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

Abstract

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 $\Gamma=$ 1.5 and a cutoff temperature of 25 keV. Residuals between 6 and 8 keV provide strong evidence of a broad Fe K$\alpha$ line. By modeling the spectrum with a relativistically blurred reflection model, we find an upper limit for the inner disk radius of $R_{\rm in}\leq2 R_{\rm ISCO}$. Consequently we find that $R_{\rm NS}\leq23$ km, assuming $M=1.4{\mbox{$\rm\,M_{\mathord\odot}$}}$ and $a=0.15$. We also find an upper limit on the magnetic field of $B\leq2\times10^8$ G.