174 research outputs found
Arylamines suppress their own activation and that of nitroarenes in V79 Chinese hamster cells by competing for acetyltransferases.
The effect of 2-aminofluorene (2-AF) on the toxicity of 2-aminoanthracene (2-AA) and 1,6-dinitropyrene (1,6-DNP) was studied in N-acetyltransferase-proficient V79-NHr1A2 cells genetically engineered for the expression of cytochrome P4501A2, and in wild-type V79-NH cells. 2-AA inhibited the growth of V79-NHr1A2 cells and induced the formation of micronuclei at concentrations of 0.1 to 1.0 microM, but was virtually without toxic effects at a concentration of 10 microM. Addition of 2-AF protected against the cytotoxic and genotoxic effects elicited by low concentrations of 2-AA. Half-maximum protection was observed at 0.2 to 0.5 microM 2-AF. The arylamine also prevented the cytotoxicity caused by 1,6-DNP in V79-NH cells and completely suppressed the formation of 1-acetylamino-6-nitropyrene from 1,6-DNP in these cells. The results indicate that arylamines and related N-hydroxyarylamines are substrates for the same acetyltransferase in V79-NH cells. In consequence, arylamines are capable of suppressing the activation of their proximate cytotoxic and genotoxic products in these cells and, presumably, in vivo
A Survey of the Northern Sky for TeV Point Sources
A search for steady TeV point sources anywhere in the northern sky has been
made with data from the Milagrito air-shower-particle detector. Over 3 x 10**9
events collected from 1997 February to 1998 May have been used in this study.
No statistically significant excess above the background from the isotropic
flux of cosmic rays was found for any direction of the sky with declination
between -5 degrees and 71.7 degrees. Upper limits are derived for the photon
flux above 1 TeV from any steady point source in the northern sky.Comment: 2 Figure
The Energy Spectrum of TeV Gamma-Rays from the Crab Nebula as measured by the HEGRA system of imaging air Cherenkov telescopes
The Crab Nebula has been observed by the HEGRA (High-Energy Gamma-Ray
Astronomy) stereoscopic system of imaging air Cherenkov telescopes (IACTs) for
a total of about 200 hrs during two observational campaigns: from September
1997 to March 1998 and from August 1998 to April 1999. The recent detailed
studies of system performance give an energy threshold and an energy resolution
for gamma-rays of 500 GeV and ~ 18%, respectively. The Crab energy spectrum was
measured with the HEGRA IACT system in a very broad energy range up to 20 TeV,
using observations at zenith angles up to 65 degrees. The Crab data can be
fitted in the energy range from 1 to 20 TeV by a simple power-law, which yields
dJg/dE = (2.79+/-0.02 +/- 0.5) 10^{-7} E^{-2.59 +/- 0.03 +/- 0.05}, ph m^{-2}
s^{-1} TeV^{-1} The Crab Nebula energy spectrum, as measured with the HEGRA
IACT system, agrees within 15% in the absolute scale and within 0.1 units in
the power law index with the latest measurements by the Whipple, CANGAROO and
CAT groups, consistent within the statistical and systematic errors quoted by
the experiments. The pure power-law spectrum of TeV gamma-rays from the Crab
Nebula constrains the physics parameters of the nebula environment as well as
the models of photon emission.Comment: to appear in ApJ, 29 pages, 6 figure
Correlated intense X-ray and TeV activity of Mrk~501 in 1998 June
We present exactly simultaneous X-ray and TeV monitoring with {\it RXTE} and
HEGRA of the TeV blazar Mrk 501 during 15 days in 1998 June. After an initial
period of very low flux at both wavelengths, the source underwent a remarkable
flare in the TeV and X-ray energy bands, lasting for about six days and with a
larger amplitude at TeV energies than in the X-ray band. At the peak of the TeV
flare, rapid TeV flux variability on sub-hour timescales is found. Large
spectral variations are observed at X-rays, with the 3--20 keV photon index of
a pure power law continuum flattening from to on a
timescale of 2--3 days. This implies that during the maximum of the TeV
activity, the synchrotron peak shifted to energies keV, a behavior
similar to that observed during the longer-lasting, more intense flare in 1997
April. The TeV spectrum during the flare is described by a power law with
photon index and an exponential cutoff at 4 TeV; an
indication for spectral softening during the flare decay is observed in the TeV
hardness ratios. Our results generally support a scenario where the TeV photons
are emitted via inverse Compton scattering of ambient seed photons by the same
electron population responsible for the synchrotron X-rays. The simultaneous
spectral energy distributions (SEDs) can be fit with a one-zone
synchrotron-self Compton model assuming a substantial increase of the magnetic
field and the electron energy by a factor of 3 and 10, respectively.Comment: Accepted for publication in ApJ, Part
Primary Proton Spectrum of Cosmic Rays measured with Single Hadrons
The flux of cosmic-ray induced single hadrons near sea level has been
measured with the large hadron calorimeter of the KASCADE experiment. The
measurement corroborates former results obtained with detectors of smaller size
if the enlarged veto of the 304 m^2 calorimeter surface is encounted for. The
program CORSIKA/QGSJET is used to compute the cosmic-ray flux above the
atmosphere. Between E_0=300 GeV and 1 PeV the primary proton spectrum can be
described with a power law parametrized as
dJ/dE_0=(0.15+-0.03)*E_0^{-2.78+-0.03} m^-2 s^-1 sr^-1 TeV^-1. In the TeV
region the proton flux compares well with the results from recent measurements
of direct experiments.Comment: 13 pages, accepted by Astrophysical Journa
Nonextensive thermal sources of cosmic rays?
The energy spectrum of cosmic rays (CR) exhibits power-like behavior with a
very characteristic "knee" structure. We consider a possibility that such a
spectrum could be generated by some specific nonstatistical temperature
fluctuations in the source of CR with the "knee" structure reflecting an abrupt
change of the pattern of such fluctuations. This would result in a generalized
nonextensive statistical model for the production of CR. The possible physical
mechanisms leading to these effects are discussed together with the resulting
chemical composition of the CR, which follows the experimentally observed
abundance of nuclei.Comment: 16 pages, 3 figures, rewritten and updated version, to be published
in Centr. Eur. J. Phy
Atmospheric neutrino flux from 3-dimensional simulation
The atmospheric muon and neutrino flux have been simulated using the same
approach which successfully accounted for the recent secondary proton, electron
and positron flux measurements in orbit by the AMS experiment. For the muon
flux, a good agreement is obtained with the CAPRICE and HEAT data for altitudes
ranging from sea level up to about 38 km. The general features of the
calculated atmospheric neutrino flux are reported and discussed. The flux
obtained at the Super-Kamiokande experiment location are reported and compared
with other calculations. For low neutrino energies the flux obtained is
significantly smaller than that used in the data analysis of underground
experiment. The simulation results for the SOUDAN experiment site are also
reported.Comment: 33 pages, 27 figures, 12 tables, final version for Phys. Rev.
Test of Hadronic Interaction Models in the Forward Region with KASCADE Event Rates
An analysis of muon and hadron rates observed in the central detector of the
KASCADE experiment has been carried out. The data are compared to CORSIKA
simulations employing the high-energy hadronic interaction models QGSJET,
DPMJET, HDPM, SIBYLL, and VENUS. In addition, first results with the new
hadronic interaction model neXus 2 are discussed. Differences of the model
predictions, both among each other and when confronted with measurements, are
observed. The hadron rates mainly depend on the inelastic cross-section and on
the contribution of diffraction dissociation. The discrepancy between
simulations and measurements at low primary energies around 5 TeV can be
reduced by increasing the non-diffractive part of the inelastic cross-section
of nucleon-air interactions. Examination of hadron multiplicities points
towards harder spectra of secondary pions and kaons needed in the calculations.Comment: 14 pages, 7 figures, corresponding author: M. Risse, accepted for
publication in J. Phys. G: Nucl. Part. Phy
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