H I Absorption Line and Anomalous Dispersion in the Radio Pulses of PSR B1937+21

We use the Five-hundred-meter Aperture Spherical radio Telescope to observe the bright millisecond pulsar PSR B1937+21 (J1939+2134) and record the data in the band from 1.0 to 1.5 GHz. We measure the neutral hydrogen (H I ) emission and absorption lines near 1420 MHz ( λ ≃ 21 cm). We derive the kinematic distance of the pulsar with the H I observation. By comparing this with the archival absorption spectra observed decades ago, we notice possible variations in the absorption spectra toward this pulsar, which correspond to a possible tiny-scale atomic structure of a few astronomical units in size. We also verify the apparent faster-than-light anomalous dispersion at the H I absorption line of this pulsar previously reported.