arXiv subjects

M. Coleman Miller

Publications and source records attributed to M. Coleman Miller.

At least 163 records · Page 9Linked to original sources

arXiv2004

Growth of Intermediate-Mass Black Holes in Globular Clusters

We present results of numerical simulations of sequences of binary-single scattering events of black holes in dense stellar environments. The simulations cover a wide range of mass ratios from equal mass objects to 1000:10:10 solar masses and compare purely Newtonian simulations to simulations in which Newtonian encounters are interspersed with gravitational wave emission from the binary. In both cases, the sequence is terminated when the binary's merger time due to gravitational radiation is less than the arrival time of the next interloper. We find that black hole binaries typically merge with a very high eccentricity (0.93 < e < 0.95 pure Newtonian; 0.85 < e < 0.90 with gravitational wave emission) and that adding gravitational wave emission decreases the time to harden a binary until merger by ~ 30% to 40%. We discuss the implications of this work for the formation of intermediate-mass black holes and gravitational wave detection.

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arXiv2004

Intermediate-Mass Black Holes

The mathematical simplicity of black holes, combined with their links to some of the most energetic events in the universe, means that black holes are key objects for fundamental physics and astrophysics. Until recently, it was generally believed that black holes in nature appear in two broad mass ranges: stellar-mass (roughly 3-20 solar masses), which are produced by the core collapse of massive stars, and supermassive (millions to billions of solar masses), which are found in the centers of galaxies and are produced by a still uncertain combination of processes. In the last few years, however, evidence has accumulated for an intermediate-mass class of black holes, with hundreds to thousands of solar masses. If such objects exist they have important implications for the dynamics of stellar clusters, the formation of supermassive black holes, and the production and detection of gravitational waves. We review the evidence for intermediate-mass black holes and discuss future observational and theoretical work that will help clarify numerous outstanding questions about these objects.

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arXiv2003

Interpreting QPOs from Accreting Neutron Stars

The high time resolution and large area of the Rossi X-ray Timing Explorer have been essential in the detection and characterization of high-frequency quasi-periodic variability in the flux from neutron stars in low-mass X-ray binaries. An unknown phenomenon prior to RXTE, kilohertz quasi-periodic oscillations (QPOs) have now been detected from more than twenty systems. Their high frequencies (up to 1330 Hz) imply that they are generated close to the neutron star, where general relativistic effects are expected to play an important role. I summarize current models for the kilohertz QPO phenomenon. In particular, I show that there is a significant domain of agreement among the models that can be used to constrain neutron star structure and look for signatures of highly curved spacetime in the properties of the QPOs.

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arXiv2003

Sonic-Point and Spin-Resonance Model of the Kilohertz QPO Pairs

KHz QPOs have now been detected in more than twenty accreting neutron stars in low-mass binary systems. Two kHz QPOs are usually detected in each star. Burst oscillations and two kHz QPOs have recently been detected in the 401 Hz accretion-powered X-ray pulsar SAX J1808.4-3658. In this star the frequency of the burst oscillation is approximately equal to the star's spin frequency nu_spin whereas the frequency separation of the two kHz QPOs is approximately nu_spin/2. If as expected the frequency of the burst oscillations in other stars is also approximately nu_spin, the frequency separation is approximately nu_spin in some stars but approximately nu_spin/2 in others. A frequency separation approximately equal to nu_spin/2 is unexplained in all existing models of the kHz QPOs. Here we propose a modified version of the sonic-point beat-frequency model that can explain within a single framework why the frequency separation is close to nu_spin in some stars but close to nu_spin/2 in others. As in the original sonic-point model, the frequency nu_QPO2 of the upper kHz QPO is close to the orbital frequency nu_orb at the radius r_{sp} of the sonic point in the disk flow. We show that magnetic and radiation fields rotating with the star will preferentially excite vertical motions in the disk at the "spin-resonance'' radius r_{sr} where nu_orb - nu_spin is equal to the vertical epicyclic frequency, producing vertical motions in the disk that modulate the X-ray flux at approximately nu_QPO2 - nu_spin or approximately nu_QPO2 - nu_spin/2, depending on whether the disk flow at r_{sr} is smooth or clumped. This sonic-point and spin-resonance model can also explain quantitatively the decrease of the kHz QPO frequency separation with increasing accretion rate that is observed in many sources.

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arXiv2003

Three-Body Encounters of Black Holes in Globular Clusters

Evidence has been mounting for the existence of black holes with masses from 10^2 to 10^4 M_Solar associated with stellar clusters. Such intermediate-mass black holes (IMBHs) will encounter other black holes in the dense cores of these clusters. The binaries produced in these interactions will be perturbed by other objects as well thus changing the orbital characteristics of the binaries. These binaries and their subsequent mergers due to gravitational radiation are important sources of gravitational waves. We present the results of numerical simulations of high mass ratio encounters, which help clarify the interactions of intermediate-mass black holes in globular clusters and help determine what types of detectable gravitational wave signatures are likely.

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arXiv2003

Formation Scenarios for Intermediate-Mass Black Holes

Black holes with hundreds to thousands of solar masses are more massive than can be formed from a single star in the current universe, yet the best candidates for these objects are not located in gas-rich environments where gradual accretion could build up the mass. Three main formation scenarios have been suggested in the literature: that intermediate-mass black holes are the remnants of the first, metal-poor, stars; that they result from direct collisions in young stellar clusters; or that they are produced by gradual interactions and mergers of compact objects in old dense clusters. We discuss each of these in turn and speculate on future observations that may help sharpen our understanding of the formation of intermediate-mass black holes.

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arXiv2002

Gravitational Radiation from Intermediate-Mass Black Holes

Recent X-ray observations of galaxies with ROSAT, ASCA, and Chandra have revealed numerous bright off-center point sources which, if isotropic emitters, are likely to be intermediate-mass black holes, with hundreds to thousands of solar masses. The origin of these objects is under debate, but observations suggest that a significant number of them currently reside in young high-density stellar clusters. There is also growing evidence that some Galactic globular clusters harbor black holes of similar mass, from observations of stellar kinematics. In such high-density stellar environments, the interactions of intermediate-mass black holes are promising sources of gravitational waves for ground-based and space-based detectors. Here we explore the signal strengths of binaries containing intermediate-mass black holes or stellar-mass black holes in dense stellar clusters. We estimate that a few to tens per year of these objects will be detectable during the last phase of their inspiral with the advanced LIGO detector, and up to tens per year will be seen during merger, depending on the spins of the black holes. We also find that if these objects reside in globular clusters then tens of sources will be detectable with LISA from the Galactic globular system in a five year integration, and similar numbers will be detectable from more distant galaxies. The signal strength depends on the eccentricity distribution, but we show that there is promise for strong detection of pericenter precession and Lense-Thirring precession of the orbital plane. We conclude by discussing what could be learned about binaries, dense stellar systems, and strong gravity if such signals are detected.

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arXiv2002

Implications of the Narrow Period Distribution of Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters

The spin periods of the ten observed Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters lie in the very narrow range 6-12 s. We use a point likelihood technique to assess the constraints this clustering imposes on the birth period and on the final period of such systems. We consider a general law for their spin evolution described by a constant braking index. We find that, for positive values of the braking index, the observed clustering requires an upper cut-off period that is very close to the maximum observed period of about 12 s. We also show that the constraint on the birth period depends very strongly on the assumed value of the braking index n, ranging from a few milliseconds for n>2 to a few seconds for n<2. We discuss the possible ways of tightening these constraints based on similarities with the population of radio pulsars and with future observations of such sources with current X-ray observatories.

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arXiv2002

Four-Body Effects in Globular Cluster Black Hole Coalescence

In the high density cores of globular clusters, multibody interactions are expected to be common, with the result that black holes in binaries are hardened by interactions. It was shown by Sigurdsson & Hernquist (1993) and others that 10 solar mass black holes interacting exclusively by three-body encounters do not merge in the clusters themselves, because recoil kicks the binaries out of the clusters before the binaries are tight enough to merge. Here we consider a new mechanism, involving four-body encounters. Numerical simulations by a number of authors suggest that roughly 20-50% of binary-binary encounters will eject one star but leave behind a stable hierarchical triple. If the orbital plane of the inner binary is strongly tilted with respect to the orbital plane of the outer object, a secular Kozai resonance, first investigated in the context of asteroids in the Solar System, can increase the eccentricity of the inner body significantly. We show that in a substantial fraction of cases the eccentricity is driven to a high enough value that the inner binary will merge by gravitational radiation, without a strong accompanying kick. Thus the merged object remains in the cluster; depending on the binary fraction of black holes and the inclination distribution of newly-formed hierarchical triples, this mechanism may allow massive black holes to accumulate through successive mergers in the cores of globular clusters. It may also increase the likelihood that stellar-mass black holes in globular clusters will be detectable by their gravitational radiation.

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arXiv2001

Suppression of Gravitational Structure Formation by Cosmological Accretion Heating

As increasingly precise information about the spectrum of the cosmic microwave background fluctuations is gathered with balloon and satellite experiments, interest has grown in foreground sources of opacity affecting these observations. One potentially important source is electron scattering produced by a post-recombination luminosity source, which would significantly attenuate the higher harmonics in the spectrum. If such an ionization source exists, then it would also heat the universe, hence increasing the Jeans mass and suppressing early gravitational structure formation. Here we consider the effects of such heating. We concentrate on one type of ionization source: luminosity generated by accretion onto primordial compact objects. We show that if such objects generate enough luminosity to affect the CMB power spectrum, then they would produce enough heat to prevent the formation of 1 sigma collapsed objects until redshifts of about 5, significantly less than the redshift at which baryonic collapse could otherwise occur. Such processes would leave signatures detectable by upcoming instruments such as NGST, SIRTF, and SWIFT.

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arXiv2001

Production of Intermediate-Mass Black Holes in Globular Clusters

The discovery of numerous non-nuclear X-ray point sources with luminosities L>10^39 erg/s in several starburst galaxies has stimulated speculation about their nature and origin. The strong variability seen in several sources points to massive black holes as the central engines. If the flux is isotropic, the luminosities range up to roughly 10^41 erg/s, implying masses of M>10^3 Msun if the luminosity is sub-Eddington. Here we explore a model for these sources. We suggest that in some tens of percent of globular clusters a very massive black hole, M>50 Msun, is formed. This black hole sinks in <10^6 yr to the center of the cluster, where in the 10^10 yr lifetime of the cluster it accretes roughly 10^3 Msun, primarily in the form of lighter black holes. Unlike less massive black holes in binaries, which are flung from clusters by recoil before they can merge gravitationally, a >50 Msun black hole has enough inertia that it remains bound to the cluster. We suggest that 10^3 Msun black holes may be common in the centers of dense globular clusters, and may therefore exist in some tens of percent of current globulars. If the cluster later merges with its host galaxy, accretion from young star clusters in molecular clouds by the black hole can generate luminosity consistent with that observed. We also consider the detectability of massive black holes in globular clusters with gravitational wave detectors such as LISA and LIGO, and speculate on future observations that may test our predictions.

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arXiv2000

Small-scale structure deduced from X- and gamma-ray timing measurements

X-ray timing observations of neutron stars and black holes are among the few available probes of ultrastrong magnetic fields, strong gravity, high densities, and the propagation of thermonuclear burning. Here we review the evidence for these effects revealed with data from the Rossi Explorer in the last five years. We also discuss the exciting prospects for making the first quantitative tests of strong-gravity general relativistic predictions with a large-area X-ray timing mission.

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arXiv2000

Implications of the PSR 1257+12 Planetary System for Isolated Millisecond Pulsars

The first extrasolar planets were discovered in 1992 around the millisecond pulsar PSR 1257+12. We show that recent developments in the study of accretion onto magnetized stars, plus the existence of the innermost, moon-sized planet in the PSR 1257+12 system, suggest that the pulsar was born with approximately its current rotation frequency and magnetic moment. If so, this has important implications for the formation and evolution of neutron star magnetic fields as well as for the formation of planets around pulsars. In particular, it suggests that some and perhaps all isolated millisecond pulsars may have been born with high spin rates and low magnetic fields instead of having been recycled by accretion.

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arXiv2000

Changing Frequency Separation of Kilohertz Quasi-Periodic Oscillations in the Sonic-Point Beat-Frequency Model

Previous work on the sonic-point beat-frequency (SPBF) model of the kilohertz quasi-periodic oscillations (QPOs) observed in the X-ray flux from neutron stars in low-mass binary systems has shown that it naturally explains many properties of these QPOs. These include the existence of just two principal QPOs in a given source, the commensurability of the frequency separation \Dnu\ of the two kilohertz QPOs and the spin frequency \nus inferred from burst oscillations, and the high frequencies, coherence, and amplitudes of these QPOs. Here we show that the SPBF model predicts that \Dnu is less than but close to \nus, consistent with the observed differences between \Dnu and \nus. It also explains naturally the decrease in \Dnu with increasing QPO frequency seen in some sources and the plateau in the QPO frequency--X-ray flux observed in 4U 1820-30. The model fits well the QPO frequency behavior observed in Sco X-1, 4U 1608-52, 4U 1728-34, and 4U 1820-30 (chisqdof = 0.4-2.1), giving masses ranging from 1.59 to 2.0 Msun and spin rates ranging from 279 to 364 Hz. In the SPBF model, the kilohertz QPOs are effects of strong-field gravity. Thus, if the model is validated, the kilohertz QPOs can be used not only to determine the properties of neutron stars but also to explore quantitatively general relativistic effects in the strong-field regime.

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arXiv2000

Beat-Frequency Models of Kilohertz QPOs

Kilohertz QPO sources are reasonably well-characterized observationally, but many questions remain about the theoretical framework for these sources and the consequent implications of the observations for disk physics, strong gravity, and dense matter. We contrast the predictions and implications of the most extensively studied class of kilohertz QPO models, the beat-frequency models, with those of alternative classes of models. We also discuss the expected impact of new observations of these sources with satellites such as Chandra, XMM, and Constellation-X.

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arXiv2000

Attenuation of Beaming Oscillations Near Neutron Stars

Observations with RXTE have revealed kilohertz quasi-periodic brightness oscillations (QPOs) from nearly twenty different neutron-star low-mass X-ray binaries (LMXBs). These frequencies often appear as a pair of kilohertz QPOs in a given power density spectrum. In many models the higher-frequency of these QPOs is a beaming oscillation at the frequency of a nearly circular orbit at some radius near the neutron star. In such models it is expected that there will also be beaming oscillations at the stellar spin frequency and at overtones of the orbital frequency, but no strong QPOs have been detected at these frequencies. We therefore examine the processes that can attenuate beaming oscillations near neutron stars, and in doing so extend the work on this subject that was initiated by the discovery of lower-frequency QPOs from LMXBs. Among our main results are (1)in a spherical scattering cloud, all overtones of rotationally modulated beaming oscillations are attenuated strongly, not just the even harmonics, and (2)it is possible to have a relatively high-amplitude modulation near the star at, e.g., the stellar spin frequency, even if no peak at that frequency is detectable in a power density spectrum taken at infinity. We discuss the application of these results to modeling of kilohertz QPOs.

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arXiv2000

Reionization Constraints on the Contribution of Primordial Compact Objects to Dark Matter

Many lines of evidence suggest that nonbaryonic dark matter constitutes roughly 30% of the critical closure density, but the composition of this dark matter is unknown. One class of candidates for the dark matter is compact objects formed in the early universe, with typical masses M between 0.1 and 1 solar masses to correspond to the mass scale of objects found with microlensing observing projects. Specific candidates of this type include black holes formed at the epoch of the QCD phase transition, quark stars, and boson stars. Here we show that accretion onto these objects produces substantial ionization in the early universe, with an optical depth to Thomson scattering out to z=1100 of approximately tau=2-4 [f_COε_{-1}(M/Msun)]^{1/2} (H_0/65)^{-1}, where ε_{-1} is the accretion efficiency ε\equiv L/{\dot M}c^2 divided by 0.1 and f_CO is the fraction of matter in the compact objects. The current upper limit to the scattering optical depth, based on the anisotropy of the microwave background, is approximately 0.4. Therefore, if accretion onto these objects is relatively efficient, they cannot be the main component of nonbaryonic dark matter.

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arXiv2000

Oscillation Waveforms and Amplitudes from Hot Spots on Neutron Stars

The discovery of high-amplitude brightness oscillations during type I X-ray bursts from six low-mass X-ray binaries has provided a powerful new tool to study the properties of matter at supranuclear densities, the effects of strong gravity, and the propagation of thermonuclear burning. There is substantial evidence that these brightness oscillations are produced by spin modulation of one or two localized hot spots confined to the stellar surface. It is therefore important to calculate the expected light curves produced by such hot spots under various physical assumptions, so that comparison with the observed light curves may most sensitively yield information about the underlying physical quantities. In this paper we make general relativistic calculations of the light curves and oscillation amplitudes produced by a rotating neutron star with one or two hot spots as a function of spot size, stellar compactness, rotational velocity at the stellar surface, spot location, orientation of the line of sight of the observer, and the angular dependence of the surface specific intensity. We find that stellar rotation and beaming of the emission tend to increase the observed oscillation amplitudes whereas greater compactness and larger spot size tend to decrease them. By applying these results to 4U 1636--536, we show that this source must have two emitting spots and place strong constraints on the neutron star's magnetic field geometry. We also show that the data on the phase lags between photons of different energies in the persistent pulsations in SAX J1808--58 can be fit well with a model in which the observed hard leads are due to Doppler beaming.

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