Settling accretion on to isolated neutron stars from interstellar medium

TL;DR

This paper models accretion onto isolated neutron stars from the interstellar medium, predicting low steady X-ray luminosity with flares, and estimates a small number of detectable sources with future X-ray surveys.

Contribution

It introduces a model of subsonic settling accretion for isolated neutron stars and predicts their X-ray emission characteristics and detectability.

Findings

Mean X-ray luminosity is 2-3 orders below Bondi estimates.

X-ray flares last about one hour with luminosity up to Bondi value.

Expected detectable AINS with future surveys is a few to ten.

Abstract

We apply the model of subsonic settling accretion on to isolated neutron stars accreting from the interstellar medium (AINS). We show that in this regime the expected mean X-ray luminosity from AINS turns out to be 2-3 orders of magnitude as small as the maximum possible Bondi value, i.e. $10^{27}-10^{28}$~erg s$^{-1}$. The intrinsically unstable character of settling accretion due to long plasma cooling time leads to regular appearance of X-ray flares with a duration of about one hour and a maximum luminosity of about the Bondi value, $\sim10^{31}$~erg~s$^{-1}$. This feature can be used to distinguished AINS from other dim X-ray sources. With the sensitivity of the forthcoming all-sky X-ray surveys the expected number of the potentially detectable AINS can be from a few to ten.