5,429 research outputs found
Mkn 463 field observed by BeppoSAX
In this work we present the observation of the Mkn 463 field performed with
the MECS instrument on-board BeppoSAX in the 1.8-10.5 keV band. The Mkn 463
field is an example of an extragalactic field crowded with absorbed X-ray
sources: apart from the Seyfert 2 galaxy Mkn 463 and the well known QSO PG
1352+183 (the only object showing no absorption), two other objects are
detected with a column density in excess to the galactic value. The first 1SAX
J1353.9+1820 is a red QSO from the BeppoSAX High Energy Large Area Survey
(HELLAS). The second 1SAX J1355.4+1815 is optically unidentified, but its X-ray
spectral characteristics indicate that it too is an AGN hidden behind a large
column density.Comment: 5 pages, 3 PostScript figures, LaTeX manuscript, new A&A file style
included, accepted for publication on Astronomy and Astrophysic
Dewey, democracy, and Malaguzzi’s vision for the schools of Reggio Emilia
Malaguzzi often refers to Dewey as one of the ‘founding fathers’,
one of the threads weaving the warp, one of the cornerstones of a much larger
building. In defining the value of these connections Malaguzzi explicitly refers
to activism, creativity and mediation between individuality and society. Central
in his vision is the need to connect individual and social dimension, build a
culture and society able to embrace both, to work for the common good while
respecting individuality. The quest for a ‘new individualism’ is strong in Dewey
as well, together with the need for qualified participation. While the questions
are mostly similar, this article aims at establishing whether the answers are
too and whether a direct link can be traced between these two thinkers. Dewey
lived a long and productive life, he died at 92, and left an incommensurable
contribution to pedagogy. His work had a wide audience in post-WW2 Italy
and became an essential reference for pedagogists. His philosophy left lasting
traces throughout the Reggio Emilia Approach (REA). This article aims at
investigating whether Dewey’s vision of democracy and the role of schools in
fostering it, inspired Malaguzzi and the schools in Reggio Emilia. Malaguzzi’s
idea of democracy is a complex, multidimensional vision, that imbues all his
work and writings since the post-war years. Contrary to Dewey, Malaguzzi
does not dwell on philosophical argumentation, he often talks about it, but
almost never in connection with the official governmental form and never
defines it. Our investigation is therefore not easy. It implies deconstructing the
concept of democracy to find clues and indicators leading back to Dewey’s
vision. We will base our analysis on Dewey’s texts available to Malaguzzi and
compare them with his own writings, focusing on two main aspects:
participation and the relation between individual and society
Isotropic inelastic and superelastic collisional rates in a multiterm atom
The spectral line polarization of the radiation emerging from a magnetized
astrophysical plasma depends on the state of the atoms within the medium, whose
determination requires considering the interactions between the atoms and the
magnetic field, between the atoms and photons (radiative transitions), and
between the atoms and other material particles (collisional transitions). In
applications within the framework of the multiterm model atom (which accounts
for quantum interference between magnetic sublevels pertaining either to the
same J-level or to different J-levels within the same term) collisional
processes are generally neglected when solving the master equation for the
atomic density matrix. This is partly due to the lack of experimental data
and/or of approximate theoretical expressions for calculating the collisional
transfer and relaxation rates (in particular the rates for interference between
sublevels pertaining to different J-levels, and the depolarizing rates due to
elastic collisions). In this paper we formally define and investigate the
transfer and relaxation rates due to isotropic inelastic and superelastic
collisions that enter the statistical equilibrium equations of a multiterm
atom. Under the hypothesis that the atom-collider interaction can be described
by a dipolar operator, we provide expressions that relate the collisional rates
for interference between different J-levels to the usual collisional rates for
J-level populations. Finally, we apply the general equations to the case of a
two-term atom with unpolarized lower term, illustrating the impact of inelastic
and superelastic collisions on scattering polarization through radiative
transfer calculations in a slab of stellar atmospheric plasma anisotropically
illuminated by the photospheric radiation field.Comment: Accepted for publication in Astronomy & Astrophysic
Theoretical formulation of Doppler redistribution in scattering polarization within the framework of the velocity-space density matrix formalism
Within the framework of the density matrix theory for the generation and
transfer of polarized radiation, velocity density matrix correlations represent
an important physical aspect that, however, is often neglected in practical
applications by adopting the simplifying approximation of complete
redistribution on velocity. In this paper, we present an application of the
Non-LTE problem for polarized radiation taking such correlations into account
through the velocity-space density matrix formalism. We consider a two-level
atom with infinitely sharp upper and lower levels, and we derive the
corresponding statistical equilibrium equations neglecting the contribution of
velocity-changing collisions. Coupling such equations with the radiative
transfer equations for polarized radiation, we derive a set of coupled
equations for the velocity-dependent source function. This set of equations is
then particularized to the case of a plane-parallel atmosphere. The equations
presented in this paper provide a complete and solid description of the physics
of pure Doppler redistribution, a phenomenon generally described within the
framework of the redistribution matrix formalism. The redistribution matrix
corresponding to this problem (generally referred to as R_I) is derived
starting from the statistical equilibrium equations for the velocity-space
density matrix and from the radiative transfer equations for polarized
radiation, thus showing the equivalence of the two approaches.Comment: Accepted for publication in Astronomy & Astrophysic
Unveiling GRB hard X-ray afterglow emission with Simbol-X
Despite the enormous progress occurred in the last 10 years, the Gamma-Ray
Bursts (GRB) phenomenon is still far to be fully understood. One of the most
important open issues that have still to be settled is the afterglow emission
above 10 keV, which is almost completely unexplored. This is due to the lack of
sensitive enough detectors operating in this energy band. The only detection,
by the BeppoSAX/PDS instrument (15-200 keV), of hard X-ray emission from a GRB
(the very bright GRB 990123), combined with optical and radio observations,
seriously challenged the standard scenario in which the dominant mechanism is
synchrotron radiation produced in the shock of a ultra-relativistic fireball
with the ISM, showing the need of a substantial revision of present models. In
this respect, thanks to its unprecedented sensitivity in the 10-80 keV energy
band, Simbol-X, through follow-up observations of bright GRBs detected and
localized by GRB dedicated experiments that will fly in the >2010 time frame,
will provide an important breakthrough in the GRB field.Comment: 4 pages, 2 figures. Paper presented at "Simbol-X: the hard X-ray
universe in focus", held in Bologna, Italy, on 14-16 May 2007. To be
published in Memorie della Societa' Astronomica Italian
- …