76 research outputs found
The new very small angle neutron scattering spectrometer at Laboratoire Leon Brillouin
The design and characteristics of the new very small angle neutron scattering
spectrometer under construction at the Laboratoire Leon Brillouin is described.
Its goal is to extend the range of scattering vectors magnitudes towards
2x10{-4} /A. The unique feature of this new spectrometer is a high resolution
two dimensional image plate detector sensitive to neutrons. The wavelength
selection is achieved by a double reflection supermirror monochromator and the
collimator uses a novel multibeam design
High Energy Cosmic Rays From Supernovae
Cosmic rays are charged relativistic particles that reach the Earth with
extremely high energies, providing striking evidence of the existence of
effective accelerators in the Universe. Below an energy around
eV cosmic rays are believed to be produced in the Milky Way while above that
energy their origin is probably extragalactic. In the early '30s supernovae
were already identified as possible sources for the Galactic component of
cosmic rays. After the '70s this idea has gained more and more credibility
thanks to the the development of the diffusive shock acceleration theory, which
provides a robust theoretical framework for particle energization in
astrophysical environments. Afterwards, mostly in recent years, much
observational evidence has been gathered in support of this framework,
converting a speculative idea in a real paradigm. In this Chapter the basic
pillars of this paradigm will be illustrated. This includes the acceleration
mechanism, the non linear effects produced by accelerated particles onto the
shock dynamics needed to reach the highest energies, the escape process from
the sources and the transportation of cosmic rays through the Galaxy. The
theoretical picture will be corroborated by discussing several observations
which support the idea that supernova remnants are effective cosmic ray
factories.Comment: Final draft of a chapter in "Handbook of Supernovae" edited by Athem
W. Alsabti and Paul Murdi
The inevitable youthfulness of known high-redshift radio galaxies
Radio galaxies can be seen out to very high redshifts, where in principle
they can serve as probes of the early evolution of the Universe. Here we show
that for any model of radio-galaxy evolution in which the luminosity decreases
with time after an initial rapid increase (that is, essentially all reasonable
models), all observable high-redshift radio-galaxies must be seen when the
lobes are less than 10^7 years old. This means that high-redshift radio
galaxies can be used as a high-time-resolution probe of evolution in the early
Universe. Moreover, this result helps to explain many observed trends of
radio-galaxy properties with redshift [(i) the `alignment effect' of optical
emission along radio-jet axes, (ii) the increased distortion in radio
structure, (iii) the decrease in physical sizes, (iv) the increase in radio
depolarisation, and (v) the increase in dust emission] without needing to
invoke explanations based on cosmology or strong evolution of the surrounding
intergalactic medium with cosmic time, thereby avoiding conflict with current
theories of structure formation.Comment: To appear in Nature. 4 pages, 2 colour figures available on request.
Also available at http://www-astro.physics.ox.ac.uk/~km
Infrared composition of the Large Magellanic Cloud
The evolution of galaxies and the history of star formation in the Universe
are among the most important topics in today's astrophysics. Especially, the
role of small, irregular galaxies in the star-formation history of the Universe
is not yet clear. Using the data from the AKARI IRC survey of the Large
Magellanic Cloud at 3.2, 7, 11, 15, and 24 {\mu}m wavelengths, i.e., at the
mid- and near-infrared, we have constructed a multiwavelength catalog
containing data from a cross-correlation with a number of other databases at
different wavelengths. We present the separation of different classes of stars
in the LMC in color-color, and color-magnitude, diagrams, and analyze their
contribution to the total LMC flux, related to point sources at different
infrared wavelengths
High energy emission from microquasars
The microquasar phenomenon is associated with the production of jets by X-ray
binaries and, as such, may be associated with the majority of such systems. In
this chapter we briefly outline the associations, definite, probable, possible,
and speculative, between such jets and X-ray, gamma-ray and particle emission.Comment: Contributing chapter to the book Cosmic Gamma-Ray Sources, K.S. Cheng
and G.E. Romero (eds.), to be published by Kluwer Academic Publishers,
Dordrecht, 2004. (19 pages
Non-thermal emission processes in massive binaries
In this paper, I present a general discussion of several astrophysical
processes likely to play a role in the production of non-thermal emission in
massive stars, with emphasis on massive binaries. Even though the discussion
will start in the radio domain where the non-thermal emission was first
detected, the census of physical processes involved in the non-thermal emission
from massive stars shows that many spectral domains are concerned, from the
radio to the very high energies.
First, the theoretical aspects of the non-thermal emission from early-type
stars will be addressed. The main topics that will be discussed are
respectively the physics of individual stellar winds and their interaction in
binary systems, the acceleration of relativistic electrons, the magnetic field
of massive stars, and finally the non-thermal emission processes relevant to
the case of massive stars. Second, this general qualitative discussion will be
followed by a more quantitative one, devoted to the most probable scenario
where non-thermal radio emitters are massive binaries. I will show how several
stellar, wind and orbital parameters can be combined in order to make some
semi-quantitative predictions on the high-energy counterpart to the non-thermal
emission detected in the radio domain.
These theoretical considerations will be followed by a census of results
obtained so far, and related to this topic... (see paper for full abstract)Comment: 47 pages, 5 postscript figures, accepted for publication in Astronomy
and Astrophysics Review. Astronomy and Astrophysics Review, in pres
Theorems on existence and global dynamics for the Einstein equations
This article is a guide to theorems on existence and global dynamics of
solutions of the Einstein equations. It draws attention to open questions in
the field. The local-in-time Cauchy problem, which is relatively well
understood, is surveyed. Global results for solutions with various types of
symmetry are discussed. A selection of results from Newtonian theory and
special relativity that offer useful comparisons is presented. Treatments of
global results in the case of small data and results on constructing spacetimes
with prescribed singularity structure or late-time asymptotics are given. A
conjectural picture of the asymptotic behaviour of general cosmological
solutions of the Einstein equations is built up. Some miscellaneous topics
connected with the main theme are collected in a separate section.Comment: Submitted to Living Reviews in Relativity, major update of Living
Rev. Rel. 5 (2002)
The Effect of Particulate Air Pollution on Emergency Admissions for Myocardial Infarction: A Multicity Case-Crossover Analysis
Recently, attention has focused on whether particulate air pollution is a specific trigger of myocardial infarction (MI). The results of several studies of single locations assessing the effects of ambient particular matter on the risk of MI have been disparate. We used a multicity case-crossover study to examine risk of emergency hospitalization associated with fine particulate matter (PM) with aerodynamic diameter < 10 μm (PM(10)) for > 300,000 MIs during 1985–1999 among elderly residents of 21 U.S. cities. We used time-stratified controls matched on day of the week or on temperature to detect possible residual confounding by weather. Overall, we found a 0.65% [95% confidence interval (CI), 0.3–1.0%] increased risk of hospitalization for MI per 10 μg/m(3) increase in ambient PM(10) concentration. Matching on apparent temperature yielded a 0.64% increase in risk (95% CI, 0.1–1.2%). We found that the effect size for PM(10) doubled for subjects with a previous admission for chronic obstructive pulmonary disease or a secondary diagnosis of pneumonia, although these differences did not achieve statistical significance. There was a weaker indication of a larger effect on males but no evidence of effect modification by age or the other diagnoses. We also found that the shape of the exposure–response relationship between MI hospitalizations and PM(10) is almost linear, but with a steeper slope at levels of PM(10) < 50 μg/m(3). We conclude that increased concentrations of ambient PM(10) are associated with increased risk of MI among the elderly
Evolution of active galactic nuclei
[Abriged] Supermassive black holes (SMBH) lurk in the nuclei of most massive
galaxies, perhaps in all of them. The tight observed scaling relations between
SMBH masses and structural properties of their host spheroids likely indicate
that the processes fostering the growth of both components are physically
linked, despite the many orders of magnitude difference in their physical size.
This chapter discusses how we constrain the evolution of SMBH, probed by their
actively growing phases, when they shine as active galactic nuclei (AGN) with
luminosities often in excess of that of the entire stellar population of their
host galaxies. Following loosely the chronological developments of the field,
we begin by discussing early evolutionary studies, when AGN represented beacons
of light probing the most distant reaches of the universe and were used as
tracers of the large scale structure. This early study turned into AGN
"Demography", once it was realized that the strong evolution (in luminosity,
number density) of the AGN population hindered any attempt to derive
cosmological parameters from AGN observations directly. Following a discussion
of the state of the art in the study of AGN luminosity functions, we move on to
discuss the "modern" view of AGN evolution, one in which a bigger emphasis is
given to the physical relationships between the population of growing black
holes and their environment. This includes observational and theoretical
efforts aimed at constraining and understanding the evolution of scaling
relations, as well as the resulting limits on the evolution of the SMBH mass
function. Physical models of AGN feedback and the ongoing efforts to isolate
them observationally are discussed next. Finally, we touch upon the problem of
when and how the first black holes formed and the role of black holes in the
high-redshift universe.Comment: 75 pages, 35 figures. Modified version of the chapter accepted to
appear in "Planets, Stars and Stellar Systems", vol 6, ed W. Keel
(www.springer.com/astronomy/book/978-90-481-8818-5). The number of references
is limited upon request of the editors. Original submission to Springer: June
201
- …