6,913 research outputs found
Particle acceleration and the origin of gamma-ray emission from Fermi Bubbles
Fermi LAT has discovered two extended gamma-ray bubbles above and below the
galactic plane. We propose that their origin is due to the energy release in
the Galactic center (GC) as a result of quasi-periodic star accretion onto the
central black hole. Shocks generated by these processes propagate into the
Galactic halo and accelerate particles there. We show that electrons
accelerated up to ~10 TeV may be responsible for the observed gamma-ray
emission of the bubbles as a result of inverse Compton (IC) scattering on the
relic photons. We also suggest that the Bubble could generate the flux of CR
protons at energies > 10^15 eV because the shocks in the Bubble have much
larger length scales and longer lifetimes in comparison with those in SNRs.
This may explain the the CR spectrum above the knee.Comment: 5 pages, 4 figures. Expanded version of the contribution to the 32nd
ICRC, Beijing, #0589. To appear in the proceeding
The Size Distribution of Kuiper Belt Objects
We describe analytical and numerical collisional evolution calculations for
the size distribution of icy bodies in the Kuiper Belt. For a wide range of
bulk properties, initial masses, and orbital parameters, our results yield
power-law cumulative size distributions, N_C propto r^{-q}, with q_L = 3.5 for
large bodies with radii of 10-100 km, and q_s = 2.5-3 for small bodies with
radii lesss than 0.1-1 km. The transition between the two power laws occurs at
a break radius of 1-30 km. The break radius is more sensitive to the initial
mass in the Kuiper Belt and the amount of stirring by Neptune than the bulk
properties of individual Kuiper Belt objects (KBOs). Comparisons with
observations indicate that most models can explain the observed sky surface
density of KBOs for red magnitudes, R = 22-27. For R 28, the model
surface density is sensitive to the amount of stirring by Neptune, suggesting
that the size distribution of icy planets in the outer solar system provides
independent constraints on the formation of Neptune.Comment: 24 pages of text, 12 figures; to appear in the Astronomical Journal,
October 200
De-Excitation Gamma-ray Line Emission from the Galactic Center
International audienceA future detection of de-excitation gamma-ray lines from the Galactic center region would provide unique information on the high-energy processes induced by the the central black hole and the physical conditions of the emitting region. We analyse the intensity of nuclear de-excitation lines in the direction of the Galactic center produced by subrelativistic protons, which are generated by star capture by the central black hole. With the metallicity two times higher than the solar one the total flux in gamma-ray lines of energies below 8 MeV is about 10â4 cmâ2 sâ1. The most promising lines for detection are those at 4.44 and 6.2 MeV, with a predicted flux in each line of 10â5 photons cmâ2 sâ1. We also analyze the possibility of detection of these lines by INTEGRAL and future missions
Planet Formation in the Outer Solar System
This paper reviews coagulation models for planet formation in the Kuiper
Belt, emphasizing links to recent observations of our and other solar systems.
At heliocentric distances of 35-50 AU, single annulus and multiannulus
planetesimal accretion calculations produce several 1000 km or larger planets
and many 50-500 km objects on timescales of 10-30 Myr in a Minimum Mass Solar
Nebula. Planets form more rapidly in more massive nebulae. All models yield two
power law cumulative size distributions, N_C propto r^{-q} with q = 3.0-3.5 for
radii larger than 10 km and N_C propto r^{-2.5} for radii less than 1 km. These
size distributions are consistent with observations of Kuiper Belt objects
acquired during the past decade. Once large objects form at 35-50 AU,
gravitational stirring leads to a collisional cascade where 0.1-10 km objects
are ground to dust. The collisional cascade removes 80% to 90% of the initial
mass in the nebula in roughly 1 Gyr. This dust production rate is comparable to
rates inferred for alpha Lyr, beta Pic, and other extrasolar debris disk
systems.Comment: invited review for PASP, March 2002. 33 pages of text and 12 figure
In-situ acceleration of subrelativistic electrons in the Coma halo and the halo's influence on the Sunyaev-Zeldovich effect
The stochastic acceleration of subrelativistic electrons from a background
plasma is studied in order to find a possible explanation of the hard X-ray
(HXR) emission detected from the Coma cluster. We calculate the necessary
energy supply as a function of the plasma temperature and of the electron
energy. We show that, for the same value of the HXR flux, the energy supply
changes gradually from its high value (when emitting particle are non-thermal)
to lower values (when the electrons are thermal). The kinetic equations we use
include terms describing particle thermalization as well as momentum diffusion
due to the Fermi II acceleration. We show that the temporal evolution of the
particle distribution function has, at its final stationary stage, a rather
specific form: it cannot be described by simple exponential or power-law
expressions. A broad transfer region is formed by Coulomb collisions at
energies between the Maxwellian and power-law parts of the distribution
function. In this region the radiative lifetime of a single electron differs
greatly from the lifetime of the distribution function as a whole. For a plasma
temperature of 8 keV, the particles emitting bremsstrahlung at 20-80 keV lie in
this quasi-thermal regime. We show that the energy supply required by
quasi-thermal electrons to produce the observed HXR flux from Coma is one or
two orders of magnitude smaller than the value derived from the assumption of a
nonthermal origin of the emitting particles. This result may solve the problem
of rapid cluster overheating by nonthermal electrons. We finally predict the
change in Coma's SZ effect caused by the distortions of the Maxwellian electron
spectrum, and we show that evidence for acceleration of subrelativistic
electrons can be derived from detailed spectral measurements.Comment: 14 pages, 11 figures, A&A in pres
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