A NuSTAR observation of the reflection spectrum of the low mass X-ray binary 4U 1728-34

الأصل · EN

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 well described by a black body 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≤2 R ISCO. Consequently we find that R NS≤23 km, assuming M=1.4M and a=0.15. We also find an upper limit on the magnetic field of B≤2×10⁸ G.