9,127 research outputs found
Production of TeV gamma-radiation in the vicinity of the supermassive black hole in the giant radiogalaxy M87
Although the giant radiogalaxy M 87 harbors many distinct regions of
broad-band nonthermal emission, the recently reported fast variability of TeV
gamma rays from M 87 on a timescale of days strongly constrains the range of
speculations concerning the possible sites and scenarios of particle
acceleration responsible for the observed TeV emission. A natural production
site of this radiation is the immediate vicinity of the central supermassive
mass black hole (BH). Because of the low bolometric luminosity, the nucleus of
M 87 is effectively transparent for gamma rays up to energy of 10 TeV, which
makes this source an ideal laboratory for study of particle acceleration
processes close to the BH event horizon. We critically analyse different
possible radiation mechanisms in this region, and argue that the observed very
high-energy gamma ray emission can be explained by the inverse Compton emission
of ultrarelativistic electron-positron pairs produced through the development
of an electromagnetic cascade in the BH magnetosphere. We demonstrate, through
detailed numerical calculations of acceleration and radiation of electrons in
the magnetospheric vacuum gap, that this ``pulsar magnetosphere like'' scenario
can satisfactorily explain the main properties of TeV gamma-ray emission of M
87.Comment: 11 pages, ApJ, in prin
Spotting Specific Agricultural Practices Impacting Desertification in Zamfara State, Nigeria
Agricultural practices are among anthropogenic factors impacting the desertification process. However, it is necessary to determine which of the practices that most affect the process. Eleven of the nineteen Nigeria’s northern states bear the characteristics of the Sudano-Sahelian region. One of them (Zamfara State) was randomly selected for the study. A structured questionnaire was administered on 500 farmers out of which 497 of them responded. Data extracted were subjected to Pearson’s correlation and stepwise regression analyses. The Pearson’s correlation showed positive association between desertification and slash and burn, (r=0.074; p=0.05) and free-range farming (r = 0.103; p = 0.011 and negative association between desertification and organic manuring (r = -0.100; p = 0.013). The stepwise regression analysis showed that there was a significant relationship between desertification and free-range farming and organic manuring at p = 0.035 and p = 0.040 respectively. It was therefore concluded that slash and burn as well as free range farming were some of the agricultural practices employed in Zamfara State that most exacerbated desertification process in the region. Keywords: Desertification, Agricultural practices, Free range farming, Organic manuring. DOI: 10.7176/JEES/12-8-02 Publication date:August 31st 202
Diffuse Extragalactic Background Radiation
Attenuation of high--energy gamma rays by pair--production with UV, optical
and IR background photons provides a link between the history of galaxy
formation and high--energy astrophysics. We present results from our latest
semi-analytic models (SAMs), based upon a CDM hierarchical structural
formation scenario and employing all ingredients thought to be important to
galaxy formation and evolution, as well as reprocessing of starlight by dust to
mid- and far-IR wavelengths. Our models also use results from recent
hydrodynamic galaxy merger simulations. These latest SAMs are successful in
reproducing a large variety of observational constraints such as number counts,
luminosity and mass functions, and color bimodality. We have created 2 models
that bracket the likely ranges of galaxy emissivities, and for each of these we
show how the optical depth from pair--production is affected by redshift and
gamma-ray energy. We conclude with a discussion of the implications of our
work, and how the burgeoning science of gamma-ray astronomy will continue to
help constrain cosmology.Comment: 12 pages, 8 figures, to be published in the Proceedings of the 4th
Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, held
July 2008 in Heidelberg, German
production and Cronin effect from p+p, d+Au and Au+Au collisions at GeV from the PHENIX experiment
We present results on identified particle production in p+p, d+Au and Au+Au
collisions at GeV at mid-rapidity measured by the PHENIX
experiment. The centrality and flavor dependence of the Cronin effect in d+Au
collisions is measured. The Cronin effect for the protons in d+Au is larger
than that for the pions, but not large enough to account for the ``anomalous''
proton to pion ratio in central Au+Au collisions.Comment: 5 pages, 3 figures, contribution to the proceedings of the 17th
International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions
(Quark Matter 2004
Modeling Gamma-Ray Attenuation in High-Redshift GeV Spectra
We present two models for the cosmological UV background light, and calculate
the opacity of GeV gamma--rays out to redshift 9. The contributors to the
background include 2 possible quasar emissivities, and output from
star--forming galaxies as determined by recent a semi--analytic model (SAM) of
structure formation. The SAM used in this work is based upon a hierarchical
build-up of structure in a CDM universe and is highly successful in
reproducing a variety of observational parameters. Above 1 Rydberg energy,
ionizing radiation is subject to reprocessing by the IGM, which we treat using
our radiative transfer code, CUBA. The two models for quasar emissivity
differing above z = 2.3 are chosen to match the ionization rates observed using
flux decrement analysis and the higher values of the line-of-sight proximity
effect. We also investigate the possibility of a flat star formation rate
density at z . We conclude that observations of gamma--rays from 10 to 100
GeV by Fermi (GLAST) and the next generation of ground based experiments should
confirm a strongly evolving opacity from z . Observation of
attenuation in the spectra of gamma--ray bursts at higher redshift could
constrain emission of UV radiation at these early times, either from a flat or
increasing star-formation density or an unobserved population of sources.Comment: 4 pages, 7 figures, To be published in the Proceedings of the 4th
Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, held
July 2008 in Heidelberg, German
HI Detection in two Dwarf S0 Galaxies in Nearby Groups: ESO384-016 and NGC 59
An \hi survey of 10 dE/dS0 galaxies in the nearby Sculptor and Centaurus A
groups was made using the Australia Telescope Compact Array (ATCA). The
observed galaxies have accurate distances derived by Jerjen et al (1998; 2000b)
using the surface brightness fluctuation technique. Their absolute magnitudes
are in the range . Only two of the ten galaxies were
detected at our detection limit ( \msol for the Centaurus
group and \msol for the Sculptor group), the two dS0
galaxies ESO384-016 in the Centaurus A Group and NGC 59 in the Sculptor Group,
with \hi masses of \msol and \msol respectively. Those two detections were confirmed using the Green
Bank Telescope. These small \hi reservoirs could fuel future generations of low
level star formation and could explain the bluer colors seen at the center of
the detected galaxies. Similarly to what is seen with the Virgo dEs, the two
objects with \hi appear to be on the outskirt of the groups.Comment: 25 pages (11 figures), accepted by A
Polymer state approximations of Schroedinger wave functions
It is shown how states of a quantum mechanical particle in the Schroedinger
representation can be approximated by states in the so-called polymer
representation. The result may shed some light on the semiclassical limit of
loop quantum gravity.Comment: 11 pages, 1 figure, Conclusions section adde
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