arXiv subjects

Michiel van der Klis

Publications and source records attributed to Michiel van der Klis.

At least 163 records · Page 9Linked to original sources

arXiv1998

Dependence of the Frequency of the Kilohertz Quasi-Periodic Oscillations on X-ray Count Rate and Colors in 4U 1608-52

We present new results based on observations carried out with the Rossi X-ray Timing Explorer during the decay of an outburst of the low-mass X-ray binary (LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at least in 4U 1608-52, one of the long-standing issues about the phenomenology of the kilohertz quasi-periodic oscillations (kHz QPOs), namely, the lack of a unique relation between the frequency of the kHz QPOs and the X-ray flux. We show that despite its complex dependence on the X-ray flux, the frequency of the kHz QPOs is monotonically related to the position of the source in the color-color diagram. Our findings strengthen the idea that, as in the case of Z sources, in the atoll sources the X-ray flux is not a good indicator of $\dot M$, and that the observed changes in the frequency of the kHz QPOs in LMXBs are driven by changes in $\dot M$. These results raise some concern about the recently reported detection of the orbital frequency at the innermost stable orbit in 4U 1820-30.

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arXiv1998

The broad-band power spectra of X-ray binaries

We analyzed the rapid aperiodic X-ray variability of different types of X-ray binaries (black hole candidates, atoll sources, the recently discovered millisecond X-ray pulsar, and Z sources) at their lowest inferred mass accretion rates. At these accretion rates, the power spectra of all sources are dominated by a strong band-limited noise component, which follows a power law with an index roughly 1 at high frequencies and breaks at a frequency between 0.02 and 32 Hz below which the spectrum is relatively flat. Superimposed on this, a broad bump (sometimes a quasi-periodic oscillation) is present with a 0.2-67 Hz centroid frequency that varies in good correlation with the frequency of the break. The black hole candidates and the low-luminosity neutron star systems (including the millisecond X-ray pulsar) have the same relation between the frequency of the bump and the frequency of the break. These similar characteristics strongly suggest that in all those different types of sources the band-limited noise and the bump are produced by the same physical mechanism. This mechanism cannot then depend on the presence or absence of either a small magnetosphere or a solid surface, so that it is most likely related to an instability in the flow in the accretion disk that modulates the accretion rate. The Z sources, which are more luminous than the other sources discussed here, follow a similar, but slightly shifted correlation between the break frequency and the frequency of the bump. The data suggest that the band-limited noise in Z sources is more complex than that in the other sources.

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arXiv1998

Discovery of a Second Kilohertz QPO in the X-ray Binary 4U 1735-44

In recent observations with the Rossi X-Ray Timing Explorer we have detected two simultaneous quasi-periodic oscillation (QPO) peaks in the low mass X-ray binary and atoll source 4U 1735-44. The lower and higher frequency QPOs have frequencies varying between 632 and 729 Hz, and 982 and 1026 Hz, respectively. The fractional rms amplitudes are 3.7 to 8.1% and 5.0 to 5.8%. The frequency separation between the two QPOs changes from 341+-7 Hz to 296+-12 Hz. The inferred mass accretion rate during our observations is relatively low compared to that during the previous observations, where only a single QPO was present. There is weak evidence that the frequency of the QPOs correlates with the mass accretion rate, as observed in other binaries. Five X-ray bursts were recorded with no detectable oscillations with upper limits for the rms fraction of 4% to 13%.

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arXiv1998

The broad-band power spectrum of SAX J1808.4-3658

We analyzed the rapid aperiodic X-ray variability of the recently discovered millisecond X-ray pulsar SAX J1808.4-3658. The power density spectrum is dominated by a strong band-limited noise component, which follows a power-law with index 1.0-1.3 at high frequencies with a break that varies between 0.25 and 1.6 Hz, below which the spectrum is relatively flat. Superimposed on this, a broad bump is present with a centroid frequency that varies well correlated with the break frequency between 2.4 and 12.0 Hz. These characteristics are very similar to what is commonly seen in other low-luminosity low-mass X-ray binaries. Between 100 and 400 Hz a third broad noise component is present similar to that recently reported in the low-luminosity low-mass X-ray binary 4U 1728-34 (Ford & van der Klis 1998). We find a similar high-frequency noise component also in other low-luminosity neutron star systems. We conclude that at any given epoch the rapid aperiodic X-ray variability of the millisecond X-ray pulsar is indistinguishable from that of other low-luminosity neutron star systems. However, contrary to what has been found in those sources the break frequency of the band-limited noise does not have a strict correlation with mass accretion rate. With decreasing mass accretion rate, the break frequency first decreased, then increased again. This previously unobserved behavior could be typical for the very low mass accretion rates we observe near the end of the outburst of SAX J1808.4-3658, or it could be related to it being a pulsar.

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arXiv1998

Difference Frequency of Kilohertz QPOs Not Equal to Half the Burst Oscillation Frequency in 4U 1636-53

We have analyzed data obtained during two observations with the Rossi X-ray Timing Explorer on January 5 and 8, 1997, of the low-mass X-ray binary (LMXB) and atoll source 4U 1636-53. We measure the frequency separation of the two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source to be 253.7 +/- 4.7 and 246.4 +/- 5.4 Hz, respectively. These values are inconsistent with being equal to 0.5 times the frequency of the 581-Hz oscillations that have been detected previously in 4U 1636-53 during type I bursts. The weighted average discrepancy is 39.5 +/- 3.5 Hz. This result shows that a simple beat-frequency interpretation of the kHz QPOs, in which the frequency of the oscillations detected during type I bursts equals the separation between the two kHz QPOs (or twice that value), is incorrect.

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arXiv1998

An asymmetric arcsecond radio jet from Circinus X-1

In observations with the Australia Telescope Compact Array we have resolved the radio counterpart of the unusual X-ray binary Cir X-1 into an asymmetric extended structure on arcsecond scales. In order to quantify the asymmetry we have redetermined as accurately as possible both the optical and radio coordinates of the source. The extended emission can be understood as a compact, absorbed core at the location of the X-ray binary, and extended emission up to 2 arcsec to the southeast of the core. The arcsec-scale extended emission aligns with the larger, more symmetric arcmin-scale collimated structures in the surrounding synchrotron nebula. This suggests that the transport of mass and/or energy from the X-ray binary to the synchroton nebula is occurring via the arcsec-scale structures. The ratio of extended flux from the southeast to that from the northwest of the core is at least 2:1. Interpreted as relativistic aberration of an intrinsically symmetric jet from the source, this implies a minimum outflow velocity of 0.1 c. Alternatively, the emission may be intrinsically asymmetric, perhaps as a result of the high space velocity of the system.

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arXiv1998

Kilohertz Quasi-Periodic Oscillation Peak Separation is not Constant in the Atoll Source 4U 1608-52

We present new Rossi X-ray Timing Explorer observations of the low-mass X-ray binary 4U 1608-52 during the decay of its 1998 outburst. We detect by a direct FFT method the existence of a second kilohertz quasi-periodic oscillation (kHz QPO) in its power density spectrum, previously only seen by means of the sensitivity-enhancing `shift and add' technique. This result confirms that 4U 1608-52 is a twin kHz QPO source. The frequency separation between these two QPO decreased significantly, from 325.5 +/- 3.4 Hz to 225.3 +/- 12.0 Hz, as the frequency of the lower kHz QPO increased from 470 Hz to 865 Hz, in contradiction with a simple beat-frequency interpretation. This change in the peak separation of the kHz QPOs is closely similar to that previously seen in Sco X-1, but takes place at a ten times lower average luminosity. We discuss this result within the framework of models that have been proposed for kHz QPO. Beat frequency models where the peak separation is identified with the neutron star spin rate, as well as the explanations previously proposed to account for the similar behavior of the QPOs in Sco X-1, are strongly challenged by this result.

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arXiv1998

Quasi-Periodic Oscillations in Black Hole Candidates as an Indicator of Transition Between Low and High States

By comparing positions on a spectral color-color diagram from 10 black hole candidates (BHCs) observed with Ginga (1354-64, 1826-24, 1630-47, LMC X-1, LMC X-3, GS 2000+25, GS 2023+33, GS 1124-68, Cyg X-1, and GX 339-4) with the observed broad-band noise (0.001-64 Hz; BBN) and quasi-periodic oscillation (QPO) variability, we find that the so-called ``Very High State'' is spectrally intermediate to the Soft/High-State and Hard/Low-State. We find a transition point in spectral hardness where the dependence of the BHC QPO centroid frequency (of GS 1124-68 and GX 339-4) on spectral hardness switches from a correlation to an anti-correlation; where the BBN variability switches from High-State to Low-State; and where the spectral hardness of the QPO relative to that of the BBN variability is a maximum. This coincidence of changing behavior in both the QPO and the broad-band variability leads us to hypothesize that the QPO is due to interaction between the physical components which dominate the behaviors of BHCs when they occupy the Hard/Low and Soft/High States. We conclude that these QPO should be observed from BHCs during transition between these two states. Comparison with QPO and BBN behavior observed during the 1996 transition of Cyg X-1 supports this hypothesis. We also report 1-3 Hz QPO observed in GS 2000+25 and Cyg X-1 in the Hard/Low State, and we compare these to the QPO observed in GS 1124-68 and GX 339-4.

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arXiv1998

Discovery of KiloHertz Quasi-Periodic Oscillations in the Z source GX 5-1

We discovered two simultaneous kHz quasi-periodic oscillations (QPOs) in the bright low-mass X-ray binary and Z source GX 5-1 with the Rossi X-ray Timing Explorer. In the X-ray color-color and hardness-intensity diagram a clear Z track is traced out, which shifted between observations. The frequencies of the two kHz QPOs increased from ~215 Hz and ~500 Hz on the left part of the horizontal branch to ~700 Hz and ~890 Hz, respectively, on the upper part of the normal branch. With increasing frequency the FWHM and rms amplitude (8.6-60 keV) of the higher-frequency kHz QPO decreased from 300 to 30 Hz, and from 6.6 to 2.4%, respectively. The FWHM and amplitude of the lower-frequency kHz QPO (50-100 Hz and 3-4% rms) did not correlate with the position of the source on the Z track. The kHz QPO separation was consistent with being constant at 298+/-11 Hz. Simultaneously with the kHz QPOs horizontal branch oscillations (HBOs) were detected with frequencies between 18 and 56 Hz.

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arXiv1998

Discovery of the first accretion-powered millisecond X-ray pulsar

The precise origins of the millisecond radio pulsars, discovered in the early 1980s, remain uncertain until this day. They plausibly evolve from accreting low magnetic-field neutron stars in X-ray binary systems. If so, these stars should spin at millisecond rates. In accordance with this idea, quasi-periodic oscillations discovered in X-ray binaries around 50 Hz and 1 kHz, and drifting oscillations at several 100 Hz in X-ray bursts have all been interpreted in terms of millisecond spins of weakly magnetized neutron stars. However, in 15 years of searching, the expected coherent millisecond signals from X-ray binaries remained elusive. In this Letter, we report the discovery of the first example of such a signal. Using the Rossi X-ray Timing Explorer we find persistent 2.49 millisecond X-ray pulsations in an X-ray binary, which we interpret to come from an accretion-powered millisecond X-ray pulsar in the system. This is the first known object of its kind. It is likely to switch on as a millisecond radio pulsar when the accretion turns off completely. The source is positionally coincident with the known transient X-ray burster SAX J1808.4-3658, which also makes this the first X-ray pulsar which exhibits thermonuclear X-ray bursts.

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arXiv1998

``Canonical'' Black Hole States in the Superluminal Source GRO J1655-40

We analyze RXTE/PCA observations of the black hole candidate and galactic superluminal source GRO J1655-40 during its recent outburst. We show that during its decay to quiescence, GRO J1655-40 goes through the high, intermediate, and low state (and that at the beginning of its decay it might have even shown signatures of a very high state), just like other black hole candidates. This is the first time that such a transition is observed in a galactic superluminal source. We discuss what are the implications of these results on the hypothesis that the spin of the black hole in superluminal sources is much higher than in other black hole candidates.

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arXiv1998

Discovery of kilohertz quasi-periodic oscillations in the Z source GX 340+0

We have discovered two simultaneous kHz quasi-periodic oscillations (QPOs) in the Z source GX 340+0 with the Rossi X-ray Timing Explorer. The X-ray hardness-intensity and color-color diagram each show a full Z-track, with an extra limb branching off the flaring branch of the Z. Both peaks moved to higher frequencies when the mass accretion rate increased. The two peaks moved from 247 +/- 6 and 567 +/- 39 Hz at the left end of the horizontal branch to 625 +/- 18 and 820 +/- 19 Hz at its right end. The higher frequency peak's rms amplitude (5-60 keV) and FWHM decreased from ~5% and 383 +/- 135 Hz to ~2%, and 145 +/- 62 Hz, respectively. The rms amplitude and FWHM of the lower peak were consistent with being constant near 2.5 % and 100 Hz. The kHz QPO separation was consistent with being constant at 325 +/- 10 Hz. Simultaneous with the kHz QPOs we detected the horizontal branch oscillations (HBO) and its second harmonic, at frequencies between 20 and 50 Hz, and 38 and 69 Hz, respectively. The normal branch oscillations were only detected on the upper and middle normal branch, and became undetectable on the lower normal branch. The HBO frequencies do not fall within the range predicted for Lense-Thirring (LT) precession, unless either the ratio of the neutron star moment of inertia to neutron star mass is at least 4, 10^45 gcm^2/M_sun, the frequencies of the HBO are in fact the sub-harmonic oscillations, or the observed kHz peak difference is half the spin frequency and not the spin frequency. During a 1.2 day gap between two observations, the Z-track in the hardness-intensity diagram moved to higher count rates by about 3.5%. Comparing data before and after this shift, we find that the HBO properties are determined by position on the Z-track and not directly by count rate or X-ray colors.

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arXiv1998

Discovery of a 57-69 Hz quasi-periodic oscillation in GX13+1

We report the discovery of a quasi-periodic oscillation (QPO) at 61.0+/-1.7 Hz with the Rossi X-Ray Timing Explorer in the low-mass X-ray binary and persistently bright atoll source GX 13+1 (4U 1811-17). The QPO had an rms amplitude of 1.7+/-0.2 % (2-13.0 keV) and a FWHM of 15.9+/-4.2 Hz. Its frequency increased with count rate and its amplitude increased with photon energy. In addition a peaked noise component was found with a cut-off frequency around 2 Hz, a power law index of around -4, and an rms amplitude of ~1.8 %, probably the well known atoll source high frequency noise. It was only found when the QPO was detected. Very low frequency noise was present with a power law index of ~1, and an rms amplitude of ~4 %. A second observation showed similar variability components. In the X-ray color-color diagram the source did not trace out the usual banana branch, but showed a two branched structure. This is the first detection of a QPO in one of the four persistently bright atoll sources in the galactic bulge. We argue that the QPO properties indicate that it is the same phenomenon as the horizontal branch oscillations (HBO) in Z sources. That HBO might turn up in the persistently bright atoll sources was previously suggested on the basis of the magnetospheric beat frequency model for HBO. We discuss the properties of the new phenomenon within the framework of this model.

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arXiv1998

Discovery of Kilohertz QPOs in the Atoll X-Ray Binary 4U 1705-44

In observations with the Rossi X-ray Timing Explorer we have discovered quasi-periodic oscillations (QPOs) near 1 kHz from 4U 1705-44, a low--mass X-ray binary with a neutron star classified as an atoll source. In six separate observations, we detect one QPO with a frequency ranging between 770 and 870 Hz and a 4% rms fraction in the full detector energy band. There is evidence for a second QPO at 1073 Hz in one interval. The separation in frequency of the two QPOs is 298+-11 Hz. The QPOs are present only in observations where the mass accretion rate is inferred to be at an intermediate level, based on the atoll source phenomenology. At the highest accretion rates, the QPOs are not detected with upper limits to the rms fraction of about 2%. At the lowest accretion rates the upper limits are about 4%. The QPO frequency increases with inferred mass accretion rate. This is expected in models where the QPO frequency is generated by motion at an inner edge of the accretion disk. An increased mass accretion rate causes the disk edge to move in, increasing the orbital frequency. Five Type-I X-ray bursts are observed with no detectable oscillations.

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arXiv1998

Discovery of KiloHertz Quasi-Periodic Oscillations in 4U 1735-44

We discovered a single kHz quasi-periodic oscillation (QPO) near 1150 Hz in the Rossi X-ray Timing Explorer X-ray light curve of the low mass X-ray binary and atoll source 4U 1735-44. The rms amplitude of this peak was 2-3%, and the FWHM 6-40 Hz. There are indications that the kHz QPO frequency decreased from 1160 Hz to 1145 Hz when the count rate increased, which would be quite different from what is observed in other atoll sources for which kHz QPOs have been discovered. In the X-ray color-color diagram and hardness-intensity diagram the source traced out the curved branch (the so-called banana branch) which has been found by previous instruments. The kHz QPO was only detected when the source was at the lowest count rates during our observations, i.e. on the lower part of the banana branch. When 4U 1735-44 was at higher count rates, i.e. on the upper part of the banana branch and at higher inferred mass accretion rate with respect to that on the lower part of the banana branch, the QPO was not detected. Besides the kHz QPO we discovered a low frequency QPO with a frequency near 67 Hz, together with a complex broad peaked noise component below 30 Hz. This 67 Hz QPO may be related to the magnetospheric beat-frequency QPO, which is observed on the horizontal branch of Z sources. This idea is supported by the (peaked) noise found in both 4U 1735-44 and Z sources at frequencies just below the QPO frequency.

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arXiv1997

GX 349+2 (Sco X-2): An odd-ball among the Z sources

We report on about 4 hrs of observations made with the Rossi X-ray Timing Explorer on 1997 May 2 of the low-mass X-ray binary and Z source GX 349+2 (Sco X-2). Initially the source was in the normal branch (NB), later it moved to the flaring branch (FB). In the NB the power spectra reveal a broad (FWHM ~16 Hz) noise component peaking near 9 Hz, with a fractional rms amplitude of ~3% (2-30 keV). This noise component does not resemble the strong quasi-periodic oscillations (QPO) usually seen in the NB of other Z sources. We set 95% confidence upper limits on the fractional rms of ~0.9% (2-30 keV) on any such QPO. In the FB the power spectrum showed a somewhat less broad noise component (FWHM ~11 Hz) peaking near 6 Hz with a fractional rms of ~4% (2-30 keV). We compare our results with previous reports, and find that the fast timing behaviour changes not only with position in the Z, but also as a function of the position of the Z track in the hardness-intensity diagram. By comparing GX 349+2 with the other Z and atoll sources, we conclude that GX 349+2 differs from the Z sources in various aspects and shows similarities to the behaviour seen in the bright atoll sources, such as GX 13+1 and GX 3+1. We also searched for kilo-Hertz QPO, similar to those present in other Z sources. We only found weak evidence (2.6 sigma confidence) for a QPO near 1020 Hz with FWHM ~40 Hz and fractional rms of ~1%. We note that this frequency and fractional rms are consistent with those expected and observed in the lower NB/FB of the Z sources Sco X-1, GX 17+2 and Cyg X-2.

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arXiv1997

KiloHertz Quasi-Periodic Oscillations in the Z sources GX 340+0, Cygnus X-2, GX 17+2, GX 5-1, and Scorpius X-1

We have discovered kiloHertz quasi-periodic oscillations (kHz QPOs) in five Z sources: GX 340+0, Cygnus X-2, GX 17+2, GX 5-1, and Scorpius X-1. In all sources the properties of these kHz QPOs are very similar and closely related to the position of the sources on the Z track traced out in the X-ray color-color diagram and the hardness-intensity diagram. The frequencies of the kHz QPOs increase when the sources move from the left end of the horizontal branch to horizontal/normal branch vertex, thus with inferred mass accretion rate. Only for Scorpius X-1 the kHz QPOs have been observed down the normal branch unto the flaring branch. The strength and the FWHM of the higher-frequency kHz QPOs decrease with mass accretion rate, but when the lower-frequency kHz QPOs are detected the strength and the FWHM of this QPO stay approximately constant with mass accretion rate. In Scorpius X-1 the frequency separation between the kHz QPOs decreases with mass accretion rate, but in the other Z sources the separation remains approximately constant, although a similar decrease in peak separation as found in Scorpius X-1 can not be excluded.

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arXiv1997

Absorption dips in GRO J1655-40: mapping the inner accretion disk

Using the RXTE PCA we discovered dips in the X-ray light curves of the black-hole candidate GRO J1655-40 during outburst. They are short (~minute) and deep (down to ~8% of the out-of-dip intensity). Similar kind of dips were found in 90s measurements of the RXTE ASM during the same outburst. The occurrences of the dips are consistent with the optically determined orbital period, and were found between photometric orbital phases 0.72 and 0.86. This constitutes the first evidence for orbital variations in X-rays for GRO J1655-40. The PCA data indicate that an absorbing medium is responsible for these dips. Using these results we are able constrain the extent of the absorbing medium and the central X-ray source. GRO J1655-40 was in the canonical high state during our PCA observations.

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