125 research outputs found
BeppoSAX observations of low power radio galaxies: possible detection of obscured nuclei
We present the first results of BeppoSAX observations of a small sample of
low brightness FRI radio galaxies. The flux of all the targets is consistent
with a thermal spectrum, as due to the presence of hot intracluster gas or
galactic corona. Moreover in three sources a non thermal absorbed spectrum can
be present in the MECS spectrum at energies larger than 7 keV, while for a
fourth object a high energy flux has been detected in the PDS instrument at
energies larger than 15 keV. This component could be related to the inner AGN
surrounded by an obscuring torus.Comment: 4 pages, LateX, 3 figures (included). Uses espcrc2.sty (included). To
appear in: "The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE", Rome,
Italy, 21-24 October, 1997, Eds.: L. Scarsi, H. Bradt, P. Giommi and F. Fior
Two-component jet simulations: II. Combining analytical disk and stellar MHD outflow solutions
Theoretical arguments along with observational data of YSO jets suggest the
presence of two steady components: a disk wind type outflow needed to explain
the observed high mass loss rates and a stellar wind type outflow probably
accounting for the observed stellar spin down.
Each component's contribution depends on the intrinsic physical properties of
the YSO-disk system and its evolutionary stage. The main goal of this paper is
to understand some of the basic features of the evolution, interaction and
co-existence of the two jet components over a parameter space and when time
variability is enforced. Having studied separately the numerical evolution of
each type of the complementary disk and stellar analytical wind solutions in
Paper I of this series, we proceed here to mix together the two models inside
the computational box. The evolution in time is performed with the PLUTO code,
investigating the dynamics of the two-component jets, the modifications each
solution undergoes and the potential steady state reached.Comment: accepted for publication in A&
BeppoSAX observation of Hercules A and MRC 0625-536
We present BeppoSAX observations of the two FR I type radio galaxies Hercules A (3C 348) and MRC 0625-536 in the energy range keV. Data analysis shows that the X-ray flux from Hercules A is consistent with a diffuse thermal plasma emitting at keV with a possible, but somewhat uncertain, contribution of a softer component at keV. The non thermal emission from the active nucleus must be significantly smaller than the thermal one, and no indication of relevant core obscuration by a surrounding torus was detected. The flux from MRC 0625-536 originates form an extended region and has been fitted to a thermal law with keV and with a column density consistent with the galactic absorption. A spatially resolved spectral analysis does not show a relevant variation of the temperature and the metallicity across the diffuse emission zone. A non thermal spectral component, related to the nuclear activity, may be present in the innermost region with some possible amount of local obscuration, contributing \lapp 10% to the total luminosity. Hard X-ray emission from MRC 0625-536 has been detected in the PDS (15 - 200 keV) that may be related either to its galactic core or to the intracluster region
Time Variability in the X-ray Nebula Powered by Pulsar B1509-58
We use new and archival Chandra and ROSAT data to study the time variability
of the X-ray emission from the pulsar wind nebula (PWN) powered by PSR B1509-58
on timescales of one week to twelve years. There is variability in the size,
number, and brightness of compact knots appearing within 20" of the pulsar,
with at least one knot showing a possible outflow velocity of ~0.6c (assuming a
distance to the source of 5.2 kpc). The transient nature of these knots may
indicate that they are produced by turbulence in the flows surrounding the
pulsar. A previously identified prominent jet extending 12 pc to the southeast
of the pulsar increased in brightness by 30% over 9 years; apparent outflow of
material along this jet is observed with a velocity of ~0.5c. However, outflow
alone cannot account for the changes in the jet on such short timescales.
Magnetohydrodynamic sausage or kink instabilities are feasible explanations for
the jet variability with timescale of ~1.3-2 years. An arc structure, located
30"-45" north of the pulsar, shows transverse structural variations and appears
to have moved inward with a velocity of ~0.03c over three years. The overall
structure and brightness of the diffuse PWN exterior to this arc and excluding
the jet has remained the same over the twelve year span. The photon indices of
the diffuse PWN and possibly the jet steepen with increasing radius, likely
indicating synchrotron cooling at X-ray energies.Comment: accepted to ApJ, 14 pages, 8 figure
The hard X-ray emission from the complex SNR MSH 15-52 observed by BeppoSAX
We present the results of a BeppoSAX observation of the Supernova Remnant MSH 15-52, associated with the pulsar PSR B1509-58, and discuss its main morphological and spectroscopic properties in the 1.6--200 keV energy range (MECS and PDS instruments). The two main structures of the remnant, the Southern Nebula, the plerion centered on the pulsar, and the Northern Nebula, are clearly visible in the MECS, with the former showing a much a harder spectrum. Furthermore, a diffuse extended emission surrounds the whole remnant up to ~ 17' from the center. Non-thermal flux is detected in the PDS up to 200 keV as well, and it appears that also in this energy range the emission is not concentrated in the central region around the pulsar. These data imply that the plerion extends up to a few tens of parsecs from the pulsar
Stellar winds, dead zones, and coronal mass ejections
Axisymmetric stellar wind solutions are presented, obtained by numerically
solving the ideal magnetohydrodynamic (MHD) equations. Stationary solutions are
critically analysed using the knowledge of the flux functions. These flux
functions enter in the general variational principle governing all axisymmetric
stationary ideal MHD equilibria. The magnetized wind solutions for
(differentially) rotating stars contain both a `wind' and a `dead' zone. We
illustrate the influence of the magnetic field topology on the wind
acceleration pattern, by varying the coronal field strength and the extent of
the dead zone. This is evident from the resulting variations in the location
and appearance of the critical curves where the wind speed equals the slow,
Alfven, and fast speed. Larger dead zones cause effective, fairly isotropic
acceleration to super-Alfvenic velocities as the polar, open field lines are
forced to fan out rapidly with radial distance. A higher field strength moves
the Alfven transition outwards. In the ecliptic, the wind outflow is clearly
modulated by the extent of the dead zone. The combined effect of a fast stellar
rotation and an equatorial `dead' zone in a bipolar field configuration can
lead to efficient thermo-centrifugal equatorial winds. Such winds show both a
strong poleward collimation and some equatorward streamline bending due to
significant toroidal field pressure at mid-latitudes. We discuss how coronal
mass ejections are then simulated on top of the transonic outflows.Comment: scheduled for Astrophys. J. 530 #2, Febr.20 2000 issue. 9 figures (as
6 jpeg and 8 eps files
Two-component jet simulations: I. Topological stability of analytical MHD outflow solutions
Observations of collimated outflows in young stellar objects indicate that
several features of the jets can be understood by adopting the picture of a
two-component outflow, wherein a central stellar component around the jet axis
is surrounded by an extended disk-wind. The precise contribution of each
component may depend on the intrinsic physical properties of the YSO-disk
system as well as its evolutionary stage. In this context, the present article
starts a systematic investigation of two-component jet models via
time-dependent simulations of two prototypical and complementary analytical
solutions, each closely related to the properties of stellar-outflows and
disk-winds. These models describe a meridionally and a radially self-similar
exact solution of the steady-state, ideal hydromagnetic equations,
respectively. By using the PLUTO code to carry out the simulations, the study
focuses on the topological stability of each of the two analytical solutions,
which are successfully extended to all space by removing their singularities.
In addition, their behavior and robustness over several physical and numerical
modifications is extensively examined. It is found that radially self-similar
solutions (disk-winds) always reach a final steady-state while maintaining all
their well-defined properties. The different ways to replace the singular part
of the solution around the symmetry axis, being a first approximation towards a
two-component outflow, lead to the appearance of a shock at the super-fast
domain corresponding to the fast magnetosonic separatrix surface. Conversely,
the asymptotic configuration and the stability of meridionally self-similar
models (stellar-winds) is related to the heating processes at the base of the
wind.Comment: Accepted for publication in A&
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