ACP Seminar (Astronomy - Cosmology - Particle Physics)

Speaker: Sharon Rapoport (Mt. Stromlo Observatory, Australian National University)
Title: Gravitational Lensing as the Source of Enhanced Strong MgII Absorption Towards Gamma-Ray Bursts
Date (JST): Thu, Nov 24, 2011, 13:30 - 15:00
Place: Seminar Room A
Related File: 528.pdf
Abstract: Sixty percent of gamma-ray bursts (GRBs) reveal strong MgII absorbing systems, which is a factor of ~2 times the rate seen along lines-of-sight to quasars. Previous studies argue that the discrepancy in the strong MgII covering factor is most likely to be the result of either quasars being obscured due to dust, or the consequence of many GRBs being strongly gravitationally lensed. We analyze observations of GRBs that show strong foreground MgII absorption. We find that GRB lines of sight pass closer to bright galaxies than would be expected for random lines of sight within the impact parameter expected for strong MgII absorption. While this cannot be explained by obscuration in the GRB sample, it is a natural consequence of gravitational lensing. Upon examining the particular configurations of galaxies near a sample of GRBs with strong MgII absorption, we find several intriguing lensing candidates. We suggest that lensing is a viable explanation of the MgII enhancement in GRBs, and outline the future observations required to test this hypothesis conclusively.