GCN Circular 14605

P. Garnavich (Notre Dame) reports:

A spectrum of the GRB 130427A afterglow was obtained with the
Large Binocular Telescope (LBT+MODS1 instrument) on 2013 May 7.15 (UT),
9.8 days after the burst. The spectrum covers 340 nm to 950 nm and
is dominated by a power-law continuum. Narrow Balmer, [OII], [OIII]
emission lines, and MgII and MgI absorption lines from the host
galaxy are present at a redshift of 0.340.

The LBT spectrum shows no obvious undulations characteristic of a
broad-lined type Ic supernova such as SN 1998bw. In contrast,
a 98bw-like supernova was detectable from GRB 030329 around
seven days after its burst (Stanek et al. 2003, ApJ, 591, L17).
This early detection was primarily due to the prominent peak
seen around 500 nm (rest frame) in broad-lined type Ic events.

Adding a pre-maximum spectrum of SN 1998bw
(Patat et al. 2001, ApJ, 555, 900) to a power-law continuum
suggests that any 98bw-like supernova is at least an R-band
magnitude fainter than the afterglow 10 days after the
GRB 130427A burst.

I thank Rick Pogge, Paul Martini and Scott Adams for help in
obtaining the spectra.
The LBT is an international collaboration among institutions in the
United States, Italy and Germany. LBT Corporation partners are:
The University of Arizona on behalf of the Arizona university system;
Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft,
Germany, representing the Max-Planck Society, the Astrophysical
Institute Potsdam, and Heidelberg University; The Ohio State University,
and The Research Corporation, on behalf of The University of Notre Dame,
University of Minnesota and University of Virginia.