arXiv subjects

M. van der Klis

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

At least 19 recordsLinked to original sources

arXiv2024

Magnetospheric Flows in X-ray Pulsars I: Instability at super-Eddington regime of accretion

Within the magnetospheric radius, the geometry of accretion flow in X-ray pulsars is shaped by a strong magnetic field of a neutron star. Starting at the magnetospheric radius, accretion flow follows field lines and reaches the stellar surface in small regions located close to the magnetic poles of a star. At low mass accretion rates, the dynamic of the flow is determined by gravitational attraction and rotation of the magnetosphere due to the centrifugal force. At the luminosity range close to the Eddington limit and above it, the flow is additionally affected by the radiative force. We construct a model simulating accretion flow dynamics over the magnetosphere, assuming that the flow strictly follows field lines and is affected by gravity, radiative and centrifugal forces only. The magnetic field of a NS is taken to be dominated by the dipole component of arbitrary inclination with respect to the accretion disc plane. We show that accretion flow becomes unstable at high mass accretion rates and tends to fluctuate quasi-periodically with a typical period comparable to the free-fall time from the inner disc radius. The inclination of a magnetic dipole with respect to the disc plane and strong anisotropy of X-ray radiation stabilise the mass accretion rate at the poles of a star, but the surface density of material covering the magnetosphere fluctuates even in this case.

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arXiv2023

A Phase Lag between Disk and Corona in GRMHD Simulations of Precessing Tilted Accretion Disks

In the course of its evolution, a black hole (BH) accretes gas from a wide range of directions. Given a random accretion event, the typical angular momentum of an accretion disc would be tilted by $\sim$60$^\circ$ relative to the BH spin. Misalignment causes the disc to precess at a rate that increases with BH spin and tilt angle. We present the first general-relativistic magnetohydrodynamic (GRMHD) simulations spanning a full precession period of highly tilted (60$^\circ$), moderately thin ($h/r=0.1$) accretion discs around a rapidly spinning ($a\simeq0.9$) BH. While the disc and jets precess in phase, we find that the corona, sandwiched between the two, lags behind by $\gtrsim 10^{\circ}$. For spectral models of BH accretion, the implication is that hard non-thermal (corona) emission lags behind the softer (disc) emission, thus potentially explaining some properties of the hard energy lags seen in Type-C low frequency quasi-periodic oscillations in X-Ray binaries. While strong jets are unaffected by this disc-corona lag, weak jets stall when encountering the lagging corona at distances $r \sim 100$ black hole radii. This interaction may quench large-scale jet formation.

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arXiv2022

H-AMR: A New GPU-accelerated GRMHD Code for Exascale Computing With 3D Adaptive Mesh Refinement and Local Adaptive Time-stepping

General-relativistic magnetohydrodynamic (GRMHD) simulations have revolutionized our understanding of black hole accretion. Here, we present a graphics processing unit (GPU) accelerated GRMHD code \hammer{} with multi-faceted optimizations that, collectively, accelerate computation by 2-5 orders of magnitude for a wide range of applications. Firstly, it introduces a spherical grid with 3D adaptive mesh refinement that operates in each of the 3 dimensions independently. This allows us to circumvent the Courant condition near the polar singularity, which otherwise cripples high-resolution computational performance. Secondly, we demonstrate that local adaptive time-stepping (LAT) on a logarithmic spherical-polar grid accelerates computation by a factor of $\lesssim10$ compared to traditional hierarchical time-stepping approaches. Jointly, these unique features lead to an effective speed of $\sim10^9$ zone-cycles-per-second-per-node on 5,400 NVIDIA V100 GPUs (i.e., 900 nodes of the OLCF Summit supercomputer). We illustrate \hammer{}'s computational performance by presenting the first GRMHD simulation of a tilted thin accretion disk threaded by a toroidal magnetic field around a rapidly spinning black hole. With an effective resolution of $13$,$440\times4$,$608\times8$,$092$ cells, and a total of $\lesssim22$ billion cells and $\sim0.65\times10^8$ timesteps, it is among the largest astrophysical simulations ever performed. We find that frame-dragging by the black hole tears up the disk into two independently precessing sub-disks. The innermost sub-disk rotation axis intermittently aligns with the black hole spin, demonstrating for the first time that such long-sought alignment is possible in the absence of large-scale poloidal magnetic fields.

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arXiv2022

Disc Tearing and Bardeen-Petterson Alignment in GRMHD Simulations of Highly Tilted Thin Accretion Discs

Luminous active galactic nuclei (AGN) and X-Ray binaries (XRBs) often contain geometrically thin, radiatively cooled accretion discs. According to theory, these are -- in many cases -- initially highly misaligned with the black hole equator. In this work, we present the first general relativistic magnetohydrodynamic simulations of very thin (h/r~0.015-0.05) accretion discs around rapidly spinning (a~0.9) black holes and tilted by 45-65 degrees. We show that the inner regions of the discs with h/r<0.03 align with the black hole equator, though out to smaller radii than predicted by analytic work. The inner aligned and outer misaligned disc regions are separated by a sharp break in tilt angle accompanied by a sharp drop in density. We find that frame-dragging by the spinning black hole overpowers the disc viscosity, which is self-consistently produced by magnetized turbulence, tearing the disc apart and forming a rapidly precessing inner sub-disc surrounded by a slowly precessing outer sub-disc. We find that the system produces a pair of relativistic jets for all initial tilt values. At small distances the black hole launched jets precess rapidly together with the inner sub-disc, whereas at large distances they partially align with the outer sub-disc and precess more slowly. If the tearing radius can be modeled accurately in future work, emission model independent measurements of black hole spin based on precession-driven quasi-periodic oscillations may become possible.

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arXiv2022

Bardeen-Petterson Alignment, Jets and Magnetic Truncation in GRMHD Simulations of Tilted Thin Accretion Discs

Prevalent around luminous accreting black holes, thin discs are challenging to resolve in numerical simulations. When the disc and black hole angular momentum vectors are misaligned, the challenge becomes extreme, requiring adaptive meshes to follow the disc proper as it moves through the computational grid. With our new high-performance general relativistic magnetohydrodynamic (GRMHD) code H-AMR we have simulated the thinnest accretion disc to date, of aspect ratio H/R~0.03, around a rapidly spinning (a=0.9375) black hole, using a cooling function. Initially tilted at 10 degrees, the disc warps inside r~5 r_g into alignment with the black hole, where r_g is the gravitational radius. This is the first demonstration of Bardeen-Petterson alignment in MHD with viscosity self-consistently generated by magnetized turbulence. The disc develops a low-density high-viscosity (alpha_eff ~ 1.0) magnetic-pressure--dominated inner region at r<25 r_g that rapidly empties itself into the black hole. This inner region may in reality, due to thermal decoupling of ions and electrons, evaporate into a radiatively inefficient accretion flow if, as we propose, the cooling time exceeds the accretion time set by the order unity effective viscosity. We furthermore find the unexpected result that even our very thin disc can sustain large-scale vertical magnetic flux on the black hole, which launches powerful relativistic jets that carry 20-50% of the accretion power along the angular momentum vector of the outer tilted disc, providing a potential explanation for the origin of jets in radio-loud quasars.

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arXiv2022

Disk Tearing Leads to Low and High Frequency Quasi Periodic Oscillations in a GRMHD Simulation of a Thin Accretion Disk

Black hole X-ray binaries (BHXRBs) display a wide range of variability phenomena, from long duration spectral state changes to short-term broadband variability and quasi-periodic oscillations (QPOs). A particularly puzzling aspect is the production of QPOs, which -- if properly understood -- could be used as a powerful diagnostic tool of black hole accretion and evolution. In this work we analyse a high resolution three-dimensional general relativistic magnetohydrodynamic simulation of a geometrically thin accretion disk which is tilted by $65^{\circ}$ with respect to the black hole spin axis. We find that the Lense-Thirring torque from the rapidly spinning 10 $M_\odot$ black hole causes several sub-disks to tear off within $\sim 10-20$ gravitational radii. Tearing occurs in cycles on timescales of seconds. During each tearing cycle the inner sub-disk precesses for 1-5 periods before it falls into the black hole. We find a precession frequency of $\sim 3\rm Hz$, consistent with observed low-frequency QPOs. In addition, we find a high frequency QPO (HFQPO) with centroid frequency of $\sim55$Hz in the power spectra of the mass-weighted radius of the inner disk. This signal is caused by radial epicyclic oscillations of a dense ring of gas at the tearing radius, which strongly suggests a corresponding modulation of the X-ray lightcurve and may thus explain some of the observed HFQPOs.

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arXiv2017

Modelling hard and soft state of Cygnus X-1 with propagating mass accretion rate fluctuations

We present a timing analysis of threeRossi X-ray Timing Explorer observations of the black hole binary Cygnus X-1 with the propagating mass accretion rate fluctuations model PROPFLUC. The model simultaneously predicts power spectra, time lags, and coherence of the variability as a function of energy. The observations cover the soft and the hard state of the source, and the transition between the two. We find good agreement between model pre-dictions and data in the hard and in the soft state. Our analysis suggests that in the soft state the fluctuations propagate in an optically thin hot flow extending up to large radii above and below a stable optically thick disc. In the hard state, our results are consistent with a truncated disc geometry, where the hot flow extends radially inside the inner radius of the disc. In the transition from soft to hard state, the characteristics of the rapid variability are too complex to be successfully described with PROPFLUC. The surface density profile of the hot flow predicted by our model and the lack of QPOs in the soft and hard state, suggest that the spin of the black hole is aligned with the inner accretion disc and therefore probably with the rotational axis of the binary system.

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arXiv2017

Cross-spectral modelling of the black hole X-ray binary XTEJ1550-564: challenges to the propagating fluctuations paradigm

Timing properties of black hole X-ray binaries in outburst can be modeled with mass accretion rate fluctuations propagating towards the black hole. Such models predict time lags between energy bands due to propagation delays. First application of a propagating fluctuations model to black hole power spectra showed good agreement with the data. Indeed, hard lags observed from these systems appear to be in agreement with this generic prediction. Our PROPFLUC code allows to simultaneously predict power spectra, time lags, and coherence of the variability as a function of energy. This was successfully applied to Swift data on the black hole MAXIJ1659-152, fitting jointly the power spectra in two energy bands and the cross-spectrum between these two bands. In the current work, we attempt to to model two high signal to noise Rossi X-ray Timing Explorer (RXTE) observations of the black hole XTE J1550-564. We find that neither observation can be adequately explained by the model even when considering, additionally to previous PROPFLUC versions, different propagation speeds of the fluctuations. After extensive exploration of model extensions, we tentatively conclude that the quantitative and qualitative discrepancy between model predictions and data is generic to the propagating fluctuations paradigm. This result encourages further investigation of the fundamental hypotheses of the propagating fluctuations model. We discuss some of these hypotheses with an eye to future works.

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arXiv2017

Detection of Very Low-Frequency Quasi-Periodic Oscillations in the 2015 Outburst of V404 Cygni

In June 2015, the black hole X-ray binary (BHXRB) V404 Cygni went into outburst for the first time since 1989. Here, we present a comprehensive search for quasi-periodic oscillations (QPOs) of V404 Cygni during its recent outburst, utilizing data from six instruments on board five different X-ray missions: Swift/XRT, Fermi/GBM, Chandra/ACIS, INTEGRAL's IBIS/ISGRI and JEM-X, and NuSTAR. We report the detection of a QPO at 18 mHz simultaneously with both Fermi/GBM and Swift/XRT, another example of a rare but slowly growing new class of mHz-QPOs in BHXRBs linked to sources with a high orbital inclination. Additionally, we find a duo of QPOs in a Chandra/ACIS observation at 73 mHz and 1.03 Hz, as well as a QPO at 136 mHz in a single Swift/XRT observation that can be interpreted as standard Type-C QPOs. Aside from the detected QPOs, there is significant structure in the broadband power, with a strong feature observable in the Chandra observations between 0.1 and 1 Hz. We discuss our results in the context of current models for QPO formation.

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arXiv2016

Modelling the cross-spectral variability of the black hole binary MAXI J1659-152 with propagating accretion rate fluctuations

The power spectrum of the X-ray fluctuations of accreting black holes often consists of two broad humps. We quantitatively investigate the hypothesis that the lower frequency hump orig- inates from variability in a truncated thin accretion disc, propagating into a large scale-height inner hot flow which, in turn, itself is the origin of the higher frequency hump. We extend the propagating mass accretion rate fluctuations model PROPFLUC to accommodate double hump power spectra in this way. Furthermore, we extend the model to predict the cross-spectrum between two energy bands in addition to their power spectra, allowing us to constrain the model using the observed time lags, which in the model result from both propagation of fluc- tuations from the disc to the hot flow, and inside the hot flow. We jointly fit soft and hard power spectrum, and the cross-spectrum between the two bands using this model for 5 Swift X-ray Telescope observations of MAXI J1659-152. The new double hump model provides a better fit to the data than the old single hump model for most of our observations. The data show only a small phase lag associated with the low frequency hump. We demonstrate quan- titatively that this is consistent with the model. We compare the truncation radius measured from our fits with that measured purely by spectral fitting and find agreement within a factor of two. This analysis encompasses the first joint fits of stellar-mass black hole cross-spectra and power spectra with a single self-consistent physical model.

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arXiv2016

eXTP -- enhanced X-ray Timing and Polarimetry Mission

eXTP is a science mission designed to study the state of matter under extreme conditions of density, gravity and magnetism. Primary targets include isolated and binary neutron stars, strong magnetic field systems like magnetars, and stellar-mass and supermassive black holes. The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV (and beyond). Key elements of the payload are: the Spectroscopic Focusing Array (SFA) - a set of 11 X-ray optics for a total effective area of about 0.9 m^2 and 0.6 m^2 at 2 keV and 6 keV respectively, equipped with Silicon Drift Detectors offering <180 eV spectral resolution; the Large Area Detector (LAD) - a deployable set of 640 Silicon Drift Detectors, for a total effective area of about 3.4 m^2, between 6 and 10 keV, and spectral resolution <250 eV; the Polarimetry Focusing Array (PFA) - a set of 2 X-ray telescope, for a total effective area of 250 cm^2 at 2 keV, equipped with imaging gas pixel photoelectric polarimeters; the Wide Field Monitor (WFM) - a set of 3 coded mask wide field units, equipped with position-sensitive Silicon Drift Detectors, each covering a 90 degrees x 90 degrees FoV. The eXTP international consortium includes mostly major institutions of the Chinese Academy of Sciences and Universities in China, as well as major institutions in several European countries and the United States. The predecessor of eXTP, the XTP mission concept, has been selected and funded as one of the so-called background missions in the Strategic Priority Space Science Program of the Chinese Academy of Sciences since 2011. The strong European participation has significantly enhanced the scientific capabilities of eXTP. The planned launch date of the mission is earlier than 2025.

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arXiv2016

Detection of the first infra-red quasi periodic oscillation in a black hole X-ray binary

We present analysis of fast variability of Very Large Telescope/ISAAC (infra-red), \textit{XMM-Newton}/OM (optical) and EPIC-pn (X-ray), and RXTE/PCA (X-ray) observations of the black hole X-ray binary GX 339-4 in a rising hard state of its outburst in 2010. We report the first detection of a Quasi Periodic Oscillation (QPO) in the infra-red band (IR) of a black hole X-ray binary. The QPO is detected at 0.08 Hz in the IR as well as two optical bands (U and V). Interestingly, these QPOs are at half the X-ray QPO frequency at 0.16 Hz, which is classified as the type-C QPO; a weak sub-harmonic close to the IR and optical QPO frequency is also detected in X-rays. The broad band sub-second time scale variability is strongly correlated in IR/X-ray bands, with X-rays leading the IR by over 100 ms. This short time delay, shape of the cross correlation function and spectral energy distribution strongly indicate that this broad band variable IR emission is the synchrotron emission from the jet. A jet origin for the IR QPO is strongly favoured, but cannot be definitively established with the current data. The spectral energy distribution indicates a thermal disc origin for the bulk of the optical emission, but the origin of the optical QPO is unclear. We discuss our findings in the context of the existing models proposed to explain the origin of variability.

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arXiv2015

Swift X-ray Telescope study of the Black Hole Binary MAXI J1659-152: Variability from a two component accretion flow

We present an energy dependent X-ray variability study of the 2010 outburst of the black hole X-ray binary MAXI J1659-152 with the Swift X-ray Telescope (XRT). The broad-band noise components and the quasi periodic oscillations (QPO) observed in the power spectra show a strong and varied energy dependence. Combining Swift XRT data with data from the Rossi X-ray Timing Explorer, we report, for the first time, an rms spectrum (fractional rms amplitude as a function of energy) of these components in the 0.5-30 keV energy range. We find that the strength of the low-frequency component (< 0.1 Hz) decreases with energy, contrary to the higher frequency components (> 0.1 Hz) whose strengths increase with energy. In the context of the propagating fluctuations model for X-ray variability, we suggest that the low-frequency component originates in the accretion disk (which dominates emission below ~ 2 keV) and the higher frequency components are formed in the hot flow (which dominates emission above ~ 2 keV). As the properties of the QPO suggest that it may have a different driving mechanism, we investigate the Lense-Thirring precession of the hot flow as a candidate model. We also report on the QPO coherence evolution for the first time in the energy band below 2 keV. While there are strong indications that the QPO is less coherent at energies below 2 keV than above 2 keV, the coherence increases with intensity similar to what is observed at energies above 2 keV in other black-hole X-ray binaries.

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arXiv2015

The Five Year Fermi/GBM Magnetar Burst Catalog

Since launch in 2008, the Fermi Gamma-ray Burst Monitor (GBM) has detected many hundreds of bursts from magnetar sources. While the vast majority of these bursts have been attributed to several known magnetars, there is also a small sample of magnetar-like bursts of unknown origin. Here we present the Fermi/GBM magnetar catalog, giving the results of the temporal and spectral analyses of 440 magnetar bursts with high temporal and spectral resolution. This catalog covers the first five years of GBM magnetar observations, from July 2008 to June 2013. We provide durations, spectral parameters for various models, fluences and peak fluxes for all the bursts, as well as a detailed temporal analysis for SGR J1550-5418 bursts. Finally, we suggest that some of the bursts of unknown origin are associated with the newly discovered magnetar 3XMM J185246.6+0033.7.

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arXiv2015

Are spectral and timing correlations similar in different spectral states in black hole X-ray binaries?

We study the outbursts of the black hole X-ray binaries MAXI J1659-152, SWIFT J1753.5--0127 and GX 339-4 with the Swift X-ray Telescope. The bandpass of the X-ray Telescope has access to emission from both components of the accretion flow: the accretion disk and the corona/hot flow. This allows a correlated spectral and variability study, with variability from both components of the accretion flow. We present for the first time, a combined study of the evolution of spectral parameters (disk temperature and radius) and timing parameters (frequency and strength) of all power spectral components in different spectral states. Comparison of the correlations in different spectral states shows that the frequency and strength of the power spectral components exhibit dependencies on the disk temperature that are different in the (low-)hard and the hard-intermediate states; most of these correlations that are clearly observed in the hard-intermediate state (in MAXI J1659-152 and GX 339-4) are not seen in the (low-)hard state (in GX 339-4 and SWIFT J1753.5-0127). Also, the responses of the individual frequency components to changes in the disk temperature are markedly different from one component to the next. Hence, the spectral-timing evolution cannot be explained by a single correlation that spans both these spectral states. We discuss our findings in the context of the existing models proposed to explain the origin of variability.

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arXiv2015

Binary evolution with LOFT

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of very faint X-ray binaries, orbital period distribution of black hole X-ray binaries and neutron star spin up. For a summary, we refer to the paper.

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arXiv2014

The Large Observatory For x-ray Timing

The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supra-nuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m 2 effective area, 2-30 keV, 240 eV spectral resolution, 1 deg collimated field of view) and a WideField Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study.

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arXiv2014

Evolution of the hot flow of MAXI J1543-564

We present a spectral and timing analysis of the black hole candidate MAXI J1543-564 during its 2011 outburst. As shown in previous work, the source follows the standard evolution of a black hole outburst. During the rising phase of the outburst we detect an abrupt change in timing behavior associated with the occurrence of a type-B quasi-periodic oscillation (QPO). This QPO and the simultaneously detected radio emission mark the transition between hard and soft intermediate state. We fit power spectra from the rising phase of the outburst using the recently proposed model propfluc. This assumes a truncated disc / hot inner flow geometry, with mass accretion rate fluctuations propagating through a precessing inner flow. We link the propfluc physical parameters to the phenomenological multi-Lorentzian fit parameters. The physical parameter dominating the QPO frequency is the truncation radius, while broad band noise characteristics are also influenced by the radial surface density and emissivity profiles of the flow. In the outburst rise we found that the truncation radius decreases from $r_o \sim 24$ to $10 R_g$, and the surface density increases faster than the mass accretion rate, as previously reported for XTE J1550-564. Two soft intermediate state observations could not be fitted with propfluc, and we suggest that they are coincident with the ejection of material from the inner regions of the flow in a jet or accretion of these regions into the BH horizon, explaining the drop in QPO frequency and suppression of broad band variability preferentially at high energy bands coincident with a radio flare.

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