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

Ludlam, R. M. and Miller, J. M. and Degenaar, N. and Sanna, A. and Cackett, E. M. and Altamirano, D. and King, A. L. (2017) Truncation of the Accretion Disk at One Third of the Eddington Limit in the Neutron Star Low-Mass X-ray Binary Aquila X-1. Astrophysical Journal, 847 (2). Art. No. 135. ISSN 1538-4357. doi:10.3847/1538-4357/aa8b1b.

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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^(+2)_(-1) 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Ludlam, R. M.0000-0002-8961-939X
Cackett, E. M.0000-0002-8294-9281
Altamirano, D.0000-0002-3422-0074
Additional Information:© 2017 The American Astronomical Society. Received 2017 July 12; revised 2017 August 30; accepted 2017 September 5; published 2017 October 3. We thank the referee for their prompt and thoughtful comments that have improved the quality of this work. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (Caltech, USA). N.D. is supported by a Vidi grant from the Netherlands Organization for Scientific Research (NWO). E.M.C. gratefully acknowledges support from the National Science Foundation through CAREER award number AST-1351222. D.A. acknowledges support from the Royal Society.
Group:NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)UNSPECIFIED
Subject Keywords:accretion, accretion disks – stars: individual (Aql X-1) – stars: neutron – X-rays: binaries
Issue or Number:2
Record Number:CaltechAUTHORS:20170907-090819705
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Official Citation:R. M. Ludlam et al 2017 ApJ 847 135
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:81237
Deposited By: Joy Painter
Deposited On:07 Sep 2017 16:46
Last Modified:15 Nov 2021 19:42

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