Dimitrios Psaltis

Sonic-point model of kilohertz quasi-periodic brightness oscillations in low-mass x-ray binaries

Quasi-periodic brightness oscillations (QPOs) with frequencies ranging from ~300 to ~1200 Hz have been discovered in the X-ray emission from 14 neutron stars in low-mass binary systems and from another neutron star in the direction of the Galactic center. These kilohertz QPOs are very strong, with rms relative amplitudes ranging up to ~15% of the total X-ray count rate, and are remarkably coherent, with frequency-to-FWHM ratios as large as ~200. Two simultaneous kilohertz QPOs differing in frequency by ~250-350 Hz have been detected in 12 of the 15 sources. Here we propose a model for these QPOs. In this model, the X-ray source is a neutron star with a surface magnetic field ~107-1010 G and a spin frequency of a few hundred hertz, accreting gas via a Keplerian disk. Some of the accreting gas is channeled by the stellar magnetic field but some remains in a Keplerian disk flow that penetrates to within a few kilometers of the stellar surface. The frequency of the higher frequency QPO in a kilohertz QPO pair is the Keplerian frequency at a radius near the sonic point at the inner edge of the Keplerian flow, whereas the frequency of the lower frequency QPO is the difference between the Keplerian frequency at a radius near the sonic point and the fundamental or first overtone of the stellar spin frequency. The difference between the frequencies of the pair of QPOs is therefore close to (but not necessarily equal to) the stellar spin frequency. The amplitudes of the QPOs at the sonic-point Keplerian frequency and at the beat frequency depend on the strength of the neutron stars magnetic field and the accretion rate, and hence one or both of these QPOs may sometimes be undetectable. Oscillations at the stellar spin frequency and its overtones are expected to be weak but may sometimes be detectable. This model is consistent with the magnetic field strengths, accretion rates, and scattering optical depths inferred from previous modeling of the X-ray spectra and rapid X-ray variability of the atoll and Z sources. It explains naturally the frequencies of the kilohertz QPOs and the similarity of these frequencies in sources with different accretion rates and magnetic fields. The model also explains the high coherence and large amplitudes of the kilohertz QPOs and the steep increase of QPO amplitude with photon energy. The increase in QPO frequency with inferred accretion rate seen in many sources is also understandable in this model. We show that if the frequency of the higher frequency QPO in a pair is an orbital frequency, as in the sonic-point model, the frequencies of these QPOs place interesting upper bounds on the masses and radii of the neutron stars in the kilohertz QPO sources and provide new constraints on the equation of state of matter at high densities. Further observations of these QPOs may provide compelling evidence for the existence of a marginally stable orbit, confirming a key prediction of general relativity in the strong-field regime.

Thermonuclear (Type I) X-Ray Bursts Observed by the Rossi X-Ray Timing Explorer

We have assembled a sample of 1187 thermonuclear (type I) X-ray bursts from observations of 48 accreting neutron stars by the Rossi X-ray Timing Explorer, spanning more than 10 years. The sample contains examples of two of the three theoretical ignition regimes (confirmed via comparisons with numerical models) and likely examples of the third. We present a detailed analysis of the variation of the burst profiles, energetics, recurrence times, presence of photospheric radius expansion, and presence of burst oscillations, as a function of accretion rate. We estimated the distance for 35 sources exhibiting radius-expansion bursts, and found that the peak flux of such bursts varies typically by 13%. We classified sources into two main groups based on the burst properties: (1) both long and short bursts (indicating mixed H/He accretion), and (2) consistently short bursts (primarily He accretion), and we calculated the mean burst rate as a function of accretion rate for the two groups. The decrease in burst rate observed at > 0.06dot MEdd (~2 × 10^37 ergs s^−1) is associated with a transition in the persistent spectral state and (as has been suggested previously) may be related to the increasing role of steady He burning. We found many examples of bursts with recurrence times <30 minutes, including burst triplets and even quadruplets. We describe the oscillation amplitudes for 13 of the 16 burst oscillation sources, as well as the stages and properties of the bursts in which the oscillations are detected. The burst properties are correlated with the burst oscillation frequency; sources spinning at <400 Hz generally have consistently short bursts, while the more rapidly spinning systems have both long and short bursts. This correlation suggests either that shear-mediated mixing dominates the burst properties, or alternatively that the nature of the mass donor (and hence the evolutionary history) has an influence on the long-term spin evolution.

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.

The Black Hole Mass Distribution in the Galaxy

We use dynamical mass measurements of 16 black holes in transient low-mass X-ray binaries to infer the stellar black hole mass distribution in the parent population. We find that the observations are best described by a narrow mass distribution at 7.8 {+-} 1.2 M{sub sun}. We identify a selection effect related to the choice of targets for optical follow-ups that results in a flux-limited sample. We demonstrate, however, that this selection effect does not introduce a bias in the observed distribution and cannot explain the absence of black holes in the 2-5 M{sub sun} mass range. On the high-mass end, we argue that the rapid decline in the inferred distribution may be the result of the particular evolutionary channel followed by low-mass X-ray binaries. This is consistent with the presence of high-mass black holes in the persistent, high-mass X-ray binary sources. If the paucity of low-mass black holes is caused by a sudden decrease of the supernova explosion energy with increasing progenitor mass, this would have observable implications for ongoing transient surveys that target core-collapse supernovae. Our results also have significant implications for the calculation of event rates from the coalescence of black hole binaries for gravitational wave detectors.

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

Probes and Tests of Strong-Field Gravity with Observations in the Electromagnetic Spectrum

Neutron stars and black holes are the astrophysical systems with the strongest gravitational fields in the universe. In this article, I review the prospect of using observations of such compact objects to probe some of the most intriguing general relativistic predictions in the strong-field regime: the absence of stable circular orbits near a compact object and the presence of event horizons around black-hole singularities. I discuss the need for a theoretical framework, within which future experiments will provide detailed, quantitative tests of gravity theories. Finally, I summarize the constraints imposed by current observations of neutron stars on potential deviations from general relativity.

ON THE MASS DISTRIBUTION AND BIRTH MASSES OF NEUTRON STARS

We investigate the distribution of neutron star masses in different populations of binaries, employing Bayesian statistical techniques. In particular, we explore the differences in neutron star masses between sources that have experienced distinct evolutionary paths and accretion episodes. We find that the distribution of neutron star masses in non-recycled eclipsing high-mass binaries as well as of slow pulsars, which are all believed to be near their birth masses, has a mean of 1.28 M ☉ and a dispersion of 0.24 M ☉. These values are consistent with expectations for neutron star formation in core-collapse supernovae. On the other hand, double neutron stars, which are also believed to be near their birth masses, have a much narrower mass distribution, peaking at 1.33 M ☉, but with a dispersion of only 0.05 M ☉. Such a small dispersion cannot easily be understood and perhaps points to a particular and rare formation channel. The mass distribution of neutron stars that have been recycled has a mean of 1.48 M ☉ and a dispersion of 0.2 M ☉, consistent with the expectation that they have experienced extended mass accretion episodes. The fact that only a very small fraction of recycled neutron stars in the inferred distribution have masses that exceed ~2 M ☉ suggests that only a few of these neutron stars cross the mass threshold to form low-mass black holes.

The Disk-Magnetosphere Interaction in the Accretion-powered Millisecond Pulsar SAX J1808.4–3658

The recent discovery of the first known accretion-powered millisecond pulsar with the Rossi X-Ray Timing Explorer provides the first direct probe of the interaction of an accretion disk with the magnetic field of a weakly magnetic (B 1010 G) neutron star. We demonstrate that the presence of coherent pulsations from a weakly magnetic neutron star over a wide range of accretion rates places strong constraints on models of the disk-magnetosphere interaction. We argue that the simple 3/7 scaling law for the Keplerian frequency at the magnetic interaction radius, widely used to model disk accretion onto magnetic stars, is not consistent with observations of SAX J1808.4-3658 for most proposed equations of state for stable neutron stars. We show that the usually neglected effects of multipole magnetic moments, radiation drag forces, and general relativity must be considered when modeling such weakly magnetic systems. Using only very general assumptions, we obtain a robust estimate of μ (1-10) × 1026 G cm3 for the dipole magnetic moment of SAX J1808.4-3658, implying a surface dipole field of ~108-109 G at the stellar equator. We therefore infer that after the end of its accretion phase, this source will become a normal millisecond radio pulsar. Finally, we compare the physical properties of this pulsar with those of the nonpulsing, weakly magnetic neutron stars in low-mass X-ray binaries and argue that the absence of coherent pulsations from the latter does not necessarily imply that these neutron stars have significantly different magnetic field strengths from SAX J1808.4-3658.

THE MASS AND RADIUS OF THE NEUTRON STAR IN EXO 1745–248

Bursting X-ray binaries in globular clusters are ideal sources for measuring neutron star masses and radii, and hence, for determining the equation of state of cold, ultradense matter. We use time-resolved spectroscopic data from EXO 1745–248 during thermonuclear bursts that show strong evidence for photospheric radius expansion to measure the Eddington flux and the apparent surface area of the neutron star. We combine this with the recent measurement of the distance to the globular cluster Terzan 5, where this source resides, to measure the neutron star mass and radius. We find tightly constrained pairs of values for the mass and radius, which are centered around M = 1.4 M ☉ and R = 11 km or around M = 1.7 M ☉ and R = 9 km. These values favor nucleonic equations of state with symmetry energy that is relatively low and has a weak dependence on density.

ANGULAR MOMENTUM TRANSPORT IN ACCRETION DISKS: SCALING LAWS IN MRI-DRIVEN TURBULENCE

We present a scaling law that predicts the values of the stresses obtained in numerical simulations of saturated MRI-driven turbulence in nonstratified shearing boxes. It relates the turbulent stresses to the strength of the vertical magnetic field, the sound speed, the vertical size of the box, and the numerical resolution and predicts accurately the results of 35 numerical simulations performed for a wide variety of physical conditions. We use our result to show that the saturated stresses in simulations with zero net magnetic flux depend linearly on the numerical resolution and would become negligible if the resolution were set equal to the natural dissipation scale in astrophysical disks. We conclude that in order for MRI-driven turbulent angular momentum transport to be able to account for the large value of the effective alpha viscosity inferred observationally, the disk must be threaded by a significant vertical magnetic field and the turbulent magnetic energy must be in near equipartition with the thermal energy. This result has important implications for the spectra of accretion disks and their stability.