32,283 research outputs found
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 Carinae) have long
been regarded as potential sources of high-energy -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 -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
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