Stochastic model of optical variability of BL Lacertae

We use optical photometric and polarimetric data of BL Lacertae that cover a period of 22 years to study the variability of the source. The long-term observations are employed for establishing parameters of a stochastic model consisting of the radiation from a steady polarized source and a number of variable components with different polarization parameters, proposed by Hagen-Thorn et al. earlier. We infer parameters of the model from the observations using numerical simulations based on a Monte Carlo method, with values of each model parameter selected from a Gaussian distribution. We determine the best set of model parameters by comparing model distributions to the observational ones using the chi-square criterion. We show that the observed photometric and polarimetric variability can be explained within a model with a steady source of high polarization, ~40%, and with direction of polarization parallel to the parsec scale jet, along with 10+-5 sources of variable polarization.Comment: 4 pages, 10 figures, published by Astronomy and Astrophysics; v2: typos correcte

Symmetry-breaking and chaos in electron transport in semiconductor superlattices

We study the motion of electrons in a single miniband of a semiconductor superlattice driven by THz electric field polarized along the growth direction. We work in the semiclassical balance-equation model, including different elastic and inelastic scattering rates, and incorporating the self-consistent electric field generated by electron motion. We explore regions of complex dynamics, which can include chaotic behaviour and symmetry-breaking. We estimate the magnitudes of dc current and dc voltage that spontaneously appear in regions of broken-symmetry for parameters characteristic of modern semiconductor superlattices. This work complements PRL 80(1998)2669 [ cond-mat/9709026 ].Comment: 4 pages, 3 figures, RevTEX, EPS

Three-dimensional MHD Simulations of Radiatively Inefficient Accretion Flows

We present three-dimensional MHD simulations of rotating radiatively inefficient accretion flows onto black holes. In the simulations, we continuously inject magnetized matter into the computational domain near the outer boundary, and we run the calculations long enough for the resulting accretion flow to reach a quasi-steady state. We have studied two limiting cases for the geometry of the injected magnetic field: pure toroidal field and pure poloidal field. In the case of toroidal field injection, the accreting matter forms a nearly axisymmetric, geometrically-thick, turbulent accretion disk. The disk resembles in many respects the convection-dominated accretion flows found in previous numerical and analytical investigations of viscous hydrodynamic flows. Models with poloidal field injection evolve through two distinct phases. In an initial transient phase, the flow forms a relatively flattened, quasi-Keplerian disk with a hot corona and a bipolar outflow. However, when the flow later achieves steady state, it changes in character completely. The magnetized accreting gas becomes two-phase, with most of the volume being dominated by a strong dipolar magnetic field from which a thermal low-density wind flows out. Accretion occurs mainly via narrow slowly-rotating radial streams which `diffuse' through the magnetic field with the help of magnetic reconnection events.Comment: 35 pages including 3 built-in plots and 14 separate jpg-plots; version accepted by Ap

Friend murine leukemia virus-immortalized myeloid cells are converted into tumorigenic cell lines by Abelson leukemia virus

Friend murine leukemia virus (Fr-MuLV) is a replication-competent murine retrovirus that induces acute nonlymphocytic leukemias in NFS/n mice. Fr-MuLV disease is divided into two stages based on the ability of the leukemia cells to grow in culture and transplant into syngeneic mice. Hematopoietic cells taken from the early stage of disease after Fr-MuLV infection grow as immortal myeloid cell lines in the presence of WEHI-3 cell-conditioned medium (CM) or interleukin 3. These growth factor-dependent cell lines do not grow in culture in the absence of CM and do not form tumors in syngeneic animals. If these Fr-MuLV-infected cells are superinfected with Abelson murine leukemia virus (Ab-MuLV), they lose their dependence on WEHI-3 CM and proliferate in culture in the absence of exogenous growth factors. Concomitant with the loss of growth factor dependence in culture, the Ab-MuLV-infected cell lines become tumorigenic in syngeneic mice. This secondary level of transformation is Ab-MuLV specific. Fr-MuLV-immortalized myeloid cell lines superinfected with Harvey murine sarcoma virus (Ha-MuSV) or amphotropic virus remain dependent on WEHI-3 CM for growth in vitro and are not tumorigenic in vivo. Neither Ab-MuLV- nor Ha-MuSV-infected normal mouse myeloid cell cultures produce growth factor-independent or tumorigenic cell lines. We conclude that at least two genetic events are needed to convert a murine myeloid precursor into a tumorigenic cell line. The first event occurs in Fr-MuLV-infected mice, generating cells that are growth factor dependent but immortal in vitro. The second event, which can be accomplished by Ab-MuLV infection, converts these immortal myeloid precursors into growth factor-independent and tumorigenic cells

Measuring Black Hole Spin by the Continuum-Fitting Method: Effect of Deviations from the Novikov-Thorne Disc Model

The X-ray spectra of accretion discs of eight stellar-mass black holes have been analyzed to date using the thermal continuum fitting method, and the spectral fits have been used to estimate the spin parameters of the black holes. However, the underlying model used in this method of estimating spin is the general relativistic thin-disc model of Novikov & Thorne, which is only valid for razor-thin discs. We therefore expect errors in the measured values of spin due to inadequacies in the theoretical model. We investigate this issue by computing spectra of numerically calculated models of thin accretion discs around black holes, obtained via three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations. We apply the continuum fitting method to these computed spectra to estimate the black hole spins and check how closely the values match the actual spin used in the GRMHD simulations. We find that the error in the dimensionless spin parameter is up to about 0.2 for a non-spinning black hole, depending on the inclination. For black holes with spins of 0.7, 0.9 and 0.98, the errors are up to about 0.1, 0.03 and 0.01 respectively. These errors are comparable to or smaller than those arising from current levels of observational uncertainty. Furthermore, we estimate that the GRMHD simulated discs from which these error estimates are obtained correspond to effective disc luminosities of about 0.4-0.7 Eddington, and that the errors will be smaller for discs with luminosities of 0.3 Eddington or less, which are used in the continuum-fitting method. We thus conclude that use of the Novikov-Thorne thin-disc model does not presently limit the accuracy of the continuum-fitting method of measuring black hole spin.Comment: 13 pages, 7 figures, accepted for publication in MNRAS. v2: fixed typo in author name, updated acknowledgment

The influence of inhaled multi-walled carbon nanotubes on the autonomic nervous system

Background: Heart rate and cardiovascular function are regulated by the autonomic nervous system. Heart rate variability (HRV) as a marker reflects the activity of autonomic nervous system. The prognostic significance of HRV in cardiovascular disease has been reported in clinical and epidemiological studies. The present study focused on the influence of inhaled multi-walled carbon nanotubes (MWCNTs) on autonomic nervous system by HRV analysis. Methods: Male Sprague–Dawley rats were pre-implanted with a telemetry device and kept in the individual cages for recovery. At week four after device implantation, rats were exposed to MWCNTs for 5 h at a concentration of 5 mg/m3 . The real-time EKGs were recorded by a telemetry system at pre-exposure, during exposure, 1 day and 7 days post-exposure. HRV was measured by root mean square of successive differences (RMSSD); the standard deviation of inter-beat (RR) interval (SDNN); the percentage of successive RR interval differences greater than 5 ms (pNN5) and 10 ms (pNN10); low frequency (LF) and high frequency (HF). Results: Exposure to MWCNTs increased the percentage of differences between adjacent R-R intervals over 10 ms (pNN10) (p \u3c 0.01), RMSSD (p \u3c 0.01), LF (p \u3c 0.05) and HF (p \u3c 0.01). Conclusions: Inhalation of MWCNTs significantly alters the balance between sympathetic and parasympathetic nervous system. Whether such transient alterations in autonomic nervous performance would alter cardiovascular function and raise the risk of cardiovascular events in people with pre-existing cardiovascular conditions warrants further study