Decomposing the Dispersion Measure of FRB 121102: Milky Way, Halo, Intergalactic Medium and Host

The repeating fast radio burst FRB 121102 has a total dispersion measure of about 557 pc per cubic centimetre and a host galaxy at redshift 0.1927, which makes it the first burst whose dispersion could be audited line by line. The audit performed here assigns about 188 pc per cubic centimetre to the Milky Way's disc, an adopted 30 to its halo, and 163 to the intergalactic medium computed from the cosmic baryon density, leaving an observer-frame residual of 176 pc per cubic centimetre, or 209 in the host's frame — a large but physically reasonable contribution from the burst's immediate environment. Across a 1 to 2 GHz observing band the total dispersion sweeps the burst by 1.73 s, which is why dedispersion must be right to milliseconds before any arrival-time physics can be done with these events. The framework examined here takes radio dispersion to be entirely a plasma effect obeying the cold-plasma law, with no vacuum contribution; the budget closes under that assumption with a host term of the size independent evidence suggests. Computed from published data; not a new measurement.