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

Clio C. Sleator, John A. Tomsick, Ashley L. King, Jon M. Miller, Steven E. Boggs, Matteo Bachetti, Didier Barret, Jérôme Chenevez, Finn Erland Christensen, William W. Craig, Charles J. Hailey, Fiona A. Harrison, Farid Rahoui, Daniel K. Stern, Dominic J. Walton, William W. Zhang

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    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 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 Rin≤2RISCO. Consequently, we find that RNS ≤ 23 km, assuming M = 1.4 M ⊙ and a = 0.15. We also find an upper limit on the magnetic field of B ≤ 2 × 108 G.
    Original languageEnglish
    Article number134
    JournalAstrophysical Journal
    Volume827
    Issue number2
    Number of pages11
    ISSN0004-637X
    DOIs
    Publication statusPublished - 2016

    Keywords

    • Accretion, accretion disks
    • Stars: neutron
    • X-rays: binaries

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