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Title: NuSTAR discovery of a luminosity dependent cyclotron line energy in Vela X-1

We present NuSTAR observations of Vela X-1, a persistent, yet highly variable, neutron star high-mass X-ray binary (HMXB). Two observations were taken at similar orbital phases but separated by nearly a year. They show very different 3-79 keV flux levels as well as strong variability during each observation, covering almost one order of magnitude in flux. These observations allow, for the first time ever, investigations on kilo-second time-scales of how the centroid energies of cyclotron resonant scattering features (CRSFs) depend on flux for a persistent HMXB. We find that the line energy of the harmonic CRSF is correlated with flux, as expected in the sub-critical accretion regime. We argue that Vela X-1 has a very narrow accretion column with a radius of around 0.4 km that sustains a Coulomb interaction dominated shock at the observed luminosities of L {sub x} ∼ 3 × 10{sup 36} erg s{sup –1}. Besides the prominent harmonic line at 55 keV the fundamental line around 25 keV is clearly detected. We find that the strengths of the two CRSFs are anti-correlated, which we explain by photon spawning. This anti-correlation is a possible explanation for the debate about the existence of the fundamental line. The ratiomore » of the line energies is variable with time and deviates significantly from 2.0, also a possible consequence of photon spawning, which changes the shape of the line. During the second observation, Vela X-1 showed a short off-state in which the power-law softened and a cut-off was no longer measurable. It is likely that the source switched to a different accretion regime at these low mass accretion rates, explaining the drastic change in spectral shape.« 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. 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. Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States)
  5. Université de Toulouse, UPS-OMP, IRAP, Toulouse (France)
  6. DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby (Denmark)
  7. Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027 (United States)
  8. Department of Astronomy, The University of Michigan, Ann Arbor, MI 48109 (United States)
  9. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
  10. NASA Goddard Space Flight Center, Astrophysics Science Division, Code 662, Greenbelt, MD 20771 (United States)
Publication Date:
OSTI Identifier:
22348249
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 780; 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; BINARY STARS; COSMIC X-RAY SOURCES; CYCLOTRONS; EV RANGE; INTERACTIONS; KEV RANGE; LUMINOSITY; MASS; NEUTRON STARS; NEUTRONS; PHOTONS; RESONANCE SCATTERING; X RADIATION