Detectability of Gamma-Ray Burst Iron Lines by Swift, Chandra, and XMM-Newton

The rapid acquisition of positions by the upcoming Swift satellite will allow monitoring for X-ray lines in gamma-ray burst (GRB) afterglows at much earlier epochs than was previously feasible. We calculate the possible significance levels of iron-line detections as a function of source redshift and observing time after the trigger for the Swift X-Ray Telescope (XRT), Chandra ACIS, and XMM-Newton EPIC detectors. For bursts with standard luminosities, decay rates, and equivalent widths of 1 keV assumed constant starting at early source-frame epochs, Swift may be able to detect lines up to z ~ 1.5 with a significance of ≳3 σ for times of t ≲ 104 s. The same lines would be detectable with ≳4 σ significance at z ≲ 6 by Chandra and at z ≲ 8 by XMM-Newton for times of t ≲ 105 s. For similar bursts with a variable equivalent width that peaks at 1 keV between 0.5 and 1 day in the source frame, Swift achieves the same significance level for z ~ 1 at t ~ 1 day, while Chandra reaches the previous detection significances around t ~ 1-2 days for z ~ 2-4; i.e., the line is detectable near the peak equivalent width times and undetectable at earlier or later times. For afterglows in the upper range of initial X-ray luminosity afterglows, which may also be typical of Population III bursts, similar significance levels are obtained out to substantially higher redshifts. A distinction between broad and narrow lines to better than 3 σ is possible with Chandra and XMM-Newton out to z ~ 2 and ~6.5, respectively, while Swift can do so up to z ~ 1 for standard burst parameters. A distinction between different energy centroid lines of 6.4 versus 6.7 keV (or 6.7 vs. Cobalt 7.2 keV) is possible up to z ≲ 0.6, 1.2, and 2 (z ≲ 1, 5, and 7.5) with Swift, Chandra, and XMM-Newton, respectively. For the higher luminosity bursts, Swift is able to distinguish at the 5 σ level between a broad and a narrow line out to z ≲ 5 and between a 6.7 versus a 7.2 keV line center out to z ≲ 5 for times of t ≲ 104 s.