Accretion Disks In X-ray Binaries Research Articles

ARTPOL: Analytical ray-tracing method for spectro-polarimetric properties of accretion disks around Kerr black holes

Spectro-polarimetric signatures of accretion disks in X-ray binaries and active galactic nuclei contain information on the masses and spins of their central black holes, as well as the geometry of matter in proximity to the compact objects. This information can be extracted by means of X-ray polarimetry. In this work, we present a fast analytical ray-tracing technique for polarized light (ARTPOL) that helps us to obtain the spinning black hole parameters from the observed properties. This technique can replace the otherwise time-consuming numerical ray-tracing calculations for any optically thick or geometrically thin accretion flow. For the purposes of illustration, we considered a standard optically thick, geometrically thin accretion disk in the equatorial plane of the Kerr black hole. We show that ARTPOL proves accurate for dimensionless spin parameter a ≤ 0.94 with a speed that is over four orders of magnitude faster than direct ray-tracing calculations. This approach opens up broader prospects for direct fittings of the spectro-polarimetric data from the Imaging X-ray Polarimetry Explorer.

Testing models of periodically modulated FRB activity

ABSTRACT The activity of the repeating FRB 20180916B is periodically modulated with a period of 16.3 d, and FRB 121102 may be similarly modulated with a period of about 160 d. In some models of this modulation, the period derivative is insensitive to the uncertain parameters; these models can be tested by measurement of or bounds on the derivative. In other models, values of the uncertain parameters can be constrained. Periodic modulation of aperiodic bursting activity may result from emission by a narrow beam wandering within a cone or funnel along the axis of a precessing disc, such as the accretion discs in X-ray binaries. The production of FRB 200428 by a neutron star that is neither accreting nor in a binary then shows universality: coherent emission occurring in a wide range of circumstances.

Longterm properties of accretion discs in X-ray Binaries - III. A search for spin-superorbital correlation in SMC X-1

Thanks to long-term X-ray monitoring, a number of interacting binaries are now known to show X-ray periodicities on timescales of tens to hundreds of binary orbits. In some systems, precession of a warped accretion disc is the leading model to explain the superorbital modulation. The High Mass X-ray Binary SMC X-1 showed two excursions in superorbital period (from ~60d to ~45d) during the 1996-2011 interval, suggesting that some characteristic of the accretion disc is varying on a timescale of years. Because its behaviour as an X-ray pulsar has also been intensively monitored, SMC X-1 offers the rare chance to track changes in both the accretion disk and pulsar behaviours over the same interval. We have used archival X-ray observations of SMC X-1 to investigate whether the evolution of its superorbital variation and pulse period are correlated. We use the 16-year dataset afforded by the RXTE~All-Sky Monitor to trace the behaviour of the warped accretion disc in this system, and use published pulse-period histories to trace the behaviour of the pulsar. While we cannot claim a strong detection of correlation, the first superorbital period excursion near MJD 50,800 does coincide with structure in SMC X-1's pulse period history. Our preferred interpretation is that the superorbital period excursion coincides with a change in the long-term spin-up rate of the SMC X-1 pulsar. In this scenario, the pulsar and the accretion disc are both responding to a change in the accretion flow, which the disc itself may regulate.

Deterministic Chaos in the X-ray Sources

Hardly any of the observed black hole accretion disks in X-Ray binaries and active galaxies shows constant flux. When the local stochastic variations of the disk occur at specific regions where a resonant behaviour takes place, there appear the Quasi-Periodic Oscillations (QPOs). If the global structure of the flow and its non-linear hydrodynamics affects the fluctuations, the variability is chaotic in the sense of deterministic chaos. Our aim is to solve a problem of the stochastic versus deterministic nature of the black hole binaries vari- ability. We use both observational and analytic methods. We use the recurrence analysis and we study the occurence of long diagonal lines in the recurrence plot of observed data series and compare it to the sur- rogate series. We analyze here the data of two X-Ray binaries - XTE J1550-564, and GX 339-4 observed by Rossi X-ray Timing Explorer. In these sources, the non-linear variability is expected because of the global conditions (such as the mean accretion rate) leading to the pos- sible instability of an accretion disk. The thermal-viscous instability and fluctuations around the fixed-point solution occurs at high accretion rate, when the radiation pressure gives dominant contribution to the stress tensor.

Simulations of magnetized discs around black holes: effects of black hole spin, disc thickness and magnetic field geometry

The standard general relativistic model of a razor-thin accretion disk around a black hole, developed by Novikov & Thorne (NT) in 1973, assumes the shear stress vanishes at the radius of the innermost stable circular orbit (ISCO) and that, outside the ISCO, the shear stress is produced by an effective turbulent viscosity. However, astrophysical accretion disks are not razor-thin, it is uncertain whether the shear stress necessarily vanishes at the ISCO, and the magnetic field, which is thought to drive turbulence in disks, may contain large-scale structures that do not behave like a simple local scalar viscosity. We describe three-dimensional general relativistic magnetohydrodynamic simulations of accretion disks around black holes with a range of spin parameters, and we use the simulations to assess the validity of the NT model. Our fiducial initial magnetic field consists of multiple (alternating polarity) poloidal field loops whose shape is roughly isotropic in the disk in order to match the isotropic turbulence expected in the poloidal plane. For a thin disk with an aspect ratio |h/r| ~ 0.07 around a non-spinning black hole, we find a decrease in the accreted specific angular momentum of 2.9% relative to the NT model and an excess luminosity from inside the ISCO of 3.5%. The deviations in the case of spinning black holes are of the same order. In addition, the deviations decrease with decreasing |h/r|. We therefore conclude that magnetized thin accretion disks in x-ray binaries in the thermal/high-soft spectral state ought to be well-described by the NT model, especially at luminosities below 30% of Eddington where we expect a very small disk thickness |h/r| <~ 0.05. We also discuss how the stress and the luminosity inside the ISCO depend on the assumed initial magnetic field geometry. (abridged)

On the limit-cycle instability in magnetized accretion discs

Observational evidence accumulated over the past decade indicates that accretion discs in X-ray binaries are viscously stable unless they accrete very close to the Eddington limit. This is at odds with the most basic standard accretion disc theory, but could be explained by either having the discs to be much cooler whereby they are not radiation pressure dominated, or by a more sophisticated viscosity law. Here we argue that the latter is taking place in practice, on the basis of a stability analysis that assumes that the magneto-rotational-instability (MRI) responsible for generating the turbulent stresses inside the discs is also the source for a magnetically dominated corona. We show that observations of stable discs in the high/soft states of black hole binaries, on the one hand, and of the strongly variable microquasar GRS 1915+105 on the other, can all be explained if the magnetic turbulent stresses inside the disc scale proportionally to the geometric mean of gas and total pressure with a constant of proportionality (viscosity parameter) having a value of a few times 10^{-2}. Implications for bright AGN are also briefly discussed

Long-term properties of accretion discs in X-ray binaries -- II. Stability of radiation-driven warping

A significant number of X-ray binaries (XRBs) are now known to exhibit long-term 'superorbital' periodicities on time-scales of ∼10-100 d. Several physical mechanisms have been proposed that give rise to such periodicities, in particular warping and/or precession of the accretion disc. Recent theoretical work predicts the stability to disc warping of XRBs as a function of the mass ratio, binary radius, viscosity and accretion efficiency, and here we examine the constraints that can be placed on such models by current observations. In Paper I we used a dynamic power spectrum (DPS) analysis of long-term X-ray data sets (Compton Gamma Ray Observatory, Rossi X-Ray Timing Explorer), focusing on the remarkable, smooth variations in the superorbital period exhibited by SMC X-1. Here we use a similar DPS analysis to investigate the stability of the superorbital periodicities in the neutron star XRBs Cyg X-2, LMC X-4 and Her X-1, and thereby confront stability predictions with observation. We find that the period and nature of superorbital variations in these sources are consistent with the predictions of warping theory. We also use a dynamic light-curve analysis to examine the behaviour of Her X-1 as it enters and leaves the 1999 anomalous low state (ALS). This reveals a significant phase shift some 15 cycles before the ALS, which indicates a change in the disc structure or profile leading into the ALS.

Long-term properties of accretion discs in X-ray binaries -- I. The variable third period in SMC X-1

Long-term X-ray monitoring data from the Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor (ASM) and the Compton Gamma Ray Observatory (CGRO) Burst And Transient Source Experiment (BATSE) reveal that the third (superorbital) period in SMC X-1 is not constant, but varies between 40 and 60 d. A dynamic power spectrum analysis indicates that the third period has been present continuously throughout the five years of ASM observations. This period changed smoothly from 60 to 45 d and then returned to its former value, on a time-scale of approximately 1600 d. During the nearly 4 yr of overlap between the CGRO and RXTE missions, the simultaneous BATSE hard X-ray data confirm and extend this variation in SMC X-1. Our discovery of such an instability in the superorbital period of SMC X-1 is interpreted in the context of recent theoretical studies of warped, precessing accretion discs. We find that the behaviour of SMC X-1 is consistent with a radiation-driven warping model.

Precessing warped accretion discs in X-ray binaries

We study the radiation-driven warping of accretion discs in the context of X-ray binaries. The latest evolutionary equations are adopted, which extend the classical alpha theory to time-dependent thin discs with non-linear warps. We also develop accurate, analytical expressions for the tidal torque and the radiation torque, including self-shadowing. We investigate the possible non-linear dynamics of the system within the framework of bifurcation theory. First, we re-examine the stability of an initially flat disc to the Pringle instability. Then we compute directly the branches of non-linear solutions representing steadily precessing discs. Finally, we determine the stability of the non-linear solutions. Each problem involves only ordinary differential equations, allowing a rapid, accurate and well resolved solution. We find that radiation-driven warping is probably not a common occurrence in low-mass X-ray binaries. We also find that stable, steadily precessing discs exist for a narrow range of parameters close to the stability limit. This could explain why so few systems show clear, repeatable `super-orbital' variations. The best examples of such systems, Her X-1, SS 433 and LMC X-4, all lie close to the stability limit for a reasonable choice of parameters. Systems far from the stability limit, including Cyg X-2, Cen X-3 and SMC X-1, probably experience quasi-periodic or chaotic variability as first noticed by Wijers & Pringle. We show that radiation-driven warping provides a coherent and persuasive framework but that it does not provide a generic explanation for the long-term variabilities in all X-ray binaries.

On the precession of accretion discs in X-ray binaries

In this Letter, recent results on the nodal precession of accretion discs in close binaries are applied to the discs in some X-ray binary systems. The ratio between the tidally forced precession period and the binary orbital period is given, as well as the condition required for the rigid precession of gaseous Keplerian discs. Hence the minimum precessional period that may be supported by a fluid Keplerian disc is determined. It is concluded that near-rigid body precession of tilted accretion discs can occur and generally reproduce observationally inferred precession periods, for reasonable system parameters. In particular, long periods in SS 433, Her X-1, LMC X-4 and SMC X-1 can be fitted by the tidal model. It is also found that the precession period that has been tentatively put forward for Cyg X-2 cannot be accommodated by a tidally precessing disc model for any realistic choice of system parameters.

The Origin of Warped, Precessing Accretion Disks in X-Ray Binaries

The radiation-driven warping instability discovered by Pringle holds considerable promise as the mechanism responsible for producing warped, precessing accretion disks in X-ray binaries. This instability is an inherently global mode of the disk, thereby avoiding the difficulties with earlier models for the precession. Here we follow up on earlier work to study the linear behavior of the instability in the specific context of a binary system. We treat the influence of the companion as an orbit-averaged quadrupole torque on the disk. The presence of this external torque allows the existence of solutions in which the direction of precession of the warp is retrograde with respect to disk rotation, in addition to the prograde solutions that exist in the absence of external torques.

Radiation-Driven Warping: The Origin of Warps and Precession in Accretion Disks

AbstractA geometrically thin, optically thick, warped accretion disk with a central source of luminosity is subject to non-axisymmetric forces due to radiation pressure; the resulting torque acts to modify the warp. Initially planar accretion disks are unstable to warping driven by radiation torque, as shown in a local analysis by Pringle (1996) and a global analysis of the stable and unstable modes by Maloney, Begelman, &amp; Pringle (1996). In general, the warp also precesses.We discuss the nature of this instability, and its possible implications for accretion disks in X-ray binaries and active galactic nuclei. Specifically, we argue that this effect provides a plausible explanation for the misalignment and precession of the accretion disks in X-ray binaries such as SS 433 and Her X–l; the same mechanism explains why the maser disk in NGC 4258 is warped.

Time-dependent X-ray emission from unstable accretion disks around black holes

The spectral evolution of accretion disks in X-ray binaries containing black holes is studied, based on the disk instability model. The thermal transition of the outer portions of the disk controls the mass flow rate into the inner portions of the disk, thus modulating the soft X-ray flux which is thought to arise from the inner disk. Calculated soft X-ray spectra are consistent with the observations of the X-ray transient A0620 - 00 and especially ASM 2000 + 25, the soft X-ray spectra of which are well fitted by blackbody radiation with a fixed inner edge of the disk, Rin, and with monotonically decreasing temperature at Rin with time. Since the gas pressure is always dominant over the radiation pressure during the decay in these models, a two-temperature region is difficult to create. Instead, it is suggested that hard X-rays are generated in a hot (kT greater than 10 keV) accretion disk corona above the cool (kT less than 1 keV) disk.

The X-ray spectral properties of accretion discs in X-ray binaries

view Abstract Citations (271) References (73) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The X-Ray Spectral Properties of Accretion Disks in X-Ray Binaries White, N. E. ; Stella, L. ; Parmar, A. N. Abstract Using EXOSAT observations, the spectral properties of the persistent emission from a number of X-ray burst sources, high-luminosity low-mass X-ray binaries (LMXRB), and galactic black hole candidates are compared with various models for X-ray emission from an accretion disk surrounding a compact object in a binary system. These models include the optically thick blackbody disk model considered by Mitsuda et al. (1984), two different disk models where the blackbody spectrum is modified by electron scattering (Shakura and Sunyaev 1973; Nannurelli, Ruffini, and Stella 1987), and a model (not yet specific to a disk geometry) where losses by the Comptonization of cool photons on hot electrons dominates the spectral formation (Sunyaev and Titarchuk 1980). We note that for accretion onto a neutron star or black hole the simple blackbody accretion disk model is not correct and that it must be modified to take into account the effects of electron scattering. Disks that emit locally as a modified blackbody require super-Eddington mass accretion rates to give high enough temperatures to account for the spectra observed from the LMXRB. Such high accretion rates are not self-consistent with the model. For the persistent emission from the lower luminosity X-ray burst sources, the modified blackbody disk models predict spectra that are much cooler than required. Only a Comptonization model provides a good fit to all the spectra considered. The spectrum of the black hole candidate LMC X-3 could be modeled equally well by either a modified blackbody disk spectrum, or by a Comptonized spectrum. The fits to the spectra of the high-luminosity LMXRB systems require an additional blackbody component with a luminosity 16% to 34% that from the Comptonized component. Blackbody emission was detected only in one out of five X-ray burst sources, with an upper limit in one case as low as 5%. No evidence was found for any dependence of the spectrum of the X-ray burst sources on system inclination. The blackbody component, when detected, may originate from an optically thick boundary layer between the accretion disk and the neutron star surface. However, the luminosity relative to the Comptonized component (assuming that the latter comes from the accretion disk) is at least a factor of ~10 less than expected. This suggests either that the neutron star in each system rotates close to equilibrium with the Keplerian disk, or that most of the boundary layer emission is not well represented by a blackbody spectrum. Publication: The Astrophysical Journal Pub Date: January 1988 DOI: 10.1086/165901 Bibcode: 1988ApJ...324..363W Keywords: Accretion Disks; Neutron Stars; Spaceborne Astronomy; Stellar Spectrophotometry; X Ray Binaries; X Ray Spectra; Black Body Radiation; Black Holes (Astronomy); Compton Effect; Emission Spectra; Molecular Clouds; Stellar Models; Astrophysics; BLACK HOLES; STARS: ACCRETION; STARS: NEUTRON; X-RAYS: BINARIES; X-RAYS: SPECTRA full text sources ADS | data products SIMBAD (17)

Radiation Pressure Driven Mass Loss from Accretion Discs in X-ray Binaries

Radiation Pressure Driven Mass Loss from Accretion Discs in X-ray Binaries