Program: GS-2012A-Q-9

We propose GMOS-South spectroscopy of galaxies near GRB020405, to determine if this GRB was gravitationally lensed. Sixty percent of GRBs reveal strong MgII absorbing systems, a factor of ~2 more than are seen towards quasars at similar redshifts. Wyithe et al 2010 has shown that lensing can explain the difference in covering factor for certain GRB luminosity functions, and we (Rapoport 2011) have shown that the distribution of galaxies towards GRBs is consistent with that expected of gravitational lensing and Wyithe's model. Rapoport 2011 also modelled each GRB, and identified the most likely lensing candidates. The most promising case is that of GRB020405, an event which showed a second, faint transient, 3" away from the GRB, and which a lensing model is consistent with all current observations. This model predicts that the GRB was lensed by a foreground group of galaxies, and makes specific predictions for these galaxies redshifts. We propose to obtain spectroscopic redshifts for these nearby galaxies, to test the gravitational lensing hypothesis for this system.