Radio and X-Ray Simultaneity in the Galactic Burst FRB 200428

On 28 April 2020 the Galactic magnetar SGR 1935+2154 produced a millisecond radio burst bright enough to count as a fast radio burst, accompanied by an X-ray burst seen by several satellites. The radio signal's dispersion sweep across the 400 to 800 MHz band amounts to 6.47 s for its dispersion measure of about 332 pc per cubic centimetre, yet after dedispersion the radio components sit within milliseconds of the X-ray emission. Adopting a conservative ten-millisecond residual offset over the roughly ten-kiloparsec path bounds the fractional difference between the radio and X-ray propagation speeds at 9.72e-15. The event's value is its cleanliness: one source, inside our own Galaxy, with the plasma term measured from the burst itself. The framework examined here predicts simultaneity after plasma removal, since it assigns all radio delay to the plasma; the observation matches that prediction. Computed from published data; not a new measurement.