arXiv subjects

M. van der Klis

Publications and source records attributed to M. van der Klis.

At least 181 records · Page 10Linked to original sources

arXiv1998

Magnetic field estimation in Cyg X-3's jet

Multi-wavelength photometric observations of Cygnus X-3 were carried out at 18 cm through to 450 um, complemented by X-ray (2-10 keV) observations. The system was mildly active with cm fluxes at 150 -- 250 mJy. We find the spectrum to be flat with a spectral index of zero. Using a modified Wolf-Rayet wind model, and assuming emission is generated in synchrotron emitting jets from the source, we find an upper-limit to the magentic field of 20 G at a distance of $5 \times 10^{12}$ cm is required.

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arXiv1998

The Beat-Frequency Interpretation of Kilohertz QPOs in Neutron Star Low-Mass X-ray Binaries

Pairs of quasi-periodic oscillations (QPOs) at kilohertz frequencies are a common phenomenon in several neutron-star low-mass X-ray binaries. The frequency separation of the QPO peaks in the pair appears to be constant in many sources and directly related to the neutron star spin frequency. However, in Sco X-1 and possibly in 4U 1608-52, the frequency separation of the QPOs decreases with increasing inferred mass accretion rate. We show that the currently available Rossi X-ray Timing Explorer data are consistent with the hypothesis that the frequency separations in all sources vary by amounts similar to the variation in Sco X-1. We discuss the implications for models of the kilohertz QPOs.

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arXiv1997

The viscous disk of GRS 1915+105

GRS 1915+105, one of the two known galactic microquasars, shows an extremely complex variability in the X-ray band, comparable to no other X-ray source in the sky. Making use of RXTE/PCA data, we have analyzed the X-ray spectral distribution throughout the variability. We find that all variations can be attributed to the rapid appearance and disappearance of the inner region of an optically-thick accretion disk. Since the time scale for each event is related to the maximum radius of the disappearing region, the difference in time structure is due to the time distribution of such radii. The observed relation between the extent of the missing inner region of the disk and the duration of an event is in remarkable agreement with the expected radius dependence of the viscous time scale of the radiation-dominated region of an accretion disk.

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arXiv1997

Thermal-viscous instabilities in the accretion disk of GRS 1915+105

We use data obtained with the PCA onboard RXTE to analyze the spectral variations of the black hole candidate and superluminal source GRS 1915+105. Our results suggest that despite the complicated structure of the X-ray light curve, all spectral changes observed in this source can be explained as a sudden disappearing of the inner part of the accretion disk around the compact object, followed by a slower re-filling of the emptied region. The duration of an event and the size of the disappearing region fit remarkably well the expected radius dependence of the viscous time scale for the radiation-pressure dominated region of an accretion disk.

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arXiv1997

A varying kHz peak separation in 4U 1608-52

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arXiv1997

Discovery of a second kHz QPO peak in 4U 1608-52

Using a new technique to improve the sensitivity to weak Quasi-Periodic Oscillations (QPO) we discovered a new QPO peak at about 1100 Hz in the March 1996 outburst observations of 4U 1608-52, simultaneous with the ~ 600-900 Hz peak previously reported from these data. The frequency separation between the upper and the lower QPO peak varied significantly from $232.7 \pm 11.5$ Hz on March 3, to $293.1 \pm 6.6$ Hz on March 6. This is the first case of a variable kHz peak separation in an atoll source. We discuss to what extent this result could be accommodated in beat-frequency models such as proposed for the kHz QPOs. We measured the rms fractional amplitude of both QPOs as a function of energy, and we found that the relation is steeper for the lower than for the upper frequency peak. This is the first source where such difference between the energy spectrum of the two kHz QPOs could be measured.

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arXiv1997

Hard X-ray Lags in Cygnus X-1

We have used the Fourier cross spectra of Cyg X-1, as obtained with BATSE during a period of almost 2000 days, to estimate the phase (or time) lags between X-ray flux variations in the 20-50 keV and the 50-100 keV bands as a function of Fourier frequency, nu. We find that these lag spectra do not depend on source state, as measured by the fractional rms variations of the X-ray flux. For frequencies well below the Nyquist frequency the variations in the 50-100 keV band lag those in the 20-50 keV band by a time interval proportional to nu^{-0.8}. Binning effects cause the observed hard X-ray time lags to decrease to zero at the Nyquist frequency. Some previous results on time lags were affected by these binning effects. At photon energies above 10 keV the time lags are approximately proportional to the logarithm of the ratio the energies of the passbands used.

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arXiv1997

Kilohertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries - a Review

This paper provides a review of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries recently discovered with NASA's Rossi X-ray Timing Explorer. It contains two tables that are up to date up to October 1st, 1997 with information about all kHz QPO phenomena so far reported. In the most straightforward interpretations, kHz QPO provide direct estimates of the millisecond spin rate of neutron stars in low-mass X-ray binaries and of the Kepler frequency at the inner edge of the accretion disk. If this is so, the accretion disk must penetrate nearly to the bottom of the potential well of these neutron stars. The kHz QPO then directly constrain the mass and radius of these neutron stars, and thereby the equation of state of high-density matter, and potentially allow the detection of major strong-field general-relativistic effects such as the existence of an innermost stable orbit, and frame dragging.

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arXiv1997

A unified model for the spectral variability in GRS 1915+105

We have analyzed the spectral variations of the superluminal black-hole X-ray binary GRS 1915+105 by using data obtained with the PCA on the Rossi XTE. We find that, despite the marked differences in the structure and the time scale of variability, all spectral changes can be attributed to the rapid disappearing of the inner region of an accretion disk, followed by a slower re-filling of the emptied region. The time scale for each event is determined by the extent of the missing part of the disk. The observed relation between the duration of an event and the radius of the disappearing region matches remarkably well the expected radius dependence of the viscous time scale for the radiation-pressure dominated region of an accretion disk.

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arXiv1997

Kilohertz QPO and Atoll Source States in 4U 0614+09

We report three RXTE/PCA observations of the low-mass X-ray binary 4U 0614+09. They show strong (~ 30% rms) band-limited noise with a cut-off frequency varying between 0.7 and 15 Hz in correlation with the X-ray flux. We observe two non-simultaneous 11-15% (rms) kHz peaks near 728 and 629 Hz in the power spectra of two of our observations when the X-ray flux is ~ 10^{-9} erg cm^{-2} s{-1} (2-10 keV), but find no QPO (< 6% rms) when the X-ray flux is half that. We suggest that count rate may not be a good measure for $\dot M$ even in sources as intrinsically weak as 4U 0614+09, and that QPO frequency and noise cutoff frequency track $\dot M$ more closely than count rate. The QPO increases in rms amplitude from ($11 \pm 1.3$) % at 3 keV to ($37 \pm 12$) % at 23 keV; the fractional amplitude of the band-limited noise is energy-independent. This suggests different sites of origin for these two phenomena. The spectrum of the oscillating flux roughly corresponds to a black body with temperature ($1.56 \pm 0.2$) keV and radius ($ 500 \pm 200$) m (other models fit as well), which might indicate the oscillations originate at a small region on the neutron star surface.

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arXiv1997

Quantifying Rapid Variability in Accreting Compact Objects

I discuss some practical aspects of the analysis of millisecond time variability X-ray data obtained from accreting neutron stars and black holes. First I give an account of the statistical methods that are at present commonly applied in this field. These are mostly based on Fourier techniques. To a large extent these methods work well: they give astronomers the answers they need. Then I discuss a number of statistical questions that astronomers don't really know how to solve properly and that statisticians may have ideas about. These questions have to do with the highest and the lowest frequency ranges accessible in the Fourier analysis: how do you determine the shortest time scale present in the variability, how do you measure steep low-frequency noise. The point is stressed that in order for any method that resolves these issues to become popular, it is necessary to retain the capabilities the current methods already have in quantifying the complex, concurrent variability processes characteristic of accreting neutron stars and black holes.

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arXiv1997

Kilohertz quasi-periodic oscillations in low-mass X-ray binaries

In early 1996 a series of discoveries begun with NASA's Rossi X-ray Timing Explorer of a new, up to then unknown astrophysical phenomenon. It turned out that accreting low magnetic-field neutron stars show quasi-periodic oscillations in their X-ray flux at rates of up to more than a kilohertz. These kHz QPO, now reported from eleven different systems, are among the fastest phenomena in the sky and can provide us with new information about the fundamental properties of neutron stars and help testing general relativity in the strong-field regime. If, for example, their frequencies can be identified with the Keplerian frequencies of matter in orbit around a 1.4 Solar-mass neutron star, then the radius of the star would have to be less than 15 km, which directly constrains the equation of state of bulk nuclear-density matter, and for an only slightly tighter orbit or slightly more massive neutron star the orbital radius would equal the Schwarzschild-geometry general-relativistic marginally stable orbit (12.5 km for a 1.4 Solar mass object). So far all models that have been put forward for explaining the new phenomenon have encountered problems. In this paper I review the relatively simple and highly suggestive phenomenology as it has emerged from the data up to now, and discuss some of the proposed models.

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arXiv1997

Ginga observations of Cygnus X-2

We analysed all X-ray data on the low-mass X-ray binary Cygnus X-2 obtained with Ginga. The analysis of the spectral and fast timing behaviour of these 4 years of data provides new insights in the behaviour of this Z source. We confirm the previously observed recurrent patterns of behaviour in the X-ray colour-colour and hardness-intensity diagrams (HIDs) consisting of shifts and shape changes in the Z track. The source behaviour is correlated with overall intensity. When the overall intensity increases, the mean velocity of the motion along the normal branch (NB) of the Z track increase, as well as the width of the NB in the HID. Contrary to previous results we find that, during different observations, when the source is at the same position in the NB of the Z track the rapid X-ray variability differs significantly. During the Kuulkers et al. (1996) ``medium'' level, a normal branch quasi-periodic oscillation (QPO) is detected, which is not seen during the ``high'' overall intensity level. During the high overall intensity level episodes the very-low frequency noise on the lower NB is very strong, whereas during the medium overall intensity level episodes this noise component at the same position in the Z track is much weaker. The explanation of the different overall intensity levels with a precessing accretion disk is difficult to reconcile with our data. Furthermore, the frequency of the horizontal branch QPO decreases when Cyg X-2 enters the upper NB, giving a model dependent upper limit on the magnetic field strength at the magnetic equator of ~8.5x10^9 G. We also report 5 bursts, with durations of ~5 s, whose occurrence are uncorrelated with location in the Z track, overall intensity level or orbital phase. The burst properties indicate that they are not regular type I bursts.

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arXiv1997

GX 17+2: X-ray spectral and timing behaviour of a bursting Z source

We investigated the properties of the Z pattern of the low-mass X-ray binary GX 17+2 in the X-ray colour-colour diagrams (CDs) and hardness-intensity diagrams (HIDs), and of the power spectra as a function of position in the Z, using all EXOSAT ME data. The Z pattern in the CD does not move to within 2 per cent between observations separated by 1 day to 1.5 years. The power spectra and the characteristics of the Z pattern both support the division of the Z sources into two sub-groups, GX 17+2 being in the group together with Sco X-1 and GX 349+2. It has been noted previously that the maximum frequency of the horizontal branch quasi-periodic oscillations in GX 17+2 is lower than for Cyg X-2, GX 5-1 and GX 340+0. We suggest that an asymmetry in the magnetic field giving rise to polar caps with different emission characteristics may be the origin of this. This may also explain the occurrence of bursts in GX 17+2. We also report the detection of two new bursts in the EXOSAT data of GX 17+2, which we interpret as type I bursts. No regular pulsations are observed during these two bursts nor during the bursts previously detected in GX 17+2 with EXOSAT, with upper limits to the pulsed amplitude of 1.5 per cent for pulsations with a frequency below 512 Hz. Three of the four bursts occurred in the normal branch, while one occurred in the lower part of the flaring branch.

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arXiv1997

An unstable central disk in the superluminal black-hole X-ray binary GRS 1915+105

We have analyzed the X-ray spectra of the microquasar GRS 1915+105, as observed with the PCA on the Rossi XTE, during periods of stable weak emission, outbursts and rapid flaring. We find that the complicated X-ray intensity curve of this source can be described by the rapid removal and replenishment of matter forming the inner part of an optically thick accretion disk, probably caused by a thermal-viscous instability analogous to that operating in dwarf novae, but here driven by the Lightman-Eardley instability. We find that the mass accretion rate in quiescence is 10^-6 solar masses per year. Only a small fraction of the energy liberated by accretion is emitted as radiation. We suggest that most of this energy is advected into the black hole in the high-viscosity state of the outburst cycle.

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arXiv1997

Correlated X-ray spectral and fast-timing behaviour of 4U 1636-53

We present a study of the persistent X-ray emission from the low-mass X-ray binary 4U 1636-53 based ont he entire archival EXOSAT ME data set. We per- formed a homogeneous analysis of all five EXOSAT observations in terms of the correlated rapid X-ray variability and X-ray spectral properties by means of power spectra, and colour-colour and hardness-intensity diagrams, respectively. Over the years we find similar patterns in the colour-colour and hardness-intensity diagrams, but small shifts in their postions occur. We find one case of an island-banana transition. On all but one occasion the differences in colour are smaller than 5%. Between the two 'island state' data sets, we find a shift of 8 % in the soft colour, which must be at least partly intrinsic. Clear correlations are present between the X-ray colours (which are governed by the mass accretion rate) and the power spectrum. With increasing mass accretion rate the fractional rms amplitude of the power-law shaped power spectral component that dominates the power spectra below ~1 Hz rises, while the band-limited noise component at higher frequencies drops. The cut-off frequency of the band-limited component increases whn its amplitude decreases. This is reminiscent of the behaviour of black hole candidates in the low state.

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arXiv1996

Discovery of 800 Hz QPO in 4U1608-52

We present results of RXTE observations of the low-mass X-ray binary and atoll source 4U 1608$-$52 made over 9 days during the decline of an X-ray intensity outburst in March 1996. A fast-timing analysis shows a strong and narrow quasi periodic oscillation (QPO) peak at frequencies between 850 and 890 Hz on March 3 and 6, and a broad peak around 690 Hz on March 9. Observations on March 12 show no significant signal. On March 3, the X-ray spectrum of the QPO is quite hard; its strength increases steadily from 5 \% at $\sim$2 to $\sim$20 \% at $\sim$12~keV. The QPO frequency varies between 850 and 890 Hz on that day, and the peak widens and its rms decreases with centroid frequency in a way very similar to the well-known horizontal branch oscillations (HBO) in Z-sources. We apply the HBO beat frequency model to atoll sources, and suggest that, whereas the model could produce QPOs at the observed frequencies, the lack of correlation we observe between QPO properties and X-ray count rate is hard to reconcile with this model.

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arXiv1996

The Wolf-Rayet counterpart of Cygnus X-3

We present orbital-phase resolved I and K-band spectroscopy of Cygnus X-3. All spectra show emission lines characteristic of Wolf-Rayet stars of the WN subclass. On time scales longer than about one day, the line strengths show large changes, both in flux and in equivalent width. In addition, the line ratios change, corresponding to a variation in spectral subtype of WN6/7 to WN4/5. We confirm the finding that at times when the emission lines are weak, they shift in wavelength as a function of orbital phase, with maximum blueshift coinciding with infrared and X-ray minimum, and maximum redshift half an orbit later. Furthermore, we confirm the prediction -- made on the basis of previous observations -- that at times when the emission lines are strong, no clear wavelength shifts are observed. We describe a simplified, but detailed model for the system, in which the companion of the X-ray source is a Wolf-Rayet star whose wind is at times ionised by the X-ray source, except for the part in the star's shadow. With this model, the observed spectral variations can be reproduced with only a small number of free parameters. We discuss and verify the ramifications of this model, and find that, in general, the observed properties can be understood. We conclude that Cyg X-3 is a Wolf-Rayet/X-ray binary.

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