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Title: NuSTAR DISCOVERY OF A CYCLOTRON LINE IN KS 1947+300

We present a spectral analysis of three simultaneous Nuclear Spectroscopy Telescope Array and Swift/XRT observations of the transient Be-neutron star binary KS 1947+300 taken during its outburst in 2013/2014. These broadband observations were supported by Swift/XRT monitoring snapshots every three days, which we use to study the evolution of the spectrum over the outburst. We find strong changes of the power-law photon index, which shows a weak trend of softening with increasing X-ray flux. The neutron star shows very strong pulsations with a period of P ≈ 18.8 s. The 0.8-79 keV broadband spectrum can be described by a power law with an exponential cutoff and a blackbody component at low energies. During the second observation we detect a cyclotron resonant scattering feature at 12.5 keV, which is absent in the phase-averaged spectra of observations 1 and 3. Pulse phase-resolved spectroscopy reveals that the strength of the feature changes strongly with pulse phase and is most prominent during the broad minimum of the pulse profile. At the same phases the line also becomes visible in the first and third observation at the same energy. This discovery implies that KS 1947+300 has a magnetic field strength of B ≈ 1.1 × 10{sup 12}(1 + z) G,more » which is at the lower end of known cyclotron line sources.« less
Authors:
; ; ;  [1] ;  [2] ;  [3] ;  [4] ;  [5] ; ; ;  [6] ;  [7] ;  [8] ;  [9] ;  [10] ;  [11]
  1. Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125 (United States)
  2. Center for Space Science and Technology, University of Maryland Baltimore County, Baltimore, MD 21250 (United States)
  3. Dr. Karl-Remeis-Sternwarte and ECAP, Sternwartstr. 7, D-96049 Bamberg (Germany)
  4. Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802 (United States)
  5. Université de Toulouse, UPS-OMP, IRAP, F-31400 Toulouse (France)
  6. Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States)
  7. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)
  8. DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby (Denmark)
  9. Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027 (United States)
  10. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
  11. NASA Goddard Space Flight Center, Astrophysics Science Division, Code 662, Greenbelt, MD 20771 (United States)
Publication Date:
OSTI Identifier:
22365935
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 784; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; EMISSION SPECTRA; INDEXES; KEV RANGE; MAGNETIC FIELDS; NEUTRON STARS; NEUTRONS; PHOTONS; PULSATIONS; RESONANCE SCATTERING; TELESCOPES; TRANSIENTS; X RADIATION