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Quasi-Periodic Oscillation in the Dipping Low-Mass X-Ray Binary

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Quasi-Periodic Oscillation in the Dipping Low-Mass X-Ray Binary UvA-DARE (Digital Academic Repository) Quasi-periodic oscillation in the dipping low-mass x-ray binary Jonker, P.G.; van der Klis, M.; Wijnands, R.A.D. DOI 10.1086/311840 Publication date 1999 Published in Astrophysical Journal Link to publication Citation for published version (APA): Jonker, P. G., van der Klis, M., & Wijnands, R. A. D. (1999). Quasi-periodic oscillation in the dipping low-mass x-ray binary. Astrophysical Journal, 511, L41-L44. https://doi.org/10.1086/311840 General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:27 Sep 2021 The Astrophysical Journal, 516:L91±L94, 1999 May 10 q 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. A VARIABLE 0.58±2.44 Hz QUASI-PERIODIC OSCILLATION IN THE ECLIPSING AND DIPPING LOW-MASS X-RAY BINARY EXO 07482676 Jeroen Homan,1 Peter G. Jonker,1 Rudy Wijnands,1 Michiel van der Klis,1 and Jan van Paradijs1,2 Received 1999 January 20; accepted 1999 March 9; published 1999 March 22 ABSTRACT We report the discovery of a quasi-periodic oscillation (QPO) in data obtained with the Rossi X-Ray Timing Explorer of the dipping and eclipsing low-mass X-ray binary EXO 07482676. The QPO had a frequency between 0.58 and 2.44 Hz changing on timescales of a few days and an rms amplitude between 8% and 12%, and it was detected in the persistent emission during dips and during type I X-ray bursts. During one observation, when the count rate was a factor of 2±3 higher than otherwise, the QPO was not detected. The strength of the QPO did not signi®cantly depend on photon energy and is consistent with being the same in the persistent emission, both during and outside the dips and during type I X-ray bursts. Frequency shifts were observed during three of the four X-ray bursts. We argue that the QPO is produced by the same mechanism as the QPO recently found by Jonker et al. in 4U 1323262. Although the exact mechanism is not clear, it is most likely related to the high inclination of both systems. An orbiting structure in the accretion disk that modulates the radiation from the central source seems the most promising mechanism. Subject headings: accretion, accretion disks Ð stars: individual (EXO 07482676) Ð stars: neutron Ð X-rays: stars 1. INTRODUCTION 2. OBSERVATION AND ANALYSIS We made use of data obtained with the proportional counter The low-mass X-ray binary (LMXB) EXO 07482676 was array (Jahoda et al. 1996) on board the RXTE (Bradt, Roth- discovered with EXOSAT by Parmar et al. (1985). It showed schild, & Swank 1993). A log of all observations is given in periodic eclipses, irregular intensity dips, and type I X-ray Table 1. Observations 2±7 were originally performed to study bursts (Parmar et al. 1986). The eclipses, because of obscuration the orbital evolution of the system. Each consisted of ®ve of the central X-ray source by the companion star, occurred at »2 ks data segments that range in orbital phase from f ¼ 0.9 a period of 3.82 hr. From the eclipse duration, an inclination tof ¼ 0.05 . All observations yielded data in the standard 1 of 757±827 was derived. Dipping activity is seen at orbital and 2 modes, which have 1/8 s time resolution in one energy phasesf » 0.8±0.2 and atf ¼ 0.65 (f 5 0 corresponds to channel (2±60 keV) and 16 s time resolution in 129 channels (2±60 keV), respectively. In addition, data were obtained in eclipse center). The dips can be as deep as 100% (Church et 212 al. 1998). It is believed (White & Swank 1982; Walter et al. modes with at least 2 s time resolution in 67 (observation 1982) that such dips are caused by obscuration of the central 1), 32 (observation 2), 256 (observations 2±7), or 64 (obser- X-ray source by a bulge in the outer accretion disk that is vations 8±10) channels, covering the 2±60 keV range. Data during eclipses were removed from further analysis. X-ray created by the impact of the gas stream from the companion bursts were studied separately. star or by the accretion stream itself if it penetrates the disk Power spectra were created in the energy bands 2±60, 2±5, further in (Frank, King, & Lasota 1987). During the eclipses, 5±8, 8±13, and 13±42 keV. The average 1/16 to 128 Hz power 4% of the 2±6 keV intensity remained, which Parmar et al. spectra of each observation were rms normalized and ®tted (1986) attributed to the presence of an accretion disk corona with a model consisting of a constant (representing the Poisson (ADC). Using their ªprogressive coveringº model, Church et level), a power law,P / n2a (the noise component), and a al. (1998) found that »70% of the out-of-dip ¯ux is contributed Lorentzian (the QPO). Errors on the parameters were deter- by this ADC, which they estimated to have a radius of mined usingDx 2 5 1 . The dependence on photon energy of (0.4±1.5) # 10 8 cm. the QPO strength was determined by ®tting the power spectra Recently, a persistent 1 Hz quasi-periodic oscillation (QPO) in four energy bands, while keeping the QPO FWHM, fre- was found with the Rossi X-Ray Timing Explorer (RXTE)in quency, and the power-law index (a) of the noise component the high-inclination (i ¼ 807 ) dipping LMXB 4U 1323262 ®xed to the values obtained in the 2±60 keV band. Unless (Jonker, van der Klis, & Wijnands 1999, hereafter JO99). JO99 stated otherwise, the ®t parameters are those in the 2±60 keV concluded that its presence is most likely related to the high band. The 95% con®dence upper limits on the presence of the inclination of the system. In this Letter, we report the discovery QPO and/or noise component were determined by ®xing the of a variable 0.58±2.44 Hz QPO in EXO 07482676 with prop- FWHM and frequency and/or by ®xing a. erties similar to those found in 4U 1323262. For observations 2±6 and 8±10, we compared power spectra obtained inside and outside the dips. A count rate level was determined for each single orbit, below which data were as- sumed to be in a dip. For observations 8±10, this level was 1 Astronomical Institute ªAnton Pannekoek,º University of Amsterdam and the persistent count rate during phases in which normally no Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ, Amsterdam, Netherlands. dipping is observed. As mentioned before, observations 2±6 2 Department of Physics, University of Alabama in Huntsville, Huntsville, were taken during phases that normally show dipping, which AL 45899. is con®rmed by increases in hardness. For these observations, L91 L92 VARIABLE 0.58±2.44 Hz QUASI-PERIODIC OSCILLATION Vol. 516 TABLE 1 Log of the RXTE Observations of EXO 07482676 and the Power Spectral Fit Parameters QPO Noise Count b c d Begin End Tobs QPO rms FWHM Frequency rms Rate Observationa (UTC) (UTC) (ks) (%) (Hz) (Hz) (%) a (counts s21) 1 ................ 1996 Mar 12 10:41 1996 Mar 12 12:54 6.0 (0.9) !2.1e ))3.8 5 0.2 0.98 5 0.12 177.2 5 0.3 10.6 2 ................ 1996 May 03 08:37 1996 May 04 00:30 12.0 (2.9) 12.120.6 0.8 5 0.1 2.44 5 0.03 5.7 5 0.4 0.43 5 0.09 72.9 5 0.3 3 ................ 1997 Jan 18 17:14 1997 Jan 19 13:18 9.6 (3.7) 8.8 5 0.7 0.16 5 0.04 0.58 5 0.01 9.6 5 0.6 0.82 5 0.07 62.7 5 0.2 4 ................ 1997 Mar 07 20:03 1997 Mar 08 12:40 9.8 (5.6) 8.9 5 0.5 0.15 5 0.02 0.68 5 0.01 10.6 5 0.4 0.81 5 0.06 59.6 5 0.2 10.8 5 ................ 1997 May 01 11:52 1997 May 02 07:25 9.5 (3.3) 9.320.6 0.36 5 0.07 1.30 5 0.02 7.2 5 0.5 0.51 5 0.09 64.9 5 0.3 Burst 1 ...... )))9.7 5 0.9 0.18 5 0.5 1.41 5 0.02 )) ) 10.8 6 ................ 1997 Jun 26 06:01 1997 Jun 26 22:03 9.7 (2.7) 9.820.6 0.34 5 0.07 1.04 5 0.02 8.8 5 0.5 0.67 5 0.08 60.9 5 0.2 10.4 7 ...............
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