30,348 research outputs found
Galactic centre star formation writ large in gamma-rays
We have modelled the high-energy astrophysics of the inner 200 pc of the
Galaxy with a view to explaining the diffuse, broad-band (radio continuum to
TeV gamma-ray), non-thermal signal detected from this region. Our modelling
pins down the ISM parameters for the environment wherein cosmic ray (CR)
electrons and ions reside in the Galactic centre (GC). We find that the
magnetic field in this region is 100-300 microG, the gas density < 60 cm^-3,
and that a powerful (> 200 km/s) 'super'-wind acts to remove > 95% of the
cosmic rays accelerated in the region before they have time to lose their
energy in situ. The ~ 10^39 erg/s carried away by the GC cosmic ray protons is
precisely enough to energise the ~GeV gamma-ray emission from the Fermi
'bubbles' recently found to extend north and south of the GC out to distances
of ~10 kpc, provided that the bubbles constitute thick targets to the GC
protons and that the situation has reached steady state. In such a situation of
'saturation' the hard, uniform spectrum of the bubbles are explained and
secondary electron synchrotron explains the non-thermal microwave emission
found in WMAP data mirroring the bubbles. Given the very low density of the
bubble plasma ( 5 Gyr. Our
scenario thus has the startling implication that a GC source of nonthermal
particles of time-averaged power 10^39 erg/s has persisted since the youth of
the Galaxy.Comment: 7 pages, 1 figure. Accepted to the Proceedings of the 25th Texas
Symposium on Relativistic Astrophysics (Heidelberg, 2010). References updates
and abstract typo corrected: "100-300 mG" -> "100-300 microG
Determination of the Thermodynamic Scaling Exponent from Static, Ambient-Pressure Quantities
An equation is derived that expresses the thermodynamic scaling exponent, g,
which superposes relaxation times and other measures of molecular mobility
determined over a range of temperatures and densities, in terms of static,
physical quantities. The latter are available in the literature or can be
measured at ambient pressure. We show for 13 materials, both molecular liquids
and polymers, that the calculated g are equivalent to the scaling exponents
obtained directly by superpositioning. The assumptions of the analysis are that
the glass transition is isochronal and that the first Ehrenfest relation is
valid; the first assumption is true by definition, while the second has been
corroborated for many glass-forming materials at ambient pressure. However, we
find that the Ehrenfest relation breaks down at elevated pressure, although
this limitation is of no consequence herein, since the appeal of the new
equation is its applicability to ambient pressure data.Comment: 9 pages, 3 figures, 1 tabl
The Galactic Centre - A Laboratory for Starburst Galaxies (?)
The Galactic centre - as the closest galactic nucleus - holds both intrinsic
interest and possibly represents a useful analogue to star-burst nuclei which
we can observe with orders of magnitude finer detail than these external
systems. The environmental conditions in the GC - here taken to mean the inner
200 pc in diameter of the Milky Way - are extreme with respect to those
typically encountered in the Galactic disk. The energy densities of the various
GC ISM components are typically ~two orders of magnitude larger than those
found locally and the star-formation rate density ~three orders of magnitude
larger. Unusually within the Galaxy, the Galactic centre exhibits
hard-spectrum, diffuse TeV (=10^12 eV) gamma-ray emission spatially coincident
with the region's molecular gas. Recently the nuclei of local star-burst
galaxies NGC 253 and M82 have also been detected in gamma-rays of such
energies. We have embarked on an extended campaign of modelling the broadband
(radio continuum to TeV gamma-ray), non- thermal signals received from the
inner 200 pc of the Galaxy. On the basis of this modelling we find that
star-formation and associated supernova activity is the ultimate driver of the
region's non-thermal activity. This activity drives a large-scale wind of hot
plasma and cosmic rays out of the GC. The wind advects the locally-accelerated
cosmic rays quickly, before they can lose much energy in situ or penetrate into
the densest molecular gas cores where star-formation occurs. The cosmic rays
can, however, heat/ionize the lower density/warm H2 phase enveloping the cores.
On very large scales (~10 kpc) the non-thermal signature of the escaping GC
cosmic rays has probably been detected recently as the spectacular 'Fermi
bubbles' and corresponding 'WMAP haze'.Comment: Invited talk to appear in Proceedings of IAU Symposium No. 284, 2011
(R.J. Tuffs & C.C. Popescu, eds.) `The Spectral Energy Distribution of
Galaxies
Thermodynamic scaling of diffusion in supercooled Lennard-Jones liquids
The manner in which the intermolecular potential u(r) governs structural
relaxation in liquids is a long standing problem in condensed matter physics.
Herein we show that diffusion coefficients for simulated Lennard-Jones m-6
liquids (8<m<36) in normal and moderately supercooled states are a unique
function of the variable rho^g/T, where rho is density and T is temperature.
The scaling exponent g is a material specific constant whose magnitude is
related to the steepness of the repulsive part of u(r), evaluated around the
distance of closest approach between particles probed in the supercooled
regime. Approximations of u(r) in terms of inverse power laws are also
discussed.Comment: 4 pages, 3 figure
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