647 research outputs found
Preliminary results on underground muon bundles observed in the Frejus proton-decay detector
The proton-decay detector installed in the Modane Underground laboratory (4400 mwe) in the Frejus tunnel (French Alps) has recorded 80 880 single muon and 2 322 multi-muon events between March '84 and March '85 (6425 hours of active time). During this period, a part of this modular detector was running, while new modules were being mounted, so that the detector size has continuously increased. The final detector has been completed in May '85
COURANTS NEUTRES ET CHARME DANS LES INTERACTIONS DE NEUTRINOS
Depuis quatre ans, la Physique du Neutrino a connu deux développements importants : l'étude des processus dus aux courants neutres faibles et l'étude de la production de particules charmées par des neutrinos. Pour illustrer les progrès expérimentaux accomplis dans ces domaines jusqu'en juin 1977, nous présentons dans cet article quelques résultats choisis parmi les plus marquants
Lognormal variability in BL Lacertae
X-ray data from the blazar BL Lac are used to investigate the nature of its
variability, and more precisely the flux dependency of the variability and the
distribution of fluxes. The variations in the flux are found to have a
lognormal distribution and the average amplitude of variability is proportional
to the flux level. BL Lac is the first blazar in which lognormal X-ray
variability is clearly detected. Lognormal variability in X-ray light curves,
probably related to accretion disk activity, has been discovered in various
compact systems, such as Seyfert galaxies and X-ray binaries. The light curve
is orders of magnitude less variable than other blazars, with few bursting
episodes. If this defines a specific state of the source, then the lognormality
might be the imprint of the accretion disk on the jet, linking for the first
time accretion and jet properties in a blazar.Comment: Accepted for Astronomy & Astrophysic
Selection and 3D-Reconstruction of Gamma-Ray-induced Air Showers with a Stereoscopic System of Atmospheric Cherenkov Telescopes
A simple 3D-reconstruction method for gamma-ray induced air showers is
presented, which takes full advantage of the assets of a system of Atmospheric
Cherenkov Telescopes combining stereoscopy and fine-grain imaging like the High
Energy Stereoscopic System (H.E.S.S.). The rich information collected by the
cameras allows to select electromagnetic showers on the basis of their
rotational symmetry with respect to the incident direction, as well as of their
relatively small lateral spread. In the framework of a 3D-model of the shower,
its main parameters -- incident direction, shower core position on the ground,
slant depth of shower maximum, average lateral spread of Cherenkov photon
origins (or ``photosphere 3D-width'') and primary energy -- are fitted to the
pixel contents of the different images. For gamma-ray showers, the photosphere
3D-width is found to scale with the slant depth of shower maximum, an effect
related to the variation of the Cherenkov threshold with the altitude; this
property allows to define a dimensionless quantity omega (the ``reduced
3D-width''), which turns out to be an efficient and robust variable to
discriminate gamma-rays from primary hadrons. In addition, the omega
distribution varies only slowly with the gamma-ray energy and is practically
independent of the zenith angle. The performance of the method as applied to
H.E.S.S. is presented. Depending on the requirements imposed to reconstructed
showers, the angular resolution at zenith varies from 0.04 to 0.1 degrees and
the spectral resolution in the same conditions from 15% to 20%.Comment: 32 pages including 38 figures. Accepted by AstroParticle Physic
Flexibility along the Neck of the Neogene Terror Bird Andalgalornis steulleti (Aves Phorusrhacidae)
BACKGROUND: Andalgalornis steulleti from the upper Miocene-lower Pliocene (≈6 million years ago) of Argentina is a medium-sized patagornithine phorusrhacid. It was a member of the predominantly South American radiation of 'terror birds' (Phorusrhacidae) that were apex predators throughout much of the Cenozoic. A previous biomechanical study suggests that the skull would be prepared to make sudden movements in the sagittal plane to subdue prey. METHODOLOGY/PRINCIPAL FINDINGS: We analyze the flexion patterns of the neck of Andalgalornis based on the neck vertebrae morphology and biometrics. The transitional cervical vertebrae 5th and 9th clearly separate regions 1-2 and 2-3 respectively. Bifurcate neural spines are developed in the cervical vertebrae 7th to 12th suggesting the presence of a very intricate ligamentary system and of a very well developed epaxial musculature. The presence of the lig. elasticum interespinale is inferred. High neural spines of R3 suggest that this region concentrates the major stresses during downstrokes. CONCLUSIONS/SIGNIFICANCE: The musculoskeletal system of Andalgalornis seems to be prepared (1) to support a particularly big head during normal stance, and (2) to help the neck (and the head) rising after the maximum ventroflexion during a strike. The study herein is the first interpretation of the potential performance of the neck of Andalgalornis in its entirety and we considered this an important starting point to understand and reconstruct the flexion pattern of other phorusrhacids from which the neck is unknown
Complex Spectral Variability from Intensive Multi-wavelength Monitoring of Mrk421 in 1998
We conducted a multi-frequency campaign for the TeV blazar Mrk~421 in 1998
April. The campaign started from a pronounced high amplitude flare recorded by
SAX and Whipple; ASCA observation started three days later. In the X-ray data,
we detected multiple flares, occuring on time scales of about one day. ASCA
data clearly reveal spectral variability. The comparison of the data from ASCA,
EUVE and RXTE indicates that the variability amplitudes in the low energy
synchrotron component are larger at higher photon energies. In TeV Gamma-rays,
large intra-day variations -- which were correlated with the X-ray flux -- were
observed when results from three Cherenkov telescopes are combined. The RMS
variability of TeV Gamma--rays was similar to that observed in hard X-rays,
above 10 keV. The X-ray light curve reveals flares which are almost symmetric
for most of cases, implying the dominant time scale is the light crossing time
through the emitting region. The structure function analysis based on the
continuous X-ray light curve of seven days indicates that the characteristic
time scale is ~0.5 day. The analysis of ASCA light curves in various energy
bands appears to show both soft (positive) and hard (negative) lags. These may
not be real, as systematic effects could also produce these lags, which are all
much smaller than an orbit. If the lags of both signs are real, these imply
that the particle acceleration and X-ray cooling time scales are similar.Comment: 10 pages, 4 figures, accepted for publication in ApJ Letter
BeppoSAX Observations of Markarian 501 in June 1999
We present the preliminary results of a long BeppoSAX observation of the BL
Lac object Mkn501 carried out in June 1999. The source was fainter than found
during the BeppoSAX pointings of 1997 and 1998, but is still detected with a
good signal-to-noise ratio up to ~40 keV. The X-ray spectrum in the energy
range 0.1-40 keV, produced through synchrotron radiation, is steeper than in
the previous years, it is clearly curved, and peaks (in nu*F_nu) at ~0.5 keV.
This energy is much lower than those at which the synchrotron component was
found to peak in 1997 and 1998. Some intraday variability suggests that
activity of the source on small time scales accompanies the large long time
scale changes of brightness and spectrum.Comment: 4 pages, Latex, 2 PostScript figures, to appear in the Proceedings of
the Conference "X-ray Astronomy '999: Stellar Endpoints, AGNs, and the
Diffuse X-ray Background" (Bologna, 6-10 September 1999
Searching for TeV dark matter by atmospheric Cerenkov techniques
There is a growing interest in the possibility that dark matter could be
formed of weakly interacting particles with a mass in the 100 GeV - 2 TeV
range, and supersymmetric particles are favorite candidates. If they constitute
the dark halo of our Galaxy, their mutual annihilations produce energetic gamma
rays that could be detected using existing atmospheric \u{C}erenkov techniques.Comment: 10 pp, LaTex (3 figures available by e-mail) PAR-LPTHE 92X
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