Fermi-LAT upper limits on gamma-ray emission from colliding wind binaries

Context: Colliding wind binaries (CWBs) are thought to give rise to a plethora of physical processes including acceleration and interaction of relativistic particles. Observation of synchrotron radiation in the radio band confirms there is a relativistic electron population in CWBs. Accordingly, CWBs have been suspected sources of high-energy gamma-ray emission since the COS-B era. Theoretical models exist that characterize the underlying physical processes leading to particle acceleration and quantitatively predict the non-thermal energy emission observable at Earth. Aims: We strive to find evidence of gamma-ray emission from a sample of seven CWB systems: WR 11, WR 70, WR 125, WR 137, WR 140, WR 146, and WR 147. Theoretical modelling identified these systems as the most favourable candidates for emitting gamma-rays. We make a comparison with existing gamma-ray flux predictions and investigate possible constraints. Methods: We used 24 months of data from the Large Area Telescope (LAT) on-board the Fermi Gamma Ray Space Telescope to perform a dedicated likelihood analysis of CWBs in the LAT energy range. Results: We find no evidence of gamma-ray emission from any of the studied CWB systems and determine corresponding flux upper limits. For some CWBs the interplay of orbital and stellar parameters renders the Fermi-LAT data not sensitive enough to constrain the parameter space of the emission models. In the cases of WR140 and WR147, the Fermi-LAT upper limits appear to rule out some model predictions entirely and constrain theoretical models over a significant parameter space. A comparison of our findings to the CWB eta Car is made.Comment: 9 pages, 3 figure

Disentangling Giant Component and Finite Cluster Contributions in Sparse Matrix Spectra

We describe a method for disentangling giant component and finite cluster contributions to sparse random matrix spectra, using sparse symmetric random matrices defined on Erdos-Renyi graphs as an example and test-bed.Comment: 7 pages, 2 multi-part figure

Spectra of Sparse Random Matrices

We compute the spectral density for ensembles of of sparse symmetric random matrices using replica, managing to circumvent difficulties that have been encountered in earlier approaches along the lines first suggested in a seminal paper by Rodgers and Bray. Due attention is payed to the issue of localization. Our approach is not restricted to matrices defined on graphs with Poissonian degree distribution. Matrices defined on regular random graphs or on scale-free graphs, are easily handled. We also look at matrices with row constraints such as discrete graph Laplacians. Our approach naturally allows to unfold the total density of states into contributions coming from vertices of different local coordination.Comment: 22 papges, 8 figures (one on graph-Laplacians added), one reference added, some typos eliminate

Equilibrium Ensemble Approach to Disordered Systems I: General Theory, Exact Results

An outline of Morita's equilibrium ensemble approach to disordered systems is given, and hitherto unnoticed relations to other, more conventional approaches in the theory of disordered systems are pointed out. It is demonstrated to constitute a generalization of the idea of grand ensembles and to be intimately related also to conventional low--concentration expansions as well as to perturbation expansions about ordered reference systems. Moreover, we draw attention to the variational content of the equilibrium ensemble formulation. A number of exact results are presented, among them general solutions for site-- and bond-- diluted systems in one dimension, both for uncorrelated, and for correlated disorder.Comment: 24 pages, Late

High-energy particle transport in 3D hydrodynamic models of colliding-wind binaries

Massive stars in binary systems (as WR140, WR147 or $\eta$ Carinae) have long been regarded as potential sources of high-energy $\gamma$-rays. The emission is thought to arise in the region where the stellar winds collide and produce relativistic particles which subsequently might be able to emit $\gamma$-rays. Detailed numerical hydrodynamic simulations have already offered insight in the complex dynamics of the wind collision region (WCR), while independent analytical studies, albeit with simplified descriptions of the WCR, have shed light on the spectra of charged particles. In this paper, we describe a combination of these two approaches. We present a 3D-hydrodynamical model for colliding stellar winds and compute spectral energy distributions of relativistic particles for the resulting structure of the WCR. The hydrodynamic part of our model incorporates the line-driven acceleration of the winds, gravity, orbital motion and the radiative cooling of the shocked plasma. In our treatment of charged particles we consider diffusive shock acceleration in the WCR and the subsequent cooling via inverse Compton losses (including Klein-Nishina effects), bremsstrahlung, collisions and other energy loss mechanisms.Comment: 28 pages, 9 figures / accepted for publication in The Astrophysical Journa

Divine Disclosure: An Introduction to Jewish Apocalyptic

Reviewed Book: Russell, D S. (David Syme). Divine Disclosure: An Introduction to Jewish Apocalyptic. Minneapolis: Augsburg Fortress; London: SCM Press, 1992

Josephus and the New Testament

Reviewed Book: Mason, Steve. Josephus and the New Testament. Peabody, Mass: Hendrickson Pubs, 1992