925 research outputs found
A Spectacular VHE Gamma-Ray Outburst from PKS 2155-304 in 2006
Since 2002 the VHE (>100 GeV) gamma-ray flux of the high-frequency peaked BL
Lac PKS 2155-304 has been monitored with the High Energy Stereoscopic System
(HESS). An extreme gamma-ray outburst was detected in the early hours of July
28, 2006 (MJD 53944). The average flux above 200 GeV observed during this
outburst is ~7 times the flux observed from the Crab Nebula above the same
threshold. Peak fluxes are measured with one-minute time scale resolution at
more than twice this average value. Variability is seen up to ~600 s in the
Fourier power spectrum, and well-resolved bursts varying on time scales of ~200
seconds are observed. There are no strong indications for spectral variability
within the data. Assuming the emission region has a size comparable to the
Schwarzschild radius of a ~10^9 solar mass black hole, Doppler factors greater
than 100 are required to accommodate the observed variability time scales.Comment: 4 pages, 3 figures; To appear in the Proceedings of the 30th ICRC
(Merida, Mexico
The new model of fitting the spectral energy distributions of Mkn 421 and Mkn 501
The spectral energy distribution (SED) of TeV blazars has a double-humped
shape that is usually interpreted as Synchrotron Self Compton (SSC) model. The
one zone SSC model is used broadly but cannot fit the high energy tail of SED
very well. It need bulk Lorentz factor which is conflict with the observation.
Furthermore one zone SSC model can not explain the entire spectrum. In the
paper, we propose a new model that the high energy emission is produced by the
accelerated protons in the blob with a small size and high magnetic field, the
low energy radiation comes from the electrons in the expanded blob. Because the
high and low energy photons are not produced at the same time, the requirement
of large Doppler factor from pair production is relaxed. We present the fitting
results of the SEDs for Mkn 501 during April 1997 and Mkn 421 during March 2001
respectively.Comment: 5 pages, 1 figures, 1table. accepted for publication in Sciences in
China --
Between overt and covert research: concealment and disclosure in an ethnographic study of commercial hospitality
This article examines the ways in which problems of concealment emerged in an ethnographic study of a suburban bar and considers how disclosure of the research aims, the recruitment of informants, and elicitation of information was negotiated throughout the fieldwork. The case study demonstrates how the social context and the relationships with specific informants determined overtness or covertness in the research. It is argued that the existing literature on covert research and covert methods provides an inappropriate frame of reference with which to understand concealment in fieldwork. The article illustrates why concealment is sometimes necessary, and often unavoidable, and concludes that the criticisms leveled against covert methods should not stop the fieldworker from engaging in research that involves covertness
Neutrino production through hadronic cascades in AGN accretion disks
We consider the production of neutrinos in active galactic nuclei (AGN)
through hadronic cascades. The initial, high energy nucleons are accelerated in
a source above the accretion disk around the central black hole. From the
source, the particles diffuse back to the disk and initiate hadronic cascades.
The observable output from the cascade are electromagnetic radiation and
neutrinos. We use the observed diffuse background X-ray luminosity, which
presumably results {}from this process, to predict the diffuse neutrino flux
close to existing limits from the Frejus experiment. The resulting neutrino
spectrum is down to the \GeV region. We discuss modifications of
this scenario which reduce the predicted neutrino flux.Comment: 12 Pages, LaTeX, TK 92 0
The local economic development processes in low-income countries: the case of the metropolis of Chegutu in Zimbabwe
Local authorities are widely regarded as catalysts accelerating localised processes of economic development in industrialised countries but in low-income countries they are perceived as dysfunctional, inefficient and ineffective in meeting and addressing societal demands. This abstract view is however, not grounded in empirical research. As such, utilising the case of the metropolis of Chegutu a survey was designed to empirically explicate the economic processes militating its economic development. The findings are useful to policy-makers, local government authorities and management scholars. The study's unique contribution lies in its examination of the processes of local economic development in a low-income country
Risk Factors for Urinary Complications After Renal Transplantation
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73788/1/j.1600-6143.2007.01790.x.pd
Tau Neutrinos Underground: Signals of Oscillations with Extragalactic Neutrinos
The appearance of high energy tau neutrinos due to
oscillations of extragalactic neutrinos can be observed by measuring the
neutrino induced upward hadronic and electromagnetic showers and upward muons.
We evaluate quantitatively the tau neutrino regeneration in the Earth for a
variety of extragalactic neutrino fluxes. Charged-current interactions of the
upward tau neutrinos below and in the detector, and the subsequent tau decay
create muons or hadronic and electromagnetic showers. The background for these
events are muon neutrino and electron neutrino charged-current and
neutral-current interactions, where in addition to extragalactic neutrinos, we
consider atmospheric neutrinos. We find significant signal to background ratios
for the hadronic/electromagnetic showers with energies above 10 TeV to 100 TeV
initiated by the extragalactic neutrinos. We show that the tau neutrinos from
point sources also have the potential for discovery above a 1 TeV threshold. A
kilometer-size neutrino telescope has a very good chance of detecting the
appearance of tau neutrinos when both muon and hadronic/electromagnetic showers
are detected.Comment: section added and two new figs; accepted for publication in Physical
Review
Observations of Mkn 421 in 2004 with H.E.S.S. at large zenith angles
Mkn 421 was observed during a high flux state for nine nights in April and
May 2004 with the fully operational High Energy Stereoscopic System (H.E.S.S.)
in Namibia. The observations were carried out at zenith angles of
60--65, which result in an average energy threshold of 1.5 TeV
and a collection area reaching 2~km at 10~TeV. Roughly 7000 photons from
Mkn~421 were accumulated with an average gamma-ray rate of 8 photons/min. The
overall significance of the detection exceeds 100 standard deviations. The
light-curve of integrated fluxes above 2~TeV shows changes of the diurnal flux
up to a factor of 4.3. For nights of high flux, intra-night variability is
detected with a decay time of less than 1 hour. The time averaged energy
spectrum is curved and is well described by a power-law with a photon index
\egamm and an exponential cutoff at \ecut~TeV and an average integral flux
above 2~TeV of 3 Crab flux units. Significant variations of the spectral shape
are detected with a spectral hardening as the flux increases. Contemporaneous
multi-wavelength observations at lower energies (X-rays and gamma-rays above
~GeV) indicate smaller relative variability amplitudes than seen
above 2~TeV during high flux state observed in April 2004.Comment: 5 pages, 4 figures, published in A&
XMM-Newton timing mode observations of Mrk 421
We present the results of a detailed temporal analysis of the bright BL Lac
object Mrk 421 using the three available long timing mode observations by the
EPIC PN camera. This detector mode is characterized by its long life time and
is largely free of photon pile-up problems. The source was found in different
intensity and variability states differing by up to more than a factor of three
in count rate. A time resolved cross correlation analysis between the soft and
hard energy bands revealed that the characteristics of the correlated emission,
with lags of both signs, change on time scales of a few thousand seconds.
Individual spectra, resolved on time scales of a few hundread seconds, can be
quite well fitted by a broken power law. We find significant spectral
variations on time scales as short as 500-1000 sec. Both the hard and the soft
band spectral indices show a non-linear correlation with the source flux. A
comparison of the observed light curves with numerical results from
relativistic hydrodynamic computer simulations of the currently favored
shock-in-jet models indicates that any determination of the jet's physical
parameters from `simple' emission models must be regarded with caution: at any
time we are seeing the emission from several emission regions distinct in space
and time, which are connected by the complex hydrodynamic evolution of the
non-uniform jet.Comment: 16 pages, 16 figures. Accepted for publication in A&
Cosmic Physics: The High Energy Frontier
Cosmic rays have been observed up to energies times larger than those
of the best particle accelerators. Studies of astrophysical particles (hadrons,
neutrinos and photons) at their highest observed energies have implications for
fundamental physics as well as astrophysics. Thus, the cosmic high energy
frontier is the nexus to new particle physics. This overview discusses recent
advances being made in the physics and astrophysics of cosmic rays and cosmic
gamma-rays at the highest observed energies as well as the related physics and
astrophysics of very high energy cosmic neutrinos. These topics touch on
questions of grand unification, violation of Lorentz invariance, as well as
Planck scale physics and quantum gravity.Comment: Topical Review Paper to be published in the Journal of Physics G, 50
page
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