Random and aperiodic quantum spin chains: A comparative study

According to the Harris-Luck criterion the relevance of a fluctuating interaction at the critical point is connected to the value of the fluctuation exponent omega. Here we consider different types of relevant fluctuations in the quantum Ising chain and investigate the universality class of the models. At the critical point the random and aperiodic systems behave similarly, due to the same type of extreme broad distribution of the energy scales at low energies. The critical exponents of some averaged quantities are found to be a universal function of omega, but some others do depend on other parameters of the distribution of the couplings. In the off-critical region there is an important difference between the two systems: there are no Griffiths singularities in aperiodic models.Comment: 4 pages RevTeX, 2 eps-figures include

Rotationally-driven VHE emission from the Vela pulsar

The recent detection of pulsed $\gamma$-ray emission from the Vela pulsar in the $\sim10$ to 100 GeV range by H.E.S.S. promises an important potential to probe into the very high energy (VHE) radiation mechanisms of pulsars. The recent detection of pulsed $\gamma$-ray emission from the Vela pulsar in the $\sim10$ to 100 GeV range by H.E.S.S. promises an important potential to probe into the very high energy (VHE) radiation mechanisms of pulsars. We analyze achievable particle energies in the magnetosphere of the Vela pulsar, and calculate the resultant emission properties. Inverse Compton up-scattering of thermal photons from the surface of the star is shown to lead a pulsed VHE contribution reaching into the TeV regime with spectral characteristics compatible with current findings. If confirmed by further observations this could be the second case where rotationally-driven processes turn out to be important to understand the VHE emission in young pulsars.Comment: 5 pages, 2 figure

A possible black hole binary in Mkn 501

A simple binary model for Mkn~501 is considered under the assumption that the TeV and X-ray periodicity of $\sim 23$ days, observed during the flaring state in 1997, may be basically interpreted as a doppler-shifted flux modulation due to the orbital motion of the relativistic jet. For the typical jet properties inferred from emission models, we find an intrinsic orbital period of $(6-14)$ yrs and a centre-of-mass distance of $(2-3.5) \times 10^{16}$ cm, the binary thus being a very close system. If the binary separation corresponds to that at which gravitational radiation becomes dominant, one may obtain a maximum allowed primary mass of $\sim 10^8 M_{\odot}$ and a corresponding maximum allowed secondary mass in the range of $\sim (1-9)\times 10^7 M_{\odot}$ assuming that gas accretion occurs on around the salpeter time scale. Interestingly such values agree with the black hole masses expected from merger scenarios.Comment: 4 pages, 2 figures; updated results of astro-ph/0005478; to appear in the Proc. of the Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, Heidelberg, June 26-30, 2000, ed. by H.J. Voelk and F. Aharonian, AIP Conf. Pro

Test particle acceleration by rotating jet magnetospheres

Centrifugal acceleration of charged test particles at the base of a rotating jet magnetosphere is considered. Based on an analysis of forces we derive the equation for the radial accelerated motion and present an analytical solution. It is shown that for particles moving outwards along rotating magnetic field lines, the energy gain is in particular limited by the breakdown of the bead-on-the-wire approximation which occurs in the vicinity of the light cylinder $r_{L}$. The corresponding upper limit for the maximum Lorentz factor $\gamma_{max}$ for electrons scales $\propto B^{2/3} r_{L}^{2/3}$, with $B$ the magnetic field strength at $r_{L}$, and is at most of the order of a $10^2-10^3$ for the conditions regarded to be typical for BL Lac objects. Such values suggest that this mechanism may provide pre-accelerated seed particles which are required for efficient Fermi-type particle acceleration at larger scales in radio jets.Comment: 4 pages, 1 figure; to appear in the Proc. of the Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, Heidelberg, June 26-30, 2000, ed. by H.J. Voelk and F. Aharonian, AIP Conf. Pro