Average Depth Of Shower Maximum Research Articles

RECENT RESULTS FROM THE PIERRE AUGER OBSERVATORY

The Pierre Auger Observatory in Argentina is the largest cosmic ray detector array ever built. Its main goal is to measure cosmic rays of energy above 1018 eV with unprecedented statistics and precision. Although the construction of its baseline design was completed in mid-2008, the Observatory has been taking data continuously since January 2004. The main results obtained with the Pierre Auger Observatory are presented, with emphasis on the energy spectrum and studies of composition and arrival directions of the ultrahigh energy cosmic rays. Features observed in the energy spectrum are discussed. Results about cosmic ray composition inferred from systematic studies of the average depth of shower maximum and its fluctuations are reviewed. Recent results of studies of arrival direction distributions and correlations with nearby extragalactic objects are presented.

On the heavy chemical composition of the ultra-high energy cosmic rays

The Pierre Auger Observatory’s (PAO) shower profile measurements can be used to constrain the chemical composition of the ultra-high energy cosmic ray (UHECR) spectrum. In particular, the PAO’s measurements of the average depth of shower maximum and the fluctuations of the depth of shower maximum indicate that the cosmic ray spectrum is dominated by a fairly narrow distribution (in charge) of heavy or intermediate mass nuclei at the highest measured energies ( E ≳ 10 19 eV ) , and contains mostly lighter nuclei or protons at lower energies ( E ∼ 10 18 eV ) . In this article, we study the propagation of UHECR nuclei with the goal of using these measurements, along with those of the shape of the spectrum, to constrain the chemical composition of the particles accelerated by the sources of the UHECRs. We find that with modest intergalactic magnetic fields, 0.3 nG in strength with 1 Mpc coherent lengths, good fits to the combined PAO data can be found for the case in which the sources accelerate primarily intermediate mass nuclei (such as nitrogen or silicon). Without intergalactic magnetic fields, we do not find any composition scenarios that can accommodate the PAO data. For a spectrum dominated by heavy or intermediate mass nuclei, the Galactic (and intergalactic) magnetic fields are expected to erase any significant angular correlation between the sources and arrival directions of UHECRs.

The dual imaging cerenkov experiment (DICE) and the elemental composition in the “Knee” region

An outstanding problem in cosmic ray physics is the cosmic ray elemental abundance in the “knee” region of the charged particle spectrum. The Dual Imaging Cerenkov Experiment (DICE) is designed to measure the mean elemental mass and energy spectrum around the “knee” energy by using composition sensitive properties of Extensive Air Showers (EAS). We present preliminary results on one EAS parameter, the average depth of shower maximum, 〈 X max 〉. We also show evidence for a change in the cosmic ray spectral index, a result which is in qualitative agreement with other experiments. A small increase in 〈 X max 〉 is present at the onset of the “knee”, implying an increase in the relative number of lighter elements. Further consistency tests of these results which are in progress are described.

Average depth of shower maximum in “knee” region as measured by the Dual Imaging Cerenkov Experiment

Preliminary results on the average depth of shower maximum (〈Xmax〉) are presented using data from the Dual Imaging Cerenkov Experiment (DICE). A change in the cosmic ray spectral slope is detected and is in qualitative agreement with other experiments. A small increase in 〈Xmax〉 is present at onset of the “knee”, consistent with an increase in the relative number of lighter elements. Further analysis which incorporates data from the Chicago Air Shower Array and Michigan Array (CASA—MIA) will test this tentative interpretation.