Time-dependent photoionization and fluorescence line emission in gamma-ray burst environments

¹ Department of Space Physics and Astronomy, Rice University, 6100 S. Main St., Houston, TX 77005-1892, U.S.A.
² E. O. Hulburt Center for Space Research, Code 7653, Naval Research Laboratory, Washington, DC 20375-5352, U.S.A.

Send offprint request to: M. Böttcher

Received: 21 January 1999
Accepted: 16 April 1999

Abstract

If γ-ray bursts (GRBs) are associated with dense star-forming regions, photoelectric absorption by the circumburster material (CBM) will occur. As the burst evolves, the surrounding material is photoionized, leading to fluorescence line emission and reduced photoelectric absorption opacity. We have numerically simulated this problem, accounting for all relevant microscopic processes. We find that if GRBs are hosted in a quasi-isotropic surrounding medium, photoionization of the CBM leads to a constant but weak level of delayed fluorescence line emission on timescales of weeks to months after the GRB. A temporally evolving Fe K edge absorption feature can serve as diagnostic tool for redshift measurements.