Astrophysics of galactic and extragalactic cosmic rays

The origin of the highest energy cosmic rays detected in the neighborhood of Earth is still an unsettled issue. Most of the low energy particles are believed to be produced by supernovae explosions in the galactic ISM, whereas some of those with higher energies (up to 10¹⁵ eV) are thought to arise from explosions of massive stars in their stellar winds. Near 3 x 10¹⁸ eV there is a rapid switch in chemical composition to H and He, and the dominant source for the rays is believed to shift from local to cosmological objects, like powerful radiogalaxies. At the highest energies (10²⁰ eV), just nearby extragalactic sources can produce the rays if they are protons or nuclei, this is the outcome of an effect known as the GZK cutoff, due to the strong propagation losses that affect the particles in their journey to Earth. Examples of CR sources:

Left: Colliding Galaxies. Right: Active galactic nucleus
Pictures obtained by HST.

A collaborative effort among the group and others researchers has been directed to the use of multifrequency observations in order to estimate the arrival energy of protons accelerated by strong shock fronts in the outer parts of southern radio galaxies, like Centaurus A and PKS 1333-33, and in the terminal shock of galactic superwinds generated in starburst galaxies, like NGC 253. In a series of papers on possible source candidates, different energy losses have been discussed for the rays as well as the associated changes in the arrival spectra.  Some of our research is directed to provide suitable predictions for the forthcoming southern Pierre Auger observatory.

The Auger Observatory is a hybrid detector, part surface array and part atmospheric fluorescence detector. The array will consist of 1700 particle detector stations or tanks distributed in a giant grid covering about 3000 square kilometers. These detectors, each containing 12 tons of pure water, will be spaced 1.5 kilometers apart. Instruments in each station will measure the density and time-of-arrival of particles in the air shower. Each tank will be self-contained -operating on solar power, communicating with its neighbors via radio, measuring time using orbiting GPS satellites, and controlled by an on-board microcomputer. Shower particles produced by a high energy cosmic ray strike the stations at nearly the same time -the small time differences observed, measured in millionths of a second, allow scientist to reconstruct the arrival direction of the cosmic ray. The number of particles observed at each tank is proportional to the total number of particles in the air shower which is in turn related to the energy of the primary cosmic ray. When shower particles strike a tank it will communicate with its neighbors to decide whether or not it has observed a portion of a much larger shower. If so, information about the shower will be transmitted by radio to a central location. There, computers will combine the measurements of particle number and time of arrival from each station to determine the direction and energy of the cosmic ray which produced the observed air shower. Read more about the Project in the Argentinian Collaboration pages at La Plata and CNEA.

Left: How Pierre Auger Observatory works. The observatory uses 1600 particle detectors spaced
uniformly over 3000 square kilometers to record cosmic ray air showers. On dark nights, sensitive light
sensors observe the faint fluorescence caused by collisions of air shower particles with air molecules in the
atmosphere. Right: southern site in Mendoza, Argentina.

The group also works on the theoretical side. Recently some members have analyzed whether there is a way out of the mystery of the GZK cutoff via the use of new neutral hadrons particles or neutrinos with new interactions. We studied the correlation between compact radio quasars and ultra-high energy cosmic rays (CRs) using an updated list of air shower detections. We estimated the level of positional correlation between both samples and the probability of pure chance association through simulations of random sets of synthetic CR events. We found that there are no reasons to claim for a physical association and that some previous results appear to be an effect of the small size of the sample used. This is also true when, instead of compact radio quasars, 3EG gamma-ray blazars are considered. Consequently, unless somehow severely deflected, we have shown that it is unlikely that the high energy CR primaries are new particles or particles with new interactions beyond the electroweak scale, produced in high-redshift active galactic nuclei. For details see the results of our study here.

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