TITLE:   GCN CIRCULAR
NUMBER:  14631
SUBJECT: GRB 130427A: Tautenburg 2nd epoch: No break, no clear SN
DATE:    13/05/14 02:39:21 GMT
FROM:    Alexander Kann at TLS Tautenburg  <kann@tls-tautenburg.de>

D. A. Kann, B. Stecklum, and C. Hoegner (TLS Tautenburg) report:

We observed the optical afterglow position (Elenin et al., GCN 14450) of
GRB 130427A (Maselli et al., GCN 14448) with the 1.34m Schmidt telescope
of the Thueringer Landessternwarte Tautenburg equipped with the 2k CCD
camera under good weather conditions. We obtained 3 x 600 sec frames in
the Rc band. The afterglow is detected in each frame.

Using the nearby star given in Rumyantsev et al. (GCN 14582) and also used
in our first epoch observations (Kann et al., GCN 14592), we derive a
preliminary magnitude of Rc = 20.37 +/- 0.07 at 15.54806 days after the
GRB.

This magnitude is only insignificantly fainter than the one we derived in
the first epoch, evidencing a clear flattening (see also Xu et al., GCN
14597). The host galaxy is expected to have about 21st magnitude in Rc
(Vega) following r' = 21.26 from SDSS (see, e.g., Watson et al., GCN
14606). Subtracting this magnitude from our detection yields a magnitude
for the optical transient of ~ 21.2 +/- 0.2.  This value agrees well with
an extrapolation of the earlier slope, implying that no further break has
occurred in the optical light curve (in agreement with the X-ray decay,
which shows a very similar slope). This implies either that the post-jet
break decay is among the most shallow known, or that a jet break has still
not occurred, pushing GRB 130427A further into the territory of hyper-
luminous events (Fan et al., arXiv:1305.1261, though see Laskar et al.,
arXiv:1305.2453).

The situation concerning a rising supernova is still unclear. Xu et al.
(GCN 14597) claimed a host-independent flattening and spectral change,
which was afterwards disputed on photometric (Watson et al., GCN 14606;
Perley & Tang, GCN 14615) and spectroscopic (Garnavich, GCN 14605; Perley
& Tang, GCN 14615) grounds. Our measurement offers no solution to this
conundrum, but it is possible that the SN, even if as luminous as SN
1998bw, will peak at a magnitude significantly fainter than the host
galaxy and afterglow (Ruffini et al., GCN 14526), making detection more
difficult than even in the case of GRB 030329/SN 2003dh.

We wish to thank T. Kruehler for discussions relating to the host galaxy.

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