H I absorption line and anomalous dispersion in the radio pulses of PSR B1937+21

TL;DR

This paper reports HI absorption observations of pulsar PSR B1937+21, revealing potential tiny-scale atomic structures and confirming anomalous dispersion effects in radio pulses, advancing understanding of interstellar medium interactions.

Contribution

It provides new HI absorption measurements, suggests tiny-scale atomic structures, and verifies anomalous dispersion effects in pulsar signals.

Findings

Detection of HI absorption lines near 1420 MHz.

Possible evidence for tiny-scale atomic structures.

Verification of anomalous dispersion in pulsar signals.

Abstract

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 (HI) emission and absorption lines near 1420 MHz ($\lambda \simeq 21$ cm). We derive the kinematic distance of the pulsar with the HI 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 HI absorption line of this pulsar previously reported.