A HARD X-RAY POWER-LAW SPECTRAL CUTOFF IN CENTAURUS X-4
Chakrabarty D, Tomsick JA, Grefenstette BW, Psaltis D, Bachetti M, Barret D, Boggs SE, Christensen FE, Craig WW, Fuerst F, Hailey CJ, Harrison FA, Kaspi VM, Miller JM, Nowak MA, Rana V, Stern D, Wik DR, Wilms J, Zhang WW (2014)
Publication Status: Published
Publication Type: Journal article
Publication year: 2014
Journal
Publisher: IOP PUBLISHING LTD
Book Volume: 797
Article Number: ARTN 92
Journal Issue: 2
DOI: 10.1088/0004-637X/797/2/92
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 1.1 x 10(33) D-kpc(2) erg s(-1), with similar or equal to 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(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 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.
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Involved external institutions
Technical University of Denmark / Danmarks Tekniske Universitet (DTU)
Denmark (DK)
University of California, Berkeley
United States (USA) (US)
Université Toulouse III - Paul Sabatier
France (FR)
University of Arizona
United States (USA) (US)
California Institute of Technology (Caltech)
United States (USA) (US)
Massachusetts Institute of Technology (MIT)
United States (USA) (US)
National Aeronautics and Space Administration (NASA)
United States (USA) (US)
University of Michigan
United States (USA) (US)
McGill University
Canada (CA)
Columbia University
United States (USA) (US)
Denmark (DK)
University of California, Berkeley
United States (USA) (US)
Université Toulouse III - Paul Sabatier
France (FR)
University of Arizona
United States (USA) (US)
California Institute of Technology (Caltech)
United States (USA) (US)
Massachusetts Institute of Technology (MIT)
United States (USA) (US)
National Aeronautics and Space Administration (NASA)
United States (USA) (US)
University of Michigan
United States (USA) (US)
McGill University
Canada (CA)
Columbia University
United States (USA) (US)
How to cite
APA:
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). https://dx.doi.org/10.1088/0004-637X/797/2/92
MLA:
Chakrabarty, Deepto, et al. "A HARD X-RAY POWER-LAW SPECTRAL CUTOFF IN CENTAURUS X-4." Astrophysical Journal 797.2 (2014).
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