118354810
Near infrared flares of Sagittarius A* - Importance of near infrared polarimetry
Context. We report on the results of new simulations of near-infrared (NIR) observations of the Sagittarius A* (Sgr A*) counterpart associated with the super-massive black hole at the Galactic Center. Aims. Our goal is to investigate and understand the physical processes behind the variability associated with the NIR flaring emission from Sgr A * . Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and CIAO NIR camera on the Subaru telescope (13 june 2004; 30 july 2005; 1 june 2006; 15 may 2007; 17 may 2007 and 28 may 2008). We used a model of synchrotron emission from relativistic electrons in the inner parts of an accretion disk. The relativistic simulations have been carried out using the Karas-Yaqoob (KY) ray-tracing code. Results. We probe the existence of a correlation between the modulations of the observed flux density light curves and changes in polarimetric data. Furthermore, we confirm that the same correlation is also predicted by the hot spot model. Correlations between intensity and polarimetric parameters of the observed light curves as well as a comparison of predicted and observed light curve features through a pattern recognition algorithm result in the detection of a signature of orbiting matter under the influence of strong gravity. This pattern is detected statistically significant against randomly polarized red noise. Expected results from future observations of VLT interferometry like GRAVITY experiment are also discussed. Conclusions. The observed correlations between flux modulations and changes in linear polarization degree and angle can be a sign that the NIR flares have properties that are not expected from purely random red-noise. We find that the geometric shape of the emission region plays a major role in the predictions of the model. From fully relativistic simulations of a spiral shape emitting region, we conclude that the observed swings of the polarization angle during NIR flares support the idea of compact orbiting spots instead of extended patterns. The effects of gravitational shearing, fast synchrotron cooling of the components and confusion from a variable accretion disk have been taken into account. Simulated centroids of NIR images lead us to the conclusion that a clear observation of the position wander of the center of NIR images with future infrared interferometers will prove the existence of orbiting hot spots in the vicinity of our Galactic super-massive black hole.
15067899
A method to determine the parameters of black holes in AGNs and galactic X-ray sources with periodic modulation of variability
We propose a simple and unambiguous way to deduce the parameters of black holes which may reside in AGNs and some types of X-ray binaries. The black-hole mass and angular momentum are determined in physical units. The method is applicable to the sources with periodic components of variability, provided one can assume the following: (i) Variability is due to a star or a stellar-mass compact object orbiting the central black hole and passing periodically through an equatorial accretion disk (variability time-scale is given by the orbital period). (ii) The star orbits almost freely, deviation of its trajectory due to passages through the disk being very weak (secular); the effect of the star on the disk, on the other hand, is strong enough to yield observable photometric and spectroscopic features. (iii) The gravitational field within the nucleus is that of the (Kerr) black hole, the star and the disk contribute negligibly.
14676201
Comparison of Relativistic Iron Line Models
The an alysis of the broad iron line profile in the X-ray spectra of active galactic nuclei and black hole X-ray binaries allows us to constrain the spin parameter of the black hole. We compare the constraints on the spin value for two X-ray sources, MCG-6-30-15 and GX 339-4, with a broad iron line using present relativistic line models in XSPEC — laor and kyrline. The laor model has the spin value set to the extremal value a = 0.9982, while the kyrline model enables direct fitting of the spin parameter. The spin value is constrained mainly by the lower boundary of the broad line, which depends on the inner boundary of the disc emission where the gravitational redshift is maximal. The position of the inner disc boundary is usually identified with the marginally stable orbit which is related to the spin value. In this way the laor model can be used to estimate the spin value. We investigate the consistency of the laor and kyrline models. We find that the spin values evaluated by both models agree within the general uncertainties when applied on the current data. However, the results are apparently distinguishable for higher quality data, such as those simulated for the International X-ray Observatory (IXO) mission. We find that the laor model tends to overestimate the spin value and furthermore, it has insufficient energy resolu- tion which affects the correct determination of the high-energy edge of the broad line.
197510338
Polarization signatures of strong gravity in AGN accretion discs
54929717
Hot-spot model for accretion disc variability as random process - II. Mathematics of the power-spectrum break frequency
Context. We study some general properties of accretion disc variability in the context of stationary random processes. In partic ular, we are interested in mathematical constraints that can be imposed on the functional form of the Fourier power-spectrum density (PSD) that exhibits a multiply broken shape and several local maxima. Aims. We develop a methodology for determining the regions of the model parameter space that can in principle reproduce a PSD shape with a given number and position of local peaks and breaks of the PSD slope. Given the vast space of possible parameters, it is an important requirement that the method is fast in estimating the PSD shape for a given parameter set of the model. Methods. We generated and discuss the theoretical PSD profiles of a sho t-noise-type random process with exponentially decaying fl ares. Then we determined conditions under which one, two, or more breaks or local maxima occur in the PSD. We calculated positions of these features and determined the changing slope of the model PSD. Furthermore, we considered the influence of the modulation by the orbi tal motion for a variability pattern assumed to result from an orbiting-spo t model. Results. We suggest that our general methodology can be useful in for describing non-monotonic PSD profiles (such as the trend seen , on different scales, in exemplary cases of the high-mass X-ray binary Cygnus X-1 and the narrow-line Seyfert galaxy Ark 564). We adopt a model where these power spectra are reproduced as a superposition of several Lorentzians with varying amplitudes in the X-ray-band light curve. Our general approach can help in constraining the model parameters and in determining which parts of the parameter space are accessible under various circumstances.
17819259
Iron lines from transient and persisting hot spots on AGN accretion disks
Aims. We model the X-ray reprocessing from a strong co-rotating flare above an accretion disk in active galactic nuclei. By performing detailed radiative transfer computations we explore the horizontal structure and evolution of the underlying hot spot. The main goal is to study how the resulting spectral features manifest themselves in short exposure time spectra. Methods. We analyze both the vertical and the horizontal spot structure and its dynamical reprocessed spectrum. To obtain the spectral evolution seen by a distant observer, we apply a general relativity ray-tracing technique. We concentrate on the energy band around the iron K-line, where the relativistic effects are most pronounced. Persistent flares lasting for a significant fraction of the orbital time scale and short, transient flares are considered. Results. In our time-resolved analysis, the spectra recorded by a distant observer depend on the position of the flare/spot with respect to the central black hole. If the flare duration significantly exceeds the light travel time across the spot, then the spot horizontal stratification is unimportant. On the other hand, if the flare duration is comparable to the light travel time across the spot radius, the lightcurves exhibit a typical asymmetry in their time profiles. The sequence of dynamical spectra proceeds from more strongly to less strongly ionized re-emission. At all locations within the spot the spectral intensity increases towards edge-on emission angles, revealing the limb brightening effect. Conclusions. Future X-ray observatories with significantly larger effective collecting areas will enable to spectroscopically map out the azimuthal irradiation structure of the accretion disk and to localize persistent flares. If the hot spot is not located too close to the marginally stable orbit of the black hole, it will be possible to probe the reflecting medium via the sub-structure of the iron K-line. Indications for transient flares will only be obtained from analyzing the observed lightcurves on the gravitational time scale of the accreting supermassive black hole.
54629650
Optimization of high-current ion beam acceleration and charge compensation in two cusps of induction linac
Results of the numerical simulation of the hollow high-current ion beam (HHCIB) dynamics in two magnet-isolated accelerating gaps separated by the drift gap are presented. The previous study has shown that the good charge and current compensations of the ion beam by the specially injected electron beam occur in the accelerating gaps of the induction linac. However in the drift gap the high positive electric potential due to the positive space charge of HHCIB was obtained because the essential difference between the electron and ion drift velocities exists under this compensation method. This disadvantage impairing the brightness of the ion beam can be considerably reduced by the additional injection of the thermal electrons into the drift region. In present report the some cases of the cold electron injection into drift gap are considered. The more optimal regime for the effective charge and current compensations of HHCIB without loss in the stability of ion beam was found.
125913694
Numerical simulation of high-current ion linear induction accelerator with additional electron beam injection
118450922
2.5D numerical simulation of relativistic electron bunches (REB) interaction with underdense and overdense plasmas
The results of REB-plasma interaction numerical simulation by means of our relativistic particle EM code-are presented. Nonlinear evolution of a single REB injected into underdense and overdense plasmas is studied in detail for cylindrical geometry. It is found that in case of comparable bunch and plasma densities an instantaneous amplitude of excited longitudinal electric field substantially depends on density ratio, relativistic factor and space density distribution of REB. Transversal electric field can considerably exceed the longitudinal one, causing more severe restrictions to initial phase and radial position of particles to be accelerated.<<ETX>>