The nature of ultrahigh-energy cosmic rays (UHECRs) at energies >10 20 eV remains a mystery 1 . They are likely to be of extragalactic origin, but should be absorbed within ~50 Mpc through interactions with the cosmic microwave background. As there are no sufficient powerful accelerators within this distance
Context. Ultracompact X-ray binaries (UCXBs) typically consist of a white dwarf donor and a neutron star or black hole accretor. The evolution of UCXBs and very low mass ratio binaries in general is poorly understood. In particular, the dynamical behavior of an accretion disk extending to a large radius (relative to the orbit) is unclear. Aims. We investigate the evolution of UCXBs in order to learn for which mass ratios and accretor types these systems can exist, and if they do, what are their orbital and neutron star spin periods, mass transfer rates and evolutionary timescales. Methods. We compute tracks of a binary containing a Roche-lobe overflowing helium white dwarf in which mass transfer is driven by gravitational wave emission. For different assumptions concerning accretion disk behavior we calculate for which system parameters dynamical instability, thermal-viscous disk instability or the propeller effect emerge. The significance of these processes during the evolution of an UCXB is considered. Results. At the onset of mass transfer, the survival of the UCXB is determined by how efficiently the accretor can eject matter in the case of a super-Eddington mass transfer rate. At later times, the evolution of systems strongly depends on the binary's capacity to return angular momentum from the disk to the orbit. We find that this feedback mechanism most likely remains effective even at very low mass ratio. In the case of steady mass transfer, the propeller effect can stop accretion onto recycled neutron stars completely at a sufficiently low mass transfer rate, based on energy considerations. However, mass transfer will likely be non-steady because disk instability allows for accretion of some of the transferred matter. Together, the propeller effect and disk instability cause the low mass ratio UCXBs to be visible a small fraction of the time at most, thereby explaining the lack of observations of such systems. Conclusions. Most likely UCXBs avoid late-time dynamically unstable mass loss from the donor and continue to evolve as the age of the Universe allows. This implies the existence of a large population of low mass ratio binaries with orbital periods ∼70-80 min, unless some other mechanism has destroyed these binaries. Even though none have been discovered yet, black hole UCXBs could also exist, at orbital periods of typically 100-110 min.
Using a large set of simulated extensive air showers, we investigate universality features of electron and positron distributions in very-highenergy cosmic-ray air showers. Most particle distributions depend only on the depth of the shower maximum and the number of particles in the cascade at this depth. We provide multi-dimensional parameterizations for the electron-positron distributions in terms of particle energy, vertical and horizontal momentum angle, lateral distance, and time distribution of the shower front. These parameterizations can be used to obtain realistic electron-positron distributions in extensive air showers for data analysis and simulations of Cherenkov radiation, fluorescence signal, and radio emission. (S. Lafebre).shown that many distributions depend only on two parameters: the number of particles in the extensive air shower and the longitudinal position in the shower evolution where this maximum occurs [5,6,7,8,9,10,11,12]. This concept, which is referred to as universality, allows us to develop parameterizations of the electron-positron distributions as a function of relevant quantities such as energy, lateral distance, and momentum angles, in terms of only a few parameters. MethodElectron and positron distributions in the atmosphere were studied through detailed Monte Carlo simulations. Unless specified otherwise, extensive air shower simulations were performed according to the specifications below.All simulations were carried out using the corsika code, version 6.5 [13]. We used the qgsjet-II-03 model [14,15] to describe high-energy interactions and the urqmd 1.3.1 code [16,17] at lower energies. Electromagnetic interactions were treated by the egs4 code [18]. We applied a low energy cutoff of 151 keV and level 10 −6 optimum thinning [19,20]. The U.S. Standard Atmosphere [21,22] was used as atmospheric model. It should be noted that, because simulations for our analysis were performed using only a single nuclear interaction model, the shape of the distributions presented may change somewhat when different models such as sibyll or qgsjet-I are employed. On the arXiv:0902.0548v1 [astro-ph.HE]
Abstract. The well known drifter PSR B0031−07 is known to exhibit drifting subpulses where the spacing between the drift bands (P 3 ) shows three distinct modes A, B and C corresponding to 12, 6 and 4 s respectively. We have investigated periodicities and polarisation properties of PSR B0031−07 for a sequence of 2700 single pulses taken simultaneously at 328 MHz and 4.85 GHz. We found that mode A occurs simultaneously at these frequencies, while modes B and C only occur at 328 MHz. However, when the pulsar is emitting in mode B at the lower frequency there is still emission at the higher frequency, hinting towards the presence of mode B emission at a weaker level. Further, we have established that modes A and B are associated with two orthogonal modes of polarisation, respectively. Based on these observations, we suggest a geometrical model where modes A and B at a given frequency are emitted in two concentric rings around the magnetic axis with mode B being nested inside mode A. Further, it is evident that this nested configuration is preserved across frequency with the higher frequency arising closer to the stellar surface compared to the lower one, consistent with the well known radius-to-frequency mapping operating in pulsars.
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