80 research outputs found
Angular momentum transport by thermal emission in black hole accretion disks
We calculate the amount of angular momentum that thermal photons carry out of
a viscous black hole accretion disc, due to the strong Doppler shift imparted
to them by the high orbital velocity of the radiating disc material. While the
emission of radiation can not drive accretion on its own, we find that it does
result in a loss of specific angular momentum, thereby contributing to an
otherwise viscosity-driven accretion flow. In particular, we show that the
fraction of the angular momentum that is lost to thermal emission at a radius r
in a standard, multi-color disc is ~ 0.4r_s/r, where r_s is the Schwarzschild
radius of the black hole. We briefly highlight the key similarities between
this effect and the closely related Poynting-Robertson effect.Comment: 6 pages, 2 figures; to appear in A
Pseudo-Newtonian Potential for Charged Particle in Kerr-Newman Geometry
We consider the equatorial circular motion of a test particle of specific
charge q/m << 1 in the Kerr-Newman geometry of a rotating charged black hole.
We find the particle's conserved energy and conserved projection of the angular
momentum on the black hole's axis of rotation as corrections, in leading order
of q/m, to the corresponding energy and angular momentum of a neutral particle.
We determine the centripetal force acting on the test particle and,
consequently, we find a classical pseudo-Newtonian potential with which one can
mimic this general relativistic problem.Comment: 8 page
Thermal Equilibria of Optically Thin, Magnetically Supported, Two-Temperature, Black Hole Accretion Disks
We obtained thermal equilibrium solutions for optically thin, two-temperature
black hole accretion disks incorporating magnetic fields. The main objective of
this study is to explain the bright/hard state observed during the bright/slow
transition of galactic black hole candidates. We assume that the energy
transfer from ions to electrons occurs via Coulomb collisions. Bremsstrahlung,
synchrotron, and inverse Compton scattering are considered as the radiative
cooling processes. In order to complete the set of basic equations, we specify
the magnetic flux advection rate. We find magnetically supported (low-beta),
thermally stable solutions. In these solutions, the total amount of the heating
via the dissipation of turbulent magnetic fields goes into electrons and
balances the radiative cooling. The low- solutions extend to high mass
accretion rates and the electron temperature is moderately cool. High
luminosities and moderately high energy cutoffs in the X-ray spectrum observed
in the bright/hard state can be explained by the low-beta solutions.Comment: 24 pages, 10 figures,accepted for publication in Astrophysical
Journa
Critical angular momentum distributions in collapsars: quiescent periods from accretion state transitions in long gamma-ray bursts
The rotation rate in pre-supernova cores is an important ingredient which can
profoundly affect the post-collapse evolution and associated energy release in
supernovae and long gamma ray bursts (LGRBs). Previous work has focused on
whether the specific angular momentum is above or below the critical value
required for the creation of a centrifugally supported disk around a black
hole. Here, we explore the effect of the distribution of angular momentum with
radius in the star, and show that qualitative transitions between high and low
angular momentum flow, corresponding to high and low luminosity accretion
states, can effectively be reflected in the energy output, leading to
variability and the possibility of quiescent times in LGRBs.Comment: 22 pages, 6 figures, 2 Tables, accepted for publication in Ap
Response of a circumbinary accretion disc to black hole mass loss
We investigate the evolution of the surface density of a circumbinary
accretion disc after the mass loss induced by the merger of two supermassive
black holes. We first introduce an analytical model, under the assumption of a
disc composed of test particles, to derive the surface density evolution of the
disc following the mass loss. The model predicts the formation of sharp density
peaks in the disc; the model also allows us to compute the typical timescale
for the formation of these peaks. To test and validate the model, we run
numerical simulations of the process using the Smoothed Particle Hydrodynamics
(SPH) code PHANTOM, taking fluid effects into account. We find good agreement
in the shape and position of the peaks between the model and the simulations.
In a fluid disc, however, the epicyclic oscillations induced by the mass loss
can dissipate, and only some of the predicted peaks form in the simulation. To
quantify how fast this dissipation proceeds, we introduce an appropriate
parameter, and we show that it is effective in explaining the differences
between the analytical, collisionless model and a real fluid disc.Comment: 10 pages, 11 figures, accepted by MNRAS; added reference in v2.
Animations of the simulations are available at http://db.tt/ceqkpmF
Analytic solutions to the accretion of a rotating finite cloud towards a central object - II. Schwarzschild spacetime
We construct a general relativistic model for the accretion flow of a
rotating finite cloud of non-interacting particles infalling onto a
Schwarzschild black hole. The streamlines start at a spherical shell, where
boundary conditions are fixed, and are followed down to the point at which they
either cross the black hole horizon or become incorporated into an equatorial
thin disc. Analytic expressions for the streamlines and the velocity field are
given, in terms of Jacobi elliptic functions, under the assumptions of
stationarity and ballistic motion. A novel approach allows us to describe all
of the possible types of orbit with a single formula. A simple numerical scheme
is presented for calculating the density field. This model is the relativistic
generalisation of the Newtonian one developed by Mendoza, Tejeda, Nagel, 2009
and, due to its analytic nature, it can be useful in providing a benchmark for
general relativistic hydrodynamical codes and for exploring the parameter space
in applications involving accretion onto black holes when the approximations of
steady state and ballistic motion are reasonable ones.Comment: 12 pages, 6 figures, references and minor changes added to match
version accepted for publication in MNRA
Trumpler 20 - an old and rich open cluster
We show that the open cluster Trumpler 20, contrary to the earlier findings,
is actually an old Galactic open cluster. New CCD photometry and
high-resolution spectroscopy are used to derive the main parameters of this
cluster. At [Fe/H]=-0.11 for a single red giant star, the metallicity is
slightly subsolar. The best fit to the color-magnitude diagrams is achieved
using a 1.3 Gyr isochrone with convective overshoot. The cluster appears to
have a significant reddening at E(B-V)=0.46 (for B0 spectral type), although
for red giants this high reddening yields the color temperature exceeding the
spectroscopic T_eff by about 200 K. Trumpler 20 is a very rich open cluster,
containing at least 700 members brighter than M_V=+4. It may extend over the
field-of-view available in our study at 20'x20'.Comment: 7 pages, 5 figures; accepted for publication in MNRA
Period-colour and amplitude-colour relations in classical Cepheid variables II: the Galactic Cepheid model
In this paper, we construct full amplitude non-linear hydrodynamical models
of fundamental mode Galactic Cepheids and analyze the resulting theoretical
period-colour and amplitude-colour relations at maximum, mean and minimum
light. These theoretical relations match the general form of the observed
relations well. This agreement is, to some extent, independent of the
mass-luminosity relations used, pulsation code, numerical techniques, details
of the input physics and methods to convert theoretical quantities, such as
bolometric luminosity and temperature, to observational quantities, such as V
band magnitudes or colours. We show that the period-colour and
amplitude-colour properties of fundamental mode Galactic Cepheids with periods
such that can be explained by a simple application of the
Stefan-Boltzmann law and the interaction of the photosphere with the hydrogen
ionization front. We discuss the implications of our results for explaining the
behavior of Galactic Cepheid period-colour, and period-luminosity relations at
mean light.Comment: 13 pages, 11 figures and 5 tables. MNRAS submitte
A microquasar classification from a disk instability perspective
The spectacular variability of microquasars has led to a long string of
efforts in order to classify their observed behaviors in a few states. The
progress made in the understanding of the Quasi-Periodic Oscillations observed
in these objects now makes it possible to develop a new way to find order in
their behavior, based on the theorized physical processes associated with these
oscillations. This will also have the interest of reuniting microquasars in a
single classification based on the physical processes at work and therefore
independent of their specificities (mass, variation timescale, outburst
history, etc.). This classification is aimed to be a tool to further our
understanding of microquasars behavior and not to replace phenomenological
states. We start by considering three instabilities that can cause accretion in
the disk. We compare the conditions for their development, and the
Quasi-Periodic Oscillations they can be expected to produce, with the spectral
states in which these Quasi-Periodic Oscillations are observed and sometimes
coexist. From the three instabilities that we proposed to explain the three
states of GRS 1915+105 we actually found the theoretical existence of four
states. We compared those four states with observations and also how those four
states can be seen in a model-independent fashion. Those four state can be used
to find an order in microquasar observations, based on the properties of the
Quasi-Periodic Oscillations and the physics of the associated instabilities.Comment: accepted by A&
A robust statistical estimation of the basic parameters of single stellar populations. I. Method
The colour-magnitude diagrams of resolved single stellar populations, such as
open and globular clusters, have provided the best natural laboratories to test
stellar evolution theory. Whilst a variety of techniques have been used to
infer the basic properties of these simple populations, systematic
uncertainties arise from the purely geometrical degeneracy produced by the
similar shape of isochrones of different ages and metallicities. Here we
present an objective and robust statistical technique which lifts this
degeneracy to a great extent through the use of a key observable: the number of
stars along the isochrone. Through extensive Monte Carlo simulations we show
that, for instance, we can infer the four main parameters (age, metallicity,
distance and reddening) in an objective way, along with robust confidence
intervals and their full covariance matrix. We show that systematic
uncertainties due to field contamination, unresolved binaries, initial or
present-day stellar mass function are either negligible or well under control.
This technique provides, for the first time, a proper way to infer with
unprecedented accuracy the fundamental properties of simple stellar
populations, in an easy-to-implement algorithm.Comment: 17 pages, 12 figures, MNRAS, in pres
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