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

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∼ 32%LEdd, 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 truncatedRin, 2016 = 11 +2 −1 Rg (where Rg = GM/c ) andRin, 2014 = 14± 2 Rg (errors quoted at the 90% confidence level). Fiducial neutron star parameters (MNS = 1.4M⊙, RNS = 10 km) give a stellar radius ofRNS = 4.85Rg; 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× 10 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 RBL, 2016 ∼ 10 Rg and RBL, 2014 ∼ 6 Rg . 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.