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Most of the X-ray emission from luminous accreting black holes emerges from within 20 gravitational radii. The effective emission radius is several times smaller if the black hole is rapidly spinning. General Relativistic effects can then be very important. Large spacetime curvature causes strong lightbending and large gravitational redshifts. The hard X-ray, power-law-emitting corona irradiates the accretion disc generating an X-ray reflection component. Atomic features in the reflection spectrum allow gravitational redshifts to be measured. Time delays between observed variations in the power-law and the reflection spectrum (reverberation) enable the physical scale of the reflecting region to be determined. The relative strength of the reflection and power-law continuum depends on light bending. All of these observed effects enable the immediate environment of the black hole where the effects of General Relativity are on display to be probed and explored.
Ultra-Luminous X-ray sources are accreting black holes that might represent strong evidence of the Intermediate Mass Black Holes (IMBH), proposed to exist by theoretical studies but with no firm detection (as a class) so far. We analyze the best X-ray timing and spectral data from the ULX in NGC 5408 provided by XMM-Newton. The main goal is to study the broad-band noise variability of the source. We found an anti-correlation of the fractional root-mean square variability versus the intensity of the source, similar to black-hole binaries during hard states.
The microquasar GX 339-4 experienced an outburst in 2010. We focus on observations that are quasi-simultaneous with those made by INTEGRAL and RXTE in March–April 2010 with radio, NIR, optical and UV data. X-ray transients are extreme systems, often harboring a black hole, known to emit throughout the whole electromagnetic spectrum in outburst. We studied the source evolution and correlated changes in all wavelengths. The bolometric flux increased from 0.8 to 2.9 × 10−8 erg cm−2 s−1 while the relative contribution of the hot medium decreased. The radio, NIR and optical emission from jets was detected and observed to fade as the source softened; reprocessing in the disc was strong at the end.
The most characteristic property of active galaxies, including quasars, are prominent broad emission lines. I will discuss an interesting possibility that dust is responsible for this phenomenon. The dust is known to be present in quasars in the form of a dusty/molecular torus which results in complexity of the appearance of active galaxies. However, this dust is located further from the black hole than the Broad Line Region. We propose that the dust is present also closer in and it is actually responsible for formation of the broad emission lines. The argument is based on determination of the temperature of the disk atmosphere underlying the Broad Line Region: it is close to 1000 K, independently from the black hole mass and accretion rate of the object. The mechanism is simple and universal but leads to a considerable complexity of the active nucleus surrounding. The understanding the formation of BLR opens a way to use it reliably - in combination with reverberation measurement of its size - as standard candles in cosmology.
We discuss the variability of winds in two low-mass X-ray binaries, GX 13+1 and 4U 1630-47. XMM-Newton observations of these sources show that strong, photoionised winds with column densities well above 1022 cm−2 can be present in both neutron star and black hole LMXBs. Such winds can fade significantly due to changes in the flux or spectral hardness of the continuum. In particular, a decrease of column density and/or an increase of ionisation are measured when the flux increases or the spectrum hardens. We show how variability studies can help us to understand what triggers changes in the wind and discuss the limitations of current instruments for such studies.
Recent observational evidence suggests the existence of two tracks in the radio-X-ray relation for X-ray binaries. Claims have also been made for deviations from the so-called fundamental plane of black hole activity due to the influence of radiative cooling on synchrotron emission from jets and the relative importance of disk and jet emission. In addition, cases of strongly boosted classes of objects, such as BL Lacs, show evidence for jet emission in their location relative to the fundamental plane. In light of the recent literature activity discussing these issues, we revisit the scaling relations expected for synchrotron emission from jet cores. We review the set of scaling laws expected for different types of emission and discuss their relevance to the new observational data, and the conditions under which breaks in the observed scaling relations should be expected. None of the canonical cases offer a satisfactory explanation for the best fit slope of the steep branch of the radio-X-ray relation in hard-state X-ray binaries.
We have performed a uniform analysis of the power spectrum densities (PSDs) of 104 nearby (z<0.4) active galactic nuclei (AGN) using 209 XMM-Newton/pn observations, including several AGN classes. These PSDs span ≃ 3 decades in temporal frequencies, ranging from minutes to days. We have fitted each PSD to two models: (1) a single power-law model and (2) a bending power-law model. A fraction of 72% show significant variability. The PSD of the majority of the variable AGN was well described by a simple power-law with a mean index of α = 2.01±0.01. In 15 sources we found that the bending power law model was preferred with a mean slope of α = 3.08±0.04 and a mean bend frequency of 〈νb〉 ≃ 2 × 10−4 Hz. Only KUG 1031+398 (RE J1034+396) shows evidence for quasi-periodic oscillations. The ‘fundamental plane’ relating variability timescale, black hole mass, and luminosity is demonstrated using the new X-ray timing results presented here together with a compilation of the previously detected timescales from the literature.
Active galactic nuclei (AGN) are powered by energetic phenomena which cannot be attributed to stars. LINERs appears to be the low power end of the AGN sequence with the lowest Eddington ratios but hosting the most massive black holes (typically 109 M⊙). Whereas variability is common for high Eddington ratio emitting sources, in the low Eddington regime data are not so clear. Recent investigations at UV (Maoz et al. 2005) and X-ray frequencies (Younes et al. 2011, González-Martín et al. 2011) point out to the long term variable nature for some particular LINERs.
In this work we add more evidence about the X-ray variability in LINERs and investigate its nature. The data set comprises X-ray archival spectroscopy from observations taken from Chandra and XMM-Newton, selecting LINERs with several observations at different epochs. Up to now we analysed two LINER 1.9 objects, NGC 1052 and NGC 4278, and checked that variability is due to different mechanisms based on the results of spectral fittings.
A major uncertainty in models for photoionised outflows in AGN is the distance of the gas to the central black hole. We present the results of a massive multiwavelength monitoring campaign on the bright Seyfert 1 galaxy Mrk 509 to constrain the location of the outflow components dominating the soft X-ray band. Mrk 509 was monitored by XMM-Newton, Integral, Chandra, HST/COS and Swift in 2009. We have studied the response of the photoionised gas to the changes in the ionising flux produced by the central regions. We were able to put tight constraints on the variability of the absorbers from day to year time scales. This allowed us to develop a model for the time-dependent photoionisation in this source. We find that the more highly ionised gas producing most X-ray line opacity is at least 5 pc away from the core; upper limits to the distance of various absorbing components range between 20 pc up to a few kpc. The more lowly ionised gas producing most UV line opacity is at least 100 pc away from the nucleus. These results point to an origin of the dominant, slow (v<1000 km s−1) outflow components in the NLR or torus-region of Mrk 509. We find that while the kinetic luminosity of the outflow is small, the mass carried away is likely larger than the 0.5 Solar mass per year accreting onto the black hole. We also determined the chemical composition of the outflow as well as valuable constraints on the different emission regions. We find for instance that the resolved component of the Fe-K line originates from a region 40–1000 gravitational radii from the black hole, and that the soft excess is produced by Comptonisation in a warm (0.2–1 keV), optically thick (τ~ 10–20) corona near the inner part of the disk.
A sudden increase in stellar luminosity may lead to the ejection of a large part of any optically thin gas orbiting the star. Test particles in circular orbits will become unbound, and will escape to infinity (if radiation drag is neglected), when the luminosity changes from zero to at least one half the Eddington value, or more generally, from L to (LEdd+L)/2 or more. Conversely, a decrease in luminosity will lead to the tightening of orbits of optically thin fluid. Even a modest fluctuation of luminosity of accreting neutron stars or black holes is expected to lead to substantial coronal heating. Luminosity fluctuations may thus account for the high temperatures of the X-ray corona in accreting black hole and neutron star systems.
The luminous accretion flares from tidally disrupted stars represent a powerful probe of the presence of supermassive black holes (SMBHs) in otherwise non-active galaxies, of accretion physics and BH spin, of jet formation, and relativistic effects. Further, the reprocessing of the continuum radiation of the flare into IR, optical and UV emission lines provides us with multiple new diagnostics of the properties of any gaseous material in the vicinity of the SMBH and in the host galaxy itself. While first events were discovered in the course of the ROSAT all-sky survey in X-rays, the last few years have seen the detection of several more flares, including in the UV, optical and radio band and via their emission-line “echoes”. A wealth of applications will become feasible in upcoming years, once flares are detected in large numbers in current and future sky surveys.
The physical origin of high-frequency QPOs (HFQPOs) in black-hole X-ray binaries remains an enigma despite many years of detailed observational studies. Although there exists a number of models for HFQPOs, many of these are simply “notions” or “concepts” without actual calculation derived from fluid or disk physics. Future progress requires a combination of numerical simulations and semi-analytic studies to extract physical insights. We review recent works on global oscillation modes in black-hole accretion disks, and explain how, with the help of general relativistic effects, the energy stored in the disk differential rotation can be pumped into global spiral density modes in the disk, making these modes grow to large amplitudes under certain conditions (“corotational instability”). These modes are robust in the presence of disk magnetic fields and turbulence. The computed oscillation mode frequencies are largely consistent with the observed values for HFQPOs in BH X-ray binaries. The approximate 2:3 frequency ratio is also expected from this model. The connection of HFQPOs with other disk properties (such as production of episodic jets) is also discussed.
The interaction between a hot corona and a cold thin disk can drive cold gas evaporating into the corona or hot gas condensing into the thin disk. The evaporation is caused by heat conduction downwards to the disk; while condensation is caused by overcooling of corona gas through inverse Compton scattering. Evaporation occurs at low accretion rates when the corona cannot efficiently radiate the viscous heat, thereby part of which is conducted down and heats up gas. Condensation occurs at high accretion rates when the Compton cooling of disk photons is strong enough to efficiently cool the corona gas. An important consequence of the evaporation is complete removal of the disk at a certain distances. In contrast, condensation can lead to a weak corona or complete collapse of the corona. This causes the observed transitions between various spectral states, at which the accretion flows are dominated respectively by ADAF, disk+ corona, and thin disk from low to high accretion rates, providing a natural explanation to the low, intermediate and high spectral states in BHXRBs, as well as their transitions. The same process can also be applied to accretion around supermassive black holes.
The emission of steady compact jets observed in the hard spectral state of X-ray binaries is likely to be powered by internal shocks caused by fluctuations of the outflow velocity. The dynamics of the internal shocks and the resulting spectral energy distribution (SED) of the jet is very sensitive to the shape of the Power Spectral Density (PSD) of the fluctuations of the jet Lorentz factor. I use Monte-Carlo simulations to investigate this dependence. It turns out that Lorentz factor fluctuations injected at the base of the jet with a flicker noise power spectrum (i.e. P(f) ∝ 1/f) naturally produce the canonical flat SED observed from radio to IR band in X-ray binary systems in the hard state. This model also predicts a strong, wavelength dependent, variability that resembles the observed one. In particular, strong sub-second variability is predicted in the infrared and optical bands.
The durations (T90) of 315 GRBs detected with Fermi/GBM (8-1000 keV) by 2011 September are calculated using the Bayesian Block method. We compare the T90 distributions between this sample and that observed with previous/current GRB missions. We show that T90 is energy-band dependent and the observed bimodal T90 distribution would be due to the instrumental selection effect.
The unmatched X-ray resolution of Chandra allows probing the gas flow near quiescent supermassive black holes (BHs). The radius of BH gravitational influence on gas, called the Bondi radius, is resolved in Sgr A* and NGC 3115. Shallow accretion flow density profiles n ∝ r−β with β=0.7–1.0 were found for Sgr A* and NGC 3115 with the help of Chandra. We construct self-consistent models with gas feeding and dynamics from near the Bondi radius to the event horizon to explain the observations. Gas is mainly supplied to the region by hot colliding stellar winds. Small-scale feedback such as conduction effectively flattens the density profile from steep β=1.5 in a Bondi flow. We further constrain density and temperature profiles using the observed radio/sub-mm radiation emitted near the event horizon. We discuss the present state of our numerical model and its qualitative features, such as the role of the galactic gravitational potential and the random motion of wind-emitting stars.
We estimate the outer radius of the accretion disk in HLX-1 from its optical brightness and from the exponential timescale of the decline in the X-ray lightcurve after an outburst. We find that the disk is an order of magnitude smaller than the semimajor axis of the orbit. If the disk size is determined by the circularization radius near periastron, the eccentricity of the binary system must be ≳ 0.95. We report on the discovery of Hα emission during the 2012 outburst, with a single-peaked, narrow profile (consistent with a nearly face-on view), and a central velocity displaced by ≈ 490 km s−1 from that of the host galaxy.
We solve the set of hydrodynamic equations for accretion disks in the spherical coordinates (rθφ) to obtain the explicit structure along the θ direction. The results display thinner, quasi-Keplerian disks for Shakura-Sunyaev Disks (SSDs) and thicker, sub-Keplerian disks for Advection Dominated Accretion Flows (ADAFs) and slim disks, which are consistent with previous popular analytical models, while an inflow region and an outflow region always exist, which supports the results of some recent numerical simulation works. Our results indicate that the outflows should be common in various accretion disks and stronger in slim disks and ADAFs.
Numerical simulations of hot accretion flows have shown that the mass accretion rate decreases with decreasing radius. Two models have been proposed to explain this result. In the adiabatic inflow-outflow solution (ADIOS), it is thought to be due to the loss of gas in outflows. In the convection-dominated accretion flow (CDAF) model, it is explained as because that the gas is locked in convective eddies. In this paper we use hydrodynamical (HD) and magnetohydrodynamical (MHD) simulations to investigate which one is physical. We calculate and compare various properties of inflow (gas with an inward velocity) and outflow (gas with an outward velocity). Systematic and significant differences are found. For example, for HD flows, the temperature of outflow is higher than inflow; while for MHD flows, the specific angular momentum of outflow is much higher than inflow. We have also analyzed the convective stability of MHD accretion flow and found that they are stable. These results suggest that systematic inward and outward motion must exist, i.e., the ADIOS model is favored. The different properties of inflow and outflow also suggest that the mechanisms of producing outflow in HD and MHD flows are buoyancy associated with the convection and the centrifugal force associated with the angular momentum transport mediated by the magnetic field, respectively. The latter mechanism is similar to the Blandford & Payne mechanism but no large-scale open magnetic field is required here. Possible observational applications are briefly discussed.