Revisiting the dispersion measure of fast radio bursts associated with gamma-ray burst afterglows

TL;DR

This paper examines the various contributions to the dispersion measure of FRBs associated with GRBs, highlighting how different physical processes and environments influence the observed DM and its implications for cosmology.

Contribution

It provides a detailed analysis of how GRB ejecta and neutron star winds affect FRB dispersion measures, offering new insights into their roles in FRB-GRB associations.

Findings

Relativistic, dense NS winds can significantly increase DM, sometimes surpassing intergalactic contributions.

Goldreich-Julian particle flux winds contribute negligible DM, correlating with absence of internal X-ray plateaus.

High-mass GRB ejecta in stellar wind environments can substantially impact the DM of associated FRBs.

Abstract

Some fast radio bursts (FRBs) are expected to be associated with the afterglow emission of gamma-ray bursts (GRBs), while a short-lived, supermassive neutron star (NS) forms during the GRBs. I investigate the possible contributions to the dispersion measure (DM) of the FRBs from the GRB ejecta and the wind blown from the precollapsing NS. On the one hand, sometimes an internal X-ray plateau afterglow could be produced by the NS wind, which indicates that a great number of electron-positron pairs are carried by the wind. If the pair-generation radius satisfies a somewhat rigorous condition, the relativistic and dense wind would contribute a high DM to the associated FRB, which can be comparable to and even exceed the DM contributed by the intergalactic medium. On the other hand, if the wind only carries a Goldreich-Julian particle flux, its DM contribution would become negligible; meanwhile, the internal plateau afterglow would not appear. Alternatively, the FRB should be associated with a GRB afterglow produced by the GRB external shock, i.e., an energy-injection-caused shallow-decay afterglow or a normal single-power-law afterglow if the impulsive energy release of the GRB is high enough. In the latter case, the DM contributed by the high-mass GRB ejecta could be substantially important, in particular, for an environment of main-sequence stellar wind. In summary, a careful assessment on the various DM contributors could be required for the cosmological application of the expected FRB-GRB association. The future DM measurements of GRB-associated FRBs could provide a constraint on the physics of NS winds.