Long-period radio pulsars: population study in the neutron star and white dwarf rotating dipole scenarios

TL;DR

This study investigates whether ultra-long period radio emitters are better explained as neutron stars or white dwarfs, finding significant challenges in both scenarios for explaining their bright radio emissions and population characteristics.

Contribution

The paper compares neutron star and white dwarf models for long-period radio pulsars using population synthesis and magnetospheric analysis, highlighting the difficulties in explaining their emission.

Findings

Neutron-star scenario predicts few ultra-long period pulsars.

White-dwarf scenario can produce many long-period radio emitters.

Neither scenario easily explains the bright coherent radio emission.

Abstract

The nature of two recently discovered radio emitters with unusually long periods of 18min (GLEAM-X J1627-52) and 21min (GPM J1839-10) is highly debated. Their bright radio emission resembles that of radio magnetars, but their long periodicities and lack of detection at other wavelengths challenge the neutron-star interpretation. In contrast, long rotational periods are common in white dwarfs but, although predicted, dipolar radio emission from isolated magnetic white dwarfs has never been unambiguously observed. In this work, we investigate these long-period objects as potential isolated neutron-star or white-dwarf dipolar radio emitters and find that both scenarios pose significant challenges to our understanding of radio emission via pair production in dipolar magnetospheres. We also perform population-synthesis simulations based on dipolar spin-down in both pictures, assuming different initial-period distributions, masses, radii, beaming fractions, and magnetic-field prescriptions, to assess their impact on the ultra-long pulsar population. In the neutron-star scenario, we do not expect a large number of ultra-long period pulsars under any physically motivated (or even extreme) assumptions for the period evolution. On the other hand, in the white-dwarf scenario, we can easily accommodate a large population of long-period radio emitters. However, no mechanism can easily explain the production of such bright coherent radio emission in either scenarios.