Modeling and implications of the transient absorption feature in GRB 990705

Details Modeling and implications of the transient absorption feature in GRB 990705 Modeling and implications of the transient absorption feature in GRB 990705

Oct 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aipc..587..190b&link_type=abstract

GAMMA 2001: Gamma-Ray Astrophysics 2001. AIP Conference Proceedings, Volume 587, pp. 190-194 (2001).

Astronomy and Astrophysics

Astrophysics

2

Gamma-Ray Sources, Gamma-Ray Bursts, X-Ray Sources, X-Ray Bursts, Atomic Processes And Interactions

Scientific paper

A transient Fe absorption edge during the prompt phase of a GRB has recently been observed for the first time. This has been interpreted as evidence for photoionization of the GRB environment by the burst radiation, which allows diagnostics of the density structure and element abundances in the vicinity of the burst. Assuming that the observed absorption feature is caused by photoelectric absorption, we model the time-dependent photoionization and X-ray radiation transport, and deduce constraints on the size and matter distribution of the photoionized region responsible for the absorption edge. If the density of the region is <~1010 cm-3, then we find that the intervening material would have to contain ~44Ω M. of iron within ~1.3 pc of the burst source, assuming the measured best-fit ~75-fold overabundance of iron. Alternatively, and more plausibly, the observed absorption feature could be caused by photoelectric absorption in dense clouds of radius rc<~1012 cm at ~1017 cm from the burst source, containing <~0.7Ω M. of iron. In such an environment, recombination would compete with photoionization, until the clouds are effectively heated to the Compton equilibrium temperature of the ionizing GRB continuum. We also briefly discuss the recent suggestion that the absorption feature may be due to resonant scattering by Fe XXVI in a highly ionized, clumpy high-velocity outflow. .

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