240 research outputs found
The upper kHz QPO: a gravitationally lensed vertical oscillation
We show that a luminous torus in the Schwarzschild metric oscillating along
its own axis gives rise to a periodically varying flux of radiation, even
though the source of radiation is steady and perfectly axisymmetric. This
implies that the simplest oscillation mode in an accretion flow, axisymmetric
up-and-down motion at the meridional epicyclic frequency, may be directly
observable when it occurs in the inner parts of accretion flow around neutron
stars and black holes. The high-frequency modulations of the X-ray flux
observed in low-mass X-ray binaries at two frequencies (twin kHz QPOs) could
then be a signature of strong gravity both because radial and meridional
oscillations have different frequencies in non-Newtonian gravity, and because
strong gravitational deflection of light rays causes the flux of radiation to
be modulated at the higher frequency.Comment: 8 p., 4 fig
Strong to weak coupling transitions of SU(N) gauge theories in 2+1 dimensions
We investigate strong-to-weak coupling transitions in D=2+1 SU(N->oo) gauge
theories, by simulating lattice theories with a Wilson plaquette action. We
find that there is a strong-to-weak coupling cross-over in the lattice theory
that appears to become a third-order phase transition at N=oo, in a manner that
is essentially identical to the Gross-Witten transition in the D=1+1 SU(oo)
lattice gauge theory. There is also evidence for a second order transition at
N=oo at approximately the same coupling, which is connected with centre
monopoles (instantons) and so analogues to the first order bulk transition that
occurs in D=3+1 lattice gauge theories for N>4. We show that as the lattice
spacing is reduced, the N=oo gauge theory on a finite 3-torus suffers a
sequence of (apparently) first-order ZN symmetry breaking transitions
associated with each of the tori (ordered by size). We discuss how these
transitions can be understood in terms of a sequence of deconfining transitions
on ever-more dimensionally reduced gauge theories.We investigate whether the
trace of the Wilson loop has a non-analyticity in the coupling at some critical
area, but find no evidence for this although, just as in D=1+1,the eigenvalue
density of a Wilson loop forms a gap at N=oo for a critical trace. The physical
implications of this are unclear.The gap formation is a special case of a
remarkable similarity between the eigenvalue spectra of Wilson loops in D=1+1
and D=2+1 (and indeed D=3+1): for the same value of the trace, the eigenvalue
spectra are nearly identical.This holds for finite as well as infinite N;
irrespective of the Wilson loop size in lattice units; and for Polyakov as well
as Wilson loops.Comment: 44 pages, 28 figures. Extensive changes and clarifications with new
results on non-analyticities and eigenvalue spectra of Wilson loops. This
version to be submitted for publicatio
Testing wind as an explanation for the spin problem in the continuum-fitting method
The continuum-fitting method is one of the two most advanced methods of
determining the black hole spin in accreting X-ray binary systems. There are,
however, still some unresolved issues with the underlying disk models. One of
them manifests as an apparent decrease in spin for increasing source
luminosity. Here, we perform a few simple tests to establish whether outflows
from the disk close to the inner radius can address this problem. We employ
four different parametric models to describe the wind and compare these to the
apparent decrease in spin with luminosity measured in the sources LMC~X-3 and
GRS~1915+105. Wind models in which parameters do not explicitly depend on the
accretion rate cannot reproduce the spin measurements. Models with mass
accretion rate dependent outflows, however, have spectra that emulate the
observed ones. The assumption of a wind thus effectively removes the artifact
of spin decrease. This solution is not unique; the same conclusion can be
obtained with a truncated inner disk model. To distinguish among valid models,
high resolution X-ray data and a realistic description of the Comptonization in
the wind will be needed.Comment: 14 pages, 11 figures, accepted by Ap
Infinite N phase transitions in continuum Wilson loop operators
We define smoothed Wilson loop operators on a four dimensional lattice and
check numerically that they have a finite and nontrivial continuum limit. The
continuum operators maintain their character as unitary matrices and undergo a
phase transition at infinite N reflected by the eigenvalue distribution closing
a gap in its spectrum when the defining smooth loop is dilated from a small
size to a large one. If this large N phase transition belongs to a solvable
universality class one might be able to calculate analytically the string
tension in terms of the perturbative Lambda-parameter. This would be achieved
by matching instanton results for small loops to the relevant large-N-universal
function which, in turn, would be matched for large loops to an effective
string theory. Similarities between our findings and known analytical results
in two dimensional space-time indicate that the phase transitions we found only
affect the eigenvalue distribution, but the traces of finite powers of the
Wilson loop operators stay smooth under scaling.Comment: 31 pages, 9 figures, typos and references corrected, minor
clarifications adde
A theoretical study of the time-lags due to Comptonization and the constraints on the X-ray corona in AGN
We study the Fourier time-lags due to the Comptonization of disc-emitted
photons in a spherical, uniform, and stationary X-ray corona, which located on
the rotational axis of the black hole. We use Monk, a general relativistic
Monte-Carlo radiative transfer code, to calculate Compton scattering of photons
emitted by a thin disc with a Novikov-Thorne temperature profile. We find that
the model time-lags due to Comptonization remain constant up to a
characteristic frequency and then rapidly decrease to zero at higher
frequencies. We provide equations which can be used to determine the time-lags
and cross spectra for a wide range of values for the corona radius,
temperature, optical depth, height, and for various accretion rates and black
hole masses. We also provide an equation for the X-ray luminosity of a single
corona, as a function of the its characteristics and location above the disc.
Remarkably, the observed X-ray time-lags of nearby, bright active galaxies can
be successfully reproduced by inverse Comptonization process of multiple
dynamic coronae.Comment: 16 pages, 17 figures; accepted for publication in MNRA
Vertical dissipation profiles and the photosphere location in thin and slim accretion disks
We calculate optically thick but geometrically thin (and slim) accretion disk
models and perform a ray-tracing of photons (in the Kerr geometry) to calculate
the observed disk spectra. Previously, it was a common practice to ray-trace
photons assuming that they are emitted from the Kerr geometry equatorial plane,
z = 0. We show that the spectra calculated with this assumption differ from
these calculated under the assumption that photons are emitted from the actual
surface of the disc, z = H(r). This implies that a knowledge of the location of
the thin disks effective photosphere is relevant for calculating the spectra.
In this paper we investigate, in terms of a simple toy model, a possible
influence of the (unknown, and therefore ad hoc assumed) vertical dissipation
profiles on the vertical structure of the disk and thus on the location of the
effective photosphere, and on the observed spectra. For disks with moderate and
high mass accretion rates (\dot m>0.01\dot m_C) we find that the photosphere
location in the inner, radiation pressure dominated, disk region (where most of
the radiation comes from) does not depend on the dissipation profile and
therefore emerging disk spectra are insensitive to the choice of the
dissipation function. For lower accretion rates the photosphere location
depends on the assumed vertical dissipation profile down to the disk inner
edge, but the dependence is very weak and thus of minor importance. We conclude
that the spectra of optically thick accretion disks around black holes should
be calculated with the ray-tracing from the effective photosphere and that,
fortunately, the choice of a particular vertical dissipation profile does not
substantially influence the calculated spectrum.Comment: 7 pages, 7 figure
- …