D. Xu (DARK/NBI), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), T.
Kruehler, D. Malesani (DARK/NBI), G. Leloudas (OKC, Stockholm and
DARK/NBI), J.P.U. Fynbo, J. Hjorth, (DARK/NBI), S. Schulze (PUC and
MCSS), P. Jakobsson, Z. Cano (U. Iceland), J. Gorosabel
We have been monitoring the optical counterpart of GRB 130427A
(Maselli et al., GCN 14448; Elenin et al., 14450) starting 12.85 hr
after the GRB trigger (Xu et al., GCN 14478), mainly using the 2.5
Nordic Optical Telescope (NOT) equipped with the ALFOSC camera.
Observations were carried out using the SDSS filters.
The light curve between ~1.0 and 5.0 days after the trigger (observer
frame) is well fit by a power law with decay index 1.3. Starting from
day 5.0, however, the light curve gradually flattens. The flattening,
albeit reduced, is still evident after subtracting the (known) flux
contribution of the host galaxy. In particular, clear flux in excess
of the afterglow and host contribution is apparent on May 5 and 6,
that is 8.6 and 9.6 days after the GRB.
Photometry in the Sloan griz filters was secured during the night of
May 6. After subtracting from the observed flux the host contribution,
and correcting for the (small) Galactic extinction, the SED clearly
deviates from a power-law, in sharp contrast with our earlier
measurements and the typical spectrum of GRB afterglows. Instead, the
griz SED shows a broad hump peaking in the i and r bands, which is
roughly consistent with the spectrum of other broad-lined SNe
associated with GRBs at comparable epochs (e.g., SN 1998bw: Patat et
al. ApJ, 555 900; SN 2006aj: Pian et al., Nat. 442,1011).
The flattening in the decay, the change of the spectral shape, and the
overall flux level are all consistent with the emergence of a SN,
though detailed spectroscopy and long-term monitoring will be required
to fully assess the nature of the flux excess.