Effective absorbing column density in the gamma-ray burst afterglow X-ray spectra

TL;DR

This paper explores how the observed X-ray absorption in GRB afterglows relates to redshift, providing scaling laws for different instruments and assessing the feasibility of redshift measurement solely from X-ray data.

Contribution

It establishes the power-law relation between absorption and redshift and offers instrument-specific scaling laws for redshift estimation from X-ray spectra.

Findings

Absorption scales with redshift as a power law with index 2.4 in ideal conditions.

Real instrument effects modify the scaling law, requiring specific adjustments.

Redshift can be estimated from X-ray data with 10% accuracy given sufficient counts.

Abstract

We investigate the scaling relation between the observed amount of absorption in the X-ray spectra of Gamma Ray Burst (GRB) afterglows and the absorber redshift. Through dedicated numerical simulations of an ideal instrument, we establish that this dependence has a power law shape with index 2.4. However, for real instruments, this value depends on their low energy cut-off, spectral resolution and on the detector spectral response in general. We thus provide appropriate scaling laws for specific instruments. Finally, we discuss the possibility to measure the absorber redshift from X-ray data alone. We find that 10^5-10^6 counts in the 0.3-10 keV band are needed to constrain the redshift with 10% accuracy. As a test case we discuss the XMM-Newton observation of GRB 090618 at z=0.54. We are able to recover the correct redshift of this burst with the expected accuracy.