A hard X-ray power-law spectral cutoff in centaurus X-4

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Abstract

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 erg s-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 kTe = 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 of 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.

Original languageEnglish (US)
Article number92
JournalAstrophysical Journal
Volume797
Issue number2
DOIs
Publication statusPublished - Dec 20 2014

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Keywords

  • accretion, accretion disks
  • binaries: close
  • stars: individual (Cen X-4)
  • stars: neutron
  • X-rays: binaries

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Chakrabarty, D., Tomsick, J. A., Grefenstette, B. W., Psaltis, D., Bachetti, M., Barret, D., ... Zhang, W. W. (2014). A hard X-ray power-law spectral cutoff in centaurus X-4. Astrophysical Journal, 797(2), [92]. https://doi.org/10.1088/0004-637X/797/2/92