353 research outputs found
A Novel Jet Model: Magnetically Collimated, Radiation-Pressure Driven Jet
Relativistic jets from compact objects are ubiquitous phenomena in the
Unvierse, but their driving mechanism has been an enigmatic issue over many
decades. Two basic models have been extensively discussed: magnetohydrodynamic
(MHD) jets and radiation-hydrodynamic (RHD) jets. Currently, the former is more
widely accepted, since magnetic field is expected to provide both the
acceleration and collimation mechanisms, whereas radiation field cannot
collimate outflow. Here, we propose a new type of jets,
radiation-magnetohydrodynamic (RMHD) jets, based on our global RMHD simulation
of luminous accretion flow onto a black hole shining above the Eddington
luminosity. The RMHD jet can be accelerated up to the relativistic speed by the
radiation-pressure force and is collimated by the Lorentz force of a magnetic
tower, inflated magnetic structure made by toroidal magnetic field lines
accumulated around the black hole, though radiation energy greatly dominates
over magnetic energy. This magnetic tower is collimated by a geometrically
thick accretion flow supported by radiation-pressure force. This type of jet
may explain relativistic jets from Galactic microquasars, appearing at high
luminosities.Comment: 5 pages, 2 figures, accepted for publication in PAS
The influence of Galactic wind upon the star formation histories of Local Group galaxies
We examine the possibility that ram pressure exerted by the galactic wind
from the Galaxy could have stripped gas from the Local Group dwarf galaxies,
thereby affecting their star formation histories. Whether gas stripping occurs
or not depends on the relative magnitudes of two counteracting forces acting on
gas in a dwarf galaxy: ram pressure force by the wind and the gravitational
binding force by the dwarf galaxy itself. We suggest that the galactic wind
could have stripped gas in a dwarf galaxy located within the distance of
kpc
(where is the surface radius and is the total binding
energy of the dwarf galaxy, respectively) from the Galaxy within a timescale of
Gyr, thereby preventing star formation there. Our result based on this Galactic
wind model explains the recent observation that dwarfs located close to the
Galaxy experienced star formation only in the early phase of their lifetimes,
whereas distant dwarfs are still undergoing star formation. The present star
formation in the Large Magellanic Cloud can also be explained through our
Galactic wind model.Comment: 7 pages LaTeX, no figures, to appear in MNRA
Spectral energy distribution of super-Eddington flows
Spectral properties of super-Eddington accretion flows are investigated by
means of a parallel line-of-sight calculation. The subjacent model, taken from
two-dimensional radiation hydrodynamic simulations by Ohsuga et al. (2005),
consists of a disc accretion region and an extended atmosphere with high
velocity outflows. The non-gray radiative transfer equation is solved,
including relativistic effects, by applying the FLD approximation.
The calculated spectrum is composed of a thermal, blackbody-like emission
from the disc which depends sensitively on the inclination angle, and of high
energy X-ray and gamma-ray emission from the atmosphere. We find mild beaming
effects in the thermal radiation for small inclination angles. If we compare
the face-on case with the edge-on case, the average photon energy is larger by
a factor of ~1.7 due mainly to Doppler boosting, while the photon number
density is larger by a factor of ~3.7 due mainly to anisotropic matter
distribution around the central black hole. This gives an explanation for the
observed X-ray temperatures of ULXs which are too high to be explained in the
framework of intermediate-mass black holes.
While the main features of the thermal spectral component are consistent with
more detailed calculations of slim accretion discs, the atmosphere induces
major changes in the high-energy part, which cannot be reproduced by existing
models. In order to interpret observational data properly, simple approaches
like the Eddington-Barbier approximation cannot be applied.Comment: 10 pages, 8 figures, accepted for publication in MNRA
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