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Time-dependent photoionization opacities in dense gamma-ray burst environments

Authors:	Davide Lazzati Rosalba Perna Gabriele Ghisellini

Institution:	¹Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA;²Harvard Society of Fellows, 78 Mt. Auburn Street, Cambridge, MA 02138, USA;³Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA;⁴Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy

Abstract:	The recent detection of a transient absorption feature in the X-ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma-ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor.

Keywords:	line: formation gamma-rays: bursts X-rays: general X-rays: ISM