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GCN Circular 13613

Subject
GRB 120714B: GROND/FORS2 detection of a supernova
Date
2012-08-09T18:34:32Z (12 years ago)
From
Sylvio Klose at TLS Tautenburg <klose@tls-tautenburg.de>
S. Klose (TLS), J. Greiner (MPE), J. Fynbo (DARK), A. Nicuesa Guelbenzu, 
S. Schmidl (both TLS), A. Rau (MPE), T. Kruehler (DARK), report on behalf 
of a larger collaboration:

GROND has been following the afterglow of GRB 120714B (Saxton et al., GCN 
13471) at z=0.398 (Fynbo et al., GCN 13477) since July 15 (Nicuesa 
Guelbenzu et al., GCN 13478). The light of the optical transient has been 
fading in all GROND optical bands during the first 1.5 days, was then 
flattening in r',i',z', and finally rising at about 3 days after the 
trigger (while it is flattening in g'). The data suggested that this 
behavior can be interpreted as an upcoming supernova component with a an 
early rise similar to the SN associated with GRB 011121 at z=0.36 (Bloom 
et al. 2002, ApJ 672, 45; Garnavich et al. 2003, ApJ 582, 924; Greiner et 
al. 2003, ApJ 599, 1223; Price et al. 2002, ApJ 572, 51).

Spectroscopy of the optical transient with VLT/FORS2 was performed on 
August 1/2, 18.3 days after the burst. Observations were done with the 
300V grism, covering the wavelength range from 445 to 865 nm, and lasted 
7200 sec. The spectrum reveals very broad features in the continuum with a 
prominent peak around 6500 A and broad lines of Si II and Ca II, in 
agreement with a broad-lined SN dominating the light of the optical 
transient. At present a decision between type Ib and Ic cannot be 
performed due to an uncomfortable presence of atmospheric features in the 
spectrum.

We thank the staff at ESO Paranal and ESO Garching for their excellent 
support and for performing the spectroscopy. GROND observations are 
continuing.
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