4,104 research outputs found
The all-particle spectrum of primary cosmic rays in the wide energy range from 10^14 eV to 10^17 eV observed with the Tibet-III air-shower array
We present an updated all-particle energy spectrum of primary cosmic rays in
a wide range from 10^14 eV to 10^17 eV using 5.5 times 10^7 events collected in
the period from 2000 November through 2004 October by the Tibet-III air-shower
array located at 4300 m above sea level (atmospheric depth of 606 g/cm^2). The
size spectrum exhibits a sharp knee at a corresponding primary energy around 4
PeV. This work uses increased statistics and new simulation calculations for
the analysis. We performed extensive Monte Carlo calculations and discuss the
model dependences involved in the final result assuming interaction models of
QGSJET01c and SIBYLL2.1 and primary composition models of heavy dominant (HD)
and proton dominant (PD) ones. Pure proton and pure iron primary models are
also examined as extreme cases. The detector simulation was also made to
improve the accuracy of determining the size of the air showers and the energy
of the primary particle. We confirmed that the all-particle energy spectra
obtained under various plausible model parameters are not significantly
different from each other as expected from the characteristics of the
experiment at the high altitude, where the air showers of the primary energy
around the knee reaches near maximum development and their features are
dominated by electromagnetic components leading to the weak dependence on the
interaction model or the primary mass. This is the highest-statistical and the
best systematics-controlled measurement covering the widest energy range around
the knee energy region.Comment: 19 pages, 20 figures, accepted by Ap
The Knee of the Cosmic Hydrogen and Helium Spectrum below 1 PeV Measured by ARGO-YBJ and a Cherenkov Telescope of LHAASO
The measurement of cosmic ray energy spectra, in particular for individual
species, is an essential approach in finding their origin. Locating the "knees"
of the spectra is an important part of the approach and has yet to be achieved.
Here we report a measurement of the mixed Hydrogen and Helium spectrum using
the combination of the ARGO-YBJ experiment and of a prototype Cherenkov
telescope for the LHAASO experiment. A knee feature at 640+/-87 TeV, with a
clear steepening of the spectrum, is observed. This gives fundamental inputs to
galactic cosmic ray acceleration models
Gamma ray flares from Mrk421 in 2008 observed with the ARGO-YBJ detector
In 2008 the blazar Markarian 421 entered a very active phase and was one of
the brightest sources in the sky at TeV energies, showing frequent flaring
episodes. Using the data of ARGO-YBJ, a full coverage air shower detector
located at Yangbajing (4300 m a.s.l., Tibet, China), we monitored the source at
gamma ray energies E > 0.3 TeV during the whole year. The observed flux was
variable, with the strongest flares in March and June, in correlation with
X-ray enhanced activity. While during specific episodes the TeV flux could be
several times larger than the Crab Nebula one, the average emission from day 41
to 180 was almost twice the Crab level, with an integral flux of (3.6 +-0.6)
10^-11 photons cm^-2 s^-1 for energies E > 1 TeV, and decreased afterwards.
This paper concentrates on the flares occurred in the first half of June.
This period has been deeply studied from optical to 100 MeV gamma rays, and
partially up to TeV energies, since the moonlight hampered the Cherenkov
telescope observations during the most intense part of the emission. Our data
complete these observations, with the detection of a signal with a statistical
significance of 3.8 standard deviations on June 11-13, corresponding to a gamma
ray flux about 6 times larger than the Crab one above 1 TeV. The reconstructed
differential spectrum, corrected for the intergalactic absorption, can be
represented by a power law with an index alpha = -2.1 extending up to several
TeV. The spectrum slope is fully consistent with previous observations
reporting a correlation between the flux and the spectral index, suggesting
that this property is maintained in different epochs and characterizes the
source emission processes.Comment: Accepted for publication on ApJ
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