and [S II] at a common
redshift of z = 0.108.

We conclude that iPTF14hvs is not related to the GRB.

The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi438020153.pdf
shows our P48 fields and the location of the optical transient in relation
to the Fermi GBM 1- and 2-sigma statistical+systematic contours and the
IPN annulus.

We thank the IPN team for promptly sharing their analysis.
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