Astron. Astrophys. 343, 111-119 (1999)

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

Markus Böttcher ¹, Charles D. Dermer ², Anthony W. Crider ¹ and Edison P. Liang <sup>1

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

Received 18 September 1998 / Accepted 2 December 1998

Abstract

If -ray bursts are associated with dense star-forming regions in young galaxies, photoelectric absorption by the dense 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 analyzed this process in detail, accounting for the time-dependent photoelectric absorption, photoionization and fluorescence line emission from the CBM. We find that even if GRBs are hosted in dense star-forming regions, photoionization of the GRB environment leads to a constant, but very weak level of delayed fluorescence line emission on timescales of weeks to years after the burst. A temporally evolving iron K edge absorption feature can serve as diagnostic tool to reveal the density structure of the CBM and may provide an opportunity for redshift measurements. We also investigated whether photoelectric absorption could be responsible for the spectral evolution of the low-energy slopes of some bright BATSE -ray bursts displaying extremely hard spectra below the peak energy, inconsistent with the optically-thin synchrotron shock model. We find that a very strong metal enrichment ( times solar-system abundances) in the -ray burst environment and a rather peculiar spatial distribution of the CBM would be necessary in order to account for the observed hard spectra below a few 100 keV and their temporal evolution.

Key words: atomic processes radiative transfer gamma rays: bursts X-rays: bursts

Send offprint requests to: M. Böttcher

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Contents

• 1. Introduction
• 2. Model assumptions and computational scheme
• 3. Absorption edges and delayed fluorescence line emission
• 4. The low-energy slopes of GRB spectra
• 5. Summary and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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