arXiv subjects

Michiel van der Klis

Publications and source records attributed to Michiel van der Klis.

At least 91 records · Page 5Linked to original sources

arXiv2007

An accreting millisecond pulsar with black hole-like X-ray variability: IGR J00291+5934

IGR J00291+5934 is one of the seven accreting millisecond pulsars (AMPs) discovered so far. We report on the aperiodic timing and color analysis of its X-ray flux, using all the RXTE observations of the 2004 outburst. Flat-top noise and two harmonically related quasi-periodic oscillations, all of them at very low frequencies (0.01-0.1 Hz), were present in the power spectra during most of the outburst as well as a very high fractional variability (~50%). These properties are atypical not only for AMPs but also for neutron star low-mass X-ray binaries (LMXBs) in general. There are instead some remarkable similarities with the variability observed in black hole systems, reinforcing the connections between these two types of LMXB, as well as some interesting differences. We note finally that the results of this paper are difficult to reconcile with interpretations where any break frequency of power density spectra scales inversely with the mass of the central object at an accuracy sufficient to distinguish between the masses of neutron stars and black holes in LMXBs.

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arXiv2006

Accreting millisecond pulsars: one on each hand

We report on the X-ray aperiodic timing analysis of two accreting millisecond pulsars: XTE J1807-294 and IGR J00291+5934. On the one hand, we discovered in XTE J1807-294 seven pairs of simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) separated in frequency by nearly the spin frequency of the neutron star. This confirms the suspected dichotomy in the frequency separation of kHz QPOs: sometimes once and sometimes half the spin frequency. On the other hand, we found an extreme behavior in the power spectra of IGR J00291+5934: very strong variability at very low frequencies. Namely, the fractional amplitude of the variability was ~50%, the highest value found so far in a neutron star system.

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arXiv2006

RXTE observations of the first transient Z source XTE J1701-462: shedding new light on mass accretion in luminous neutron star LMXBs

(Abridged) We report on ten weeks of RXTE observations of the X-ray transient XTE J1701-462. Comparisons with other sources suggest it had all the characteristics of the neutron star Z sources (the brightest persistent neutron star LMXBs). These include Z tracks in X-ray color diagrams and typical variability components detected in the power spectra. XTE J1701-462 is the first transient Z source and provides unique insights into mass accretion rate (Mdot) and luminosity dependencies in neutron star LMXBs. As its overall luminosity decreased, we observed a switch between two types of Z-source behavior, with most of the branches of the Z-track changing their shape and/or orientation. We interpret this switch as an extreme case of the longterm changes seen in the persistent Z sources and suggest that these result from changes in Mdot. We also suggest that the Cyg-like Z sources (Cyg X-2, GX 5-1, and GX 340+0) might be more luminous (> 50%) than the Sco-like Z sources (Sco X-1, GX 17+2, and GX 349+2). Adopting a possible explanation for the behavior of kHz QPOs, which involves a prompt as well as a filtered response to changes in Mdot, we propose that changes in Mdot can explain both movement along the Z track and changes in the shape of the Z track. We discuss some consequences of this and consider the possibility that the branches of the Z will evolve into the branches observed in the X-ray color diagrams of the less luminous atoll sources, but not in a way that was previously suggested.

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arXiv2005

Discovery of kHz QPOs and shifted frequency correlations in the accreting millisecond pulsar XTE J1807--294

We report the discovery of twin kHz QPOs in the X ray flux of XTE J1807--294, the fourth accreting millisecond pulsar (AMP). This is the second AMP exhibiting twin kHz QPOs. In contrast to the first case, SAX J1808.4--3658, the frequency separation $Δν$ between the kHz QPOs is consistent with the pulse frequency (190.6 Hz), not with half that value, confirming for the first time from pulsation measurements the inference, based on burst oscillations, that 'slow rotators' (spin frequency less than 400 Hz) have $Δν$ approximately equal to the spin frequency. While the QPOs move in frequency together over a range of more than 200 Hz, $Δν$ remains constant with an average value of 205$\pm$6 Hz. Variability components were found in the 5--130 Hz range similar to those seen in other LMXBs. The correlations between the QPO and noise frequencies are also similar to those in other sources, but shifted by a factor of 1.59 in kHz QPO frequencies, similar to the factor 1.45 shift found for SAX J1808.4--3658. Our results argue in favour of a spin-related formation mechanism for twin kHz QPOs and against a spin-related cause of the shift in the frequency correlations.

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arXiv2005

RXTE observations of the dipping low-mass X-ray binary 4U 1624-49

We analyse ~ 360 ks of archival data from the Rossi X-Ray Timing Explorer (RXTE) of the 21 hr orbital period dipping low-mass X-ray binary 4U 1624-49. We find that outside the dips the tracks in the colour-colour and hardness-intensity diagrams (CDs and HIDs) are reminiscent of those of atoll sources in the middle and upper parts of the banana branch. The tracks show secular shifts up to ~ 10%. We study the power spectrum of 4U 1624-49 as a function of the position in the CD. This is the first time power spectra of this source are presented. No quasi-periodic oscillations (QPOs) are found. The power spectra are dominated by very low frequency noise (VLFN), characteristic for atoll sources in the banana state, and band limited noise (BLN) which is not reliably detected but may, uncharacteristically, strengthen and increase in frequency with spectral hardness. The VLFN fits to a power law, which becomes steeper when the source moves to the harder part of the CD. We conclude that 4U 1624-49 is an atoll source which in our observations is in the upper banana branch. Combining this with the high (0.5-0.7 L_Edd) luminosity, the long-term flux stability of the source as seen with the RXTE All-Sky Monitor (ASM), and with the fact that it is an X-ray dip source, we conclude that 4U 1624-49 is most likely a GX atoll source such as GX 3+1 and GX 9+9, but seen edge on.

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arXiv2004

Relations Between Timing Features and Colors in Accreting Millisecond Pulsars

We have studied the aperiodic X-ray timing and color behavior of the accreting millisecond pulsars SAX J1808.4-3658, XTE J1751-305, XTE J0929-314, and XTE J1814-338 using large data sets obtained with the Rossi X-ray Timing Explorer. We find that the accreting millisecond pulsars have very similar timing properties to the atoll sources and the low luminosity bursters. Based on the correlation of timing and color behavior SAX J1808.4-3658 can be classified as an atoll source, and XTE J0929-314 and XTE J1814-338 are consistent with being atoll sources, but the color behavior of XTE J1751-305 is different. Unlike in other atoll sources the hard color in the extreme island state of XTE J1751-305 is strongly correlated with soft color and intensity, and is not anti-correlated with any of the characteristic frequencies. We found previously, that the frequencies of the variability components of atoll sources follow a universal scheme of correlations. The frequency correlations of the accreting millisecond pulsars are similar, but in the case of SAX J1808.4-3658 and XTE J0929-314 shifted compared to those of the other atoll sources in a way that is most easily described as a shift in upper and lower kilohertz QPO frequency by a factor close to 1.5. Although, we note that the shift in lower kilohertz QPO frequency is based on only one observation for SAX J1808.4-3658. XTE J1751-305 and XTE J1814-338, as well as the low luminosity bursters show no or maybe small shifts.

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arXiv2004

Correlation between Hard X-ray Peak Flux and Soft X-ray Peak Flux in the Outburst Rise of Low Mass X-ray Binaries

We have analyzed {\it Rossi} X-ray timing explorer (RXTE) pointed observations of the outbursts of black hole and neutron star soft X-ray transients in which an initial low/hard state or `island' state, followed by a transition to a softer state, was observed. In three sources, the black hole transient XTE J1550-564, the neutron star transient Aquila X-1 and a quasi-persistent neutron star low mass X-ray binary (LMXB) 4U 1705-44, two such outbursts were found. We find that the flux of the soft X-ray peak, which lags the hard X-ray peak by a few days to several weeks, scales with the flux of the hard X-ray peak. We conclude that we are able to predict the soft X-ray outburst peak flux based on the `preceding' hard X-ray peak flux, implying an early set up of the outbursts. We also find that the X-ray luminosity corresponding to the peak of the hard X-ray flux, which corresponds to the X-ray luminosity of the start of the hard-to-soft state transition, varies by a factor of about 2. If the accretion geometry early in the outburst rise is composed of two flows (e.g. a hot sub-Keplerian halo flow and a Keplerian disk flow, or an outflow and a Keplerian disk flow), the correlation indicates that the two flows are initially related, probably due to processes in the outer part of the accretion disk. We discuss constraints on a single flow model and a disk-jet model from these observations.

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arXiv2004

High- and low-frequency quasi-periodic oscillations in the X-ray light curves of the black hole transient H1743-322

We present a variability study of the black hole candidate and X-ray transient H1743-322 during its 2003/2004 outburst. We analyzed five Rossi X-ray Timing Explorer observations that were performed as part of a multi-wavelength campaign, as well as six observations from the early rise of the outburst. The source was observed in several black hole states and showed various types of X-ray variability, including high frequency quasi-periodic oscillations (QPOs) at 240 Hz and 160 Hz (i.e. with a 3:2 frequency ratio), several types of low frequency QPOs, and strong variations on a time scale of a few hundred seconds. The discovery of high frequency QPOs in H1743-322 supports predictions that these QPOs should be more easily observed in high inclination systems. In one of our observations we observed a transition in count rate and color, during which we were able to follow the smooth evolution of the low-frequency QPOs from type-B to type-A. We classify the X-ray observations and QPOs, and briefly discuss the QPOs in terms of recently proposed models.

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arXiv2004

Long Term Spectral and Timing Behavior of the Black Hole Candidate XTE J1908+094

We present the long term X-ray light curves, detailed spectral and timing analyses of XTE J1908+094 using the Rossi X-ray Timing Explorer Proportional Counter Array observations covering two outbursts in 2002 and early 2003. At the onset of the first outburst, the source was found in a spectrally low/hard state lasting for ~40 days, followed by a three day long transition to the high/soft state. The source flux (in 2$-$10 keV) reached $\sim$100 mCrab on 2002 April 6, then decayed rapidly. In power spectra, we detect strong band-limited noise and varying low-frequency quasi periodic oscillations that evolved from ~0.5 Hz to ~5 Hz during the initial low/hard state of the source. We find that the second outburst closely resembled the spectral evolution of the first. The X-ray transient's overall outburst characteristics lead us to classify XTE J1908+094 as a black-hole candidate. Here we also derive precise X-ray position of the source using Chandra observations which were performed during the decay phase of the first outburst and following the second outburst.

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arXiv2004

X-ray Sources and their Optical Counterparts in the Globular Cluster M4

We report on the Chandra X-ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the core and 19 more within the cluster half-mass radius. The limiting luminosity of this observation is Lx~10e29 erg/sec for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify 6 X-ray sources with known objects and use ROSAT observations to show that the brightest X-ray source is variable. Archival data from the Hubble Space Telescope allow us to identify optical counterparts to 16 X-ray sources. Based on the X-ray and optical properties of the identifications and the information from the literature, we classify two (possibly three) sources as cataclysmic variables, one X-ray source as a millisecond pulsar and 12 sources as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass.

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arXiv2004

X-ray spectral and timing properties of the 2001 superburst of 4U 1636-536

Preliminary results are reported on the spectral and timing properties of the spectacular 2001 superburst of 4U 1636-536 as seen by the RXTE/PCA. The (broad-band) power-spectral and hardness properties during the superburst are compared to those just before and after the superburst. Not all of the superburst emission can be fitted by pure black-body radiation. We also gathered BeppoSAX/WFC and RXTE/ASM data, as well as other RXTE/PCA data, obtained days to months before and after the superburst to investigate the normal X-ray burst behavior around the time of the superburst. The first normal X-ray burst after the 2001 superburst was detected 23 days later. During inspection of all the RXTE/ASM data we found a third superburst. This superburst took place on June 26, 1999, which is ~2.9 yrs after the 1996 superburst and ~1.75 yrs before the 2001 superburst. The above findings are the strongest constraints observed so far on the duration of the cessation of normal X-ray bursts after a superburst and the superburst recurrence times.

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arXiv2004

An ultra-relativistic outflow from a neutron star accreting gas from a companion

Collimated relativistic outflows, or jets, are amongst the most energetic and relativistic phenomena in the Universe. They are associated with supermassive black holes in distant active galactic nuclei (AGN), accreting black holes and neutron stars in binary systems and are believed to be responsible for gamma-ray bursts. The physics of these jets,however, remains something of a mystery in that their bulk velocities, compositions and energetics remain poorly determined. Here we report the discovery of an ultra-relativistic outflow from a binary accreting neutron star accreting gas within a binary system. The velocity of the outflow is comparable to the fastest-moving flows observed from active galactic nuclei, and its strength is modulated by the rate of accretion of material onto the neutron star. Shocks are energized further downstream in the flow, which are themselves moving with mildly relativistic bulk velocities and are the sites of the observed synchrotron emission from the jet. We conclude that the generation of such highly relativistic outflows does not require properties unique to black holes, such as an event horizon.

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arXiv2003

A transient variable 6 Hz QPO from GX 339-4

We report the results of an observation with the Rossi X-ray Timing Explorer of the black hole candidate GX 339-4 during its 2002/2003 outburst. This observation took place during a spectral transition from the hard to the soft state. A strong (6% rms) transient quasi-periodic oscillation (QPO) appears suddenly in the power density spectrum during this observation. The QPO centroid is ~6 Hz, but it varies significantly between 5 and 7 Hz with a characteristic time scale of ~10 seconds, correlated with the 2-30 keV count rate. The appearance of the QPO is related to spectral hardening of the flux, due to a change in the relative contribution of the soft and hard spectral components. We compare this peculiar behavior with results from other systems that show similar low frequency QPO peaks, and discuss the results in terms of possible theoretical models for QPO production.

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arXiv2003

High frequency features in the 1998 outburst of 4U 1630-47

We report on the detection with RXTE of variable high frequency features in the power spectra of the black hole candidate 4U 1630-47 during its 1998 outburst. We find high frequency features during the rise and the early decay of the outburst in the range of 100-300 Hz. The features are usually found to be broad, and have rms amplitudes of 2-8; their frequencies are correlated with count rate and anti-correlated with spectral hardness. Also, for the first time twin high frequency features are detected simultaneously, with a ratio in frequencies consistent with 1:4. Although some of the high frequency features are less coherent and stronger than found in other black hole sources, we show that they behave very similar to the high frequency features found in for instance XTE J1550-564.

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arXiv2003

Evidence of a decrease of kHz QPO peak separation towards low frequencies in 4U 1728-34 (GX 354-0)

We have produced the colour-colour diagram of all the observations of 4U 1728-34 available in the Rossi X-ray Timing Explorer public archive (from 1996 to 2002) and found observations filling in a previously reported 'gap' between the island and the banana X-ray states. We have made timing analysis of these gap observations and found, in one observation, two simultaneous kHz quasi-periodic oscillations (QPOs). The timing parameters of these kHz QPOs fit in the overall trend of the source. The 'lower' kHz QPO has a centroid frequency of ~308 Hz. This is the lowest 'lower' kHz QPO frequency ever observed in 4U 1728-34. The peak frequency separation between the 'upper' and the 'lower' kHz QPO is 274+/-11 Hz, significantly smaller than the constant value of ~350 Hz found when the 'lower' kHz QPO frequency is between ~500 and 800 Hz. This is the first indication in this source for a significant decrease of kHz QPO peak separation towards low frequencies. We briefly compare the result to theoretical models for kHz QPO production.

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arXiv2003

Simultaneous radio and X-ray observations of the low-mass X-ray binary GX 13+1

We present the results of two simultaneous X-ray/radio observations of the low-mass X-ray binary GX 13+1, performed in July/August 1999 with the Rossi X-ray Timing Explorer and the Very Large Array. In X-rays the source was observed in two distinct spectral states; a soft state, which had a corresponding 6 cm flux density of ~0.25 mJy, and a hard state, which was much brighter at 1.3-7.2 mJy. For the radio bright observation we measured a delay between changes in the X-ray spectral hardness and the radio brightness of ~40 minutes, similar to what has been found in the micro-quasar GRS 1915+105. We compare our results with those of GRS 1915+105 and the atoll/Z-type neutron star X-ray binaries. Although it has some properties that do not match with either atoll or Z sources, GX 13+1 seems more similar to the Z sources.

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arXiv2003

Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar

The relativistic plasma flows onto neutron stars that are accreting material from stellar companions can be used to probe strong-field gravity as well as the physical conditions in the supranuclear-density interiors of neutron stars. Plasma inhomogeneities orbiting a few kilometres above the stars are observable as X-ray brightness fluctuations on the millisecond dynamical timescale of the flows. Two frequencies in the kilohertz range dominate these fluctuations: the twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the origins of these oscillations (based on orbital motions) all predict that they should be related to the stellar spin frequency, but tests have been difficult because the spins were not unambiguously known. Here we report the detection of kHz QPOs from a pulsar whose spin frequency is known. Our measurements establish a clear link between kHz QPOs and stellar spin, but one not predicted by any current model. A new approach to understanding kHz QPOs is now required. We suggest that a resonance between the spin and general relativistic orbital and epicyclic frequencies could provide the observed relation between QPOs and spin.

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arXiv2003

Chandra Observations of the Faintest Low-Mass X-ray Binaries

There exists a group of persistently faint galactic X-ray sources that, based on their location in the galaxy, high L_x/L_opt, association with X-ray bursts, and absence of low frequency X-ray pulsations, are thought to be low-mass X-ray binaries (LMXBs). We present results from Chandra observations for eight of these systems: 4U 1708-408, 2S 1711-339, KS 1739-304, SLX 1735-269, GRS 1736-297, SLX 1746-331, 1E 1746.7-3224, and 4U 1812-12. Locations for all sources, excluding GRS 1736-297, SLX 1746-331, and KS 1739-304 (which were not detected) were improved to 0.6" error circles (90% confidence). Our observations support earlier findings of transient behavior of GRS 1736-297, KS 1739-304, SLX 1746-331, and 2S 1711-339 (which we detect in one of two observations). Energy spectra for 4U 1708-408, 2S 1711-339, SLX 1735-269, 1E 1746.7-3224, and 4U 1812-12 are hard, with power law indices typically 1.4-2.1, which are consistent with typical faint LMXB spectra.

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