[vsnet-grb-info 5760] GRB080310, unusual slow decay in the optical

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7414
SUBJECT: GRB080310, unusual slow decay in the optical
DATE:    08/03/12 14:17:59 GMT
FROM:    Peter Garnavich at U of Notre Dame  <pgarnavi at nd.edu>

P. Garnavich (Notre Dame), J. L. Prieto, R. Pogge (OSU) report:

Beginning March 12.48 (UT), we again observed the position of
GRB 080310 (Chornock et al. GCN 7381; Cummings et al., GCN 7382)
with the Large Binocular Telescope and LBC red and blue cameras.
The seeing was 0.7 arcsec in the r-Sloan filter.

The brightness of the afterglow 2.12 days after the burst is measured
to be r=21.67+/-0.05 mag, or only 0.3 mag fainter than the previous
night. This corresponds to a power-law decay index of only 0.42
and confirms the slowing in the decay rate noted in GCN 7409.

It is unlikely that the slow fade is due to contamination from
a host galaxy or a galaxy along the line of sight. To explain the
observed decay rate, a galaxy would have to contribute at a
magnitude near r=22.5, but nothing at this brightness is detected
in the SDSS at the position of the afterglow. Also, the afterglow is
consistent with a point source in the LBT images obtained in good seeing.

We conclude that the shallow decay rate is intrinsic to the afterglow
and this suggests either a continuing energy input or that the shock has
encountered a change in density of the ambient medium. (We must note
another possibility: the afterglow is being gravitationally
microlensed.)

The LBT is an international collaboration among institutions in the
United States, Italy and Germany. The 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
*  The Research Corporation, on behalf of The University of Notre Dame,
University of Minnesota and University of Virginia

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