2,908 research outputs found
Integral and Light Dark Matter
The nature of Dark Matter remains one of the outstanding questions of modern
astrophysics. The success of the Cold Dark Matter cosmological model argues
strongly in favor of a major component of the dark matter being in the form of
elementary particles, not yet discovered. Based on earlier theoretical
considerations, a possible link between the recent SPI/INTEGRAL measurement of
an intense and extended emission of 511 keV photons (positron annihilation)
from the central Galaxy, and this mysterious component of the Universe, has
been established advocating the existence of a light dark matter particle at
variance with the neutralino, in general considered as very heavy. We show that
it can explain the 511 keV emission mapped with SPI/INTEGRAL without
overproducing undesirable signals like high energy gamma-rays arising from
decays, and radio synchrotron photons emitted by high energy
positrons circulating in magnetic fields. Combining the annihilation line
constraint with the cosmological one (i.e. that the relic LDM energy density
reaches about 23% of the density of the Universe), one can restrict the main
properties of the light dark matter particle. Its mass should lie between 1 and
100 MeV, and the required annihilation cross section, velocity dependent,
should be significantly larger than for weak interactions, and may be induced
by the virtual production of a new light neutral spin 1 boson . On
astrophysical grounds, the best target to validate the LDM proposal seems to be
the observation by SPI/INTEGRAL and future gamma ray telescopes of the
annihilation line from the Sagittarius dwarf galaxy and the Palomar-13 globular
cluster, thought to be dominated by dark matter.Comment: 7 pages, 0 figures. To appear in the Proceedings of the 5th INTEGRAL
Workshop: "The INTEGRAL Universe", February 16-20, 2004, Munich, German
Neutron-rich nuclei in cosmic rays and Wolf-Rayet stars
Wolf-Rayet stars figure prominently in astrophysical research. As a bonus, they seem to offer, in the recent past, an interesting connection between classical astronomy and high energy astrophysics due to their unusual composition and their huge mechanical power. The material flowing from WC stars (carbon-rich WR stars) contains gas which has been processed through core-helium burning, i.e., considerably enriched into 12C,16O, 22Ne, and 25,26Mg. This composition is reminiscent of the cosmic ray source anomalies. Encouraging agreement is obtained with observation in the mass range 12 A 26 assuming acceleration of wind particles at the shock that delineates the WR cavity, and adequate dilution with normal cosmic rays, but silicon poses
Distinguishing an ejected blob from alternative flare models at the Galactic centre with GRAVITY
The black hole at the Galactic centre exhibits regularly flares of radiation,
the origin of which is still not understood. In this article, we study the
ability of the near-future GRAVITY infrared instrument to constrain the nature
of these events. We develop realistic simulations of GRAVITY astrometric data
sets for various flare models. We show that the instrument will be able to
distinguish an ejected blob from alternative flare models, provided the blob
inclination is >= 45deg, the flare brightest magnitude is 14 <= mK <= 15 and
the flare duration is >= 1h30.Comment: 11 pages, 9 figures, accepted by MNRA
Continuum gamma-ray emission from light dark matter positrons and electrons
The annihilation of light dark matter was recently advocated as a possible
explanation of the large positron injection rate at the Galactic center deduced
from observations by the SPI spectrometer aboard INTEGRAL. The modelling of
internal Bremsstrahlung and in-flight annihilation radiations associated to
this process drastically reduced the mass range of this light dark matter
particle. We estimate critically the various energy losses and radiations
involved in the propagation of the positron before its annihilation --in-
flight or at rest.
Using a simple model with mono-energetic positrons injected and confined to
the Galactic bulge, we compute energy losses and gamma-ray radiations caused by
ionization, Bremsstrahlung interactions as well as in-flight and at rest
annihilation and compare these predictions to the available observations, for
various injection energies.
Confronting the predictions with observations by the CGRO/EGRET,
CGRO/COMPTEL, INTEGRAL/SPI and INTEGRAL/IBIS/ISGRI instruments, we deduce a
mass upper bound of 3 to 7.5 MeV/c^2 for the hypothetical light dark matter
particle. The most restrictive limit is in agreement with the value previously
found by Beacom and Yuksel and was obtained under similar assumptions, while
the 7.5 MeV/c^2 value corresponds to more conservative choices and to a
partially ionized propagation medium. We stress how the limit depends on the
degree of ionization of the propagation medium and how its precision could be
improved by a better appraisal of data uncertainties.Comment: Revision of the section dedicated to the observational
interpretation. 11 pages, 8 figures, 1 table. Accepted for publication in
Phys. Rev.
Accretion-ejection instability in magnetized disks: Feeding the corona with Alfven waves
We present a detailed calculation of the mechanism by which the
Accretion-Ejection Instability can extract accretion energy and angular
momentum from a magnetized disk, and redirect them to its corona. In a disk
threaded by a poloidal magnetic field of the order of equipartition with the
gas pressure, the instability is composed of a spiral wave (analogous to
galactic ones) and a Rossby vortex. The mechanism detailed here describes how
the vortex, twisting the footpoints of field lines threading the disk,
generates Alfven waves propagating to the corona. We find that this is a very
efficient mechanism, providing to the corona (where it could feed a jet or a
wind) a substantial fraction of the accretion energy.Comment: accepted by A&
Predicted rotation signatures in MHD disc winds and comparison to DG Tau observations
Motivated by the first detections of rotation signatures in the DG Tau jet
(Bacciotti et al. 2002), we examine possible biases affecting the relation
between detected rotation signatures and true azimuthal velocity for
self-similar MHD disc winds, taking into account projection, convolution as
well as excitation gradients effects. We find that computed velocity shifts are
systematically smaller than the true underlying rotation curve. When outer
slower streamlines dominate the emission, we predict observed shifts increasing
with transverse distance to the jet axis, opposite to the true rotation
profile. Determination of the full transverse rotation profile thus requires
high angular resolution observations (< 5 AU) on an object with dominant inner
faster streamlines. Comparison of our predictions with HST/STIS observations of
DG Tau clearly shows that self-similar, warm MHD disc wind models with lambda =
13 and an outer radius of the disc ~ 3 AU are able to reproduce detected
velocity shifts, while cold disc wind models (lambda > 50) are ruled out for
the medium-velocity component in the DG Tau jet.Comment: 4 Pages, 4 Figures, to be published in A&
Rossby Wave Instability and three-dimensional vortices in accretion disks
Context. The formation of vortices in accretion disks is of high interest in
various astrophysical contexts, in particular for planet formation or in the
disks of compact objects. But despite numerous attempts it has thus far not
been possible to produce strong vortices in fully three-dimensional simulations
of disks. Aims. The aim of this paper is to present the first 3D simulation of
a strong vortex, established across the vertically stratified structure of a
disk by the Rossby Wave Instability. Methods. Using the Versatile Advection
Code (VAC), we set up a fully 3D cylindrical stratified disk potentially prone
to the Rossby Wave Instability. Results. The simulation confirms the basic
expectations obtained from previous 2D analytic and numerical works. The
simulation exhibits a strong vortex that grows rapidly and saturates at a
finite amplitude. On the other hand the third dimension shows unexpected
additional behaviours that could be of strong importance in the astrophysical
roles that such vortices can play.Comment: Accepted by Astronomy and Astrophysic
The effect of a stellar magnetic variation on the jet velocity
Stellar jets are normally constituted by chains of knots with some
periodicity in their spatial distribution, corresponding to a variability of
order of several years in the ejection from the protostar/disk system. A widely
accepted theory for the presence of knots is related to the generation of
internal working surfaces due to variations in the jet ejection velocity. In
this paper we study the effect of variations in the inner disk-wind radius on
the jet ejection velocity. We show that a small variation in the inner
disk-wind radius produce a variation in the jet velocity large enough to
generate the observed knots. We also show that the variation in the inner
radius may be related to a variation of the stellar magnetic field.Comment: 5 pages, 3 figures, accepted for publication in Ap
On the Phenomenology of Hydrodynamic Shear Turbulence
The question of a purely hydrodynamic origin of turbulence in accretion disks
is reexamined, on the basis of a large body of experimental and numerical
evidence on various subcritical (i.e., linearly stable) hydrodynamic flows.
One of the main points of this paper is that the length scale and velocity
fluctuation amplitude which are characteristic of turbulent transport in these
flows scale like , where is the minimal Reynolds number for
the onset of fully developed turbulence. From this scaling, a simple
explanation of the dependence of with relative gap width in subcritical
Couette-Taylor flows is developed. It is also argued that flows in the shearing
sheet limit should be turbulent, and that the lack of turbulence in all such
simulations performed to date is most likely due to a lack of resolution, as a
consequence of the effect of the Coriolis force on the large scale fluctuations
of turbulent flows.
These results imply that accretion flows should be turbulent through
hydrodynamic processes. If this is the case, the Shakura-Sunyaev
parameter is constrained to lie in the range in accretion
disks, depending on unknown features of the mechanism which sustains
turbulence. Whether the hydrodynamic source of turbulence is more efficient
than the MHD one where present is an open question.Comment: 31 pages, 3 figures. Accepted for publication in Ap
- …