Transient discs around isolated accreting neutron stars

Abstract

Mature, isolated neutron stars can accrete from the interstellar medium. Due to turbulence, the accreted gas can have a substantial angular momentum and form a disc around the compact object. In this paper, we perform a population synthesis of isolated neutron stars in the Milky Way that specifically tracks the possibility of disc formation, which typically requires a low spatial velocity of the compact object ($\lesssim 40$ km s$^{-1}$). In general, weak magnetic fields favour disc formation as the magnetosphere occupies a smaller volume. Still, even in the case of fields decaying exponentially over a characteristic timescale of $\sim 1.5$ Gyr, we find that for several realistic models of propeller spin-down, only a small fraction of accretors (down to $\sim 0.02$ %) attain discs. However, disc-accreting isolated neutron stars are relatively numerous among the brightest sources. We estimate that their number can reach a few hundred at X-ray fluxes of $\gtrsim 10^{-14}$ erg s$^{-1}$ cm$^{-2}$. We speculate that disc-accreting isolated neutron stars can manifest as long-period radio transient sources via electron cyclotron maser emission, predicting spectra and critical conditions for quenching.