33,683 research outputs found
Forming circumnuclear disks and rings in galactic nuclei: a competition between supermassive black hole and nuclear star cluster
We investigate the formation of circumnuclear gas structures from the tidal
disruption of molecular clouds in galactic nuclei, by means of smoothed
particle hydrodynamics simulations. We model galactic nuclei as composed of a
supermassive black hole (SMBH) and a nuclear star cluster (NSC) and consider
different mass ratios between the two components. We find that the relative
masses of the SMBH and the NSC have a deep impact on the morphology of the
circumnuclear gas. Extended disks form only inside the sphere of influence of
the SMBH. In contrast, compact rings naturally form outside the SMBH's sphere
of influence, where the gravity is dominated by the NSC. This result is in
agreement with the properties of the Milky Way's circumnuclear ring, which
orbits outside the SMBH sphere of influence. Our results indicate that compact
circumnuclear rings can naturally form outside the SMBH sphere of influence.Comment: Accepted for publication in ApJ. 12 pages, 6 figures, 3 tables.
Comments welcom
Solar wind test of the de Broglie-Proca's massive photon with Cluster multi-spacecraft data
Our understanding of the universe at large and small scales relies largely on
electromagnetic observations. As photons are the messengers, fundamental
physics has a concern in testing their properties, including the absence of
mass. We use Cluster four spacecraft data in the solar wind at 1 AU to estimate
the mass upper limit for the photon. We look for deviations from Amp\`ere's
law, through the curlometer technique for the computation of the magnetic
field, and through the measurements of ion and electron velocities for the
computation of the current. We show that the upper bound for lies
between and kg, and thereby discuss
the currently accepted lower limits in the solar wind.Comment: The paper points out that actual photon mass upper limits (in the
solar wind) are too optimistic and model based. We instead perform a much
more experiment oriented measurement. This version matches that accepted by
Astroparticle Physic
Short intervals asymptotic formulae for binary problems with primes and powers, I: density 3/2
We prove that suitable asymptotic formulae in short intervals hold for the problems of representing an integer as a sum of a prime and a square, or a prime square. Such results are obtained both assuming the Riemann Hypothesis and in the unconditional case
Short intervals asymptotic formulae for binary problems with primes and powers, II: density 1
We prove that suitable asymptotic formulae in short intervals hold for the problems of representing an integer as a sum of a prime square and a square, or a prime square. Such results are obtained both assuming the Riemann Hypothesis and in the unconditional case
Colour image segmentation by the vector-valued Allen-Cahn phase-field model: a multigrid solution
We propose a new method for the numerical solution of a PDE-driven model for
colour image segmentation and give numerical examples of the results. The
method combines the vector-valued Allen-Cahn phase field equation with initial
data fitting terms. This method is known to be closely related to the
Mumford-Shah problem and the level set segmentation by Chan and Vese. Our
numerical solution is performed using a multigrid splitting of a finite element
space, thereby producing an efficient and robust method for the segmentation of
large images.Comment: 17 pages, 9 figure
Improved Soundness for QMA with Multiple Provers
We present three contributions to the understanding of QMA with multiple
provers:
1) We give a tight soundness analysis of the protocol of [Blier and Tapp,
ICQNM '09], yielding a soundness gap Omega(1/N^2). Our improvement is achieved
without the use of an instance with a constant soundness gap (i.e., without
using a PCP).
2) We give a tight soundness analysis of the protocol of [Chen and Drucker,
ArXiV '10], thereby improving their result from a monolithic protocol where
Theta(sqrt(N)) provers are needed in order to have any soundness gap, to a
protocol with a smooth trade-off between the number of provers k and a
soundness gap Omega(k^2/N), as long as k>=Omega(log N). (And, when
k=Theta(sqrt(N)), we recover the original parameters of Chen and Drucker.)
3) We make progress towards an open question of [Aaronson et al., ToC '09]
about what kinds of NP-complete problems are amenable to sublinear
multiple-prover QMA protocols, by observing that a large class of such examples
can easily be derived from results already in the PCP literature - namely, at
least the languages recognized by a non-deterministic RAMs in quasilinear time.Comment: 24 pages; comments welcom
Satellite measurement of the Hannay angle
The concept of a measurement of the yet unevaluated Hannay angle, by means of
an Earth-bound satellite, adiabatically driven by the Moon, is shown herein.
Numerical estimates are given for the angles, the orbital displacements, the
shortening of the orbital periods, for different altitudes. It is concluded
that the Hannay effect is measurable in high Earth orbits, by means of atomic
clocks, accurate Time & Frequency transfer system and precise positioning.Comment: Lette
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