4 v 3 1 8 Ja n 20 05 Detectability of GRB Iron Lines by Swift , Chandra and XMM

The rapid acquisition of positions by the upcoming Swift satellite will allow the monitoring for X-ray lines in 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 XRT, Chandra ACIS and XMM 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 > ∼ 3σ for times t < ∼ 10 4 s. The same lines would be detectable with > ∼ 4σ significance at z < ∼ 6 by Chandra, and at z < ∼ 8 by XMM, for times of t < ∼ 10 5 s. For similar bursts with a variable equivalent width which peaks at 1 keV between 0.5 and 1 days 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 luminosites afterglows, which may also be typical of pop. III bursts, similar significance levels are obtained out to substantially higher redshifts. A distinction between broad and narrow lines to better than 3σ is possible with Chandra and XMM 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 keV vs. 6.7 KeV (or 6.7 keV vs. Cobalt 7.2 keV) is possible up to z < ∼ 0.6, 1.2, and 2 (z < ∼ 1, 5, 7.5), with Swift, Chandra, and XMM respectively. For the higher luminosity bursts, Swift is able to distinguish at the 5σ level between a broad and a narrow line out to z < ∼ 5, and between a 6.7 keV vs. a 7.2 keV line center out to z < ∼ 5 for times of t < ∼ 10 4 s. – 2 –