9,156 research outputs found
On Lifshitz scaling and hyperscaling violation in string theory
We explore string/M-theory constructions of holographic theories with
Lifshitz scaling exponent and hyperscaling violation exponent ,
finding a range of -values. Some of these arise as effective metrics
from dimensional reduction of certain kinds of null deformations of
spacetimes appearing in the near horizon geometries of extremal D3-, M2- and
M5-brane theories. The solution in particular gives rise to
in (boundary) space dimensions. Other solutions arise as the IIA D2- and
D4-brane solutions with appropriate null deformations, and we discuss the phase
structure of these systems.Comment: Latex, 15pgs, v2: references, minor clarifications adde
A New Model for Black Hole Soft X-ray Transients in Quiescence
We present models of the soft X-ray transients, A0620-00, V404 Cyg, and X-ray
Nova Mus 1991, in quiescence. In each source, we postulate that there is an
outer region, extending outward from about 3000 Schwarzschild radii, where the
accretion flow is in the form of a standard thin disk. The outer disk produces
most of the radiation we observe in the infrared, optical and UV bands. We
propose that the disk undergoes an instability at its inner edge, perhaps by
the mechanism described recently by Meyer \& Meyer-Hofmeister for cataclysmic
variables. The accreting gas is thereby converted into a hot corona which flows
into the black hole as a nearly virial two-temperature flow. We describe the
hot inner flow by means of a recently discovered stable solution of optically
thin advection-dominated accretion. In this flow, most of the thermal energy
released by viscous dissipation is advected into the black hole and only a
small fraction, , of the energy is radiated. The radiation
is in the form of Comptonized synchrotron and bremsstrahlung emission, and has
a broad spectrum extending from optical to soft gamma-rays. The models we
present are consistent with all the available data in the three sources. In
particular, the X-ray emission from the hot inner flow fits the observed flux
and spectral index of A0620-00. We derive a mass accretion rate of
\sim10^{-11}\msyr in A0620-00 and Nova Mus, and \sim{\rm
few}\times10^{-10}\msyr in V404 Cyg. The best fit to the data is obtained for
a viscosity parameter in the hot flow. The models predict
that all three sources must have substantial flux in hard X-rays and soft
-rays. This prediction is testable in the case of V404 Cyg with current
instruments. A necessary feature of our proposal is that most of the viscousComment: 32 Pages, 6 Figures included, Compressed Postscript, To Appear in
Astrophysical Journa
Formation and dynamics of self-sustained neutron haloes in disk accreting sources
It has been recognized long ago that the presence of hot plasma in the inner
accretion disks around black holes could lead to the neutron production via
dissociation of helium nuclei. We show that, for a broad range of accretion
parameters, neutrons effectively decouple from protons and pile up in the inner
disk leading to the formation of self-sustained halo. This means that new
neutrons in the halo are supplied mainly by the splitting of helium nuclei in
their collisions with existing neutrons. Once formed, such a halo can exist
even if the proton temperature is much lower than the energy threshold of
helium dissociation. We show that neutron haloes can be the natural source of
relativistic electrons and positrons, providing characteristic comptonization
spectra and hard spectral tails observed in many black hole candidates, and
also giving rise to relativistic outflows. Deuterium gamma-ray line at 2.2 MeV
resulting from neutron capture is also expected at a level detectable by future
INTEGRAL mission. Furthermore, the presence of a neutron halo strongly affects
the dynamics of accretion and leads to the rich variety of transient dynamical
regimes.Comment: 10 pages, submitted to Astronomy and Astrophysic
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