An in-depth study of a neutron star accreting at low Eddington rate: On the possibility of a truncated disk and an outflow

TL;DR

This study investigates low-level accretion in a neutron star binary, revealing a truncated disk, possible outflows, and magnetic effects that influence accretion and outflow phenomena at very low Eddington rates.

Contribution

It provides detailed spectral analysis and evidence for disk truncation and outflows in a neutron star accreting at 0.1% Eddington, highlighting magnetic inhibition effects.

Findings

Inner disk located beyond 100 GM/c2, indicating truncation.

Detection of potential outflow signatures and reflection features.

Constraints on neutron star magnetic field and spin period.

Abstract

Due to observational challenges our knowledge of low-level accretion flows around neutron stars is limited. We present Nustar, Swift and Chandra observations of the low-mass X-ray binary IGR J17062-6143, which has been persistently accreting at ~0.1 per cent of the Eddington limit since 2006. Our simultaneous Nustar/Swift observations show that the 0.5-79 keV spectrum can be described by a combination of a power law with a photon index of Gamma~2, a black body with a temperature of kT_bb~0.5 keV (presumably arising from the neutron star surface), and disk reflection. Modeling the reflection spectrum suggests that the inner accretion disk was located at R_in>100 GM/c2 (>225 km) from the neutron star. The apparent truncation may be due to evaporation of the inner disk into a radiatively-inefficient accretion flow, or due to the pressure of the neutron star magnetic field. Our Chandra gratings data reveal possible narrow emission lines near 1 keV that can be modeled as reflection or collisionally-ionized gas, and possible low-energy absorption features that could point to the presence of an outflow. We consider a scenario in which this neutron star has been able to sustain its low accretion rate through magnetic inhibition of the accretion flow, which gives some constraints on its magnetic field strength and spin period. In this configuration, IGR J17062-6143 could exhibit a strong radio jet as well as a (propeller-driven) wind-like outflow.