Twinkling pulsar wind nebulae in the synchrotron cut-off regime and the gamma-ray flares in the Crab Nebula

Synchrotron radiation of ultra-relativistic particles accelerated in a pulsar wind nebula may dominate its spectrum up to gamma-ray energies. Because of the short cooling time of the gamma-ray emitting electrons, the gamma-ray emission zone is in the immediate vicinity of the acceleration site. The particle acceleration likely occurs at the termination shock of the relativistic striped wind, where multiple forced magnetic field reconnections provide strong magnetic fluctuations facilitating Fermi acceleration processes. The acceleration mechanisms imply the presence of stochastic magnetic fields in the particle acceleration region, which cause stochastic variability of the synchrotron emission. This variability is particularly strong in the steep gamma-ray tail of the spectrum, where modest fluctuations of the magnetic field lead to strong flares of spectral flux. In particular, stochastic variations of magnetic field, which may lead to quasi-cyclic gamma-ray flares, can be produced by the relativistic cyclotron ion instability at the termination shock. Our model calculations of the spectral and temporal evolution of synchrotron emission in the spectral cut-off regime demonstrate that the intermittent magnetic field concentrations dominate the gamma-ray emission from highest energy electrons and provide fast, strong variability even for a quasi-steady distribution of radiating particles. The simulated light curves and spectra can explain the very strong gamma-ray flares observed in the Crab nebula and the lack of strong variations at other wavelengths. The model predicts high polarization in the flare phase, which can be tested with future polarimetry observations.Comment: 5 pages, 3 figures, MNRAS in pres

Continuation of connecting orbits in 3D-ODEs: (I) Point-to-cycle connections

We propose new methods for the numerical continuation of point-to-cycle connecting orbits in 3-dimensional autonomous ODE's using projection boundary conditions. In our approach, the projection boundary conditions near the cycle are formulated using an eigenfunction of the associated adjoint variational equation, avoiding costly and numerically unstable computations of the monodromy matrix. The equations for the eigenfunction are included in the defining boundary-value problem, allowing a straightforward implementation in AUTO, in which only the standard features of the software are employed. Homotopy methods to find connecting orbits are discussed in general and illustrated with several examples, including the Lorenz equations. Complete AUTO demos, which can be easily adapted to any autonomous 3-dimensional ODE system, are freely available.Comment: 18 pages, 10 figure

Hard X-ray Emission Clumps in the gamma-Cygni Supernova Remnant: an INTEGRAL-ISGRI View

Spatially resolved images of the galactic supernova remnant G78.2+2.1 (gamma-Cygni) in hard X-ray energy bands from 25 keV to 120 keV are obtained with the IBIS-ISGRI imager aboard the International Gamma-Ray Astrophysics Laboratory INTEGRAL. The images are dominated by localized clumps of about ten arcmin in size. The flux of the most prominent North-Western (NW) clump is (1.7 +/- 0.4) 10^{-11} erg/cm^2/s in the 25-40 keV band. The observed X-ray fluxes are in agreement with extrapolations of soft X-ray imaging observations of gamma-Cygni by ASCA GIS and spatially unresolved RXTE PCA data. The positions of the hard X-ray clumps correlate with bright patches of optical line emission, possibly indicating the presence of radiative shock waves in a shocked cloud. The observed spatial structure and spectra are consistent with model predictions of hard X-ray emission from nonthermal electrons accelerated by a radiative shock in a supernova interacting with an interstellar cloud, but the powerful stellar wind of the O9V star HD 193322 is a plausible candidate for the NW source as well.Comment: 5 pages, 5 figures, Astronomy and Astrophysics Letter

A population of isolated hard X-ray sources near the supernova remnant Kes 69

Recent X-ray observations of the supernova remnant IC443 interacting with molecular clouds have shown the presence of a new population of hard X-ray sources related to the remnant itself, which has been interpreted in terms of fast ejecta fragment propagating inside the dense environment. Prompted by these studies, we have obtained a deep {\sl XMM-Newton} observation of the supernova remnant (SNR) Kes 69, which also shows signs of shock-cloud interaction. We report on the detection of 18 hard X-ray sources in the field of Kes 69, a significant excess of the expected galactic source population in the field, spatially correlated with CO emission from the cloud in the remnant environment. The spectra of 3 of the 18 sources can be described as hard power laws with photon index <2 plus line emission associated to K-shell transitions. We discuss the two most promising scenarios for the interpretation of the sources, namely fast ejecta fragments (as in IC443) and cataclysmic variables. While most of the observational evidences are consistent with the former interpretation, we cannot rule out the latter.Comment: 9 pages, 5 figures, A&A in pres