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Published December 10, 2013 | Published + Submitted
Journal Article Open

Constraints on the Neutron Star and Inner Accretion Flow in Serpens X-1 Using NuSTAR


We report on an observation of the neutron star low-mass X-ray binary Serpens X-1, made with NuSTAR. The extraordinary sensitivity afforded by NuSTAR facilitated the detection of a clear, robust, relativistic Fe K emission line from the inner disk. A relativistic profile is required over a single Gaussian line from any charge state of Fe at the 5σ level of confidence, and any two Gaussians of equal width at the same confidence. The Compton back-scattering "hump" peaking in the 10-20 keV band is detected for the first time in a neutron star X-ray binary. Fits with relativistically blurred disk reflection models suggest that the disk likely extends close to the innermost stable circular orbit (ISCO) or stellar surface. The best-fit blurred reflection models constrain the gravitational redshift from the stellar surface to be z_(NS) ≥ 0.16. The data are broadly compatible with the disk extending to the ISCO; in that case, z_(NS) ≥ 0.22 and R_(NS) ≤ 12.6 km (assuming M_(NS) = 1.4 M_☉ and a = 0, where a = cJ/GM^2). If the star is as large or larger than its ISCO, or if the effective reflecting disk leaks across the ISCO to the surface, the redshift constraints become measurements. We discuss our results in the context of efforts to measure fundamental properties of neutron stars, and models for accretion onto compact objects.

Additional Information

© 2013 American Astronomical Society. Received 2013 October 2; accepted 2013 October 18; published 2013 November 21. This work was supported under NASA Contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA.

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Published - 2041-8205_779_1_L2.pdf

Submitted - 1310.5776v1.pdf


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