The analysis of eight EXOSAT X-ray lightcurves of six active galactic nuclei by nonlinear prediction methods indicates that the observed variability is truly stochastic and is not caused by deterministic chaos. This result favors X-ray emission models with mutliple centers of production of hot, possibly relativistic electrons.

X-ray pulse duration control is critical to exploring the ultra-fast science at an x-ray freeelectron laser (FEL) facility such as the Linac Coherent Light Source (LCLS). The pulse length of a typical electron bunch that drives a self-amplified spontaneous emission (SASE) FEL is limited to the order of 100~200 fs, due to short-bunch collective interactions. Many novel schemes have been proposed...

[1] Recent observations of terrestrial gamma-ray flashes (TGFs) (Smith et al., 2005) suggest the need for a new mechanism of TGF production. Consideration of relativistic runaway electron (RRE) avalanche driven by electromagnetic impulses (EMP) radiated by rapidly moving lightning return strokes indicates that TGFs can be produced by discharges with peak return stroke currents Ip > 450–700 kA w...

The excess of antiprotons (P) observed in cosmic rays has been attributed to their production in supernova_(SN) envelopes expanding in dense clouds. While creating P, y-rays are also produced and these clouds would shine as y-ray sources. The evolution of the Y-ray spectrum has been calculated for clouds of nH = 104 and 105/cm 3.

The energy spectra of TeV gamma-rays from blazars, after being corrected for intergalatic absorption in the Extragalactic Background Light (EBL), appear unusually hard, a fact that poses challenges to the conventional models of particle acceleration in TeV blazars and/or to the EBL models. In this paper we show that the internal absorption of gamma-rays caused by interactions with dense narrow-...

We show that the luminosity of a star forming galaxy is capped by the production and subsequent expulsion of cosmic rays from its interstellar medium. By defining an Eddington luminosity in cosmic rays, we show that the star formation rate of a given galaxy is limited by its mass content and the cosmic ray mean free path. When the cosmic ray luminosity and pressure reaches a critical value as a...

We suggest that the highest energy > ∼ 10eV cosmic ray primaries may be relativistic magnetic monopoles. Motivations for this hypothesis are that conventional primaries are problematic, while monopoles are naturally accelerated to E ∼ 10eV by galactic magnetic fields. By matching the cosmic monopole production mechanism to the observed highest energy cosmic ray flux we estimate the monopole mas...

The sources of very high energy cosmic rays should also be sources of neutrinos. While the complete scheme of ultrahigh energy cosmic ray production is not yet understood [1], ancillary to cosmic ray production are neutrino fluxes, both at the source of the cosmic rays and through cosmic ray propagation and scattering with the microwave background. Given that neutrinos are unabsorbed and undefl...

The transient emission produced behind internal shocks that are driven by overtaking collisions of a magnetized, relativistic outflow is considered. A self-consistent model capable of describing the structure and dynamics of the shocks and the time evolution of the pair-and gamma-ray distribution functions is developed and applied to gamma-ray flares in blazars, in the case in which gamma-ray p...

Indirect detection of particle dark matter relies upon pair annihilation of Weakly Interaction Massive Particles (WIMPs), which is complementary to the well known techniques of direct detection (WIMP-nucleus scattering) and collider production (WIMP pair production). Pair annihilation of WIMPs results in the production of gamma-rays, neutrinos, and anti-matter. Of the various experiments sensit...