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 5 \sigma 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 } \geq 0.16 .
The data are broadly compatible with the disk extending to the ISCO ; in that case , z _ { NS } \geq 0.22 and R _ { NS } \leq 12.6 km ( assuming M _ { NS } = 1.4 ~ { } M _ { \odot } 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 .