A NuSTAR Observation of the Reflection Spectrum of the Low-mass X-Ray Binary 4U 1728-34

We report on a simultaneous NuSTAR and Swift observation of the neutron star low-mass X-ray binary 4U 1728-34. We identified and removed four Type I X-ray bursts during the observation in order to study the persistent emission. The continuum spectrum is hard and described well by a blackbody with kT = 1.5 keV and a cutoff power law with Γ = 1.5, and a cutoff temperature of 25 keV. Residuals between 6 and 8 keV provide strong evidence of a broad Fe Kα line. By modeling the spectrum with a relativistically blurred reflection model, we find an upper limit for the inner disk radius of R_(in) ⩽ 2R_(ISCO). Consequently, we find that R_(NS) ⩽ 23 km, assuming M = 1.4 M_⊙ and a = 0.15. We also find an upper limit on the magnetic field of B ⩽ 2 × 10^8 G