The Low-Frequency Spectra of Radio Pulsars

TL;DR

The paper addresses the diversity of pulsar radio spectra at low frequencies, where turnovers and high frequency features encode both emission physics and ISM propagation. It surveys spectral morphologies, empirical fitting forms, and physical turnover mechanisms including SSA, FFA, and coherent curvature radiation, and it analyzes turnover statistics across a large pulsar sample. Key findings reveal that many spectra exhibit low frequency turnover from a mix of intrinsic and environmental effects, GPS pulsars are often linked to dense environments, and MSPs show distinct high-frequency behavior; a tri-modal distribution of turnover peaks and a correlation between peak frequency and spectral steepness emerge from the data. The work underscores the need to disentangle intrinsic emission from ISM effects to leverage upcoming SKA-Low data, improve ISM corrections, and advance magnetospheric and plasma physics insights in pulsars.

Abstract

Low-frequency spectral studies of radio pulsars represent a key method for uncovering their emission mechanisms, magnetospheric structure, and signal interactions with the surrounding interstellar medium (ISM). In recent years, more next-generation low-frequency radio telescopes (e.g., LOFAR, LWA and MWA) have enriched the observational window below 350 MHz, enabling more detailed explorations of the ISM effects, such as absorption and scattering, resulting in diverse spectral behaviors observed across different pulsars. This paper reviews the morphology of pulsar radio spectra, advances in spectral modeling, and the key physical processes governing the low-frequency emission. Looking ahead, next-generation instruments such as SKA-Low - with their unprecedented sensitivity - are expected to resolve outstanding questions in pulsar emission processes, offering insights into the extreme physical regimes governing these exotic objects.

Figures (6)

• Figure 1: Top: Flux densities from Bondonneau2020 (25$\sim$80 MHz) compared to extrapolated values from high-frequency spectral indices Bilous2016. Blue dots indicate pulsars with known spectral turnovers, while red triangles represent those modeled with a simple power law. Bottom: Consistency check between Bondonneau2020 and LOFAR Core LBA Bilous2020 measurements, with green lines and shaded regions indicating systematic uncertainties. Red triangles show upper limits for pulsars detected by only one instrument.
• Figure 2: This figure illustrates the typical characteristics of GPS pulsars, with their spectral nature approximated using a free-free thermal absorption model (solid dark line) and the corresponding 1-$\sigma$ envelopes for the model fits (dotted lines) Rozko2021.
• Figure 3: The emission spectrum of a particle undergoing acceleration in an electric field that increases linearly with altitude above the surface of the pulsar Kontorovich2013.
• Figure 4: Spectral fitting of pulsars showing different morphological types.
• Figure 5: The distribution of the spectral composition in different forms based on pulsar_spectra.