Delayed Nickel Decay in Gamma‐Ray Bursts

Recently observed emission lines in the X-ray afterglow of gamma ray burstssuggest that iron group elements are either produced in the gamma ray burst, orare 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 nickelremains partially ionized, this prevents the electron capture reaction normallyassociated with the decay of Nickel-56, dramatically increasing the decaytimescale. Here we examine the consequences of rapid ejection of a fraction ofa 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 andtherefore the ejection process. Future observations of iron, nickel and cobaltlines can be used to diagnose the origin of these elements and to betterunderstand the astrophysical site of gamma ray bursts. In this model, the X-raylines of these iron-group elements could be detected in suspected hypernovaethat did not produce an observable gamma ray burst due to beaming