Publications (26)102.68 Total impact
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Article: Quasi-periodic Oscillations and Spectral States in GRS 1915+105
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ABSTRACT: We present results from the analysis of X-ray energy spectra and quasi-periodic oscillations (QPOs) from a set of observations that samples a broad range of time variability in GRS 1915+105. We first demonstrate that the frequency and integrated amplitude of a 0.5-10 Hz QPO is correlated with the apparent temperature of the accretion disk for the majority of observations. We then show that the behavior of GRS 1915+105 exhibits two distinct modes of accretion. In the first mode, the QPO is present between 0.5 and 10 Hz and variability in the source luminosity is dominated by the power-law component. In the second mode, the QPO is absent and the changes in the luminosity are dominated by thermal emission from the accretion disk. We find that the color radius and temperature of the inner accretion disk are empirically related by Rcol T + const. We discuss these results in terms of ongoing efforts to explain the origin of both the QPOs and the hard X-ray component in the spectrum of GRS 1915+105.The Astrophysical Journal 01/2009; 527(1):321. · 6.02 Impact Factor -
Article: Radio Emission and the Timing Properties of the Hard X-Ray State of GRS 1915+105
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ABSTRACT: We combine a complete sample of 113 pointed observations taken with the Rossi X-Ray Timing Explorer between 1996 and 1999, monitoring observations taken with the Ryle Telescope and the Green Bank Interferometer, and selected observations with the Very Large Array to study the radio and X-ray properties of GRS 1915+105 when its X-ray emission is hard and steady. We establish that radio emission always accompanies the hard-steady state of GRS 1915+105 but that the radio flux density at 15.2 GHz and the X-ray flux between 2 and 200 keV are not correlated. Therefore, we study the X-ray spectral and timing properties of GRS 1915+105 using three approaches: first by describing in detail the properties of three characteristic observations, then by displaying the time evolution of the timing properties during periods of both faint and bright radio emission, and lastly by plotting the timing properties as a function of the radio flux density. We find that as the radio emission becomes brighter and more optically thick, (1) the frequency of a ubiquitous 0.5-10 Hz quasi-periodic oscillation (QPO) decreases, (2) the Fourier phase lags between hard (11.5-60 keV) and soft (2-4.3 keV) in the frequency range of 0.01-10 Hz change sign from negative to positive, (3) the coherence between hard and soft photons at low frequencies decreases, and (4) the relative amount of low-frequency power in hard photons compared to soft photons decreases. We discuss how these results reflect upon basic models from the literature describing the accretion flow around black holes and the possible connection between Comptonizing electrons and compact radio jets.The Astrophysical Journal 12/2008; 556(2):515. · 6.02 Impact Factor -
Article: The Burst Behavior of the Eclipsing Low-Mass X-Ray Binary MXB 1659–298
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ABSTRACT: We present a detailed study of the correlations between the burst properties and the inferred mass accretion rate for the X-ray transient MXB 1659-298. The bursts that exhibited oscillations were observed when the source was at a relatively high mass accretion rate, similar to what has been seen for other sources. However, owing to the limited number of observations at lower mass accretion rates, no bursts were observed at such accretion rates, and it is still possible that when MXB 1659-298 accretes at such low-mass accretion rates, bursts can occur that might still exhibit burst oscillations. No clear correlations were found between the different burst properties and the accretion rate, in contrast to what has been found for KS 1731-260 and 4U 1728-34, but similar to what has been reported for Aql X-1. However, this lack of correlation for MXB 1659-298 and Aql X-1 might be due to the limited range of the mass accretion rate observed for those sources compared to KS 1731-260 and 4U 1728-34.The Astrophysical Journal 12/2008; 566(2):1060. · 6.02 Impact Factor -
Article: Characterizing the Quasi-periodic Oscillation Behavior of the X-Ray Nova XTE J1550–564
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ABSTRACT: For all 209 Rossi X-Ray Timing Explorer observations of the X-ray nova XTE J1550-564 during its major outburst of 1998-1999, we have analyzed the X-ray power spectra, phase lags, and coherence functions. These observations constitute one of the richest and most complete data sets obtained for any black hole X-ray nova. The phase lags and coherence measures of low-frequency quasi-periodic oscillations (QPOs; ν < 20 Hz) are used to distinguish three types (one more than those reported by Wijnands, Homan, & van der Klis). For the most common type ("C"), the phase lag is correlated with both the QPO frequency and the amplitude. The physical significance of the QPO types is evident in the relationships between QPO properties and the apparent temperature and flux from the accretion disk. There is also a clear pattern in how the QPO types relate to the presence of high-frequency QPOs (ν > 100 Hz). In general, the amplitude of low-frequency QPOs decreases when high-frequency oscillations appear, and when both low-frequency and high-frequency QPOs are present, their Q-values (ν/FWHM) are anticorrelated. We speculate that this opposition between low-frequency and high-frequency QPOs arises from competing structures in a perturbed accretion disk. However, we find that the frequencies of slow and fast QPOs are not correlated. In addition, we encounter systematic problems in attempting to reliably compare the QPO frequencies with broad features in the power continuum, since there are a variable number of features or spectral breaks in the power spectra. These results cast some doubt on the reported global relationship between QPOs from neutron stars and those from black hole systems.The Astrophysical Journal 12/2008; 564(2):962. · 6.02 Impact Factor -
Article: Evidence for Harmonic Relationships in the High-Frequency Quasi-periodic Oscillations of XTE J1550–564 and GRO J1655–40
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ABSTRACT: We continue to investigate the X-ray properties of the black hole binary XTE J1550-564. By grouping observations (1998-1999) according to the type of low-frequency quasiperiodic oscillation (LFQPO) identified in a previous paper, we show evidence that two high-frequency QPOs (HFQPOs) occur simultaneously near 184 and 276 Hz. We can model the QPO profiles while assuming that the central frequencies are related by a 3 : 2 ratio. In one group, there is some evidence of a broad feature at the fundamental frequency of 92 Hz. We also investigate the 2000 April outburst, and we confirm the suggestion of Miller et al. that a 270 Hz QPO is accompanied by a second feature near 180 Hz. The histogram for the 28 individual HFQPO detections in XTE J1550-564 shows two peaks near 184 and 276 Hz, while there is a notable exception in the 143 Hz QPO detected on 1998 October 15. Similarly, all of the 13 HFQPO detections in the black hole binary GRO J1655-40 occur at two frequencies that are related by a 3 : 2 ratio. We next investigate all of the energy spectra for XTE J1550-564, and we find a systematic increase in the strength of the power-law component as the stronger of the two HFQPOs shifts from 276 to 184 Hz. A strikingly similar result is seen in the spectra of GRO J1655-40 when the stronger HFQPO shifts from 450 to 300 Hz. The fundamental HFQPO frequencies for the two X-ray sources scale as M-1, which is consistent with the hypotheses that these HFQPOs represent some kind of oscillation rooted in general relativity (GR) and that the two black holes have similar values of the dimensionless spin parameter. We discuss physical mechanisms that may explain these HFQPOs. A resonance between orbital and radial coordinate frequencies is one possibility suggested by Abramowicz & Kluzniak. For XTE J1550-564, this would imply moderate values for the dimensionless spin parameter (0.1 < a* < 0.6), with similar results for GRO J1655-40. A resonance between polar and radial coordinate frequencies allows additional values for a* above 0.9. There remain serious uncertainties regarding the physical mechanism whereby resonances in coordinate frequencies may produce HFQPOs. We also discuss models for "diskoseismic" oscillations. In this case, the concept that the inner disk behaves as a resonance cavity in GR has certain attractions for explaining HFQPOs, but integral harmonics are not predicted for the three types of diskoseismic modes derived for adiabatic perturbations in a thin accretion disk.The Astrophysical Journal 12/2008; 580(2):1030. · 6.02 Impact Factor -
Article: Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
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ABSTRACT: We have assembled a sample of 1187 thermonuclear (type-I) X-ray bursts from 48 accreting neutron stars by the Rossi X-ray Timing Explorer, spanning more than ten years. The sample contains examples of two of the three theoretical ignition regimes 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: both long and short bursts (indicating mixed H/He accretion), and consistently short bursts (primarily He accretion). The decrease in burst rate observed for both groups at >0.06 Mdot_Edd (>~2E37 erg/s) 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 examples of bursts separated by <30 min, 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 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 that the nature of the mass donor (and hence the evolutionary history) has an influence on the long-term spin evolution. Comment: 70 pages (emulateapj format), 22 figures, 11 tables, accepted by ApJS. The burst sample has been expanded with data made public since the previous submission; the discussion has been revised and substantially expanded following the referee team's report. Data tables will be available online in the usual places or from http://tinyurl.com/5rrg7q08/2006; -
Article: Eddington-limited X-ray Bursts as Distance Indicators. II. Possible Compositional Effects in Bursts from 4U 1636-536
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ABSTRACT: We analyzed 123 thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer from the low-mass X-ray binary 4U 1636-536. All but two of the 40 radius-exansion bursts in this sample reached peak fluxes which were normally distributed about a mean of 6.4e-8 ergs/cm^2/s, with a standard deviation of 7.6%. The remaining two radius-expansion bursts reached peak fluxes a factor of 1.69+/-0.13 lower than this mean value; as a consequence, the overall variation in the peak flux of the radius-expansion bursts was a factor of ~2. This variation is comparable to the range of the Eddington limit between material with solar H-fraction (X=0.7) and pure He. Such a variation may arise if, for the bright radius-expansion bursts, most of the accreted H is eliminated either by steady hot CNO burning or expelled in a radiatively-driven wind. However, steady burning cannot exhaust the accreted H for solar composition material within the typical ~2 hr burst recurrence time, nor can it result in sufficient elemental stratification to allow selective ejection of the H only. An additional stratification mechanism appears to be required to separate the accreted elements and thus allow preferential ejection of the hydrogen. We found no evidence for a gap in the peak flux distribution between the radius-expansion and non-radius expansion bursts, previously observed in smaller samples. Assuming that the faint radius-expansion bursts reached the Eddington limit for H-rich material (X~0.7), and the brighter bursts the limit for pure He (X=0), we estimate the distance to 4U 1636-536 (for a canonical neutron star with M_NS=1.4M_sun, R_NS=10 km) to be 6.0+/-0.5 kpc, or for M_NS=2M_sun at most 7.1 kpc. (Abstract abridged) Comment: 6 pages, 2 figures, accepted by ApJ11/2005; -
Article: A Lack of Radio Emission from Neutron Star Low Mass X-ray Binaries
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ABSTRACT: We report strict upper limits to the radio luminosities of three neutron star low-mass X-ray binaries obtained with the Very Large Array while they were in hard X-ray states as observed with the Rossi X-ray Timing Explorer: 1E 1724-307, 4U 1812-12, and SLX 1735-269. We compare these upper limits to the radio luminosities of several black hole binaries in very similar hard states, and find that the neutron star systems are as faint as or fainter than all of the black hole candidates. The differences in luminosities can partly be attributed to the lower masses of the neutron star systems, which on theoretical and observational grounds are expected to decrease the radio luminosities as M^0.8. However, there still remains a factor of 30 scatter in the radio luminosities of black hole and neutron star X-ray binaries, particularly at X-ray luminosities of a few percent Eddington. We find no obvious differences in the X-ray timing and spectral properties that can be correlated with the radio luminosity. We discuss the implications of these results on current models for the relationship between accretion and jets. Comment: 9 pages, 5 figures, two of which are color. Submitted to ApJ11/2004; -
Article: Millisecond Oscillations During Thermonuclear X-ray Bursts
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ABSTRACT: I review the basic phenomenology and theory of the millisecond brightness oscillations observed during thermonuclear X-ray bursts from 13 of approximately 70 accreting neutron stars in low-mass X-ray binaries. Compelling observations indicate that the oscillations are produced by surface brightness patterns on the rapidly rotating neutron stars. However, it remains to be understood (1) why the brightness patterns producing them persist for up to 15 s during an X-ray burst, whereas the burning should cover the entire surface in less than 1 s, and (2) why the frequencies drift upward by about 5 Hz during the course of the burst. These peculiarities can probably be explained by taking into account the expansion of the surface layers caused by the burning, zonal flows that form due to pressure gradients between the equator and poles, and Rossby-Alfven modes that are excited in the surface ocean. Further progress toward understanding how burning progresses on the surface of the neutron star can be made with a next-generation X-ray timing mission, which would provide a larger sample of sources with oscillations, detect sideband signals produced by the spectrum of modes that should be excited in the neutron star ocean, and measure harmonic structure in the profiles of the oscillations. These observations would be crucial for measuring the distribution of the rotation rates of neutron stars, the progression of unstable nuclear burning in the accreted ocean, and the curvature of the space-time around the neutron star. Comment: Review article for "X-Ray Timing 2003: Rossi and Beyond", ed. P. Kaaret, F. K. Lamb, & J. H. Swank (Melville, NY: American Institute of Physics). 6 pages, including 5 figures03/2004; -
Article: The Effect of Neutron Star Rotation on the Properties of Thermonuclear X-ray Bursts
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ABSTRACT: Previously, observations with the Rossi X-ray Timing Explorer showed that millisecond oscillations occur preferentially in thermonuclear X-ray bursts with photospheric radius expansion from sources rotating near 600 Hz, while they occur with equal likelihood in X-ray bursts with and without radius expansion for sources rotating near 300 Hz. With a larger sample of data than in previous studies, we find that the detectability of the oscillations is not directly determined by the properties of the X-ray bursts. Instead, we find that (1) the oscillations are observed almost exclusively when the accretion rate onto the neutron star is high, but that (2) radius expansion is only observed at high accretion rates from the 600 Hz sources, whereas it occurs only at low accretion rates in the 300 Hz sources. The persistent millisecond pulsars provide the only apparent exceptions to these trends. The first result might be explained if the oscillation amplitudes are attenuated at low accretion rates by an extended electron corona. The second result indicates that the rotation period of the neutron star determines how the burst properties vary with accretion rate, possibly through the differences in the effective surface gravity or the strength of the Coriolis force. Comment: 5 pages, including 2 figures. To appear in 20 June 2004 issue of ApJ. Added references and made minor changes to discussion10/2003; -
Article: Nuclear-powered millisecond pulsars and the maximum spin frequency of neutron stars.
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ABSTRACT: 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.Nature 08/2003; 424(6944):42-4. · 36.28 Impact Factor -
Article: Nuclear-powered millisecond pulsars and the maximum spin frequency of neutron stars
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ABSTRACT: Millisecond pulsars are neutron stars that are thought to have been spun-up by mass accretion from a stellar companionNature 07/2003; 424(6944):42-44. · 36.28 Impact Factor -
Article: The Energy Dependence of Millisecond Oscillations in Thermonuclear X-ray Bursts
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ABSTRACT: We examine the energy-resolved pulse profiles of 51 flux oscillations observed during the decline of thermonuclear X-ray bursts from accreting weakly- magnetized neutron stars with the Rossi X-ray Timing Explorer. We find that the fractional rms amplitudes of the oscillations increase as a function of energy by 0.25%/keV to 0.9%/keV between 3-20 keV, and are as large as 20% in the 13-20 keV band. We also show that the pulses observed in the higher energy bands generally lag behind those in lower energy bands by 0.002 cycles/keV to 0.007 cycles/keV between 3-20 keV. This amounts to total delays of 0.03-0.12 cycles between the lowest and highest energy bands, or time delays that range from 100-200 mus. We then model the oscillations as flux variations arising from temperature patterns on the surfaces of rapidly rotating neutron stars. In this framework, we find that the increase in the pulse amplitude with photon energy can be explained if the cooler regions on the neutron star emit in the lower energy bands, reducing the flux variations there. On the other hand, the Doppler shifts caused by the rapid rotation of the neutron star should cause the hard pulses to precede the soft pulses by about 0.05 cycles (100 mus), in contrast to the observations. This suggests that the photons originating from the stellar surface are reprocessed by a hot corona of electrons before they reach the observer. Comment: 10 pages, including 5 figures and 4 tables. Submitted to ApJ02/2003; -
Article: X-ray QPOs in Black-Hole Binary Systems
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ABSTRACT: We briefly review the properties and physical consequences of quasiperiodic oscillations (QPOs) seen on many occasions in the X-ray emission from black-hole binary systems. We also report on the discovery of QPOs at 164 and 328 Hz in GRS1915+105. These appear to be analogous to the harmonic systems of high-frequency QPOs seen in XTEJ1550-564 and GROJ1655-40.09/2002; -
Article: Eddington-limited X-ray Bursts as Distance Indicators. I. Systematic Trends and Spherical Symmetry in Bursts from 4U 1728-34
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ABSTRACT: We investigate the limitations of thermonuclear X-ray bursts as a distance indicator for the weakly-magnetized accreting neutron star 4U 1728-34. We measured the unabsorbed peak flux of 81 bursts in public data from the Rossi X-Ray Timing Explorer (RXTE). The distribution of peak fluxes was bimodal: 66 bursts exhibited photospheric radius expansion and were distributed about a mean bolometric flux of 9.2e-8 erg/cm^2/s, while the remaining (non-radius expansion) bursts reached 4.5e-8 erg/cm^2/s, on average. The peak fluxes of the radius-expansion bursts were not constant, exhibiting a standard deviation of 9.4% and a total variation of 46%. These bursts showed significant correlations between their peak flux and the X-ray colors of the persistent emission immediately prior to the burst. We also found evidence for quasi-periodic variation of the peak fluxes of radius-expansion bursts, with a time scale of approximately 40 d. The persistent flux observed with RXTE/ASM over 5.8 yr exhibited quasi-periodic variability on a similar time scale. We suggest that these variations may have a common origin in reflection from a warped accretion disk. Once the systematic variation of the peak burst fluxes is subtracted, the residual scatter is only approximately 3%, roughly consistent with the measurement uncertainties. The narrowness of this distribution strongly suggests that i) the radiation from the neutron star atmosphere during radius-expansion episodes is nearly spherically symmetric, and ii) the radius-expansion bursts reach a common peak flux which may be interpreted as a standard candle intensity.Adopting the minimum peak flux for the radius-expansion bursts as the Eddington flux limit, we derive a distance for the source of 4.4-4.8 kpc. Comment: 9 pages, 7 figures, accepted by ApJ. Minor referee's revisions, also includes 9 newly public X-ray bursts08/2002; -
Article: The Amplitude Evolution and Harmonic Content of Millisecond Oscillations in Thermonuclear X-ray Bursts
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ABSTRACT: We present a comprehensive observational and theoretical analysis of the amplitudes and profiles of oscillations that occur during thermonuclear X-ray bursts from weakly-magnetized neutron stars in low mass X-ray binaries. Our sample contains 59 oscillations from six sources observed with the Rossi X-ray Timing Explorer. The oscillations that we examined occurred primarily during the decaying portions of bursts, and lasted for several seconds each. We find that the oscillations are as large as 15% during the declines of the bursts, and they appear and disappear due to intrinsic variations in their fractional amplitudes. However, the maxima in the amplitudes are not related to the underlying flux in the burst. We derive folded profiles for each oscillation train to study the pulse morphologies. The mean rms amplitudes of the oscillations are 5%, although the eclipsing source MXB 1659-298 routinely produces 10% oscillations in weak bursts. We also produce combined profiles from all of the oscillations from each source. Using these pulse profiles, we place upper limits on the fractional amplitudes of harmonic and half-frequency signals of 0.3% and 0.6%, respectively (95% confidence). We then compare the pulse morphologies to theoretical profiles from models with one or two antipodal bright regions on the surface of a rotating neutron star. We find that if one bright region is present on the star, it must either lie near the rotational pole or cover nearly half the neutron star in order to be consistent with the observed lack of harmonic signals. If an antipodal pattern is present, the hot regions must form very near the rotational equator. We discuss how these geometric constraints challenge current models for the production of brightness variations on the surface of a neutron star. (abridged) Comment: 12 pages, including 8 figures. To appear in ApJ v 581 No 1, Dec 10 2002. Revised to include discussion of scattering on harmonic amplitudes04/2002; -
Article: The Frequency Stability of Millisecond Oscillations in Thermonuclear X-Ray Bursts
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ABSTRACT: We analyze the frequency evolution of millisecond oscillations observed during type I X-ray bursts with the Rossi X-ray Timing Explorer in order to establish the stability of the mechanism underlying the oscillations. Our sample contains 68 pulse trains detected in a search of 159 bursts from 8 accreting neutron stars. As a first step, we confirm that the oscillations usually drift upward in frequency by about 1% toward an apparent saturation frequency. Previously noted anomalies, such as drifts toward lower frequencies as the oscillations disappear (``spin-down'' episodes) and instances of two signals present simultaneously at frequencies separated by a few Hz, occur in 5% of oscillations. Having verified the generally accepted description of burst oscillations, we proceed to study the coherence of the oscillations during individual bursts, and the dispersion in the asymptotic frequencies in bursts observed over five years. On short time scales, we find that 30% of the oscillation trains do not appear to evolve smoothly in phase. This suggests either that two signals are present simultaneously with a frequency difference too small to resolve (< 1 Hz), that the frequency evolution is discontinuous, or that discrete phase jumps occur. On time scales of years, the maximum frequencies of the oscillations exhibit fractional dispersions of < 0.004. In the case of 4U 1636-536, this dispersion is uncorrelated with the known orbital phase, which indicates that a mechanism besides orbital Doppler shifts prevents the oscillations from appearing perfectly stable. We interpret these results under the assumption that the oscillations originate from anisotropies in the emission from the surfaces of these rotating neutron stars. (Abridged) Comment: 13 pages, including 10 figures. To be published in ApJ v580 No. 2, December, 1 2002. Some changes and clarifications04/2002; -
Article: Evidence for Harmonic Relationships in the High Frequency QPOs of XTE J1550-564 and GRO J1655-40
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ABSTRACT: We continue to investigate the X-ray timing and spectral properties of the black hole binary, XTE J1550-564. For three different groups of observations, we show evidence that two high-frequency QPOs (HFQPOs) occur simultaneously near 184 and 276 Hz. In one group, there is also evidence of a broad feature at 92 Hz. In each case, we can model the QPO profiles while assuming that the central frequencies are related as integral harmonics of a single frequency. We next investigate the X-ray energy spectra, and we find a systematic increase in the strength of the power-law component as the stronger of the two HFQPOs shifts from 276 to 184 Hz. A strikingly similar result is seen for GRO J1655-40 when the stronger HFQPO shifts from 450 to 300 Hz. The fundamental HFQPO frequencies for the two X-ray sources scale inversely with black hole mass, which is consistent with the hypotheses that these HFQPOs represent some oscillation rooted in general relativity (GR) and that the two black holes have similar values of the dimensionless spin parameter. We discuss physical mechanisms that may explain these HFQPOs. In particular, a resonance between the orbital and radial coordinate frequencies in the innner accretion disk, as proposed by Abramowicz & Kluzniak, would imply moderate values for the dimensionless spin parameter (0.1 < a < 0.6) for both black holes. Comment: 31 pages, including 9 figures and 1 Table; submitted to The ApJ02/2002; -
Article: How Do Z and Atoll X-ray Binaries Differ?
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ABSTRACT: Low-mass X-ray binaries containing weakly magnetized neutron stars may be divided into two classes, Z and atoll sources, based in large part upon the patterns which they trace in plots of two X-ray colors. In this paper, we examine color-color diagrams of nine atoll sources and five Z sources using data from the Rossi X-ray Timing Explorer. The five-year span of data we have examined is significantly longer than those of previous studies. We find that the previous clear distinction between atoll and Z sources may be an artifact of incomplete sampling, as those atoll sources which are sampled over a wide dynamic range in intensity (greater than a factor of 80) trace three-branched color-color patterns similar to the tracks for which Z sources are named. This suggests that the two classes are more intimately related than previously realized, which also has been suggested based upon recent timing studies. However, detailed differences in the spectral behavior of the two classes argue against any simple unification. Comment: Five pages, including one table and three figures. Submitted to ApJ letters11/2001; -
Article: Characterizing the QPO Behavior of the X-ray Nova XTE J1550--564
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ABSTRACT: For all 209 RXTE observations of the X-ray nova XTE J1550-564 during its major outburst of 1998-1999, we have analyzed the X-ray power spectra, phase lags, and coherence functions. The phase lags and coherence measures are used to distinguish three types of low-frequency QPOs (one more than those reported by Wijnands, Homan, & van der Klis 1999). For the most common type, the phase lag is correlated with both the QPO frequency and the amplitude. The physical significance of the QPO types is evident in the relationships between QPO properties and the apparent temperature and flux from the accretion disk. There is also a clear pattern in how the QPO types relate to the presence of high-frequency QPOs. In general, both the amplitude and the Q value (freq./FWHM) of low-frequency QPOs decrease as the high-frequency oscillations increase in frequency (100 to 284 Hz) and in Q value. We speculate that the antagonism between low-frequency and high-frequency QPOs arises from competing structures in a perturbed accretion disk. However, we find that the frequencies of slow (< 20 Hz) and fast (> 100 Hz) QPOs are not correlated. In addition, we encounter systematic problems in attempting to reliably compare the QPO frequencies with broad features in the power continuum, since there are a variable number of such features in the power spectra. These results cast some doubt on the reported global relationship between QPOs from neutrons stars and those from black hole systems. Comment: 27 pages, including 2 tables and 7 figures. submitted to ApJ; rewrite of astro-ph/000421505/2001;
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Institutions
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2002–2008
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Massachusetts Institute of Technology
- Kavli Institute for Astrophysics and Space Research
Cambridge, MA, USA
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