Delayed Nickel Decay in Gamma‐Ray Bursts

Abstract

Recently observed emission lines in the X-ray afterglow of � -ray bursts suggest that iron-group elements are either produced in the � -ray burst or are present nearby. If this material is the product of a thermonuclear burn, then such material would be expected to be rich in nickel-56. If the nickel remains partially ionized, this prevents the electron capture reaction normally associated with the decay of nickel-56, dramatically increasing the decay timescale. Here we examine the consequences of rapid ejection of a fraction of a solar mass of iron-group material from the center of a collapsar/hypernova. The exact rate of decay then depends on the details of the ionization and, therefore, the ejection process. Future observations of iron, nickel, and cobalt lines can be used to diagnose the origin of these elements and to better understand the astrophysical site of � ray bursts. In this model, the X-ray lines of these iron-group elements could be detected in suspected hypernovae that did not produce an observable � -ray burst due to beaming. Subject headings: gamma rays: bursts — line: profiles — nuclear reactions, nucleosynthesis, abundances — supernovae: general