Counterexample to an additivity conjecture for output purity of quantum channels

A conjecture arising naturally in the investigation of additivity of classical information capacity of quantum channels states that the maximal purity of outputs from a quantum channel, as measured by the p-norm, should be multiplicative with respect to the tensor product of channels. We disprove this conjecture for p>4.79. The same example (with p=infinity) also disproves a conjecture for the multiplicativity of the injective norm of Hilbert space tensor products.Comment: 3 pages, 3 figures, revte

Numerical studies of planar closed random walks

Lattice numerical simulations for planar closed random walks and their winding sectors are presented. The frontiers of the random walks and of their winding sectors have a Hausdorff dimension $d_H=4/3$. However, when properly defined by taking into account the inner 0-winding sectors, the frontiers of the random walks have a Hausdorff dimension $d_H\approx 1.77$.Comment: 15 pages, 15 figure

Weak magnetic fields in white dwarfs and their direct progenitors?

We have carried out a re-analysis of polarimetric data of central stars of planetary nebulae, hot subdwarfs, and white dwarfs taken with FORS1 (FOcal Reducer and low dispersion Spectrograph) on the VLT (Very Large Telescope), and added a large number of new observations in order to increase the sample. A careful analysis of the observations using only one wavelength calibration for the polarimetrically analysed spectra and for all positions of the retarder plate of the spectrograph is crucial in order to avoid spurious signals. We find that the previous detections of magnetic fields in subdwarfs and central stars could not be confirmed while about 10% of the observed white dwarfs have magnetic fields at the kilogauss level.Comment: 6 pages, Proceedings of the 18th European White Dwarf Workshop, ASP Conference Serie

A non-pulsating neutron star in the supernova remnant HESS J1731-347 / G353.6-0.7 with a carbon atmosphere

Context: The CCO candidate in the center of the supernova remnant shell HESS J1731-347 / G353.6-0.7 shows no pulsations and exhibits a blackbody-like X-ray spectrum. If the absence of pulsations is interpreted as evidence for the emitting surface area being the entire neutron star surface, the assumption of the measured flux being due to a blackbody emission translates into a source distance that is inconsistent with current estimates of the remnant's distance. Aims: With the best available observational data, we extended the pulse period search down to a sub-millisecond time scale and used a carbon atmosphere model to describe the X-ray spectrum of the CCO and to estimate geometrical parameters of the neutron star. Methods: To search for pulsations we used data of an observation of the source with XMM-Newton performed in timing mode. For the spectral analysis, we used earlier XMM-Newton observations performed in imaging mode, which permits a more accurate treatment of the background. The carbon atmosphere models used to fit the CCO spectrum are computed assuming hydrostatic and radiative equilibria and take into account pressure ionization and the presence of spectral lines. Results: Our timing analysis did not reveal any pulsations with a pulsed fraction above ~8% down to 0.2 ms. This finding further supports the hypothesis that the emitting surface area is the entire neutron star surface. The carbon atmosphere model provides a good fit to the CCO spectrum and leads to a normalization consistent with the available distance estimates of the remnant. The derived constraints on the mass and radius of the source are consistent with reasonable values of the neutron star mass and radius. After the CCO in Cas A, the CCO in HESS J1731-347 / G353.6-0.7 is the second object of this class for which a carbon atmosphere model provides a consistent description of X-ray emission.Comment: 6 pages, 5 figures, accepted for publication in Astronomy&Astrophysic

Ensuring prudent use of antimicrobials in human medicine in the European Union, 2005

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Analysis of enhanced tan(beta) corrections in MFV GUT scenarios

We analyse a minimal supersymmetric standard model (MSSM) taking a minimal flavour violation (MFV) structure at the GUT scale. We evaluate the parameters at the electroweak scale taking into account the full flavour structure in the evolution of the renormalization group equations. We concentrate mainly on the decay Bs -> mu mu and its correlations with other observables like b -> s gamma, b -> s l l, Delta M_Bs and the anomalous magnetic moment of the muon. We restrict our analysis to the regions in parameter space consistent with the dark matter constraints. We find that the BR(Bs -> mu mu) can exceed the current experimental limit in the regions of parameter space which are allowed by all other constraints thus providing an additional bound on supersymmetric parameters. This holds even in the constrained MSSM. Assuming an hypothetical measurement of BR(Bs -> mu mu) ~ 10^-7 we analyse the predicted MSSM spectrum and flavour violating decay modes of supersymmetric particles which are found to be small.Comment: 47 pages, 16 figures (best viewed printed or in pdf format), updated lattice inputs used, version submitted to PR

A future very-high-energy view of our Galaxy

The survey of the inner Galaxy with H.E.S.S. was remarkably successful in detecting a wide range of new very-high-energy gamma-ray sources. New TeV gamma-ray emitting source classes were established, although several of the sources remain unidentified, and progress has been made in understanding particle acceleration in astrophysical sources. In this work, we constructed a model of a population of such very-high-energy gamma-ray emitters and normalised the flux and size distribution of this population model to the H.E.S.S.-discovered sources. Extrapolating that population of objects to lower flux levels we investigate what a future array of imaging atmospheric telescopes (IACTs) such as AGIS or CTA might detect in a survey of the Inner Galaxy with an order of magnitude improvement in sensitivity. The sheer number of sources detected together with the improved resolving power will likely result in a huge improvement in our understanding of the populations of galactic gamma-ray sources. A deep survey of the inner Milky Way would also support studies of the interstellar diffuse gamma-ray emission in regions of high cosmic-ray density. In the final section of this paper we investigate the science potential for the Galactic Centre region for studying energy-dependent diffusion with such a future array.Comment: Proceeding of "Heidelberg International Symposium on High Energy Gamma-Ray Astronomy", held in Heidelberg, 7-11 July 2008, submitted to AIP Conference Proceedings. 4 pages, 4 figure

On the origin of white dwarfs with carbon-dominated atmospheres: the case of H1504+65

We explore different evolutionary scenarios to explain the helium deficiency observed in H1504+65, the most massive known PG1159 star. We concentrate mainly on the possibility that this star could be the result of mass loss shortly after the born-again and during the subsequent evolution through the [WCL] stage. This possibility is sustained by recent observational evidence of extensive mass-loss events in Sakurai's object and is in line with the recent finding that such mass losses give rise to PG1159 models with thin helium-rich envelopes and large rates of period change, as demanded by the pulsating star PG1159-035. We compute the post born again evolution of massive sequences by taking into account different mass-loss rate histories. Our results show that stationary winds during the post-born-again evolution fail to remove completely the helium-rich envelope so as to explain the helium deficiency observed in H1504+65. Stationary winds during the Sakurai and [WCL] stages only remove at most half of the envelope surviving the violent hydrogen burning during the born-again phase. In view of our results, the recently suggested evolutionary connection born-again stars --> H1504+65 --> white dwarfs with carbon-rich atmospheres is difficult to sustain unless the whole helium-rich envelope could be ejected by non-stationary mass-loss episodes during the Sakurai stage.Comment: 5 pages, 2 figures. To be published in Astronomy & Astrophysic