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Correlations in Quasi-Periodic Oscillation and Noise Frequencies Among Neutron-Star and Black-Hole X-Ray Binaries

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Correlations in Quasi-Periodic Oscillation and Noise Frequencies Among Neutron-Star and Black-Hole X-Ray Binaries View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server Correlations in Quasi-Periodic Oscillation and Noise Frequencies Among Neutron-Star and Black-Hole X-ray Binaries Dimitrios Psaltis Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, U.S.A.; [email protected] Tomaso Belloni, and Michiel van der Klis Astronomical Institute \Anton Pannekoek", University of Amsterdam and Center for High Energy Astrophysics, Kruislaan 403, NL-1098 SJ Amsterdam, The Netherlands; tmb, [email protected] ABSTRACT We study systematically the 0:1 1200 Hz quasi-periodic oscillations (QPOs) and broad noise components observed' in− the power spectra of non-pulsing neutron-star and black-hole low-mass X-ray binaries. We show that among these components we can identify two, occurring over a wide range of source types and luminosities, whose frequencies follow a tight correlation. The variability components involved in this cor- relation include neutron-star kilohertz QPOs and horizontal-branch oscillations, as well as black-hole QPOs and noise components. Our results suggest that the same types of variability may occur in both neutron-star and black-hole systems over three orders of magnitude in frequency and with coherences that vary widely but systematically. Confirmation of this hypothesis will strongly constrain theoretical models of these phe- nomena and provide additional clues to understanding their nature. Subject headings: accretion, accretion disks | black hole physics | stars: neutron | X-rays: stars To appear in the Astrophysical Journal 1 1. INTRODUCTION of which may or may not be constant ( 67 Hz in GRS 1915+105: Morgan, Remillard,' & Greiner Fast quasi-periodic oscillations (QPOs) in the 1997; 300 Hz in GRO 1655 40: Remillard et X-ray brightness of neutron-star and black-hole al. 1999a;' 160 220 Hz in− XTE J1550-564: X-ray binaries provide a useful probe into the in- Remilard et∼ al. 1999b).− Broad noise components ner accretion flows around such compact objects. are also prominent in the power-density spectra Since the original discovery of 20 50 Hz QPOs of black-hole X-ray binaries and show many sim- in the luminous neutron-star' binary− GX 5 1 − ilarities with those of neutron-star sources (van (van der Klis et al. 1985), a variety of additional der Klis 1994a, 1994b; Wijnands & van der Klis QPOs as well as broad noise components have 1999). been observed with properties that depend on A large variety of theoretical models have been the spectral state of the sources (see van der Klis proposed for the different QPOs in neutron-star 1995, 1998 for reviews). and black-hole systems. The rms amplitudes of The Z sources, which are luminous neutron- almost all QPOs increase with increasing photon star low-mass X-ray binaries (Hasinger & van der energy up to 10 30 keV and their frequencies Klis 1989), typically show four distinct types of usually depend' strongly− on mass accretion rate QPOs. In current nomenclature, these are the (see, however, Jonker, van der Klis, & Wijnands 5 20 Hz normal branch oscillation (NBO), 1999). For these and other reasons, such QPOs the' − 15 60 Hz horizontal branch oscillation ' − are thought to originate close to the compact (HBO; see van der Klis 1989 for a review of these objects and their frequencies have been identi- low frequency QPOs), and the 200 1200 Hz ' − fied with various characteristic frequencies in the kilohertz QPOs that typically occur in pairs (van inner accretion flows. For example, theoretical der Klis et al. 1996). The atoll sources, which models attribute some of the observed QPOs to are less luminous neutron-star low-mass X-ray Keplerian orbital frequencies in the disk (e.g., Al- binaries, typically show 500 1250 Hz kilo- ∼ − par et al. 1992; Miller, Lamb, & Psaltis 1998), to hertz QPOs that occur in pairs (Strohmayer et the beat of such frequencies with the stellar spin al. 1996), as well as 20 60 Hz QPOs and ∼ − (e.g., Alpar & Shaham 1985; Lamb et al. 1985; broad noise components that have been identi- Miller et al. 1998), to radiation-hydrodynamic fied as possibly similar to horizontal branch os- (Fortner, Lamb, & Miller 1989; Klein et al. 1996) cillations (see, e.g., Hasinger & van der Klis 1989; or oscillatory disk and stellar modes (e.g., Nowak Homan et al. 1998). All of these QPOs have cen- & Wagoner 1991; Bildsten & Cutler 1995; Kane- troid frequencies that increase with inferred mass take, Takauti, & Fukue 1995; Alpar & Yilmaz accretion rate. In several atoll sources, nearly 1998; Titarchuk, Lapidus, & Muslimov 1998), to coherent 300 600 Hz oscillations have also ' − general relativistic effects (Ipser 1996; Stella & been detected during thermonuclear Type I X- Vietri 1998, 1999), etc. Some of these models ray bursts (see, e.g., Strohmayer et al. 1996). depend explicitly on the existence of a hard sur- The phenomenology of QPOs in black-hole face or a large-scale magnetic field and therefore X-ray binaries has not been developed to the are valid only for QPOs in neutron-star systems, same extent so far. Several low-frequency ( whereas others are applicable only to black-hole 1 ∼ 10− 10 Hz) QPOs have been detected with systems or to both. − frequencies that depend on inferred mass accre- The detection of several QPOs at the same tion rate (see, e.g., Chen, Swank, & Taam 1997; time in a single source provides multiple probes Morgan, Remillard, & Greiner 1997), as well as to the inner accretion flow around the compact three cases of QPOs with higher frequencies, two object. Such a simultaneous detection is often at- 2 tributed to actions of distinct mechanisms affect- nent, and attempt to identify the various power- ing the X-ray brightness of the source simultane- spectral components with QPOs and noise com- ously in different ways, whereas in other models ponents in other sources on the basis of, primar- all QPOs correspond to different modes of the ily, their frequencies. We restrict our study to same fundamental mechanism (e.g., Titarchuk & phenomena with frequencies > 0:1Hztoavoid Muslimov 1997). When several types of QPO are the very complicated timing behavior∼ of the mi- observed in the X-ray brightness of a system si- croquasars at these low frequencies (see, e.g., multaneously, their frequencies, which commonly Remillard et al. 1999a). We do not consider in increase with mass accretion rate, are often tightly our study the peculiar 1HzQPOinthedipper correlated (see, e.g., van der Klis et al. 1996; Ford 4U 1323 62 (Jonker et' al. 1999) that has prop- & van der Klis 1998; Psaltis et al. 1998, 1999). erties very− different compared to all other QPOs In some cases, these correlations appear to de- in non-pulsing neutron-star sources. We will also pend only weakly on the other properties of the exclude from our study QPOs observed in accre- sources. For example, the frequencies of the up- tion powered pulsars, most of which are thought per and lower kHz QPOs, as well as the frequen- to be strongly magnetic ( 1012 G) and hence cies of the upper kHz QPO and the HBO are to have different inner accretion∼ flow properties consistent with following very similar relations than the non-pulsing neutron star sources. The in all Z sources (Psaltis et al. 1998, 1999). QPO and noise properties in the only known mil- Here we study the systematics of QPOs and lisecond accretion powered pulsar, SAX J1808.4- peaked broad noise components observed in non- 3658, have been compared to those of the non- pulsing neutron-star and black-hole low-mass X- pulsing sources by Wijnands & van der Klis ray binaries. We find tight correlations among (1999) and are also excluded in the present study. them. In particular, we find an indication that two types of variability, showing up in some sys- 2.1. Neutron-Star Sources tems as an HBO and a kHz QPO, may occur, Z sources.|In the power spectra of Z sources, at widely different coherences and over a wide the identification of QPOs and noise components range of frequencies, in both neutron-star and is unambiguous. For example, four distinct QPO black-hole systems. We caution, however, that peaks that are not harmonically related are often the striking correlations we report here might be observed simultaneously in Sco X-1 (van der Klis an artifact produced by the accidental line-up of et al. 1996). In this as well as the other Z sources, multiple, independent correlations. If future ob- these QPO peaks are identified as NBOs, HBOs, servation confirm our results, this will strongly or kHz QPOs, based on their frequencies and constrain theoretical models of QPOs in neutron- occurrence in different spectral branches. Fig- star and black-hole systems. ure 1 shows a power-density spectrum of Sco X-1 (based on the data described in van der Klis et 2. IDENTIFICATION OF QPOs IN al. 1997), in which an HBO and its harmonic, as NEUTRON-STAR AND BLACK-HOLE well as a lower and an upper kHz QPO are ev- SYSTEMS ident (notice that frequency shifts were applied to the power spectra shown in Fig. 1). In the five Quasi-periodic oscillations and peaked broad Z sources in which HBOs have been observed si- noise components have been detected in both multaneously with two kHz QPOs, the frequency neutron star and black hole systems. In this sec- of the HBO is tightly correlated to the frequency tion we discuss observations of such systems in of the upper kHz QPO (Wijnands et al. 1998a, which at least one QPO has been detected to- 1998b; Homan et al.
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