Measuring A Truncated Disk in Aquila X-1

TL;DR

This study uses NuSTAR and Swift observations to measure the inner radius of the accretion disk in Aquila X-1 during an outburst, revealing disk truncation likely caused by magnetic or boundary layer effects.

Contribution

First measurement of the truncated inner disk radius in Aquila X-1 using relativistic reflection modeling during an outburst.

Findings

Inner disk radius is approximately 15 gravitational radii.

Disk truncation possibly caused by magnetic field or boundary layer.

Material still accreting onto the neutron star evidenced by X-ray burst.

Abstract

We present NuSTAR and Swift observations of the neutron star Aquila X-1 during the peak of its July 2014 outburst. The spectrum is soft with strong evidence for a broad Fe K\alpha line. Modeled with a relativistically broadened reflection model, we find that the inner disk is truncated with an inner radius of 15+/-3 R_G. The disk is likely truncated by either the boundary layer and/or a magnetic field. Associating the truncated inner disk with pressure from a magnetic field gives an upper limit of B<5+/-2x10^8G. Although the radius is truncated far from the stellar surface, material is still reaching the neutron star surface as evidenced by the X-ray burst present in the t NuSTAR observation.