2017ApJ...847..135L

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_in,2016=11_–1⁺² R_g (where R_g = GM/c²) and R_in,2014=14±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 x 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_BL,2016∼10 R_g and R_BL,2014∼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.