2,546 research outputs found
Non-linear shock acceleration and high energy gamma rays from clusters of galaxies
Merger and accretion shocks in clusters of galaxies can accelerate particles
via first order Fermi process. Since this mechanism is believed to be
intrinsically efficient, shocks are expected to be modified by the backreaction
of the accelerated particles. Such a modification might induce appreciable
effects on the non--thermal emission from clusters and a suppression of the
heating of the gas at strong shocks. Here we consider in particular the gamma
ray emission and we discuss the capability of Cherenkov telescopes such as HESS
to detect clusters at TeV energies.Comment: 6 pages, to appear in the proceedings of the Gamma 2004 Symposium on
High Energy Gamma Ray Astronomy, Heidelberg, July 2004 (AIP Proceedings
Series
Dynamics and High Energy Emission of the Flaring HST-1 Knot in the M 87 Jet
Stimulated by recent observations of a radio-to-X-ray synchrotron flare from
HST-1, the innermost knot of the M 87 jet, as well as by a detection of a very
high energy gamma-ray emission from M 87, we investigated the dynamics and
multiwavelength emission of the HST-1 region. We study thermal pressure of the
hot interstellar medium in M 87 and argue for a presence of a gaseous
condensation in its central parts. Interaction of the jet with such a feature
is likely to result in formation of a converging reconfinement shock in the
innermost parts of the M 87 jet. We show that for a realistic set of the
outflow parameters, a stationary and a flaring part of the HST-1 knot located
\~100 pc away from the active center can be associated with the decelerated
portion of the jet matter placed immediately downstream of the point where the
reconfinement shock reaches the jet axis. We discuss a possible scenario
explaining a broad-band brightening of the HST-1 region related to the variable
activity of the central core. We show that assuming a previous epoch of the
high central black hole activity resulting in ejection of excess particles and
photons down along the jet, one may first expect a high-energy flare of HST-1
due to inverse-Comptonisation of the nuclear radiation, followed after a few
years by an increase in the synchrotron continuum of this region. If this is
the case, then the recently observed increase in the knot luminosity in all
spectral bands could be regarded as an unusual echo of the outburst that had
happened previously in the active core of the M 87 radio galaxy.Comment: 30 pages, 7 figures included. Accepted for publication in MNRA
Magnetic excitations in vanadium spinels
We study magnetic excitations in vanadium spinel oxides AVO (A=Zn,
Mg, Cd) using two models: first one is a superexchange model for vanadium S=1
spins, second one includes in addition spin-orbit coupling, and crystal
anisotropy. We show that the experimentally observed magnetic ordering can be
obtained in both models, however the orbital ordering is different with and
without spin-orbit coupling and crystal anisotropy. We demonstrate that this
difference strongly affects the spin-wave excitation spectrum above the
magnetically ordered state, and argue that the neutron measurement of such
dispersion is a way to distinguish between the two possible orbital orderings
in AVO.Comment: accepted in Phys. Rev.
Spin excitations used to probe the nature of the exchange coupling in the magnetically ordered ground state of PrCaMnO
We have used time-of-flight inelastic neutron scattering to measure the spin
wave spectrum of the canonical half-doped manganite
PrCaMnO, in its magnetic and orbitally ordered phase. The
data, which cover multiple Brillouin zones and the entire energy range of the
excitations, are compared with several different models that are all consistent
with the CE-type magnetic order, but arise through different exchange coupling
schemes. The Goodenough model, i.e. an ordered state comprising strong nearest
neighbor ferromagnetic interactions along zig-zag chains with antiferromagnetic
inter-chain coupling, provides the best description of the data, provided that
further neighbor interactions along the chains are included. We are able to
rule out a coupling scheme involving formation of strongly bound ferromagnetic
dimers, i.e. Zener polarons, on the basis of gross features of the observed
spin wave spectrum. A model with weaker dimerization reproduces the observed
dispersion but can be ruled out on the basis of discrepancies between the
calculated and observed structure factors at certain positions in reciprocal
space. Adding further neighbor interactions results in almost no dimerization,
i.e. recovery of the Goodenough model. These results are consistent with
theoretical analysis of the degenerate double exchange model for half-doping,
and provide a recipe for how to interpret future measurements away from
half-doping, where degenerate double exchange models predict more complex
ground states.Comment: 14 pages, 11 figure
Radiative acceleration and transient, radiation-induced electric fields
The radiative acceleration of particles and the electrostatic potential
fields that arise in low density plasmas hit by radiation produced by a
transient, compact source are investigated. We calculate the dynamical
evolution and asymptotic energy of the charged particles accelerated by the
photons and the radiation-induced electric double layer in the full
relativistic, Klein-Nishina regime. For fluxes in excess of , the radiative force on a diluted plasma
(n\la 10^{11} cm) is so strong that electrons are accelerated rapidly
to relativistic speeds while ions lag behind owing to their larger inertia. The
ions are later effectively accelerated by the strong radiation-induced double
layer electric field up to Lorentz factors , attainable in the
case of negligible Compton drag. The asymptotic energies achieved by both ions
and electrons are larger by a factor 2--4 with respect to what one could
naively expect assuming that the electron-ion assembly is a rigidly coupled
system. The regime we investigate may be relevant within the framework of giant
flares from soft gamma-repeaters.Comment: 14 pages, 7 figures, ApJ, in press (tentatively scheduled for the v.
592, 2003 issue
The broad-band X-ray spectrum of the blazar PKS B1830-211 by Chandra and INTEGRAL
In this paper we present a broad-band study of the X-ray emission of the
blazar PKS1830-211 based on Chandra and Integral observations. Notwithstanding
the high redshift (z=2.507), it is a bright X-ray source (F(2-10 keV)~10^{-11}
erg cm^{-2} s^{-1}), due to gravitational lensing by an intervening galaxy at
z=0.89. Previous X-ray observations attribute the observed absorption at E<2
keV to the lensing galaxy. Our analysis, although not in contrast with this
hypothesis, suggests also the possibility of an intrinsic (ionized) absorption,
taking place at the front side of the jet. This scenario is also supported by
some evidence, in the same data, of a feature observed at 2.15 keV which can be
interpreted as a blueshifted iron line (v/c ~ 0.18). The SED of PKS1830-211 can
be well modelled by combining a Synchrotron Self-Compton component and an
external source of photons to be scattered up to \gamma-ray energies by
relativistic electrons moving outward in the jet. The main source of low energy
photons is a dust torus at the temperature of 10^3 K as expected in MeV
blazars.Comment: Accepted for publication in A&
- …