1,160 research outputs found
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 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 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<sup>+ </sup>concentration is believed to increase significantly and diffusive K<sup>+ </sup>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<sup>+ </sup>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<sup>+</sup>] 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<sup>+</sup>] 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
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