P. Miranda

Solar Neutron Event in Association with a Large Solar Flare on 2000 November 24

Solar neutrons have been detected using the neutron monitor located at Mount Chacaltaya, Bolivia, in association with a large solar flare on 2000 November 24. This is the first detection of solar neutrons by a neutron monitor that has been reported so far in solar cycle 23. The statistical significance of the detection is 5.5 σ. In this flare, the intense emission of hard X-rays and γ-rays has been observed by the Yohkoh Hard X-ray Telescope (HXT) and Gamma Ray Spectrometer (GRS), respectively. The production time of solar neutrons is better correlated with those of hard X-rays and γ-rays than with the production time of soft X-rays. The observations of the solar neutrons on the ground have been limited to solar flares with soft X-ray class greater than X8 in former solar cycles. In this cycle, however, neutrons were detected associated with an X2.3 solar flare on 2000 November 24. This is the first report of the detection of solar neutrons on the ground associated with a solar flare with an X-ray class smaller than X8.

Long-lived solar neutron emission in comparison with electron-produced radiation in the 2005 September 7 solar flare

Strong signals of neutral emissions were detected in association with a solar flare that occurred on 2005 September 7. They were produced by both relativistic ions and electrons. In particular, relativistic neutrons were observed with the solar neutron telescopes (SNTs) located at Mount Chacaltaya in Bolivia and Mount Sierra Negra in Mexico and with neutron monitors (NMs) at Chacaltaya and Mexico City with high statistical significances. At the same time, hard X-rays and γ-rays, which were predominantly emitted by high-energy electrons, were detected by the Geotail and the INTEGRAL satellites. We found that a model of the impulsive neutron emission at the time of the X-ray/γ-ray peak can explain the main peaks of all the detected neutron signals, but failed to explain the long tailed decaying phase. An alternative model, in which the neutron emission follows the X-ray/γ-ray profile, also failed to explain the long tail. These results indicate that the acceleration of ions began at the same time as the electrons but that ions were continuously accelerated or trapped longer than the electrons in the emission site. We also demonstrate that the neutron data observed by multienergy channels of SNTs put constraints on the neutron spectrum.

The Energy Spectrum and the Chemical Composition of Primary Cosmic Rays with Energies from 1014 to 1016 eV

We have measured extensive air showers with primary energies above 6 TeV at Mount Chacaltaya in Bolivia. The data were collected by an air shower array called the Minimum Air Shower (MAS) array starting in 2000 March. We applied an equi-intensity analysis method to the extensive air showers extended over the region of their maximum development. We varied the mixture of protons and iron in our simulations and compared these to the data to determine the mixing ratio of protons as a function of the primary energy. Using this, we derived the primary energy spectrum from 1014 to 5 × 1016 eV. Consequently, we conclude that the power-law index of the spectrum changes gradually around 1015.5 eV and that the obtained proton ratio decreases with increasing energy. We directly measured the longitudinal development of air showers generated by primaries with energies around the knee. We found that the average mass number of primary cosmic rays shows a steady increase with energy above 1014.5 eV and that the dominant component around the knee is not protons.

Chemical composition of primary cosmic rays with energies from 1015 to 1016.5 eV

Abstract The chemical composition of primary cosmic rays with energies from 1015 to 1016.5 eV, so called “knee” region, is examined. We have observed the time structures of air Cerenkov light associated with air showers at Mt. Chacaltaya, Bolivia, since 1995. The distribution of a parameter that characterizes the observed time structures is compared with that calculated with a Monte Carlo technique for various chemical compositions. Then the energy dependence of the average logarithmic mass numbers 〈 ln A 〉 of the primary cosmic rays is determined. The present result at 1015.3 eV is almost consistent with the result of JACEE (A≃12) and shows gradual increase in 〈 ln A 〉 as a function of the primary energy (A≃24 at 1016 eV). Form the comparison of the observational results with several theoretical models, we conclude that the supernova explosion of massive stars is a plausible candidate for the origin of cosmic rays around the “knee” region.

Searches for Ultra-Ehigh-Energy Gamma Rays from Sn 1987A and Centaurus X-3

Searches for gamma rays above 100 TeV from SN 1987A and above 50 TeV from Cen X-3 were carried out with an air shower array at Mount Chacaltaya over a period of 3 years. The array, which consists of 13 4 m2 and 28 1 m2 scintillation detectors, was located on a slope of about 30i facing southward to e†ectively observe air showers coming from the directions of these sources. In the analysis, a selection by shower age parameter was applied to reduce the background due to showers initiated by protons or nuclei. No signi-cant excess of gamma rays (and/or neutral particles) from these sources was found; our upper limits on the Nux of gamma rays above 100 TeV from SN 1987A and above 50 TeV from Cen X-3 are 2.2 ) 10~13 and 3.5 ) 10~13 photons cm~2 s~1 (95% con-dence level), respectively. Subject headings: gamma rays: observations E stars: individual (Centaurus X-3) E supernovae: individual(SN 1987A)