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Title: THE SPECTRAL-TIMING PROPERTIES OF UPPER AND LOWER kHz QPOs

Abstract

Soft lags from the emission of the lower kilohertz quasi-periodic oscillations (kHz QPOs) of neutron star low-mass X-ray binaries have been reported from 4U1608-522 and 4U1636-536. Those lags hold prospects for constraining the origin of the QPO emission. In this paper, we investigate the spectral-timing properties of both the lower and upper kHz QPOs from the neutron star binary 4U1728-34, using the entire Rossi X-Ray Timing Explorer archive on this source. We show that the lag-energy spectra of the two QPOs are systematically different: while the lower kHz QPO shows soft lags, the upper kHz QPO shows either a flat lag-energy spectrum or hard variations lagging softer variations. This suggests two different QPO-generation mechanisms. We also performed the first spectral deconvolution of the covariance spectra of both kHz QPOs. The QPO spectra are consistent with Comptonized blackbody emission, similar to the one found in the time-averaged spectrum, but with a higher seed-photon temperature, suggesting that a more compact inner region of the Comptonization layer (boundary/spreading layer, corona) is responsible for the QPO emission. Considering our results together with other recent findings, this leads us to the hypothesis that the lower kHz QPO signal is generated by coherent oscillations of themore » compact boundary layer region itself. The upper kHz QPO signal may then be linked to less-coherent accretion-rate variations produced in the inner accretion disk, and is then detected when they reach the boundary layer.« less

Authors:
;  [1];  [2]
  1. Université de Toulouse, UPS-OMP, IRAP, Toulouse (France)
  2. Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam (Netherlands)
Publication Date:
OSTI Identifier:
22525351
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 811; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; BINARY STARS; BOUNDARY LAYERS; ENERGY SPECTRA; KHZ RANGE; MASS; NEUTRON STARS; OSCILLATIONS; PERIODICITY; PHOTON TEMPERATURE; STELLAR CORONAE; TIMING PROPERTIES; X RADIATION

Citation Formats

Peille, Philippe, Barret, Didier, and Uttley, Phil, E-mail: philippe.peille@irap.omp.eu. THE SPECTRAL-TIMING PROPERTIES OF UPPER AND LOWER kHz QPOs. United States: N. p., 2015. Web. doi:10.1088/0004-637X/811/2/109.
Peille, Philippe, Barret, Didier, & Uttley, Phil, E-mail: philippe.peille@irap.omp.eu. THE SPECTRAL-TIMING PROPERTIES OF UPPER AND LOWER kHz QPOs. United States. doi:10.1088/0004-637X/811/2/109.
Peille, Philippe, Barret, Didier, and Uttley, Phil, E-mail: philippe.peille@irap.omp.eu. Thu . "THE SPECTRAL-TIMING PROPERTIES OF UPPER AND LOWER kHz QPOs". United States. doi:10.1088/0004-637X/811/2/109.
@article{osti_22525351,
title = {THE SPECTRAL-TIMING PROPERTIES OF UPPER AND LOWER kHz QPOs},
author = {Peille, Philippe and Barret, Didier and Uttley, Phil, E-mail: philippe.peille@irap.omp.eu},
abstractNote = {Soft lags from the emission of the lower kilohertz quasi-periodic oscillations (kHz QPOs) of neutron star low-mass X-ray binaries have been reported from 4U1608-522 and 4U1636-536. Those lags hold prospects for constraining the origin of the QPO emission. In this paper, we investigate the spectral-timing properties of both the lower and upper kHz QPOs from the neutron star binary 4U1728-34, using the entire Rossi X-Ray Timing Explorer archive on this source. We show that the lag-energy spectra of the two QPOs are systematically different: while the lower kHz QPO shows soft lags, the upper kHz QPO shows either a flat lag-energy spectrum or hard variations lagging softer variations. This suggests two different QPO-generation mechanisms. We also performed the first spectral deconvolution of the covariance spectra of both kHz QPOs. The QPO spectra are consistent with Comptonized blackbody emission, similar to the one found in the time-averaged spectrum, but with a higher seed-photon temperature, suggesting that a more compact inner region of the Comptonization layer (boundary/spreading layer, corona) is responsible for the QPO emission. Considering our results together with other recent findings, this leads us to the hypothesis that the lower kHz QPO signal is generated by coherent oscillations of the compact boundary layer region itself. The upper kHz QPO signal may then be linked to less-coherent accretion-rate variations produced in the inner accretion disk, and is then detected when they reach the boundary layer.},
doi = {10.1088/0004-637X/811/2/109},
journal = {Astrophysical Journal},
issn = {0004-637X},
number = 2,
volume = 811,
place = {United States},
year = {2015},
month = {10}
}