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 (NS) low-mass X-ray binary Aquila X-1 taken with NuSTAR during the 2016 August outburst and compare with the 2014 July outburst. The source was captured at ~32% L 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 ${R}_{\mathrm{in},2016}={11}_{-1}^{+2}\ {R}_{g}$ (where R g = GM/c 2) and ${R}_{\mathrm{in},2014}=14\pm 2\ {R}_{g}$ (errors quoted at the 90% confidence level). Fiducial NS parameters (M NS = 1.4 M ⊙, R NS = 10 km) give a stellar radius of R NS = 4.85 R g ; our measurements rule out a disk extending to that radius at more than the 6σ level of confidence. We are able to place an upper limit on the magnetic field strength of B ≤ 3.0–4.5 × 109 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 NS surface, we estimate a boundary layer with a maximum extent of ${R}_{\mathrm{BL},2016}\sim 10\ {R}_{g}$ and ${R}_{\mathrm{BL},2014}\sim 6\ {R}_{g}$. Additionally, we compare the magnetic field strength inferred from the Fe line profile of Aquila X-1 and other NS low-mass X-ray binaries to known accreting millisecond X-ray pulsars.

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ORCID for D. Altamirano: orcid.org/0000-0002-3422-0074

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