Please use this identifier to cite or link to this item: http://hdl.handle.net/2289/7365
Title: A Hard X-Ray Power-Law Spectral Cutoff in Centaurus X-4
Authors: Chakrabarty, Deepto
Tomsick, John A.
Rana, V
+17 Co-Authors
Keywords: accretion
accretion disks
binaries: close
stars: individual: Cen X-4
stars: neutron
X-rays: binaries
Issue Date: 20-Dec-2014
Publisher: IOP Sciences for The American Astronomical Society
Citation: The Astrophysical Journal, 2014, Vol.797, p15
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× 1033 D^2_kpc erg s-1, with sime60% 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.
Description: Open Access.
URI: http://hdl.handle.net/2289/7365
ISSN: 0004-637X
1538-4357(Online)
Alternative Location: https://ui.adsabs.harvard.edu/abs/2014ApJ...797...92C/abstract
https://arxiv.org/abs/1403.6751
http://dx.doi.org/10.1088/0004-637X/797/2/92
Copyright: 2014, The American Astronomical Society
Appears in Collections:Research Papers (A&A)

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