Scaling law in target-hunting processes

We study the hunting process for a target, in which the hunter tracks the goal by smelling odors it emits. The odor intensity is supposed to decrease with the distance it diffuses. The Monte Carlo experiment is carried out on a 2-dimensional square lattice. Having no idea of the location of the target, the hunter determines its moves only by random attempts in each direction. By sorting the searching time in each simulation and introducing a variable $x$ to reflect the sequence of searching time, we obtain a curve with a wide plateau, indicating a most probable time of successfully finding out the target. The simulations reveal a scaling law for the searching time versus the distance to the position of the target. The scaling exponent depends on the sensitivity of the hunter. Our model may be a prototype in studying such the searching processes as various foods-foraging behavior of the wild animals.Comment: 7 figure

Superdiffusion in a Model for Diffusion in a Molecularly Crowded Environment

We present a model for diffusion in a molecularly crowded environment. The model consists of random barriers in percolation network. Random walks in the presence of slowly moving barriers show normal diffusion for long times, but anomalous diffusion at intermediate times. The effective exponents for square distance versus time usually are below one at these intermediate times, but can be also larger than one for high barrier concentrations. Thus we observe sub- as well as super-diffusion in a crowded environment.Comment: 8 pages including 4 figure

The Nature of the Emission Components in the Quasar/NLS1 PG1211+143

We present the study of the emission properties of the quasar PG1211+143, which belongs to the class of Narrow Line Seyfert 1 galaxies. On the basis of observational data analyzed by us and collected from the literature, we study the temporal and spectral variability of the source in the optical/UV/X-ray bands and we propose a model that explains the spectrum emitted in this broad energy range. In this model, the intrinsic emission originating in the warm skin of the accretion disk is responsible for the spectral component that is dominant in the softest X-ray range. The shape of reflected spectrum as well as Fe K line detected in hard X-rays require the reflecting medium to be mildly ionized (xi~500). We identify this reflector with the warm skin of the disk and we show that the heating of the skin is consistent with the classical alpha P_{tot} prescription, while alpha P_{gas} option is at least two orders of magnitude too low to provide the required heating. We find that the mass of the central black hole is relatively small (M_BH~10^7- 10^8 Msun, which is consistent with the Broad Line Region mapping results and characteristic for NLS1 class.Comment: 22 pages, 10 figures, accepted to Ap

Discovery of superstrong, fading, iron line emission and double-peaked Balmer lines of the galaxy SDSSJ0952+2143 - the light echo of a huge flare

We report the discovery of superstrong, fading, high-ionization iron line emission in the galaxy SDSSJ095209.56+214313.3 (SDSSJ0952+2143 hereafter), which must have been caused by an X-ray outburst of large amplitude. SDSSJ0952+2143 is unique in its strong multiwavelength variability; such a broadband emission-line and continuum response has not been observed before. The strong iron line emission is accompanied by unusual Balmer line emission with a broad base, narrow core and double-peaked narrow horns, and strong HeII emission. These lines, while strong in the SDSS spectrum taken in 2005, have faded away significantly in new spectra taken in December 2007. Comparison of SDSS, 2MASS, GALEX and follow-up GROND photometry reveals variability in the NUV, optical and NIR band. Taken together, these unusual observations can be explained by a giant outburst in the EUV--X-ray band, detected even in the optical and NIR. The intense and variable iron, Helium and Balmer lines represent the ``light echo'' of the flare, as it traveled through circumnuclear material. The outburst may have been caused by the tidal disruption of a star by a supermassive black hole. Spectroscopic surveys such as SDSS are well suited to detect emission-line light echoes of such rare flare events. Reverberation-mapping of these light echoes can then be used as a new and efficient probe of the physical conditions in the circumnuclear material in non-active or active galaxies.Comment: ApJ Letters, 678, L13 (May 1 issue); incl. 4 colour figures. This and related papers on tidal disruption flares also available at http://www.xray.mpe.mpg.de/~skomossa

Status of the XMM-Newton cross-calibration with SASv6.5.0

Further achievements of the XMM-Newton cross-calibration - XMM internal as well as with other X-ray missions - are presented. We explain the major changes in the new version SASv6.5 of the XMM-Newton science analysis system. The current status of the cross-calibration of the three EPIC cameras is shown. Using a large sample of blazars, the pn energy redistribution at low energy could be further calibrated, correcting the overestimation of fluxes in the lowest energy regime. In the central CCDs of the MOSs, patches were identified at the bore-sight positions, leading to an underestimation of the low energy fluxes. The further improvement in the understanding of the cameras resulted in a good agreement of the EPIC instruments down to lowest energies. The latest release of the SAS software package already includes corrections for both effects as shown in several examples of different types of sources. Finally the XMM internal cross-calibration is completed by the presentation of the current cross-calibration status between EPIC and RGS instruments. Major efforts have been made in cross-calibrations with other X-ray missions, most importantly with Chandra, of course, but also with currently observing satellites like Swift.Comment: 6 pages, 23 figures. To appear in the proceedings of "The X-Ray Universe 2005" conference, 2005 Sept 26-30, El Escorial, Madrid, Spai

The Soft X-Ray Properties of a Complete Sample of Optically Selected Quasars II. Final Results

We present the final results of a ROSAT PSPC program to study the soft X-ray emission properties of a complete sample of low $z$ quasars. The main results are: 1. There is no evidence for significant soft excess emission or excess foreground absorption by cold gas in 22 of the 23 quasars. 2. The mean 0.2-2 keV continuum of quasars agrees remarkably well with an extrapolation of the mean 1050-350A continuum recently determined by Zheng et al. (1996), indicating that there is no steep soft component below 0.2 keV. 3. The occurrence of warm absorbers in quasars is rather rare, in sharp contrast to lower luminosity AGN. 4. The strongest correlation found is between the spectral slope, alpha_x, and the Hb FWHM. This remarkably strong correlation may result from a dependence of alpha_x on L/L_Edd, as seen in Galactic black hole candidates. 5. There appears to exist a distinct class of ``X-ray weak'' quasars. These may be quasars where the direct X-ray source is obscured, and only scattered X-rays are observed. 6. Thin accretion disk models cannot reproduce the observed optical to soft X-ray spectral shape. An as yet unknown physical mechanism maintains a strong correlation between the optical and soft X-ray emission. 7. The well known difference in alpha_x between radio-loud and radio-quiet quasars may be due only to their different Hb FWHM. 8. The agreement of the 21 cm and X-ray columns implies that He in the diffuse H II component of the Galactic ISM is ionized to He II or He III (shortened abstract).Comment: 19 pages of text only, uses aas2pp4.sty file, to appear in ApJ vol. 447, 3/1/97, complete postscript version of 34 pages including 5 tables and 8 figures available at http://physics.technion.ac.il/~laor/rosat/paper.p

Curvature-coupling dependence of membrane protein diffusion coefficients

We consider the lateral diffusion of a protein interacting with the curvature of the membrane. The interaction energy is minimized if the particle is at a membrane position with a certain curvature that agrees with the spontaneous curvature of the particle. We employ stochastic simulations that take into account both the thermal fluctuations of the membrane and the diffusive behavior of the particle. In this study we neglect the influence of the particle on the membrane dynamics, thus the membrane dynamics agrees with that of a freely fluctuating membrane. Overall, we find that this curvature-coupling substantially enhances the diffusion coefficient. We compare the ratio of the projected or measured diffusion coefficient and the free intramembrane diffusion coefficient, which is a parameter of the simulations, with analytical results that rely on several approximations. We find that the simulations always lead to a somewhat smaller diffusion coefficient than our analytical approach. A detailed study of the correlations of the forces acting on the particle indicates that the diffusing inclusion tries to follow favorable positions on the membrane, such that forces along the trajectory are on average smaller than they would be for random particle positions.Comment: 16 pages, 8 figure

Observation of Parity Nonconservation in Moller Scattering

We report a measurement of the parity-violating asymmetry in fixed target electron-electron (Moller) scattering: A_PV = -175 +/- 30 (stat.) +/- 20 (syst.) parts per billion. This first direct observation of parity nonconservation in Moller scattering leads to a measurement of the electron's weak charge at low energy Q^e_W = -0.053 +/- 0.011. This is consistent with the Standard Model expectation at the current level of precision: sin^2\theta_W(M_Z)_MSbar = 0.2293 +/- 0.0024 (stat.) +/- 0.0016 (syst.) +/- 0.0006 (theory).Comment: Version 3 is the same as version 2. These versions contain minor text changes from referee comments and a change in the extracted value of Q^e_W and sin^2\theta_W due to a change in the theoretical calculation of the bremsstrahulung correction (ref. 16

Anomalous ion diffusion within skeletal muscle transverse tubule networks

<p>Abstract</p> <p>Background</p> <p>Skeletal muscle fibres contain transverse tubular (t-tubule) networks that allow electrical signals to rapidly propagate into the fibre. These electrical signals are generated by the transport of ions across the t-tubule membranes and this can result in significant changes in ion concentrations within the t-tubules during muscle excitation. During periods of repeated high-frequency activation of skeletal muscle the t-tubule K⁺concentration is believed to increase significantly and diffusive K⁺transport from the t-tubules into the interstitial space provides a mechanism for alleviating muscle membrane depolarization. However, the tortuous nature of the highly branched space-filling t-tubule network impedes the diffusion of material through the network. The effective diffusion coefficient for ions in the t-tubules has been measured to be approximately five times lower than in free solution, which is significantly different from existing theoretical values of the effective diffusion coefficient that range from 2–3 times lower than in free solution. To resolve this discrepancy, in this paper we study the process of diffusion within electron microscope scanned sections of the skeletal muscle t-tubule network using mathematical modelling and computer simulation techniques. Our model includes t-tubule geometry, tautness, hydrodynamic and non-planar network factors.</p> <p>Results</p> <p>Using our model we found that the t-tubule network geometry reduced the K⁺diffusion coefficient to 19–27% of its value in free solution, which is consistent with the experimentally observed value of 21% and is significantly smaller than existing theoretical values that range from 32–50%. We also found that diffusion in the t-tubules is anomalous for skeletal muscle fibres with a diameter of less than approximately 10–20 μm as a result of obstructed diffusion. We also observed that the [K⁺] within the interior of the t-tubule network during high-frequency activation is greater for fibres with a larger diameter. Smaller skeletal muscle fibres are therefore more resistant to membrane depolarization. Because the t-tubule network is anisotropic and inhomogeneous, we also found that the [K⁺] distribution generated within the network was irregular for fibres of small diameter.</p> <p>Conclusion</p> <p>Our model explains the measured effective diffusion coefficient for ions in skeletal muscle t-tubules.</p