GCN Circular 14666

Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC),
William H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori
Fox (UCB), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino
Cucchiara (UCSC), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), Jos� A. de Diego (UNAM), Leonid Georgiev (UNAM),
Jes�s Gonz�lez (UNAM), Carlos Rom�n-Z��iga (UNAM), Neil Gehrels (GSFC),
and Harvey Moseley (GSFC) report:

We have continued to monitor GRB 130427A with the Reionization and
Transients Infrared Camera (RATIR; www.ratir.org) on the 1.5m Harold
Johnson Telescope at the Observatorio Astron�mico Nacional on Sierra San
Pedro M�rtir, obtaining homogenous photometry in griZYJH. We have
photometry for every night except 2013 May 6. On most nights our
photometric uncertainties in gri are about 2%.

As we reported earlier in Watson et al. (GCN Circular 14606
Not found
 
 
), the
optical afterglow during the first day is well-fitted by a power law
with a temporal index of -1. However, around T+1d there is a break, and
the power law steepens. From T+2.5d to T+14.9d our gri photometry is
well-fitted by a power law with a temporal index close to -1.5 plus a
constant component consistent with the presumed SDSS host galaxy.

However, our observations at T+15.9d, T+16.9d, and T+17.9d are
systematically brighter than this fit. Adding a new component starting
at T+15.5d with zero colors and constant magnitude significantly
improves the fit (with a confidence level of better than 99.5%). The
constant component has

  g = r = i = 24.53 � 0.25.

We do not mean to suggest that the new component actually has zero color
or constant magnitude. However, at this moment our data cannot usefully
constrain anything other than a characteristic brightness.

Our data, model, and residuals are shown at

  https://dl.dropboxusercontent.com/u/528672/GCN/2013-05-16-GRB-130427A.pdf

Assuming a distance modulus of 41.26, the new component corresponds to
an absolute magnitude of -16.7 � 0.25. If the new component is a Type 1c
supernova, as suggested by de Ugarte Postigo et al. (GCN Circular
14646
Not found
 
 
), we might expect the peak extinction-corrected absolute magnitude to be
around -18 (Drout et al. 2011, ApJ, 741, 97). Thus, depending on the
host galaxy extinction, we might be seeing this possible supernova at or just
before its peak.

We caution that the new component is currently about 2 magnitudes
fainter than the afterglow component, which at 18.0 days is predicted to
have

  i = 22.21 � 0.04

and even fainter then the galaxy, which is predicted to have

  i = 21.23 � 0.03.

The relative brightnesses of the new component, the fading afterglow,
and the host galaxy also have significant implications for unveiling the
spectrum of the possible supernova.

We further caution that from our data alone we cannot exclude the
possibility that the new component might simply be a significant
flattening of the late afterglow component.

The largest residuals of our observations from the model (with or
without the new component) are at the level of 0.05 magnitudes. We do
not see the large variations reported by Trotter et al. (GCN Circular
14662
Not found
 
 
).

Further observations are planned.

We thank the staff of the Observatorio Astron�mico Nacional in San Pedro
M�rtir.