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Kilohertz Quasi-periodic Oscillation Peak Separation Is Not Constant in the Atoll Source 4U 1608_52

Méndez, M. and van der Klis, M. and Wijnands, R. and Ford, E. C. and van Paradijs, J. and Vaughan, B. A. (1998) Kilohertz Quasi-periodic Oscillation Peak Separation Is Not Constant in the Atoll Source 4U 1608_52. Astrophysical Journal, 505 (1). L23-L26. ISSN 0004-637X. doi:10.1086/311600.

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We present new Rossi X-Ray Timing Explorer observations of the low-mass X-ray binary 4U 1608-52 during the decay of its 1998 outburst. We detect, by a direct fast Fourier transform method, the existence of a second kilohertz quasi-periodic oscillation (kHz QPO) in its power density spectrum, which was previously only seen by means of the sensitivity-enhancing "shift and add" technique. This result confirms that 4U 1608-52 is a twin kHz QPO source. The frequency separation between these two QPOs decreased significantly, from 325.5±3.4 to 225.3±12.0 Hz, as the frequency of the lower kHz QPO increased from 470 to 865 Hz, in contradiction with a simple beat-frequency interpretation. This change in the peak separation of the kHz QPOs is closely similar to that previously seen in Scorpius X-1 but takes place at a 10 times lower average luminosity. We discuss this result within the framework of models that have been proposed for kHz QPO. Beat-frequency models where the peak separation is identified with the neutron star spin rate, as well as the explanations previously proposed to account for the similar behavior of the QPOs in Sco X-1, are strongly challenged by this result.

Item Type:Article
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URLURL TypeDescription Paper
van der Klis, M.0000-0003-0070-9872
Additional Information:© 1998. The American Astronomical Society. Received 1998 June 12; accepted 1998 July 27; published 1998 August 24. We would like to thank Professor Hale Bradt for his comments that helped us to improve the original manuscript. This work was supported in part by the Netherlands Organization for Scientific Research (NWO) under grant PGS 78-277 and by the Netherlands Foundation for research in astronomy (ASTRON) under grant 781-76-017. M. M. is a fellow of the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina. J. v. P. acknowledges support from the National Aeronautics and Space Administration through contracts NAG 5-3269 and 5-4482. M. v. d. K. gratefully acknowledges the Visiting Miller Professor Program of the Miller Institute for Basic Research in Science (UCB).
Group:Space Radiation Laboratory
Funding AgencyGrant Number
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)PGS 78-277
Netherlands Institute for Radio Astronomy (ASTRON)781-76-017
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)UNSPECIFIED
NASANAG 5-3269
NASANAG 5-4482
Miller Institute for Basic Research in ScienceUNSPECIFIED
Subject Keywords:accretion, accretion disks; stars: individual (4U 1608–52) ; stars: neutron; X-rays: stars
Issue or Number:1
Record Number:CaltechAUTHORS:20170408-172308274
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Official Citation:M. Méndez et al 1998 ApJ 505 L23
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76336
Deposited By: 1Science Import
Deposited On:13 Mar 2018 20:54
Last Modified:15 Nov 2021 16:59

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