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A Hard X-Ray Power-law Spectral Cutoff in Centaurus X-4

Chakrabarty, Deepto and Tomsick, John A. and Grefenstette, Brian W. and Psaltis, Dimitrios and Bachetti, Matteo and Barret, Didier and Boggs, Steven E. and Christensen, Finn E. and Craig, William W. and Fürst, Felix and Hailey, Charles J. and Harrison, Fiona A. and Kaspi, Victoria M. and Miller, Jon M. and Nowak, Michael A. and Rana, Vikram R. and Stern, Daniel and Wik, Daniel R. and Wilms, Jörn and Zhang, William W. (2014) A Hard X-Ray Power-law Spectral Cutoff in Centaurus X-4. Astrophysical Journal, 797 (2). Art. No. 92. ISSN 0004-637X. doi:10.1088/0004-637X/797/2/92.

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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^2_(kpc 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 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Chakrabarty, Deepto0000-0001-8804-8946
Tomsick, John A.0000-0001-5506-9855
Grefenstette, Brian W.0000-0002-1984-2932
Bachetti, Matteo0000-0002-4576-9337
Barret, Didier0000-0002-0393-9190
Boggs, Steven E.0000-0001-9567-4224
Christensen, Finn E.0000-0001-5679-1946
Fürst, Felix0000-0003-0388-0560
Harrison, Fiona A.0000-0003-2992-8024
Kaspi, Victoria M.0000-0001-9345-0307
Nowak, Michael A.0000-0001-6923-1315
Rana, Vikram R.0000-0003-1703-8796
Stern, Daniel0000-0003-2686-9241
Wik, Daniel R.0000-0001-8952-676X
Wilms, Jörn0000-0003-2065-5410
Zhang, William W.0000-0002-1426-9698
Additional Information:© 2014 The American Astronomical Society. Received 2014 March 26; accepted 2014 October 8; published 2014 December 3. We thank the referee, Craig Heinke, for several suggestions that greatly improved our paper. D.C. thanks Herman Marshall, Sera Markoff, Caroline D’Angelo, Stephen Reynolds, Federico Bernardini, Phil Charles, and Chris Done for useful discussions and Luca Zampieri and Roberto Soria for sharing their XSPEC additive table model zamp. We also thank Thorsten Brand for help with evaluating the level of photon pileup in the XMM-Newton data. This work was supported in part under NASA contract NNG08FD60C and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA. We thank the NuSTAR Operations, Software, and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS), jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). J.A.T. acknowledges partial support from the XMM-Newton Guest Observer program through NASA grant NNX13AB47G. Facilities: NuSTAR, XMM
Group:NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
Subject Keywords:accretion, accretion disks; binaries: close; stars: individual (Cen X-4); stars: neutron; X-rays: binaries
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2015-89
Issue or Number:2
Record Number:CaltechAUTHORS:20141211-081434965
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Official Citation:A Hard X-Ray Power-law Spectral Cutoff in Centaurus X-4 Deepto Chakrabarty et al. 2014 ApJ 797 92
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:52571
Deposited By: Ruth Sustaita
Deposited On:11 Dec 2014 21:46
Last Modified:10 Nov 2021 19:42

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