596 research outputs found
Effects of guanidine on synaptic transmission in the spinal cord of the frog
The effects of guanidine on motoneurons of the isolated frog spinal cord were studied by adding the drug to the solution bathing the cord during intracellular recording. Guanidine (5·10–4 M) did not alter the membrane potential of motoneurons.
The main effect was a marked increase of the amplitudes and frequencies of small spontaneously occurring inhibitory postsynaptic potentials. The hyperpolarizing component of postsynaptic potentials evoked by stimulation of dorsal roots was also enhanced by guanidine. Higher concentrations of guanidine (5·10–3 M) resulted in a very large and irreversible increase of the small spontaneously occurring inhibitory potentials, which now appeared in a regular, rhythmic pattern.
The effects of guanidine could easily be blocked by increasing the magnesium ions (15 mM) in the bath solution.
These results indicate that guanidine facilitates the release of an inhibitory transmitter in afferent terminals of the frog spinal cord either by a direct action on these terminals or indirectly by an action on nerve endings impinging on inhibitory interneurons
Off-Center Collisions between Clusters of Galaxies
We present numerical simulations of off-center collisions between galaxy
clusters made using a new hydrodynamical code based on the piecewise-parabolic
method (PPM) and an isolated multigrid Poisson solver. We have performed three
gas-only high-resolution simulations of collisions between equal-mass clusters
with different values of the impact parameter (0, 5, and 10 times the core
radius). With these simulations we have studied the observational appearance of
the merging clusters and the variation in equilibration time, luminosity
enhancement during the collision, and structure of the merger remnant with
varying impact parameter. Observational evidence of an ongoing collision is
present for 1-2 sound crossing times after the collision, but only for special
viewing angles. The remnant actually requires at least five crossing times to
reach virial equilibrium. Since the sound crossing time can be as large as 1-2
Gyr, the equilibration time can thus be a large fraction of the age of the
universe. The final merger remnant is very similar for impact parameters of
zero and five core radii. It possesses a roughly isothermal core, with central
density and temperature twice the initial values. Outside the core the
temperature drops as r^-1, and the density roughly as r^-3.8. The core radius
shows a small increase due to shock heating during the merger. For an impact
parameter of ten core radii the core of the remnant possesses a more flattened
density profile, with a steeper dropoff outside the core. In both off-center
cases the merger remnant rotates, but only for the ten-core-radius case does
this appear to have an effect on the structure of the remnant.Comment: 26 pages, 15 figures included, submitted to ApJ; for color figures
and movies see http://www.astro.virginia.edu/~pmr7u/paper_offctr.htm
On the jet structure and magnetic field configuration of GRB 020813
The polarization curve of GRB 020813 is discussed and compared to different
models for the structure, evolution and magnetisation properties of the jet and
the interstellar medium onto which the fireball impacts. GRB 020813 is best
suited for this kind of analysis for the smoothness of its afterglow light
curve, ensuring the applicability of current models. The polarization dataset
allows us to rule out the standard GRB jet, in which the energy and Lorentz
factor have a well defined value inside the jet opening angle and the magnetic
field is generated at the shock front. We explore alternative models finding
that a structured jet or a jet with a toroidal component of the magnetic field
can fit equally well the polarization curve. Stronger conclusions cannot be
drawn due to the incomplete sampling of the polarization curve. A more dense
sampling, especially at early times, is required to pin down the structure of
the jet and the geometry of its magnetic field.Comment: 7 pages, 5 postscript figures, minor revisions according to the
referee comments. A&A in pres
GRB 020813: polarization in the case of a smooth optical decay
We present the results of a VLT polarimetric monitoring campaign of the GRB
020813 optical afterglow carried out in three nights, from 0.88 to 4.20 days
after the gamma-ray event. The mean values of the degree of linear polarization
(P) and its position angle (Theta) resulting from our campaign are
=1.18+/-0.10% and Theta = 148.7+/-2.3 deg after correcting for Galactic
interstellar polarization. Our VLT data set is most naturally described by a
constant degree of linear polarization and position angle, nonetheless a slow
Theta evolution cannot be entirely ruled out by our data. The VLT monitoring
campaign did not reveal either a significant Theta rotation respect to the Keck
spectropolarimetric observations performed ~0.25 days after the GRB (Barth et
al. 2003). However, is a factor of two lower than the polarization degree
measured from Keck. Additionally, the VLT polarization data allowed us to
construct an accurate V-band light curve. The V-band photometric data revealed
a smooth light curve with a break located between the last Keck and our first
VLT polarimetric measurement, 0.33 < t_(break, V) < 0.88 days after the GRB.
The typical magnitude fluctuations of the VLT V-band lightcurve are 0.003 mag,
0.010 mag and 0.016 mag for our three observing nights, respectively. We
speculate that the stability of Theta might be related to the smoothness of the
light curve.Comment: 7 pages, 2 figures, accepted for publication in A&A. Minor changes
included with respect to the previously posted versio
Identifying the Location in the Host Galaxy of the Short GRB 111117A with the Chandra Sub-arcsecond Position
We present our successful Chandra program designed to identify, with
sub-arcsecond accuracy, the X-ray afterglow of the short GRB 111117A, which was
discovered by Swift and Fermi. Thanks to our rapid target of opportunity
request, Chandra clearly detected the X-ray afterglow, though no optical
afterglow was found in deep optical observations. The host galaxy was clearly
detected in the optical and near-infrared band, with the best photometric
redshift of z=1.31_{-0.23}^{+0.46} (90% confidence), making it one of the
highest known short GRB redshifts. Furthermore, we see an offset of 1.0 +- 0.2
arcseconds, which corresponds to 8.4 +- 1.7 kpc, between the host and the
afterglow position. We discuss the importance of using Chandra for obtaining
sub-arcsecond X-ray localizations of short GRB afterglows to study GRB
environments.Comment: 17 pages, 11 figures, accepted for publication in Ap
Simulation techniques for cosmological simulations
Modern cosmological observations allow us to study in great detail the
evolution and history of the large scale structure hierarchy. The fundamental
problem of accurate constraints on the cosmological parameters, within a given
cosmological model, requires precise modelling of the observed structure. In
this paper we briefly review the current most effective techniques of large
scale structure simulations, emphasising both their advantages and
shortcomings. Starting with basics of the direct N-body simulations appropriate
to modelling cold dark matter evolution, we then discuss the direct-sum
technique GRAPE, particle-mesh (PM) and hybrid methods, combining the PM and
the tree algorithms. Simulations of baryonic matter in the Universe often use
hydrodynamic codes based on both particle methods that discretise mass, and
grid-based methods. We briefly describe Eulerian grid methods, and also some
variants of Lagrangian smoothed particle hydrodynamics (SPH) methods.Comment: 42 pages, 16 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 12; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
The dynamical state of the Coma cluster with XMM-Newton
We present in this paper a substructure and spectroimaging study of the Coma
cluster of galaxies based on XMM-Newton data. XMM-Newton performed a mosaic of
observations of Coma to ensure a large coverage of the cluster. We add the
different pointings together and fit elliptical beta-models to the data. We
subtract the cluster models from the data and look for residuals, which can be
interpreted as substructure. We find several significant structures: the
well-known subgroup connected to NGC4839 in the South-West of the cluster, and
another substructure located between NGC 4839 and the centre of the Coma
cluster. Constructing a hardness ratio image, which can be used as a
temperature map we see that in front of this new structure the temperature is
significantly increased (higher or equal 10 keV). We interpret this temperature
enhancement as the result of heating as this structure falls onto the Coma
cluster. We furthermore reconfirm the filament-like structure South-East of the
cluster centre. This region is significantly cooler than the mean cluster
temperature. We estimate the temperature of this structure to be equal or below
1keV. A possible scenario to explain the observed features is stripping caused
by the infall of a small group of galaxies located around the two galaxies
NGC4921 and NGC4911 into the Coma cluster with a non-zero impact parameter. We
also see significant X-ray depressions North and South-East of NGC4921, which
might either be linked to tidal forces due to the merger with the Western
structure or connected to an older cluster merger.Comment: 12 pages, 4 figures accepted for publication in A&
Mean ergodicity and spectrum of the Cesà ro operator on weighted c0 spaces
[EN] A detailed investigation is made of the continuity, the compactness and the spectrum of the Cesà ro operator C acting on the weighted Banach sequence space c0(w) for a bounded, strictly positive weight w. New features arise in the weighted setting (e.g. existence of eigenvalues, compactness, mean ergodicity) which are not present in the classical setting of c0.The research of the first two authors was partially supported by the Projects MTM2013-43540-P, GVA Prometeo II/2013/013 and ACOMP/2015/186 (Spain).Albanese, AA.; Bonet Solves, JA.; Ricker, WJ. (2016). Mean ergodicity and spectrum of the Cesà ro operator on weighted c0 spaces. Positivity. 20:761-803. https://doi.org/10.1007/s11117-015-0385-xS76180320Akhmedov, A.M., Başar, F.: On the fine spectrum of the Cesà ro operator in c 0 . Math. J. Ibaraki Univ. 36, 25–32 (2004)Akhmedov, A.M., Başar, F.: The fine spectrum of the Cesà ro operator C 1 over the sequence space b v p , ( 1 ≤ p < ∞ ) . Math. J. Okayama Univ. 50, 135–147 (2008)Albanese, A.A., Bonet, J., Ricker, W.J.: Convergence of arithmetic means of operators in Fréchet spaces. J. Math. Anal. Appl. 401, 160–173 (2013)Albanese, A.A., Bonet, J., Ricker, W.J.: Spectrum and compactness of the Cesà ro operator on weighted ℓ p spaces. J. Aust. Math. Soc. 99, 287–314 (2015)Albanese, A.A., Bonet, J., Ricker, W.J.: The Cesà ro operator in the Fréchet spaces ℓ p + and L p - . Glasg. Math. J (to appear)Ansari, S.I., Bourdon, P.S.: Some properties of cyclic operators. Acta Sci. Math. Szeged 63, 195–207 (1997)Brown, A., Halmos, P.R., Shields, A.L.: Cesà ro operators. Acta Sci. Math. Szeged 26, 125–137 (1965)Curbera, G.P., Ricker, W.J.: Spectrum of the Cesà ro operator in ℓ p . Arch. Math. 100, 267–271 (2013)Curbera, G.P., Ricker, W.J.: Solid extensions of the Cesà ro operator on ℓ p and c 0 . Integr. Equ. Oper. Theory 80, 61–77 (2014)Curbera, G.P., Ricker, W.J.: The Cesà ro operator and unconditional Taylor series in Hardy spaces. Integr. Equ. Oper. Theory 83, 179–195 (2015)Diestel, J.: Sequences and Series in Banach Spaces. Springer, New York (1984)Dowson, H.R.: Spectral Theory of Linear Operators. Academic Press, London (1978)Dunford, N., Schwartz, J.T.: Linear Operators I: General Theory, 2nd Printing. Wiley Interscience Publ, New York (1964)Emilion, R.: Mean-bounded operators and mean ergodic theorems. J. Funct. Anal. 61, 1–14 (1985)Goldberg, S.: Unbounded Linear Operators: Theory and Applications. Dover Publ, New York (1985)Hille, E.: Remarks on ergodic theorems. Trans. Am. Math. Soc. 57, 246–269 (1945)Jarchow, H.: Locally Convex Spaces. Teubner, Stuttgart (1981)Krengel, U.: Ergodic Theorems. de Gruyter, Berlin (1985)Leibowitz, G.: Spectra of discrete Cesà ro operators. Tamkang J. Math. 3, 123–132 (1972)Lin, M.: On the uniform ergodic theorem. Proc. Am. Math. Soc. 43, 337–340 (1974)Megginson, R.E.: An Introduction to Banach Space Theory. Springer, New York (1998)Mureşan, M.: A Concrete Approach to Classical Analysis. Springer, Berlin (2008)Okutoyi, J.I.: On the spectrum of C 1 as an operator on b v 0 . J. Aust. Math. Soc. Ser. A 48, 79–86 (1990)Radjavi, H., Tam, P.-W., Tan, K.-K.: Mean ergodicity for compact operators. Studia Math. 158, 207–217 (2003)Reade, J.B.: On the spectrum of the Cesà ro operator. Bull. Lond. Math. Soc. 17, 263–267 (1985)Rhoades, B.E., Yildirim, M.: The spectra and fine spectra of factorable matrices on c 0 . Math. Commun. 16, 265–270 (2011)Taylor, A.E.: Introduction to Functional Analysis. Wiley, New York (1958
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure
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