Radio-continuum spectra of pulsars with free-free thermal absorption

¹ CONICET-Universidad Nacional de Córdoba, Instituto de Astronomía Teórica y Experimental (IATE), Laprida 854, X5000BGR, Córdoba, Argentina
² Observatorio Astronómico, Universidad Nacional de Córdoba, Laprida 854, X5000BGR, Córdoba, Argentina
³ Instituto de Astronomía y Física del Espacio (IAFE), Ciudad Universitaria - Pabellón 2, Intendente Güiraldes 2160 (C1428EGA), Ciudad Autónoma de Buenos Aires, Argentina
⁴ Remote Sensing Division, Naval Research Laboratory, Code 7213, 4555 Overlook Ave SW, Washington, DC, 20375, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 September 2025
Accepted: 15 November 2025

Abstract

The radio continuum spectra of pulsars (PSRs) exhibit a wide variety of shapes, which are interpreted as pure and broken power laws, power laws with turnovers or cutoffs, and logarithmic-parabolic profiles. A notable fraction of these have well-defined power laws with ν^−2.1 exponential turnovers, indicative of free-free thermal absorption along the line of sight. We analysed a sample of 63 PSRs with such spectral shapes, compiled from four previously published studies, to investigate their statistical properties. We normalised each spectrum to a characteristic frequency and flux density of its own, facilitating a consistent treatment across the four subsamples. We show that these two fitted parameters are correlated by a power law, with its slope reflecting the median spectral index (∼−2.0) of PSR emission. We find that the turnover frequencies in our sample are typically high, clustering around 558 MHz, which implies notably high emission measures (∼10⁵ pc cm⁻⁶) for an inferred thermal absorbing medium with an electron temperature of 8000 K. Moreover, by combining these emission measures with dispersion measures derived from pulse time delays, we break the degeneracy between the electron density and the path length of the absorbers. This reveals a discrete near-in population of absorbers characterised by small sizes (∼0.1 pc) and high electron densities (∼10³ cm⁻³), which exhibit a clear size-density anti-correlation reminiscent of that observed in Galactic and extragalactic H ii regions.