XMM-Newton observation of the double pulsar system J0737-3039

We report on a 50 ksec XMM-Newton observation of the double pulsar system J0737-3039 performed on April 2004. We present results of the spectral analysis of these data combined with the much shorter Chandra pointing performed on January 2004. Black body emission with effective temperature of 0.20^{+0.02}_{-0.02} keV (90% confidence level) and emission radius 75^{+30}_{-9} m for a distance of 0.5 kpc (implying a 0.5-10 keV luminosity \~6x10^{29} erg/s) is a viable interpretation, calling for a stream of particles accelerated in the magnetosphere of PSR J0737-3039A and depositing their kinetic energy in the magnetic polar cap of PSR J0737-3039A or of the companion PSR J0737-3039B. A single power-law emission model implies a very steep photon index Gamma=4.2^{+2.1}_{-1.2} and a suspiciously high hydrogen column density, whereas a photon index Gamma=2 does not provide an adequate description of the XMM-Newton and Chandra data. A two component model (a black body plus a power-law with Gamma=2) is statistically acceptable, but the additional power-law component is not required by the data.Comment: Accepted for publication on ApJ

Lepton Flavour Violation in a Left-Right Symmetric Model

We consider in this paper a Left-Right symmetric gauge model in which a global lepton-number-like symmetry is introduced and broken spontaneously at a scale that could be as low as 10^4 GeV or so. The corresponding physical Nambu-Goldstone boson, which we call majoron and denote J, can have tree-level flavour-violating couplings to the charged fermions, leading to sizeable majoron-emitting lepton-flavour-violating weak decays. We consider explicitly a leptonic variant of the model and show that the branching ratios for \mu -> e+J, \tau -> e + J and \tau -> \mu + J decays can be large enough to fall within the sensitivities of future \mu and \tau factories. On the other hand the left-right gauge symmetry breaking scale may be as low as few TeV.Comment: LaTeX, 16 pages, 3 PS figures, uses JHEP.cls, published versio

Pairing and polarization in systems with retarded interactions

In a system where a boson (e.g, a phonon) of finite frequency $\omega_0$ is coupled to electrons, two phenomena occur as the coupling is increased: electron pairing and polarization of the boson field. Within a path integral formalism and a Dynamical Mean-Field approach, we introduce {\it ad hoc} distribution function which allow us to pinpoint the two effects. When $\omega_0$ is smaller than the bandwidth $D$, pairing and polarization occur for fairly similar couplings for all considered temperatures. When $\omega_0 > D$, the two phenomena tend to coincide only for $T \gg \omega_0$, but are no longer tied for low temperatures so that a state of paired particles without finite polarization is stabilized.Comment: 4 pages, 2 figure

Direct calculation of the critical Casimir force in a binary fluid

We show that critical Casimir effects can be accessed through direct simulation of a model binary fluid passing through the demixing transition. We work in the semi grand canonical ensemble, in slab geometry, in which the Casimir force appears as the excess of the generalized pressure, $P_{\bot}-n\mu$. The excesses of the perpendicular pressure, $P_{\bot}$, and of $n\mu$, are individually of much larger amplitude. A critical pressure anisotropy is observed between forces parallel and perpendicular to the confinement direction, which collapses onto a universal scaling function closely related to that of the critical Casimir force

Quantum scalar field in D-dimensional static black hole space-times

An Euclidean approach for investigating quantum aspects of a scalar field living on a class of D-dimensional static black hole space-times, including the extremal ones, is reviewed. The method makes use of a near horizon approximation of the metric and $\zeta$-function formalism for evaluating the partition function and the expectation value of the field fluctuations $$. After a review of the non-extreme black hole case, the extreme one is considered in some details. In this case, there is no conical singularity, but the finite imaginary time compactification introduces a cusp singularity. It is found that the $\zeta$-function regularized partition function can be defined, and the quantum fluctuations are finite on the horizon, as soon as the cusp singularity is absent, and the corresponding temperature is T=0.Comment: 9 pages, LaTe