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}{in,2016}={11}-1+2 {R}g (where R g = GM/c 2) 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 × 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2017
- DOI:
- 10.3847/1538-4357/aa8b1b
- arXiv:
- arXiv:1709.01559
- Bibcode:
- 2017ApJ...847..135L
- Keywords:
-
- accretion;
- accretion disks;
- stars: individual: Aql X-1;
- stars: neutron;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Accepted for publication in ApJ, 7 pages, 2 Tables, 5 Figures