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Title: The smooth cyclotron line in Her X-1 as seen with nuclear spectroscopic telescope array

Her X-1, one of the brightest and best studied X-ray binaries, shows a cyclotron resonant scattering feature (CRSF) near 37 keV. This makes it an ideal target for a detailed study with the Nuclear Spectroscopic Telescope Array (NuSTAR), taking advantage of its excellent hard X-ray spectral resolution. We observed Her X-1 three times, coordinated with Suzaku, during one of the high flux intervals of its 35 day superorbital period. This paper focuses on the shape and evolution of the hard X-ray spectrum. The broadband spectra can be fitted with a power law with a high-energy cutoff, an iron line, and a CRSF. We find that the CRSF has a very smooth and symmetric shape in all observations and at all pulse phases. We compare the residuals of a line with a Gaussian optical-depth profile to a Lorentzian optical-depth profile and find no significant differences, strongly constraining the very smooth shape of the line. Even though the line energy changes dramatically with pulse phase, we find that its smooth shape does not. Additionally, our data show that the continuum only changes marginally between the three observations. These changes can be explained with varying amounts of Thomson scattering in the hot coronamore » of the accretion disk. The average, luminosity-corrected CRSF energy is lower than in past observations and follows a secular decline. The excellent data quality of NuSTAR provides the best constraint on the CRSF energy to date.« less
Authors:
; ; ; ; ;  [1] ; ;  [2] ; ; ;  [3] ; ;  [4] ; ;  [5] ;  [6] ;  [7] ;  [8] ;  [9] ;  [10]
  1. Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125 (United States)
  2. Institut für Astronomie und Astrophysik, Universität Tübingen (IAAT), D-72076 Tübingen (Germany)
  3. Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States)
  4. Université de Toulouse, UPS-OMP, IRAP, F-31028 Toulouse (France)
  5. DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby (Denmark)
  6. Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027 (United States)
  7. CRESST, UMBC, and NASA GSFC, Code 661, Greenbelt, MD 20771 (United States)
  8. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
  9. Dr. Karl-Remeis-Sternwarte and ECAP, Sternwartstr. 7, D-96049 Bamberg (Germany)
  10. NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
Publication Date:
OSTI Identifier:
22348489
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 779; Journal Issue: 1; 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; COMPARATIVE EVALUATIONS; CYCLOTRONS; HARD X RADIATION; IRON; LIMITING VALUES; LUMINOSITY; NEUTRONS; PULSARS; RESOLUTION; RESONANCE SCATTERING; SPECTRA; STARS; SYMMETRY; TELESCOPES; THOMSON SCATTERING