Showing papers by "M. Coleman Miller published in 2000"
TL;DR: In this article, the authors analyzed the brightness oscillations in the initial 0.75 s of the five bursts that were used in a previous search for a brightness oscillation at the expected ~290 Hz spin frequency.
Abstract: The discovery of nearly coherent brightness oscillations during thermonuclear X-ray bursts from six neutron star low-mass X-ray binaries has opened up a new way to study the propagation of thermonuclear burning and may ultimately lead to greater understanding of thermonuclear propagation in other astrophysical contexts, such as in Type Ia supernovae. Here, we report detailed analyses of the ~580 Hz brightness oscillations during bursts from 4U 1636-536. We investigate the bursts as a whole and, in more detail, the initial portions of the bursts. We analyze the ~580 Hz oscillations in the initial 0.75 s of the five bursts that were used in a previous search for a brightness oscillation at the expected ~290 Hz spin frequency. We find that, although it is appropriate to use an arbitrarily complicated model of the ~580 Hz oscillations to generate a candidate waveform for the ~290 Hz oscillations, models with more than two parameters are not required by the data. For the bursts as a whole, we show that the characteristics of the brightness oscillations vary greatly from burst to burst. We find, however, that in at least one of the bursts, and possibly in three of the four that have strong brightness oscillations throughout the burst, the oscillation frequency decreases well after the peak of the burst. This behavior poses a challenge to the standard burning layer expansion explanation for the frequency changes.
25 citations
TL;DR: In this paper, the authors examined the processes that can attenuate beaming oscillations near neutron stars, and in doing so they extend the work on this subject that was initiated by the discovery of lower frequency QPOs from LMXBs.
Abstract: Observations made with the Rossi X-Ray Timing Explorer 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. It is extremely likely that the frequency of the higher frequency of these QPOs is the orbital frequency of gas at some radius near the neutron star. It is also likely that the QPOs are caused by the movement of bright arcs or luminous clumps around the star, which produce a modulation in the observed X-ray intensity as they are periodically occulted by the star or as they present a different viewing aspect to the observer at infinity. If this picture is correct, it means that this type of QPO is a beaming oscillation. 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 we extend the work on this subject that was initiated by the discovery of lower frequency QPOs from LMXBs. We consider attenuation by scattering, attenuation by light deflection, and the decrease in modulation caused by integration over the visible surface of the neutron star. 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, (2) the amount of attenuation is diminished, and hence the observed modulation amplitude is increased, by the presence of a central, finite-sized star, even if the scattering cloud is much larger than the star, and (3) if the specific intensity of radiating points on the star has a large angular width, then even with zero optical depth from the stellar surface to the observer, and even in the approximation of straight-line photon propagation, the modulation amplitude seen at infinity is decreased significantly by integration over the visible portion of the surface. We also compare the modulation of flux as seen at infinity with the modulation near the star and show that (4) 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.
10 citations
TL;DR: In this article, it was shown that accretion onto these objects produces substantial ionization in the early universe, with an optical depth to Thomson scattering out to z ~ 1100 of τ ≈ 2 - 4[fCO-1(M/M☉)]1/2(H0/65)-1, where -1 is the accretion efficiency ≡ L/c2 divided by 0.1, and fCO is the fraction of matter in the compact objects.
Abstract: Many lines of evidence suggest that nonbaryonic dark matter constitutes ~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 ~ 0.1-1 M☉, 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 τ ≈ 2 - 4[fCO-1(M/M☉)]1/2(H0/65)-1, where -1 is the accretion efficiency ≡ L/c2 divided by 0.1, and fCO 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 ≈0.4. Therefore, if accretion onto these objects is relatively efficient, they cannot be the main component of nonbaryonic dark matter.
9 citations
TL;DR: In this paper, it was shown 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_CO\epsilon{-1}(M/Msun)]^{1/2} (H_0/65)^{-1}, where f_CO is the accretion efficiency
Abstract: 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_CO\epsilon_{-1}(M/Msun)]^{1/2} (H_0/65)^{-1}, where \epsilon_{-1} is the accretion efficiency \epsilon\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.
9 citations
TL;DR: In this article, it was shown that 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.
Abstract: 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
us 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
us, consistent with the observed differences between \Dnu and
us. 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.
5 citations
TL;DR: In this paper, the authors compare 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.
Abstract: 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.
2 citations
TL;DR: The first extrasolar planets were discovered in 1992 around the millisecond pulsar PSR 1257+12 as discussed by the authors, and it was shown that the pulsar was born with approximately its current rotation frequency and magnetic moment.
Abstract: 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.