4,493 research outputs found
An application of Khovanov homology to quantum codes
We use Khovanov homology to define families of LDPC quantum error-correcting
codes: unknot codes with asymptotical parameters
[[3^(2l+1)/sqrt(8{\pi}l);1;2^l]]; unlink codes with asymptotical parameters
[[sqrt(2/2{\pi}l)6^l;2^l;2^l]] and (2,l)-torus link codes with asymptotical
parameters [[n;1;d_n]] where d_n>\sqrt(n)/1.62.Comment: 20 page
A hybrid approach to black hole perturbations from extended matter sources
We present a new method for the calculation of black hole perturbations
induced by extended sources in which the solution of the nonlinear
hydrodynamics equations is coupled to a perturbative method based on
Regge-Wheeler/Zerilli and Bardeen-Press-Teukolsky equations when these are
solved in the frequency domain. In contrast to alternative methods in the time
domain which may be unstable for rotating black-hole spacetimes, this approach
is expected to be stable as long as an accurate evolution of the matter sources
is possible. Hence, it could be used under generic conditions and also with
sources coming from three-dimensional numerical relativity codes. As an
application of this method we compute the gravitational radiation from an
oscillating high-density torus orbiting around a Schwarzschild black hole and
show that our method is remarkably accurate, capturing both the basic
quadrupolar emission of the torus and the excited emission of the black hole.Comment: 12 pages, 4 figures. Phys. Rev. D, in pres
Global General Relativistic Magnetohydrodynamic Simulations of Accretion Tori
This paper presents an initial survey of the properties of accretion flows in
the Kerr metric from three-dimensional, general relativistic
magnetohydrodynamic simulations of accretion tori. We consider three fiducial
models of tori around rotating, both prograde and retrograde, and nonrotating
black holes; these three fiducial models are also contrasted with axisymmetric
simulations and a pseudo-Newtonian simulation with equivalent initial
conditions to delineate the limitations of these approximations.Comment: Submitted to ApJ. 30 pages, 21 figures. Animations and
high-resolution version of figures available at
http://www.astro.virginia.edu/~jd5
Panchromatic spectral energy distributions of Herschel sources
(abridged) Far-infrared Herschel photometry from the PEP and HerMES programs
is combined with ancillary datasets in the GOODS-N, GOODS-S, and COSMOS fields.
Based on this rich dataset, we reproduce the restframe UV to FIR ten-colors
distribution of galaxies using a superposition of multi-variate Gaussian modes.
The median SED of each mode is then fitted with a modified version of the
MAGPHYS code that combines stellar light, emission from dust heated by stars
and a possible warm dust contribution heated by an AGN. The defined Gaussian
grouping is also used to identify rare sources. The zoology of outliers
includes Herschel-detected ellipticals, very blue z~1 Ly-break galaxies,
quiescent spirals, and torus-dominated AGN with star formation. Out of these
groups and outliers, a new template library is assembled, consisting of 32 SEDs
describing the intrinsic scatter in the restframe UV-to-submm colors of
infrared galaxies. This library is tested against L(IR) estimates with and
without Herschel data included, and compared to eight other popular methods
often adopted in the literature. When implementing Herschel photometry, these
approaches produce L(IR) values consistent with each other within a median
absolute deviation of 10-20%, the scatter being dominated more by fine tuning
of the codes, rather than by the choice of SED templates. Finally, the library
is used to classify 24 micron detected sources in PEP GOODS fields. AGN appear
to be distributed in the stellar mass (M*) vs. star formation rate (SFR) space
along with all other galaxies, regardless of the amount of infrared luminosity
they are powering, with the tendency to lie on the high SFR side of the "main
sequence". The incidence of warmer star-forming sources grows for objects with
higher specific star formation rates (sSFR), and they tend to populate the
"off-sequence" region of the M*-SFR-z space.Comment: Accepted for publication in A&A. Some figures are presented in low
resolution. The new galaxy templates are available for download at the
address http://www.mpe.mpg.de/ir/Research/PEP/uvfir_temp
GRMHD simulations of accretion onto Sgr A*: How important are radiative losses?
We present general relativistic magnetohydrodynamic (GRMHD) numerical
simulations of the accretion flow around the supermassive black hole in the
Galactic centre, Sagittarius A* (Sgr A*). The simulations include for the first
time radiative cooling processes (synchrotron, bremsstrahlung, and inverse
Compton) self-consistently in the dynamics, allowing us to test the common
simplification of ignoring all cooling losses in the modeling of Sgr A*. We
confirm that for Sgr A*, neglecting the cooling losses is a reasonable
approximation if the Galactic centre is accreting below ~10^{-8} Msun/yr i.e.
Mdot < 10^{-7} Mdot_Edd. But above this limit, we show that radiative losses
should be taken into account as significant differences appear in the dynamics
and the resulting spectra when comparing simulations with and without cooling.
This limit implies that most nearby low-luminosity active galactic nuclei are
in the regime where cooling should be taken into account.
We further make a parameter study of axisymmetric gas accretion around the
supermassive black hole at the Galactic centre. This approach allows us to
investigate the physics of gas accretion in general, while confronting our
results with the well studied and observed source, Sgr A*, as a test case. We
confirm that the nature of the accretion flow and outflow is strongly dependent
on the initial geometry of the magnetic field. For example, we find it
difficult, even with very high spins, to generate powerful outflows from discs
threaded with multiple, separate poloidal field loops.Comment: Resubmitted to MNRAS, including modifications in response to referee
report. 13 pages, 15 figure
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