Doppler tomography of relativistic accretion disks

Spectral lines from a source orbiting around a compact object are studied. Time variations of observed frequency and count rate due to motion of the source and gravitational lensing are considered. Gravitational field of the central object is described by the Kerr metric. It is shown that: (i) simultaneous temporal and frequency resolution enables us to restrict parameters of the model (inclination angle, position of the source, angular momentum of the black hole); (ii) techniques of image restoration, familiar from other fields of astronomy, can be applied to study inner regions of active galactic nuclei. This contribution is relevant for extremely variable X-ray sources with high parameter of efficiency, such as Seyfert 1 galaxy PHL 1092 observed by ROSAT.Comment: 12 pages, to appear in Publications of the Astronomical Society of Japan, Vol. 48 (October 1996), figures available upon request from the authors, or at http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.ht

Reflection nebulae in the Galactic Center: the case for soft X-ray imaging polarimetry

The origin of irradiation and fluorescence of the 6.4 keV bright giant molecular clouds surrounding Sgr A*, the central supermassive black hole of our Galaxy, remains enigmatic. Testing the theory of a past active period of Sgr A* requires X-ray polarimetry. In this paper, we show how modern imaging polarimeters could revolutionize our understanding of the Galactic Center. Through Monte Carlo modeling, we produce a 4-8 keV polarization map of the Galactic Center, focusing on the polarimetric signature produced by Sgr B1, Sgr B2, G0.11-0.11, Bridge E, Bridge D, Bridge B2, MC2, MC1, Sgr C3, Sgr C2, and Sgr C1. We estimate the resulting polarization, include polarized flux dilution by the diffuse plasma emission detected toward the GC, and simulate the polarization map that modern polarimetric detectors would obtain assuming the performances of a mission prototype. The eleven reflection nebulae investigated in this paper present a variety of polarization signatures, ranging from nearly unpolarized to highly polarized (about 77%) fluxes. A major improvement in our simulation is the addition of a diffuse, unpolarized plasma emission that strongly impacts soft X-ray polarized fluxes. The dilution factor is in the range 50% - 70%, making the observation of the Bridge structure unlikely even in the context of modern polarimetry. The best targets are the Sgr B and Sgr C complexes, and the G0.11-0.11 cloud. An exploratory observation of a few hundred kilo-seconds of the Sgr B complex would allow a significant detection of the polarization and be sufficient to derive hints on the primary source of radiation. A more ambitious program (few Ms) of mapping the giant molecular clouds could then be carried out to probe with great precision the turbulent history of Sgr A*, and place important constraints on the composition and three-dimensional position of the surrounding gas.Comment: 7 pages, 3 figures, 2 tables, accepted for publication in A&

Non-linear resonance in nearly geodesic motion in low-mass X-ray binaries

We have explored the ideas that parametric resonance affects nearly geodesic motion around a black hole or a neutron star, and that it may be relevant to the high frequency (twin) quasi-periodic oscillations occurring in some low-mass X-ray binaries. We have assumed the particles or fluid elements of an accretion disc to be subject to an isotropic perturbation of a hypothetical but rather general form. We find that the parametric resonance is indeed excited close to the radius where epicyclic frequencies of radial and meridional oscillations are in a 2:3 ratio. The location and frequencies of the highest amplitude excitation vary with the strength of the perturbation. These results agree with actual frequency ratios of twin kHz QPOs that have been reported in some black hole candidates, and they may be consistent also with correlation of the twin peaks in Sco X-1.Comment: 5 pages; accepted for publication in PAS

QPO in RE J1034+396: model constraints from observed trends

We analyze the time variability of the X-ray emission of RE J1034+396, an active galactic nucleus with the first firm detection of a quasi-periodic oscillations (QPO). Based on the results of a wavelet analysis, we find a drift in the QPO central frequency. The change inthe QPO frequency correlates with the change in the X-ray flux with a short time delay. Linear structures such as shocks, spiral waves, orvery distant flares seem to be a favored explanation for this particular QPO event.Comment: to appear in the proceedings to "The Central Kiloparsec in Galactic Nuclei (AHAR2011)", Journal of Physics: Conference Series (JPCS), IOP Publishin

Conditions for the Thermal Instability in the Galactic Centre Mini-spiral region

We explore the conditions for the thermal instability to operate in the mini-spiral region of the Galactic centre (Sgr A*), where both the hot and cold media are known to coexist. The photoionisation Cloudy calculations are performed for different physical states of plasma. We neglect the dynamics of the material and concentrate on the study of the parameter ranges where the thermal instability may operate, taking into account the past history of Sgr A* bolometric luminosity. We show that the thermal instability does not operate at the present very low level of the Sgr A* activity. However, Sgr A* was much more luminous in the past. For the highest luminosity states the two-phase medium can be created up to 1.4 pc from the centre. The presence of dust grains tends to suppress the instability, but the dust is destroyed in the presence of strong radiation field and hot plasma. The clumpiness is thus induced in the high activity period, and the cooling/heating timescales are long enough to preserve later the past multi-phase structure. The instability enhances the clumpiness of the mini-spiral medium and creates a possibility of episodes of enhanced accretion of cold clumps towards Sgr A*. The mechanism determines the range of masses and sizes of clouds; under the conditions of Sgr A*, the likely values come out $1$ - $10^2M_{\oplus}$ for the cloud typical mass.Comment: Accepted for publication in MNRAS, 10 pages, 7 figure

The financial instruments market – an institutional approach

In response to the last financial crisis new institutional reforms were implemented. The aim for these reforms is to save and secure the functioning of markets in financial instruments. It seems though that these efforts lack the clarity of the basic notion, which is the term “institution”. This weakness my cause interpretational problems on both theoretical and practical level. The aim of this article is to clarify the understanding of the notion of “institution” in finance. One of ways to achieve this goal is to present the institutional structure of the market in financial instruments, to specify the characteristics of both individual institutions and the whole environment in which they act. And, lastly, to outline an institutional transformation process which is driven by innovations. The classification of institutions is also proposed. As a result of analysis, following types of institutions are singled out: institutions in a broad meaning (established through institutional contracts), institutions in a narrow sense (norms and social rules). Additionally, the formal and informal institutions are distinguished

Stability of an Erratic Tailings Deposit

150 million cubic meters of erratic colliery spoils were deposited between 1971 and 1983 close to the Kardia lignite fields, in Greece. The heap finally reached satisfactorily an escalating height of 73 m. Field reconnaissance, collation of local experience and geotechnical investigations were performed during midstage of deposition. They aimed at an optimal configuration of the fill in progress, within the possibilities allowed by extensive earth moving operations. Present paper out- 1ines the approach and the methodology used to arrive at strength parameters and slope design of a highly inhomogeneous massive comprising variegated soils, marls and encaptured lignites