380 research outputs found
Identification of Shocks in the Spectra from Black Holes
We study the spectral properties of a low angular momentum flow as a function
of the shock strength, compression ratio, accretion rate and flow geometry. In
the absence of a satisfactory description of magnetic fields inside the
advective disk, we consider the presence of only stochastic fields and use the
ratio of the field energy to the gravitational energy density as a parameter.
We not only include `conventional' synchrotron emission and Comptonization by
Maxwell-Bolzmann electrons in the gas, but we also compute these effects due to
power-law electrons. For strong shocks, a bump is produced due to the
post-shock flow. A power-law spectral components due to the thermal and
non-thermal electrons appear after this bump.Comment: 8 pages, 5 figures, Astronomy and Space Science (in press),
Proceedings of the Hong Kong Conference (2004) Edited by Cheng and Romer
Dissipative accretion flows around a rotating black hole
We study the dynamical structure of a cooling dominated rotating accretion
flow around a spinning black hole. We show that non-linear phenomena such as
shock waves can be studied in terms of only three flow parameters, namely, the
specific energy (), the specific angular momentum () and the
accretion rate () of the flow. We present all possible accretion
solutions. We find that a significant region of the parameter space in the
plane allows global accretion shock solutions. The effective
area of the parameter space for which the Rankine-Hugoniot shocks are possible
is maximum when the flow is dissipation free. It decreases with the increase of
cooling effects and finally disappears when the cooling is high enough. We show
that shock forms further away when the black hole is rotating compared to the
solution around a Schwarzschild black hole with identical flow parameters at a
large distance. However, in a normalized sense, the flow parameters for which
the shocks form around the rotating black holes are produced shocks closer to
the hole. The location of the shock is also dictated by the cooling efficiency
in that higher the accretion rate (), the closer is the shock
location. We believe that some of the high frequency quasi-periodic
oscillations may be due to the flows with higher accretion rate around the
rotating black holes.Comment: 9 pages, 7 figures. To appear in MNRA
Satellite observations of thought experiments close to a black hole
Since black holes are `black', methods of their identification must
necessarily be indirect. Due to very special boundary condition on the horizon,
the advective flow behaves in a particular way, which includes formation of
centrifugal pressure dominated boundary layer or CENBOL where much of the
infall energy is released and outflows are generated. The observational aspects
of black holes must depend on the steady and time-dependent properties of this
boundary layer. Several observational results are written down in this review
which seem to support the predictions of thought experiments based on this
advective accretion/outflow model. In future, when gravitational waves are
detected, some other predictions of this model could be tested as well.Comment: Published in Classical and Quantum Gravity, v. 17, No. 12, p. 2427,
200
Radiatively Driven Plasma Jets around Compact Objects
Matter accreting onto black holes may develop shocks due to the centrifugal
barrier. A part of inflowing matter in the post-shock flow is deflected along
the axis in the form of jets. Post-shock flow which behaves like a Compton
cloud has `hot' electrons emiting high energy photons. We study the effect of
these `hot' photons on the outflowing matter. Radiation from this region could
accelerate the outflowing matter but radiation pressure should also slow it
down. We show that the radiation drag restricts the flow from attaining a very
high velocity. We introduce the concept of an `equilibrium velocity' () which sets the upper limit of the terminal velocity achieved by a
cold plasma due to radiation deposition force in the absence of gravity. If the
injection energy is , then we find that the terminal velocity
satisfies a relation v_\infty^2 \lsim v_{eq}^2 + 2 E_{in}.Comment: Accepted for publication in MNRA
Radiatively driven electron-positron jets from two component accretion flows
Matter accreting onto black holes has long been known to have standing or
oscillating shock waves. The post-shock matter puffs up in the form of a torus,
which intercepts soft photons from the outer Keplerian disc and inverse
Comptonizes to produce hard photons. The post-shock region also produces jets.
We study the interaction of both hard photons and soft photons, with on-axis
electron-positron jets. We show that the radiation from post-shock torus
accelerates the flow to relativistic velocities, while that from the Keplerian
disc has marginal effect. We also show that, the velocity at infinity or
terminal velocity , depends on the shock location in the disc.Comment: 24 pages, 8 figures, accepted in MNRA
Accretion Disks Around Black Holes: Twenty Five Years Later
We study the progress of the theory of accretion disks around black holes in
last twenty five years and explain why advective disks are the best bet in
explaining varied stationary and non-stationary observations from black hole
candidates. We show also that the recently proposed advection dominated flows
are incorrect.Comment: 30 Latex pages including figures. Kluwer Style files included.
Appearing in `Observational Evidence for Black Holes in the Universe', ed.
Sandip K. Chakrabarti, Kluwer Academic Publishers (DORDRECHT: Holland
Particle acceleration in ultra-relativistic oblique shock waves
We perform Monte Carlo simulations of diffusive shock acceleration at highly
relativistic oblique shock waves. High upstream flow Lorentz gamma factors are
used, which are relevant to models of ultra relativistic particle shock
acceleration in Active Galactic Nuclei (AGN) central engines and relativistic
jets and Gamma Ray Burst (GRB) fireballs. We investigate numerically the
acceleration properties -in the ultra relativistic flow regime of - such as angular distribution, acceleration time constant, particle
energy gain versus number of crossings and spectral shapes. We perform
calculations for sub-luminal and super-luminal shocks, using two different
approaches respectively. The energization for the first crossing
cycle and the significantly large energy gain for subsequent crossings as well
as the high 'speed up' factors found, are important in supporting the Vietri
and Waxman models on GRB ultra-high energy cosmic ray, neutrino, and gamma-ray
output.Comment: 24 pages, 35 figures, accepted for publication in Astroparticle
Physic
Scalar and Spinor Perturbation to the Kerr-NUT Spacetime
We study the scalar and spinor perturbation, namely the Klein-Gordan and
Dirac equations, in the Kerr-NUT space-time. The metric is invariant under the
duality transformation involving the exchange of mass and NUT parameters on one
hand and radial and angle coordinates on the other. We show that this
invariance is also shared by the scalar and spinor perturbation equations.
Further, by the duality transformation, one can go from the Kerr to the dual
Kerr solution, and vice versa, and the same applies to the perturbation
equations. In particular, it turns out that the potential barriers felt by the
incoming scalar and spinor fields are higher for the dual Kerr than that for
the Kerr. We also comment on existence of horizon and singularity.Comment: 31 pages including 20 figures, RevTeX style: Final version to appear
in Classical and Quantum Gravit
Radiation Transfer of Models of Massive Star Formation. I. Dependence on Basic Core Properties
Radiative transfer calculations of massive star formation are presented.
These are based on the Turbulent Core Model of McKee & Tan and
self-consistently included a hydrostatic core, an inside-out expansion wave, a
zone of free-falling rotating collapse, wide-angle dust-free outflow cavities,
an active accretion disk, and a massive protostar. For the first time for such
models, an optically thick inner gas disk extends inside the dust destruction
front. This is important to conserve the accretion energy naturally and for its
shielding effect on the outer region of the disk and envelope. The simulation
of radiation transfer is performed with the Monte Carlo code of Whitney,
yielding spectral energy distributions (SEDs) for the model series, from the
simplest spherical model to the fiducial one, with the above components each
added step-by-step. Images are also presented in different wavebands of various
telescope cameras, including Spitzer IRAC and MIPS, SOFIA FORCAST and Herschel
PACS and SPIRE. The existence of the optically thick inner disk produces higher
optical wavelength fluxes but reduces near- and mid-IR emission. The presence
of outflow cavities, the inclination angle to the line of sight, and the
thickness of the disk all affect the SEDs and images significantly. For the
high mass surface density cores considered here, the mid-IR emission can be
dominated by the outflow cavity walls, as has been suggested by De Buizer. The
effect of varying the pressure of the environment bounding the surface of the
massive core is also studied. With lower surface pressures, the core is larger,
has lower extinction and accretion rates, and the observed mid-IR flux from the
disk can then be relatively high even though the accretion luminosity is lower.
In this case the silicate absorption feature becomes prominent, in contrast to
higher density cores forming under higher pressures.Comment: 19 pages, 14 figures, 2 tables, accepted for publication in Ap
Accretion Disc Theory: From the Standard Model Until Advection
Accretion disc theory was first developed as a theory with the local heat
balance, where the whole energy produced by a viscous heating was emitted to
the sides of the disc. One of the most important new invention of this theory
was a phenomenological treatment of the turbulent viscosity, known as ''alpha''
prescription, when the (r) component of the stress tensor was
approximated by ( P) with a unknown constant . This
prescription played the role in the accretion disc theory as well important as
the mixing-length theory of convection for stellar evolution. Sources of
turbulence in the accretion disc are discussed, including nonlinear
hydrodynamical turbulence, convection and magnetic field role. In parallel to
the optically thick geometrically thin accretion disc models, a new branch of
the optically thin accretion disc models was discovered, with a larger
thickness for the same total luminosity. The choice between these solutions
should be done of the base of a stability analysis. The ideas underlying the
necessity to include advection into the accretion disc theory are presented and
first models with advection are reviewed. The present status of the solution
for a low-luminous optically thin accretion disc model with advection is
discussed and the limits for an advection dominated accretion flows (ADAF)
imposed by the presence of magnetic field are analysed.Comment: Roceeding of the Int. Workshop "Observational Evidence for Black
Holes in the Universe". Calcutta, 11-17 January 1998. Kluwer Acad. Pu
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