Gabriela Raluca Mocanu

Cosmological evolution of finite temperature Bose-Einstein condensate dark matter

Once the temperature of a bosonic gas is smaller than the critical, density dependent, transition temperature, a Bose-Einstein condensation process can take place during the cosmological evolution of the Universe. Bose-Einstein condensates are very strong candidates for dark matter, since they can solve some major issues in observational astrophysics, like, for example, the galactic core/cusp problem. The presence of the dark matter condensates also drastically affects the cosmic history of the Universe. In the present paper we analyze the effects of the finite dark matter temperature on the cosmological evolution of the Bose-Einstein condensate dark matter systems. We formulate the basic equations describing the finite temperature condensate, representing a generalized Gross-Pitaevskii equation that takes into account the presence of the thermal cloud in thermodynamic equilibrium with the condensate. The temperature dependent equations of state of the thermal cloud and of the condensate are explicitly obtained in an analytical form. By assuming a flat Friedmann-Robertson-Walker geometry, the cosmological evolution of the finite temperature dark matter filled Universe is considered in detail in the framework of a two interacting fluid dark matter model, describing the transition from the initial thermal cloud to the low temperature condensate state. The dynamics of the cosmological parameters during the finite temperature dominated phase of the dark matter evolution are investigated in detail, and it is shown that the presence of the thermal excitations leads to an overall increase in the expansion rate of the Universe.

Stochastic oscillations of general relativistic discs

We analyze the general relativistic oscillations of thin accretion disks around compact astrophysical objects interacting with the surrounding medium through non-gravitational forces. The interaction with the external medium (a thermal bath) is modeled via a friction force, and a random force, respectively. The general equations describing the stochastically perturbed disks are derived by considering the perturbations of trajectories of the test particles in equatorial orbits, assumed to move along the geodesic lines. By taking into account the presence of a viscous dissipation and of a stochastic force we show that the dynamics of the stochastically perturbed disks can be formulated in terms of a general relativistic Langevin equation. The stochastic energy transport equation is also obtained. The vertical oscillations of the disks in the Schwarzschild and Kerr geometries are considered in detail, and they are analyzed by numerically integrating the corresponding Langevin equations. The vertical displacements, velocities and luminosities of the stochastically perturbed disks are explicitly obtained for both the Schwarzschild and the Kerr cases.

Generalized Langevin equation with colored noise description of the stochastic oscillations of accretion disks

We consider a description of the stochastic oscillations of the general relativistic accretion disks around compact astrophysical objects interacting with their external medium based on a generalized Langevin equation with colored noise and on the fluctuation–dissipation theorems. The former accounts for the general memory and retarded effects of the frictional force. The presence of the memory effects influences the response of the disk to external random interactions, and it modifies the dynamical behavior of the disk, as well as the energy dissipation processes. The generalized Langevin equation of the motion of the disk in the vertical direction is studied numerically, and the vertical displacements, velocities, and luminosities of the stochastically perturbed disks are explicitly obtained for both the Schwarzschild and the Kerr cases. The power spectral distribution of the disk luminosity is also obtained. As a possible astrophysical application of the formalism we investigate the possibility that the intra-day variability of the active galactic nuclei may be due to the stochastic disk instabilities. The perturbations due to colored/nontrivially correlated noise induce a complicated disk dynamics, which could explain some astrophysical observational features related to disk variability.

Generalized Langevin Equation Description of Stochastic Oscillations of General Relativistic Disks

We consider a description of the stochastic oscillations of the general relativistic accretion disks around compact astrophysical objects based on the generalized Langevin equation, which accounts for the general retarded effects of the frictional force, and on the fluctuation–dissipation theorems. The vertical displacements, velocities and luminosities of the stochastically perturbed disks are explicitly obtained for both the Schwarzschild and Kerr cases. The power spectral distribution of the luminosity is also obtained, and it is shown that it has non-standard values. The theoretical predictions of the model are compared with the observational data for the luminosity time variation of the BL Lac S5 0716+714 object.

Appearance of an accretion disc perturbed by fractional Brownian Motion density

This paper aims to investigate/map the effects that perturbations applied to an accretion disk might produce on the registered Light Curves (LC). The case of accretion disks around supermassive active black holes (AGNs) is studied with the goal to explain some of the statistical properties of the observed IntraDay Variability (IDV). The region producing optical IDV is perturbed by allowing it to develop a mass density of a fractional Brownian Motion-like type. The light curves and spectral slopes are calculated and compared to observational data for different Hurst parameters. The spectral slopes of the simulated light curves vary in the range

Hydropower Structures Monitoring System with Hierarchically Distributed Smart Sensor Network

(0.4,2.5)

Radiation emitted by a charged particle undergoing Brownian motion in a magnetic field

. The agreement with observational data shows that a magnetized disk subjected to stochastic perturbations can produce some of the features observed in the light curves.

Standing waves in a solar periodic structures

Traditionally, a remote monitoring system for the geotechnical instruments intended for surveillance of a hydropower structure is made using large concentrated data acquisition stations with a capacity of up to 256 measurement channels, connected to a local data acquisition server. This paper proposes a new hydropower structure monitoring system, based on a distributed architecture of smart sensor, organized as hierarchical sensor networks. Several developments are required in order to implement such a new system: development of a new family of network interfacing modules, implementation of a group of new smart wired and wireless sensors, implementation of a software application for configuring the sensor network, acquiring, visualizing and archiving data from sensors.

MHD cellular automata simulations: Application to GRB X-ray afterglows

The physical setting in which a charged particle undergoes Brownian motion in a magnetic field is studied and the results are discussed in an astrophysical context. The mathematical description associated to this physical setting is based on Langevin equations. These are solved numerically, for both thermal and explosive initial conditions. Special attention is devoted to the Light Curve generated during the motion, its statistical moments and its Power Spectral Density.

On the algorithmic behaviour of complex physical systems

The spatial structuring of solar and space plasmas is known to have a dispersive effect on incident waves. Many solar features posses a periodic structure with structures having alternating properties. This study analyzes the effect of periodic alternation of magnetic slabs on wave propagation using the Bendickson and Dowling model (1996). We have obtained the criteria for the appearance of standing waves taking into account the spatial scaling of the system and the strength of the magnetic field. We have obtained the correlation between the observed standing waves and the number of magnetic slabs in plume/interplume regions.