The Optical Contents of the IPN1 Localization of GRB 790325B: The Discovery of a Probable QSO Association

Abstract

We report the results of the deepest imaging survey yet carried out of the IPN1 localization of gamma-ray burst (GRB) 790325b. While this is one of the smallest of the IPN1 GRB localizations, it has been previously surveyed only to shallow limits because of the nearby bright (V=4.97) star 104 Herculis. We have used a camera with a 9 mag neutral density region in front of a Tek 2048² CCD to apodize the light of 104 Her and allow deep UBVI imaging of the localization. The observations, obtained at the US Naval Observatory 1.55 m telescope to a detection limit of V~22.9, reveal 20 objects clearly within or at the edge of the localization. Near the center of the localization is a QSO candidate that is unlikely (~97% confidence) to be a random occurrence based on known QSO statistics. Combining these new statistics with earlier summed statistics linking QSOs/blue galaxies with IPN1 localizations yields a marginal (~96% confidence) overall correlation. We also compare our observational results for this field with another recent imaging survey of GRB 790325b based on HST data and discuss the differences in the results.