Identifying Gamma‐Ray Burst Remnants in Nearby Galaxies

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Abstract
We study the spectral signatures arising from cooling and recombination of an interstellar medium whose equilibrium state has been altered over ~100 pc by the radiation of a gamma-ray burst (GRB) and its afterglow. We identify signatures in the line diagnostics that are indicative of a photoionized GRB remnant which is 5 × 104 yr old. We estimate that at least a few such remnants should be detectable in the Virgo Cluster of galaxies. If the gamma-ray emission from GRBs is beamed to a fraction fb of their sky, then the expected number of Virgo remnants is larger by a factor of f. Virgo remnants can be resolved with arcsecond imaging and are likely to be center-filled using narrowband filters of high-ionization lines (such as [O III] λ5007 or He II λ4686) and limb-brightened for low-ionization lines (such as [S II] λ6717). The nonrelativistic blast wave might be visible separately, since it does not reach the outer edge of these young photoionized remnants. The remnants should show evidence for ionization cones if the prompt or afterglow UV emission from GRBs is beamed.
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