On gigahertz spectral turnovers in pulsars

TL;DR

This paper explains gigahertz spectral turnovers in pulsars using a thermal absorption model, linking environmental conditions to observed spectral features and suggesting many pulsars in dense regions could exhibit GPS-like spectra.

Contribution

It introduces a thermal absorption model that accounts for GHz spectral turnovers in pulsars, connecting environmental factors to spectral behavior and implications for pulsar populations.

Findings

Model successfully fits known GPS pulsar spectra.

Normal pulsars could show GPS-like spectra in dense environments.

Implications for detecting pulsars in the Galactic center.

Abstract

Pulsars are known to emit non-thermal radio emission that is generally a power-law function of frequency. In some cases, a turnover is seen at frequencies around 100~MHz. Kijak et al. have reported the presence of a new class of ''Gigahertz Peaked Spectrum'' (GPS) pulsars that show spectral turnovers at frequencies around 1 GHz. We apply a model based on free-free thermal absorption to explain these turnovers in terms of surrounding material such as the dense environments found in HII regions, Pulsar Wind Nebulae (PWNe), or in cold, partially ionized molecular clouds. We show that the turnover frequency depends on the electron temperature of the environment close to the pulsar, as well as the emission measure along the line of sight. We fitted this model to the radio fluxes of known GPS pulsars and show that it can replicate the GHz turnover. From the thermal absorption model, we demonstrate that normal pulsars would exhibit a GPS-like behaviour if they were in a dense environment. We discuss the application of this model in the context of determining the population of neutron stars within the central parsec of the Galaxy. We show that a non-negligible fraction of this population might exhibit high-frequency spectral turnovers, which has implications on the detectability of these sources in the Galactic centre.