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Title: A HARD X-RAY POWER-LAW SPECTRAL CUTOFF IN CENTAURUS X-4

The low-mass X-ray binary (LMXB) Cen X-4 is the brightest and closest (<1.2 kpc) quiescent neutron star transient. Previous 0.5-10 keV X-ray observations of Cen X-4 in quiescence identified two spectral components: soft thermal emission from the neutron star atmosphere and a hard power-law tail of unknown origin. We report here on a simultaneous observation of Cen X-4 with NuSTAR (3-79 keV) and XMM-Newton (0.3-10 keV) in 2013 January, providing the first sensitive hard X-ray spectrum of a quiescent neutron star transient. The 0.3-79 keV luminosity was 1.1×10{sup 33} D{sub kpc}{sup 2} erg s{sup –1}, with ≅60% in the thermal component. We clearly detect a cutoff of the hard spectral tail above 10 keV, the first time such a feature has been detected in this source class. We show that thermal Comptonization and synchrotron shock origins for the hard X-ray emission are ruled out on physical grounds. However, the hard X-ray spectrum is well fit by a thermal bremsstrahlung model with kT{sub e} = 18 keV, which can be understood as arising either in a hot layer above the neutron star atmosphere or in a radiatively inefficient accretion flow. The power-law cutoff energy may be set by the degree ofmore » Compton cooling of the bremsstrahlung electrons by thermal seed photons from the neutron star surface. Lower thermal luminosities should lead to higher (possibly undetectable) cutoff energies. We compare Cen X-4's behavior with PSR J1023+0038, IGR J18245–2452, and XSS J12270–4859, which have shown transitions between LMXB and radio pulsar modes at a similar X-ray luminosity.« less
Authors:
;  [1] ; ; ;  [2] ; ; ; ;  [3] ;  [4] ; ;  [5] ;  [6] ;  [7] ;  [8] ;  [9] ;  [10] ; ;  [11] ;  [12]
  1. MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)
  2. Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States)
  3. Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125 (United States)
  4. Department of Astronomy, University of Arizona, Tucson, AZ 85721 (United States)
  5. Observatoire Midi-Pyrénées, Université de Toulouse III - Paul Sabatier, F-31400 Toulouse (France)
  6. Division of Astrophysics, National Space Institute, Technical University of Denmark, DK-2800 Lyngby (Denmark)
  7. Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027 (United States)
  8. Department of Physics, McGill University, Montreal, PQ H3A 2T8 (Canada)
  9. Department of Astronomy, University of Michigan, Ann Arbor, MI 48109 (United States)
  10. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
  11. Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
  12. Dr. Karl-Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics, Universität Erlangen-Nürnberg, D-96049 Bamberg (Germany)
Publication Date:
OSTI Identifier:
22364853
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 797; 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; BREMSSTRAHLUNG; COMPARATIVE EVALUATIONS; ELECTRONS; HARD X RADIATION; KEV RANGE; LUMINOSITY; MASS; NEUTRON STARS; NEUTRONS; PHOTON EMISSION; PHOTONS; PULSARS; STELLAR ATMOSPHERES; TRANSIENTS