Truncation of the Accretion Disk at One Third of the Eddington Limit in the Neutron Star Low-Mass X-ray Binary Aquila X-1

Abstract

We perform a reflection study on a new observation of the neutron star low-mass X-ray binary Aquila X-1 taken with NuSTAR during the August 2016 outburst and compare with the July 2014 outburst. The source was captured at $\sim 32\%L_{\mathrm{Edd}}$, which is over four times more luminous than the previous observation during the 2014 outburst. Both observations exhibit a broadened Fe line profile. Through reflection modeling, we determine that the inner disk is truncated $R_{in,2016}=11_{-1}^{+2}{R}_g$ (where $R_g=GM/c^2$) and $R_{in,2014}=14\pm 2R_g$ (errors quoted at the 90% confidence level). Fiducial neutron star parameters (M${}_{NS}=1.4$ M${}_{\odot }$, $R_{NS}=10$ km) give a stellar radius of $R_{NS}=4.85R_g$; our measurements rule out a disk extending to that radius at more than the $6\sigma$ level of confidence. We are able to place an upper limit on the magnetic field strength of $B\le 3.0-4.5\times 10^9$ G at the magnetic poles, assuming that the disk is truncated at the magnetospheric radius in each case. This is consistent with previous estimates of the magnetic field strength for Aquila X-1. However, if the magnetosphere is not responsible for truncating the disk prior to the neutron star surface, we estimate a boundary layer with a maximum extent of $R_{BL,2016}\sim 10R_g$ and $R_{BL,2014}\sim 6R_g$. Additionally, we compare the magnetic field strength inferred from the Fe line profile of Aquila X-1 and other neutron star low-mass X-ray binaries to known accreting millisecond X-ray pulsars.

Accepted for publication in ApJ, 7 pages, 2 Tables, 5 Figures

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