458 research outputs found
Spectral variability in transonic discs around black holes
Transonic discs with accretion rates relevant to intrinsically bright
Galactic X-ray sources (-) exhibit
a time dependent cyclic behaviour due to the onset of a thermal instability
driven by radiation pressure. In this paper we calculate radiation spectra
emitted from thermally-unstable discs to provide detailed theoretical
predictions for observationally relevant quantities. The emergent spectrum has
been obtained by solving self-consistently the vertical structure and radiative
transfer in the disc atmosphere. We focus on four particular stages of the disc
evolution, the maximal evacuation stage and three intermediate stages during
the replenishment phase. The disc is found to undergo rather dramatic spectral
changes during the evolution, emitting mainly in the 1-10 keV band during
outburst and in the 0.1-1 keV band off-outburst. Local spectra, although
different in shape from a blackbody at the disc effective temperature, may be
characterized in terms of a hardening factor . We have found that is
rather constant both in radius and in time, with a typical value .Comment: 10 pages Latex with 11 ps figures. Accepted for publication in MNRA
Is the Bursting Radio-source GCRT J1745-3009 a Double Neutron Star Binary ?
GCRT J1745-3009 is a peculiar transient radio-source in the direction of the
Galactic Center. It was observed to emit a series of ~ 1 Jy bursts at 0.33 GHz,
with typical duration ~ 10 min and at apparently regular intervals of ~ 77 min.
If the source is indeed at the distance of the Galactic Center as it seems
likely, we show that its observational properties are compatible with those
expected from a double neutron star binary, similar to the double pulsar system
J0737-3039. In the picture we propose the (coherent) radio emission comes from
the shock originating in the interaction of the wind of the more energetic
pulsar with the magnetosphere of the companion. The observed modulation of the
radio signal is the consequence of an eccentric orbit, along which the
separation between the two stars varies. This cyclically drives the shock
inside the light cylinder radius of the less energetic pulsar.Comment: 5 pages, 3 figures, accepted for publication in The Astrophysical
Journal Letters, comment on geodetic precession adde
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
Advection-dominated Inflow/Outflows from Evaporating Accretion Disks
In this Letter we investigate the properties of advection-dominated accretion
flows (ADAFs) fed by the evaporation of a Shakura-Sunyaev accretion disk (SSD).
In our picture the ADAF fills the central cavity evacuated by the SSD and
extends beyond the transition radius into a coronal region. We find that,
because of global angular momentum conservation, a significant fraction of the
hot gas flows away from the black hole forming a transsonic wind, unless the
injection rate depends only weakly on radius (if , ). The Bernoulli number of the inflowing gas is negative
if the transition radius is Schwarzschild radii, so matter
falling into the hole is gravitationally bound. The ratio of inflowing to
outflowing mass is , so in these solutions the accretion rate is
of the same order as in standard ADAFs and much larger than in
advection-dominated inflow/outflow models (ADIOS). The possible relevance of
evaporation-fed solutions to accretion flows in black hole X-ray binaries is
briefly discussed.Comment: 5 pages Latex with 2 ps figures. Accepted for publication in ApJ
Letter
Phosphorus recovery from a pilot-scale grate furnace: influencing factors beyond wet chemical leaching conditions
Phosphorus is a non-renewable resource going to exhaustion in the future. Sewage sludge ash is a promising secondary raw material due to its high phosphorus content. In this work, the distribution of 19 elements in bottom and cyclone ashes from pilot-scale grate furnace have been monitored to determine the suitability for the phosphorus acid extraction. Moreover, the influence of some parameters beyond wet chemical leaching conditions were investigated. Experimental results showed that bottom ash presented lower contamination in comparison to cyclone ash and low co-dissolution of heavy metals (especially Cr, Pb and Ni), while high phosphorus extraction efficiencies (76-86%) were achieved. High Al content in the bottom ash (9.4%) negatively affected the phosphorus extraction efficiency as well as loss on ignition, while the particle size reduction was necessary for ensuring a suitable contact surface. The typology of precipitating agents did not strongly affect the phosphorus precipitation, while pH was the key parameter. At pH 3.5-5, phosphorus precipitation efficiencies higher than 90% were achieved, with a mean phosphorus content in the recovered material equal to 16-17%, comparable to commercial fertilizers. Instead, the co-precipitation of Fe and Al had a detrimental effect on the recovered material, indicating the need for additional treatments
A Note on Bimodal Accretion Disks
The existence of bimodal disks is investigated. Following a simple argument
based on energetic considerations we show that stationary, bimodal accretion
disk models in which a Shakura--Sunyaev disk (SSD) at large radii matches an
advection dominated accretion flow (ADAF) at smaller radii are never possible
using the standard slim disk approach, unless some extra energy flux is
present. The same argument, however, predicts the possibility of a transition
from an outer Shapiro--Lightman--Eardley (SLE) disk to an ADAF, and from a SLE
disk to a SSD. Both types of solutions have been found.Comment: 9 pages including 9 figures, accepted for publication in The
Astrophysical Journa
Compton Scattering in Static and Moving Media. II. System-Frame Solutions for Spherically Symmetric Flows
I study the formation of Comptonization spectra in spherically symmetric,
fast moving media in a flat spacetime. I analyze the mathematical character of
the moments of the transfer equation in the system-frame and describe a
numerical method that provides fast solutions of the time-independent radiative
transfer problem that are accurate in both the diffusion and free-streaming
regimes. I show that even if the flows are mildly relativistic (V~0.1, where V
is the electron bulk velocity in units of the speed of light), terms that are
second-order in V alter the emerging spectrum both quantitatively and
qualitatively. In particular, terms that are second-order in V produce
power-law spectral tails, which are the dominant feature at high energies, and
therefore cannot be neglected. I further show that photons from a static source
are upscattered by the bulk motion of the medium even if the velocity field
does not converge. Finally, I discuss these results in the context of radial
accretion onto and outflows from compact objects.Comment: 28 pages, 9 figures; minor changes, to appear in the Astrophysical
Journa
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