Time variation in the low frequency spectrum of Vela-like pulsar B1800-21

TL;DR

This study tracks the evolving low-frequency radio spectrum of pulsar B1800-21 over years, revealing significant spectral changes linked to dense interstellar medium structures affecting pulsar observations.

Contribution

It provides the first detailed long-term observation of spectral evolution in a GPS pulsar and links these changes to specific interstellar medium structures and absorption mechanisms.

Findings

Spectral turnover frequency increased over several years

Dense interstellar structures cause observed spectral evolution

Reversion of spectrum to initial shape observed in recent measurements

Abstract

We report the flux measurement of the Vela like pulsar B1800-21 at the low radio frequency regime over multiple epochs spanning several years. The spectrum shows a turnover around the GHz frequency range and represents a typical example of gigahertz-peaked spectrum (GPS) pulsar. Our observations revealed that the pulsar spectrum show a significant evolution during the observing period with the low frequency part of the spectrum becoming steeper, with a higher turnover frequency, for a period of several years before reverting back to the initial shape during the latest measurements. The spectral change over times spanning several years requires dense structures, with free electron densities around 1000--20000 cm$^{-3}$ and physical dimensions ~220 AU, in the interstellar medium (ISM) traversing across the pulsar line of sight. We look into the possible sites of such structures in the ISM and likely mechanisms particularly the thermal free-free absorption as possible explanations for the change.