Michiel van der Klis

A Unified Description of the Timing Features of Accreting X-Ray Binaries

We study an empirical model for a unified description of the power spectra of accreting neutron stars and black holes. This description is based on a superposition of multiple Lorentzians and offers the advantage that all quasi-periodic oscillation and noise components are dealt with in the same way, without the need of deciding in advance the nature of each component. This approach also allows us to compare frequencies of features with high and low coherences in a consistent manner and greatly facilitates comparison of power spectra across a wide range of source types and states. We apply the model to six sources: the low-luminosity X-ray bursters 1E 1724-3045, SLX 1735-269, and GS 1826-24; the high-latitude transient XTE J1118+480; the bright system Cir X-1; and the Z source GX 17+2. We find that it provides a good description of the observed spectra without the need for a scale-free (1/f) component. We update previously reported correlations between characteristic frequencies of timing features in the light of this new approach and discuss similarities between different types of systems that may point toward similar underlying physics.

Correlated X-ray Spectral and Timing Behavior of the Black Hole Candidate XTE J1550-564: A New Interpretation of Black Hole States

We present an analysis of data of the black hole candidate and X-ray transient XTE J1550-564, taken with the Rossi X-Ray Timing Explorer between 1998 November 22 and 1999 May 20. During this period the source went through several different states, which could be divided into soft and hard states based on the relative strength of the high-energy spectral component. These states showed up as distinct branches in the color-color and hardness-intensity diagrams, connecting to form a structure with a comblike topology, the branch corresponding to the soft state forming the spine and the branches corresponding to the various hard states forming the teeth of the comb. The power spectral properties of the source were strongly correlated with its position on the branches. The broadband noise became stronger and changed from power law-like to band-limited, as the spectrum became harder. Three types of quasi-periodic oscillations (QPOs) were found: 1-18 Hz and 102-284 Hz QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. The 1-18 Hz QPOs on the hard branches could be divided into three subtypes. The frequencies of the high- and low-frequency QPOs on the hard branches were correlated with each other and were anticorrelated with spectral hardness. The changes in QPO frequency suggest that the inner disk radius only increases by a factor of 3-4 as the source changes from a soft to a hard state. Our results on XTE J1550-564 strongly favor a two-dimensional description of black hole behavior, where the regions near the spine of the comb in the color-color diagram can be identified with the high state, and the teeth with transitions from the high state, via the intermediate state (which includes the very high state) to the low state, and back. The two physical parameters underlying this two-dimensional behavior vary to a large extent independently and could for example be the accretion rate through the disk and the size of the Comptonizing region causing the hard tail. The difference between the various teeth is then associated with the mass accretion rate through the disk, suggesting that high state ↔ low state transitions can occur at any disk mass accretion rate and that these transitions are primarily caused by another, independent parameter. We discuss how this picture could tie in with the canonical, one-dimensional behavior of black hole candidates that has usually been observed.

Correlations in quasi-periodic oscillation and noise frequencies among neutron star and black hole x-ray binaries

We study systematically the ^0.1¨1200 Hz quasi-periodic oscillations (QPOs) and broad noise com- ponents observed in the power spectra of nonpulsing 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 correlation 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 3 orders of magnitude in frequency and with coherences that vary widely but systematically. Con—rmation of this hypothesis will strongly con- strain theoretical models of these phenomena and provide additional clues to understanding their nature. Subject headings: accretion, accretion disksblack hole physicsstars: neutron ¨ stars: oscillationsX-rays: stars

Nuclear-powered millisecond pulsars and the maximum spin frequency of neutron stars

Millisecond pulsars are neutron stars that are thought to have been spun-up by mass accretion from a stellar companion. It is not known whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many neutron stars that are accreting mass from a companion star exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces. Millisecond-period brightness oscillations during bursts from ten neutron stars (as distinct from other rapid X-ray variability that is also observed) are thought to measure the stellar spin, but direct proof of a rotational origin has been lacking. Here we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting neutron stars, corroborating earlier evidence. The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most neutron-star models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars.

Dynamical Evidence for a Black Hole in the Microquasar XTE J1550-564

Optical spectroscopic observations of the companion star (type G8 IV to K4 III) in the microquasar system XTE J1550-564 reveal a radial velocity curve with a best-fitting spectroscopic period of Psp = 1.552 ± 0.010 days and a semiamplitude of K2 = 349 ± 12 km s-1. The optical mass function is f(M) = 6.86 ± 0.71 M☉ (1 σ). We tentatively measure the rotational velocity of the companion star to be Vrot sin i = 90 ± 10 km s-1, which when taken at face value implies a mass ratio of Q ≡ M1/M2 = 6.6 (1 σ), using the above value of K2. We derive constraints on the binary parameters from simultaneous modeling of the ellipsoidal light and radial velocity curves. We find 1 σ ranges for the photometric period (1.5430 days ≤ Pph ≤ 1.5440 days), K-velocity (350.2 ≤ K2 ≤ 368.6 km s-1), inclination (670 ≤ i ≤ 774), mass ratio (Q ≥ 12.0), and orbital separation (11.55 R☉ ≤ a ≤ 12.50 R☉). Given these geometrical constraints, we find that the most likely value of the mass of the compact object is 9.41 M☉ with a 1 σ range of 8.36 M☉ ≤ M1 ≤ 10.76 M☉. If we apply our tentative value of Vrot sin i = 90 ± 10 km s-1 as an additional constraint in the ellipsoidal modeling, we find 1 σ ranges of 1.5432 days ≤ Pph ≤ 1.5441 days for the photometric period, 352.2 ≤ K2 ≤ 370.1 km s-1 for the K-velocity, 708 ≤ i ≤ 754 for the inclination, 6.7 ≤ Q ≤ 11.0 for the mass ratio, and 12.35 R☉ ≤ a ≤ 13.22 R☉ for the orbital separation. These geometrical constraints imply the most likely value of the mass of the compact object of 10.56 M☉ with a 1 σ range of 9.68 M☉ ≤ M1 ≤ 11.58 M☉. In either case the mass of the compact object is well above the maximum mass of a stable neutron star, and we therefore conclude that XTE J1550-564 contains a black hole.

The broadband power spectra of x-ray binaries

We analyze the rapid, aperiodic X-ray variability of different types of X-ray binaries (black hole candidates, atoll sources, the recently discovered millisecond X-ray pulsar, and Z sources) at their lowest inferred mass accretion rates. At these accretion rates, the power spectra of all sources are dominated by a strong band-limited noise component, which follows a power law with an index of roughly 1 at high frequencies and breaks at a frequency between 0.02 and 32 Hz, below which the spectrum is relatively flat. Superimposed on this, a broad bump (sometimes a quasi-periodic oscillation) is present, with a 0.2-67 Hz centroid frequency that varies in good correlation with the frequency of the break. The black hole candidates and the low-luminosity neutron star systems (including the millisecond X-ray pulsar) have the same relation between the frequency of the bump and the frequency of the break. These similar characteristics strongly suggest that in all those different types of sources, the band-limited noise and the bump are produced by the same physical mechanism. This mechanism cannot then depend on the presence or absence of either a small magnetosphere or a solid surface, so that it is most likely related to an instability in the flow in the accretion disk that modulates the accretion rate. The Z sources, which are more luminous than the other sources discussed here, follow a similar, but slightly shifted correlation between the break frequency and the frequency of the bump. The data suggest that the band-limited noise in Z sources is more complex than that in the other sources.

Dynamical Formation of Close Binary Systems in Globular Clusters

We know from observations that globular clusters are very efficient catalysts in forming unusual short-period binary systems or their offspring, such as low-mass X-ray binaries (LMXBs; neutron stars accreting matter from low-mass stellar companions), cataclysmic variables (white dwarfs accreting matter from stellar companions), and millisecond pulsars (rotating neutron stars with spin periods of a few milliseconds). Although there has been little direct evidence, the overabundance of these objects in globular clusters has been attributed by numerous authors to the high densities in the cores, which leads to an increase in the formation rate of exotic binary systems through close stellar encounters. Many such close binary systems emit X-radiation at low luminosities (LX 1034 ergs s-1) and are being found in large numbers through observations with the Chandra X-Ray Observatory. Here we present conclusive observational evidence of a link between the number of close binaries observed in X-rays in a globular cluster and the stellar encounter rate of the cluster. We also make an estimate of the total number of LMXBs in globular clusters in our Galaxy.

The magnetic nature of disk accretion onto black holes

Although disk accretion onto compact objects—white dwarfs, neutron stars and black holes—is central to much of high-energy astrophysics, the mechanisms that enable this process have remained observationally difficult to determine. Accretion disks must transfer angular momentum in order for matter to travel radially inward onto the compact object. Internal viscosity from magnetic processes and disk winds can both in principle transfer angular momentum, but hitherto we lacked evidence that either occurs. Here we report that an X-ray-absorbing wind discovered in an observation of the stellar-mass black hole binary GRO J1655 - 40 (ref. 6) must be powered by a magnetic process that can also drive accretion through the disk. Detailed spectral analysis and modelling of the wind shows that it can only be powered by pressure generated by magnetic viscosity internal to the disk or magnetocentrifugal forces. This result demonstrates that disk accretion onto black holes is a fundamentally magnetic process.

KILOHERTZ QUASI-PERIODIC OSCILLATION PEAK SEPARATION IS NOT CONSTANT IN SCORPIUS X-1

Wereportonaseriesof20,110 5 countss 21 ,0.125mstime-resolutionRossiX-RayTimingExplorerobservations of theZ-source and low-mass X-ray binary Scorpius X-1. Twin kilohertz quasi-periodic oscillation (QPO) peaks are obvious in nearly all observations. We find that the peak separation is not constant, as expected in some beat-frequency models, but instead varies from1310 to1230 Hz when the centroid frequency of the higher frequency peak varies from 1875 to 11085 Hz. We detect none of the additional QPO peaks at higher frequencies predicted in the photon bubble model (PBM), with best-case upper limits on the peaks’ power ratio of 0.025. We do detect, simultaneously with the kilohertz QPO, additional QPO peaks near 45 and 90 Hz whose frequency increases with mass accretion rate. We interpret these asfirst and second harmonics of the so-called horizontal-branch oscillations that are well known from otherZ-sources and usually interpreted in terms of the magnetosphericbeat-frequencymodel(BFM).WeconcludethatthemagnetosphericBFMandthePBMarenow unlikelytoexplainthekilohertzQPOinScoX-1.Inordertosucceedindoingso,anyBFMinvolvingtheneutron starspin(unseeninScoX-1)willhavetopostulateatleastoneadditionalunseenfrequency,beatingwiththespin to produce one of the kilohertz peaks. Subject headings: stars: individual (Scorpius X-1)—stars: neutron—pulsars: general

The Atoll Source States of 4U 1608-52

We have studied the atoll source 4U 1608-52 using a large data set obtained with the Rossi X-Ray Timing Explorer. We find that the timing properties of 4U 1608-52 are almost exactly identical to those of the atoll sources 4U 0614+09 and 4U 1728-34 despite the fact that contrary to these sources 4U 1608-52 is a transient covering 2 orders of magnitude in luminosity. The frequencies of the variability components of these three sources follow a universal scheme when plotted versus the frequency of the upper kilohertz quasi-periodic oscillation, suggesting a very similar accretion flow configuration. If we plot the Z sources on this scheme, only the lower kilohertz quasi-periodic oscillation and horizontal branch oscillation follow identical relations. Using the mutual relations between the frequencies of the variability components, we tested several models: the transition layer model, the sonic point beat frequency model, and the relativistic precession model. None of these models described the data satisfactory. Recently, it has been suggested that the atoll sources (among them 4U 1608-52) trace out similar three-branch patterns as the Z sources in the color-color diagram. We have studied the relation between the power spectral properties and the position of 4U 1608-52 in the color-color diagram and conclude that the timing behavior is not consistent with the idea that 4U 1608-52 traces out a three-branched Z shape in the color-color diagram along which the timing properties vary gradually, as Z sources do.