arXiv subjects

Michiel van der Klis

Publications and source records attributed to Michiel van der Klis.

At least 145 records · Page 8Linked to original sources

arXiv2000

The power spectral properties of the Z-source GX 340+0

We present an analysis of ~390 ksec of data of the Z source GX 340+0 taken during 24 observations with the Rossi X-ray Timing Explorer satellite. We report the discovery of a new broad component in the power spectra. The frequency of this component varied between 9 and 14 Hz, and remained close to half that of the horizontal branch quasi-periodic oscillations (HBO). Its rms amplitude was consistent with being constant around ~5%, while its FWHM increased with frequency from 7 to 18 Hz. If this sub-HBO component is the fundamental frequency, then the HBO and its second harmonic are the second and fourth harmonic component, while the third harmonic was not detected. This is similar to what was recently found for the black hole candidate XTE J1550-564. The profiles of both the horizontal- and the normal branch quasi-periodic oscillation peaks were asymmetric when they were strongest. We describe this in terms of a shoulder component at the high frequency side of the quasi-periodic oscillation peak, whose rms amplitudes were approximately constant at ~4% and \~3%, respectively. The peak separation between the twin kHz quasi-periodic oscillations was consistent with being constant at 339+-8 Hz but a trend similar to that seen in, e.g. Sco X-1 could not be excluded. We discuss our results within the framework of the various models which have been proposed for the kHz QPOs and low frequency peaks.

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arXiv2000

Relations Between Timing Features and Colors in the X-Ray Binary 4U 0614+09

We study the correlations between timing and X-ray spectral properties in the low mass X-ray binary 4U 0614+09 using a large (265-ks) data set obtained with the Rossi X-ray Timing Explorer. We find strong quasi-periodic oscillations (QPOs) of the X-ray flux, like the kilohertz QPOs in many other X-ray binaries with accreting neutron stars, with frequencies ranging from 1329 Hz down to 418 Hz and, perhaps, as low as 153 Hz. We report the highest frequency QPO yet from any low mass X-ray binary at 1329+-4 Hz, which has implications for neutron star structure. This QPO has a 3.5-sigma single-trial significance, for an estimated 40 trials the significance is 2.4-sigma. Besides the kilohertz QPOs, the Fourier power spectra show four additional components: high frequency noise (HFN), described by a broken power-law with a break frequency between 0.7 and 45 Hz, very low frequency noise (VLFN), which is fitted as a power-law below 1 Hz, and two broad Lorentzians with centroid frequencies varying from 6 to 38 Hz and 97 to 158 Hz, respectively. We find strong correlations between the frequencies of the kilohertz QPOs, the frequency of the 6 to 38 Hz broad Lorentzian, the break frequency of the HFN, the strength of both the HFN and the VLFN and the position of the source in the hard X-ray color vs. intensity diagram. The frequency of the 97 to 158 Hz Lorentzian does not correlate with these parameters. We also find that the relation between power density and break frequency of the HFN is similar to that established for black hole candidates in the low state. We suggest that the changing mass accretion rate is responsible for the correlated changes in all these parameters.

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arXiv1999

No QPO time lags from Sco X-1 as seen with EXOSAT: a comparison with Cyg X-2

We have measured the phase-delay and rms amplitude spectra of CygX-2 and ScoX-1. Using EXOSAT data from the normal branch of CygX-2 we confirm earlier (Ginga) results, showing that at an energy near 6keV there is both a minimum in the QPO rms amplitude spectrum and a 150 degree phase jump in the quasi-periodic oscillations (QPO) phase-delay spectrum. Surprisingly, using EXOSAT and Ginga data, we find no evidence for a phase jump of this kind in the phase-delay spectrum of ScoX-1 on either the normal or flaring branch. Upper limits (90% confidence) of 42 degrees can be set on any phase jump in the energy range 2-10keV on the normal branch, and 88 degrees on the flaring branch. The QPO rms amplitude spectrum of ScoX-1 increases steeply with energy on both the normal and flaring branches. These results suggest that the X-ray spectrum pivots about an energy of <~2keV or >~10keV or that normal branch QPO of ScoX-1 does not have a pivoting spectrum. We discuss the implications of these results in terms of the radiation-hydrodynamic model for normal branch QPO.

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arXiv1999

The rapid X-ray variability of V4641 Sagittarii (SAX J1819.3-2525 = XTE J1819-254)

We report on the rapid X-ray variability of the variable star and X-ray transient V4641 Sagittarii (SAX J1819.3-2525; XTE J1819-254) as observed on 15 Sept. 1999 by the proportional counter array (PCA) on board the Rossi X-ray Timing Explorer (RXTE). During the first ~900 s of the first PCA observation, V4641 Sgr showed very strong X-ray fluctuations by a factor of 4 on time scales of seconds to about 500 on time scales of minutes. The spectrum of the source during this flaring episode became harder when the count rate decreased. After this flaring episode, V4641 Sgr entered a quiescent state in which it remained for the rest of this, and the subsequent PCA observations. The X-ray spectrum was considerably softer in this quiescent state than during the flaring episode. The intrinsic X-ray luminosity (both during the flaring episode and the quiescent state) and the rapid X-ray variability do not strongly constrain the nature of the compact object (neutron star or black hole) in the system, although a black hole seems to be more likely. The very short duration of the bright X-ray phase of V4641 Sgr and its likely close distance suggest that many similar objects could be present in our galaxy, most of which are not noticed when they are in X-ray outburst due to the short duration of these outbursts. A considerable number of the black holes present in our galaxy might be contained in systems similar to V4641 Sgr.

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arXiv1999

Discovery of a ~1 Hz quasi-periodic oscillation in the low-mass X-ray binary 4U 1746-37

We have discovered a ~1 Hz quasi-periodic oscillation (QPO) in the persistent X-ray emission and during type I X-ray bursts of the globular cluster source, dipper and low-mass X-ray binary (LMXB) 4U 1746-37. The QPO properties resemble those of QPOs found recently in the LMXB dippers 4U 1323-62, and EXO 0748-676, which makes 4U 1746-37 the third source known to exhibit this type of QPOs. We present evidence for X-ray spectral changes in this source similar to those observed in LMXBs referred to as atoll sources. We detect two states, a low intensity and spectrally hard state, and a higher intensity and spectrally soft state. This may explain the different spectral characteristics reported for 4U 1746-37 earlier. The high intensity state resembles the banana branch state of atoll sources. The QPOs are only seen in the low intensity state, and are absent when the source is in the banana branch. This strongly suggests that either the accretion disk or an extended central source change shape between the low intensity state and the banana branch. Twelve bursts were detected, of which 5 took place while the source was on the banana branch and 7 when the source was in the low intensity state. The bursts occurring on the banana branch had an e-folding time ~3 times longer than those which occurred in the low intensity state. Whereas previously detected dips showed only a decrease in count rate of ~15%, we found in one observation a dip in which the count rate dropped from ~200 counts per second to ~20 counts per second. This dip lasted only ~250 seconds, during which clear spectral hardening occured. This is the first time strong evidence for spectral changes during a dip are reported for this source.

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arXiv1999

The Complex Phase Lag Behavior of the 3-12 Hz Quasi-Periodic Oscillations during the Very High State of XTE J1550-564

We present a study of the complex phase lag behavior of the low-frequency (<20 Hz) quasi-periodic oscillations (QPOs) in the X-ray transient and black-hole candidate XTE J1550-564 during its very high state. We distinguish two different types of low-frequency QPOs, based on their coherence and harmonic content. The first type is characterized by a 6 Hz QPO with a Q (the QPO frequency divided by the QPO width) of <3 and with a harmonic at 12 Hz. The second type of QPO is characterized by a 6 Hz QPO with a Q value of >6 and with harmonics at 3, 12, 18, and possibly at 9 Hz. Not only the Q values and the harmonic content of the two types are different, but also their phase lag behavior. For the first type of QPO, the low energy photons (<5 keV) of both the 6 Hz QPO and its harmonic at 12 Hz lag the hard energy photons (>5 keV) by as much as 1.3 radian. The phase lags of the second type of QPO are more complex. The soft photons (<5 keV) of the 3 and the 12 Hz QPOs lag the hard photons (>5 keV) by as much as 1.0 radian. However, the soft photons of the 6 Hz QPO precede the hard ones by as much as 0.6 radian. This means that different harmonics of this type of QPO have different signs for their phase lags. This unusual behavior is hard to explain when the lags are due to light-travel-time differences between the photons at different energies, e.g., in a Comptonizing region surrounding the area where the QPOs are formed.

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arXiv1999

The Timing Properties of Sco X-1 along its Z track as seen with EXOSAT

We present a systematic, homogeneous analysis of all the EXOSAT ME, high time resolution data on Sco X-1. We investigate all power spectral properties of the $<$100 Hz quasi-periodic oscillations (QPO) and noise of Sco X-1 as a function of position on the Z-shaped track traced out in the X-ray colour-colour diagram. Generally, the properties of Sco X-1 vary smoothly as a function of position along the Z track. However, some variability parameters change abruptly at either of the vertices of the Z track indicating that the branches of the Z track represent distinct source states not only in spectral state but also in rapid variability characteristics. We show that the very low frequency noise (VLFN) is consistent with being solely due to motion along the Z track. The QPO show a remarkably rapid change in frequency at or just before the normal-branch/flaring-branch (NB/FB) vertex. This transition happens within 1.5% of the entire extent of the Z track. The QPO themselves are visible for 17% of the Z. We find a new sort of behaviour near the NB/FB vertex, i.e., rapid excursions from the NB into the FB and back again taking only a few minutes.

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arXiv1999

X-ray timing behaviour of Cygnus X-2 at low intensities

It is known that the overall (mean) intensity of the low-mass X-ray binary Cyg X-2 varies on time scales from a day to months, independently from the variations on time scales of hours to a day by which the source moves between the horizontal, normal and flaring branches. We present RXTE PCA observations of Cyg X-2, taken when its overall intensity was near its lowest values, in October 1996 and September 1997. For the first time we perform a study of the fast timing behaviour at such low intensities. During the October 1996 observations the source was in the left part of the horizontal branch, and during the September 1997 observations most likely in the lower parts of the normal branch and flaring branch. We find that the properties of the very low frequency noise during the September 1997 observations are consistent with a monotonic decrease in its strength and power-law index as a function of overall intensity. In contrast, the strength of the ~6 Hz normal branch quasi-periodic oscillations do not vary monotonically with overall intensity. They are strongest at medium overall intensity and weaker both when the overall intensity is low and when the overall intensity is high.

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arXiv1999

The discovery of a 7-14 Hz Quasi-Periodic Oscillation in the X-ray Transient XTE J1806-246

We have studied the correlated X-ray spectral and X-ray timing behavior of the X-ray transient XTE J1806-246 using data obtained with the proportional counter array onboard the Rossi X-ray Timing Explorer. In the X-ray color-color diagram two distinct patterns are traced out. The first pattern is a curved branch, which is observed during the rise and the decay of the outburst. This pattern resembles the so-called banana branch of those low-luminosity neutron star low-mass X-ray binaries (LMXBs) which are referred to as atoll sources. The power spectrum of XTE J1806-246 on this curved branch consisted of a power law and a cutoff power law component. The presence of these components and their dependence on position of the source on the branch is also identical to the behavior of atoll sources on the banana branch. Near the end of it outburst XTE J1806-246 formed patches in the color-color diagram, the spectrum was harder, and the power spectrum showed strong band limited noise, characteristic of the atoll sources in the island state. A second pattern was traced out during the only observation at the peak of the outburst. It consists of a structure which we interpret as formed by two distinct branches. This pattern resembles the normal-flaring branches of the high-luminosity neutron star LMXBs (the Z sources). The discovery of a 7-14 Hz QPO during this observation strengthens this similarity. We conclude that if XTE J1806-246 is a neutron star, it is most likely an atoll source that only at the peak of its outburst reached a luminosity level sufficiently high to show the type of QPO that Z sources who on their normal and flaring branches.

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arXiv1999

The X-ray Timing Behavior of the X-ray Burst Source SLX 1735-269

We report for the first time on the rapid X-ray variability of the galactic bulge source and X-ray burster SLX 1735-269. The power spectrum as observed with the Rossi X-ray Timing Explorer is characterized by a strong band-limited noise component which is approximately flat below a 0.1-2.3 Hz break frequency; above this frequency the power spectrum declines as a power law of index 0.9. At the highest observed count rate a broad bump is superimposed on this band-limited noise. The power spectrum is very similar to that of other low-luminosity neutron-star low-mass X-ray binaries (LMXBs) and to black-hole candidates when these types of source accrete at their lowest observed mass accretion rates. However, we identify one unusual aspect of the X-ray variability of SLX 1735-269: the break frequency increases when the inferred mass accretion rate decreases. This is the opposite to what is normally observed in other sources. The only source for which the same behavior has been observed is the accretion-powered millisecond X-ray pulsar SAX J1808.4-3658. No coherent millisecond pulsations were observed from SLX 1735-269 with an upper limit on the amplitude of 2.2% rms. Observing this behavior in SLX 1735-269 increases the similarities between SAX J1808.4-3658 and the other neutron star LMXBs for which so far no coherent pulsations have been observed. We expect that other sources will show the same behavior when these sources are studied in detail at their lowest mass accretion rates.

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arXiv1999

Precise Measurements of the Kilohertz Quasi-Periodic Oscillations in 4U 1728-34

We have analyzed seventeen observations of the low-mass X-ray binary and atoll source 4U 1728-34, carried out by the Rossi X-ray Timing Explorer in 1996 and 1997. We obtain precise measurements of the frequencies of the two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source. We show that the frequency separation between the two QPO, $Δν$, is always significantly smaller than the frequency of the nearly-coherent oscillations seen in this source during X-ray bursts, even at the lowest inferred mass accretion rate, when $Δν$ seems to reach its maximum value. We also find that $Δν$ decreases significantly, from $349.3 \pm 1.7$ Hz to $278.7 \pm 11.6$ Hz, as the frequency of the lower frequency kHz QPO increases from 615 to 895 Hz. This is the first time that variations of the kHz QPO peak separation are measured in a source which shows nearly-coherent oscillations during bursts.

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arXiv1999

A variable 0.58--2.44 Hz quasi-periodic oscillation in the eclipsing and dipping low-mass X-ray binary EXO 0748-676

We report the discovery of a quasi-periodic oscillation (QPO) in data obtained with the Rossi X-ray Timing Explorer of the dipping and eclipsing low-mass X-ray binary EXO 0748-676. The QPO had a frequency between 0.58 and 2.44 Hz changing on time scales of a few days, an rms amplitude between 8% and 12%, and was detected in the persistent emission, during dips and during type I X-ray bursts. During one observation, when the count rate was a factor 2 to 3 higher than otherwise, the QPO was not detected. The strength of the QPO did not significantly depend on photon energy, and is consistent with being the same in the persistent emission, both during and outside the dips, and during type I X-ray bursts. Frequency shifts were observed during three of the four X-ray bursts. We argue that the QPO is produced by the same mechanism as the QPO recently found by Jonker et al. (1999) in 4U 1323-62. Although the exact mechanism is not clear, it is most likely related to the high inclination of both systems. An orbiting structure in the accretion disc that modulates the radiation from the central source seems the most promising mechanism.

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arXiv1999

On the Magnetospheric Beat-Frequency and Lense-Thirring Interpretations of the Horizontal Branch Oscillation in the Z Sources

We study the horizontal branch oscillations (HBO) and the two simultaneous kilohertz QPOs comparing their properties in five Z sources with the predictions of the magnetospheric beat-frequency and Lense-Thirring precession models. We find that the variation of the HBO frequency with accretion rate predicted by the magnetospheric beat-frequency model for a purely dipolar stellar magnetic field and a radiation-pressure-dominated inner accretion disk agrees well with the observed variation. This model implies that the neutron stars in the Z sources are near magnetic spin equilibrium, that their magnetic field strengths are ~10^9-10^10 G, and that the critical fastness parameter for these sources is >0.8. If the frequency of the upper kilohertz QPO is an orbital frequency in the accretion disk, the magnetospheric beat-frequency model requires that a small fraction of the gas in the disk does not couple strongly to the stellar magnetic field at 3-4 stellar radii but instead drifts slowly inward in nearly circular orbits until it is within a few kilometers of the neutron star surface. The Lense-Thirring precession model is consistent with the observed magnitudes of the HBO frequencies only if the moments of inertia of the neutron stars in the Z sources are 4-5 times larger than the largest values predicted by realistic neutron-star equations of state. The trend of the correlation between the HBO and upper kHz QPO frequencies is not consistent with this model. We argue that the change in the slope of the correlation between the frequency of the horizontal branch oscillation and the frequency of the upper kilohertz QPO, when the latter is greater than 850 Hz, is directly related to the varying frequency separation of the kilohertz QPOs.

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arXiv1999

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 components observed in the power spectra of non-pulsing 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 three orders of magnitude in frequency and with coherences that vary widely but systematically. Confirmation of this hypothesis will strongly constrain theoretical models of these phenomena and provide additional clues to understanding their nature.

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arXiv1999

Discovery of twin kHz quasi-periodic oscillations in the high galactic la titude X-ray transient XTE J2123--058

We report the discovery of twin kHz quasi-periodic oscillations (QPOs) in the persistent X-ray flux of the new X-ray transient XTE J2123-058. We detected the QPOs in data taken with the proportional counter array on board the Rossi X-ray Timing Explorer on 1998 July 14, when the source count rate was at about 50% of its outburst peak level. The frequencies of the QPOs were 853+/-4 Hz and 1129+/-8 Hz. The peaks had widths of 32 (+11/-8) Hz and 50 (+17/-13) Hz and the rms amplitudes were 6.1% (+0.7/-0.6) and 6.5% (+0.9/-0.8), respectively. The QPO frequencies increased marginally with count rate and the amplitudes showed a small increase with photon energy. On the basis of the appearance in the color-color and hardness-intensity diagrams, the low-frequency power spectrum, and the strength of the kHz QPOs, the system can be classified as an atoll source. The source showed five type I X-ray bursts. The distance to the source is estimated to be about 10 (5-15) kpc, which combined with the high Galactic latitude of 36.2 degrees indicates the source is a located in the Galactic halo.

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arXiv1998

Discovery of a ~7 Hz Quasi-Periodic Oscillation in the low-luminosity low-mass X-ray binary 4U 1820-30

We have discovered a 7.06+-0.08 Hz quasi-periodic oscillation (QPO) in the X-ray flux of the low-luminosity low-mass X-ray binary (LMXB) and atoll source 4U 1820-30. This QPO was only observable at the highest observed mass accretion rate, when the source was in the uppermost part of the banana branch, at a 2-25 keV luminosity of 5.4x10^37 erg/s (for a distance of 6.4 kpc). The QPO had a FWHM of only 0.5+-0.2 Hz during small time intervals (32-s of data), and showed erratic shifts in the centroid frequency between 5.5 and 8 Hz. The rms amplitude over the energy range 2-60 keV was 5.6%+-0.2%. The amplitude increased with photon energy from 3.7%+-0.5% between 2.8 and 5.3 keV to 7.3%+-0.6% between 6.8 and 9.3 keV, above which it remained approximately constant at ~7%. The time lag of the QPO between 2.8-6.8 and 6.8-18.2 keV was consistent with being zero (-1.2+-3.4 ms). The properties of the QPO (i.e., its frequency and its presence only at the highest observed mass accretion rate) are similar to those of the 5-20 Hz QPO observed in the highest luminosity LMXBs (the Z sources) when they are accreting near the Eddington mass accretion limit. If this is indeed the same phenomenon, then models explaining the 5-20 Hz QPO in the Z sources, which require the near-Eddington accretion rates, will not hold. Assuming isotropic emission, the 2-25 keV luminosity of 4U 1820-30 at the time of the 7 Hz QPOs is at maximum only 40% (for a companion star with cosmic abundances), but most likely ~20% (for a helium companion star) of the Eddington accretion limit.

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arXiv1998

Measurement of Hard Lags and Coherences in the X-Ray Flux of Accreting Neutron Stars and Comparison with Accreting Black Holes

Using the Rossi X-ray Timing Explorer we have measured lags of the 9 to 33 keV photons relative to the 2 to 9 keV photons in the timing noise between 0.01 and 100 Hz in the accreting neutron stars 4U 0614+091 and 4U 1705-44. We performed similar measurements on the accreting black hole candidates Cyg X-1 and GX 339-4 as a comparison. During the observations these sources were all in low (hard) states. We find phase lags of between 0.03 and 0.2 radians in all these sources, with a variation in frequency much less than expected for a lag constant in time. We also measure a coherence consistent with unity in all sources. As already noted for the black hole candidates, these data are inconsistent with simple Comptonization models invoking a constant time delay. Comptonization in a non-uniform medium can perhaps explain the lags. However, the magnitudes of the lags imply that the hot electron gas extends to more than 1000 Schwarzschild radii. This may constitute an energy problem. We argue that while a large hot cloud is possible for black holes which may hide some of their accretion energy in advection, such a distribution may not be possible neutron stars where all the accretion energy is eventually released at the neutron star surface. This casts doubt on the Comptonization model, though the energy problem may be resolved for example by a wind from the inner disk.

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arXiv1998

A persistent ~1 Hz quasi-periodic oscillation in the dipping low-mass X-ray binary 4U 1323-62

We have discovered a ~1 Hz quasi-periodic oscillation (QPO) in the persistent-emission, the dips, and the type I X-ray bursts of the low-mass X-ray binary 4U 1323-62. The rms amplitude of the QPO is approximately 9%, only weakly depending on photon energy. The amplitude is consistent with being constant throughout the persistent-emission, the dips and the bursts in all but one observation, where it is much weaker during one dip. These properties suggest that we have observed a new type of QPO, which is caused by quasi-periodic obscuration of the central X-ray source by a structure in the accretion disk. This can only occur when the binary inclination is high, consistent with the fact that 4U 1323-62 is a dipping source. The quasi-periodic obscuration could take place by partial covering of an extended central X-ray source by a near-opaque medium, or by covering of a point source by a medium of suitable characteristics to produce the relatively energy-independent oscillations.

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