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Post Info TOPIC: Cosmic gamma rays
Blobrana


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RE: Cosmic gamma rays
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Title: Multi-wavelength identification of high-energy sources
Authors: R.P. Mignani (UVL-MSSL)

The nature of most of the ~300 high-energy gamma-ray sources discovered by the EGRET instrument aboard the Gamma-ray Observatory (GRO) between 1991 and 1999 is one of the greatest enigmas in high-energy astrophysics. While about half of the extragalactic sources have been optically identified with Active Galactic Nuclei (AGN), only a meagre 10% of the galactic sources have a reliable identification. This low success rate has mainly to be ascribed to the local crowding of potential optical counterparts and to the large gamma-ray error boxes (of the order of one degree in radius) which prevented a straightforward optical identification. Indeed, a multi-wavelength identification strategy, based on a systematic coverage of the gamma-ray error boxes, has been the only do-able approach. The situation is now greatly improving thanks to the observations performed by the Fermi Gamma-ray Space Telescope which, thanks to the LAT instrument, provides a factor of 50 improvement in sensitivity and a factor of 10 improvement in positional accuracy. However, while the smaller error boxes will make the multi-wavelength follow-ups easier, the larger sensitivity will enormously increase the number of detected gamma-ray sources, requiring an even larger effort in the multi-wavelength follow-ups. This effort can not be obviously sustained by targeted observations only and it would greatly benefit from multi-wavelength data and advanced data products available world wide through the science data centres and interfaced by the Virtual Observatory (VO) tools. In this contribution, I outline the science case, the multi-wavelength observation synergies, and the requirements for both the science data centres and the VO.

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Blobrana


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Cosmic Rays
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By the time cosmic rays hit Earth, they have journeyed through so many magnetic fields and other perturbations that they arrive nearly uniformly from all directions.
So when a detector in New Mexico began registering streams of charged particles coming from the general direction of the Orion nebula and about 500 light-years from Earth -- a neighbour by astronomical measures -- scientists took note.
Scientists aren't sure what causes cosmic rays, which are charged particles, namely protons and electrons, moving at high speeds due to unknown events. The list of candidates includes supernova explosions and quasars.

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Blobrana


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RE: Cosmic gamma rays
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Title: Is the Universe More Transparent to Gamma Rays Than Previously Thought?
Authors: Floyd W. Stecker, Sean T. Scully

The MAGIC collaboration has recently reported the detection of the strong gamma-ray blazar 3C279 during a 1-2 day flare. They have used their spectral observations to draw conclusions regarding upper limits on the opacity of the Universe to high energy gamma-rays and, by implication, upper limits on the extragalactic mid-infrared background radiation. In this paper we examine the effect of gamma-ray absorption by the extragalactic infrared radiation on intrinsic spectra for this blazar and compare our results with the observational data on 3C279. We find agreement with our previous results, contrary to the recent assertion of the MAGIC group that the Universe is more transparent to \gray s than our calculations indicate. Our analysis indicates that in the energy range between ~80 and ~500 GeV, 3C279 has a best-fit intrinsic spectrum with a spectral index ~2.18 using our fast evolution model and ~1.78 using our baseline model. However, we also find that spectral indices in the range of 0.0 to 3.0 are almost as equally acceptable as the best fit spectral indices. Our analysis method also allows us to determine the luminosity of the flare observed by MAGIC. Assuming the same intrinsic spectral index for this flare as for the 1991 flare from 3C279 observed by EGRET, viz., 2.02, which lies between our best fit indices, we estimate that the MAGIC flare was ~3 times brighter than the EGRET flare observed 15 years earlier.

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Blobrana


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Ultra-high-energy cosmic rays (UHECRs)
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The most energetic particles in the universe have regained some of their former mystery. Last year, it seemed that the origin of these particles had finally been tracked down to a set of giant black holes in nearby galaxies, but a new study casts doubt on that conclusion.

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Blobrana


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RE: Cosmic gamma rays
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Final results from the University of Utah's High Resolution Fly's Eye cosmic ray observatory show that the most energetic particles in the universe rarely reach Earth at full strength because they come from great distances, so most of them collide with radiation left over from the birth of the universe.
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Blobrana


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Title: Cosmic Ray Astronomy
Authors: P Sommers, S Westerhoff

Cosmic ray astronomy attempts to identify and study the sources of ultrahigh energy cosmic rays. It is unique in its reliance on charged particles as the information carriers. While no discrete source of ultrahigh energy cosmic rays has been identified so far, a new generation of detectors is acquiring the huge exposure that is needed at the highest energies, where deflection by magnetic fields is minimised and the background from distant sources is eliminated by pion photoproduction. In this paper, we summarise the status of cosmic ray astronomy, describing the detectors and the analysis techniques.

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 Final results from the University of Utahs High Resolution Flys Eye cosmic ray observatory show that the most energetic particles in the universe rarely reach Earth at full strength because they come from great distances, so most of them collide with radiation left over from the birth of the universe.
The findings are based on nine years of observations at the now-shuttered observatory on the U.S. Armys Dugway Proving Ground. They confirm a 42-year-old prediction known as the Greisen-Zatsepin-Kuzmin (GZK) cutoff, limit or suppression about the behaviour of ultrahigh-energy cosmic rays, which carry more energy than any other known particle.
The idea is that most but not all cosmic ray particles with energies above the GZK cutoff cannot reach Earth because they lose energy when they collide with cosmic microwave background radiation, which was discovered in 1965 and is the afterglow of the big bang physicists believe formed the universe 13 billion years ago.
The journal Physical Review Letters published the results Friday, March 21.

 Located at the University of Utah, the High Resolution Fly's Eye (HiRes) is an experiment to study the highest energy cosmic rays to determine the energy, direction, and chemical composition of the incident particle.
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Blobrana


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A star that died 340,000 years ago has provided fresh evidence that the energetic particles known as cosmic rays are being fired out by supernovae.
It is seems likely that a supernova's shock wave can boost protons to the sort of huge energies found in cosmic rays. Direct evidence for this is hard to find, however, as cosmic rays are deflected by magnetic fields and so cannot be easily traced back to their source.

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Blobrana


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Cosmic rays
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Faint, fleeting blue flashes of radiation emitted by particles that travel faster than the speed of light through the atmosphere may help scientists solve one of the oldest mysteries in astrophysics.
For nearly a century, scientists have wondered about the origin of cosmic rayssubatomic particles of matter that stream in from outer space.

Where exactly, we dont know. Theyre raining down on the atmosphere of the Earth, tens of thousands of particles per second per square meter - Scott Wakely, Assistant Professor in Physics at the University of Chicago.

Recent results from the Pierre Auger Cosmic Ray Observatory suggest that the highest-energy cosmic rays may come from the centres of active galaxies. But the vast majority of the cosmic rays seen at Earth originate from its own galaxy, from sources that are still unknown. Tracking down these sources is crucial to developing a comprehensive understanding of the phenomenon.

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Blobrana


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RE: Cosmic gamma rays
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Cosmic-ray source still in doubt
High-energy cosmic rays might not be from active galactic nuclei after all.
The origin of ultra-high-energy cosmic rays is one of the great mysteries of cosmology. Last week a team reported that they thought they had tracked down the source. But the first check on that answer, by another team with different data, has failed to support it.
The latest work has not yet been published, so it is hard for the community to know what to make of the results. But the conclusion hints that the mystery of high-energy cosmic rays remains open.

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Blobrana


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Ultra high-energy cosmic rays
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Monster black holes power highest-energy cosmic rays
Enormous black holes in galaxies millions of light years away are pelting us with energetic particles. The finding, from a telescope array 10 times the size of Paris, solves a long-standing mystery about the origins of the most energetic cosmic rays that strike the Earth's atmosphere.
Cosmic rays are charged particles such as protons and atomic nuclei that constantly rain down on Earth's atmosphere. Most come from the Sun and other sources within our galaxy, such as supernova remnants.
But the origins of the highest-energy particles, which travel within a whisker of the speed of light, have been puzzling. A single proton can have as much energy as a tennis ball served at 100 kilometres per hour.

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