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 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 This message may be cited.