v 1 2 3 A ug 2 00 4 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 standard burst luminosities and decay rates and equivalent widths of 1 keV as previously reported, depending on the source-frame epoch at which the lines appear, Swift may be able to detect lines up to z ∼ 7 with a significance of > ∼ 3σ. Equivalent widths of 0.5 keV should be detectable with > ∼ 4.5σ at z< ∼ 8 by Chandra, and at z< ∼ 11 by XMM. For higher initial luminosities, as seen in some GRBs and as might be the case in population III bursts, similar significance levels are obtained out to substantially higher redshifts. For standard burst parameters, Chandra can distinguish between broad and narrow lines to better than 3σ out to z ∼ 6, while Swift can do so up to z ∼ 0.5. For the higher luminosities, Swift is able to distinguish at the 5σ level between a broad and a narrow line out to z< ∼ 3, and between a 6.7 kev vs. a 6.4 keV line center out to z< ∼ 5.