Thursday, November 15, 2007

AGN's and Cosmic Rays

Last week the phenomenon of ultra-high energy cosmic rays was linked to giant Black Holes. The most energetic particles in the Universe, more powerful than those produced by human accelerators, are in cosmic rays. Scientists don't even know how energetic these particles can get. Ultra-high energy cosmic rays have been nicknamed "Oh My God" particles.

Using the Auger Observatory in Argentina, astronomers have linked the origin of some high-energy cosmic rays to active galactic nuclei. AGN's are energetic cores of galaxies powered by supermassive Black Holes. Scientists still don't know how the galactic cores are formed. Size of a primordial singularity is limited by a horizon distance related to the speed of light. Supermassive Primordial Black Holes ar more indications of a changing speed of light.

The most energetic cosmic rays don't reach Auger at all, but are stopped by Earth's upper atmosphere. Here they scatter into a shower of secondary particles. Source of the electrical discharges in lightning storms has been another mystery. Since cosmic rays fall on Earth constantly, some have suggested that they are the source of lightning. If true, it is one more example of how are lives are intertwined with Space.

To observe the most energetic cosmic rays, we must go beyond the atmosphere. Spacecraft like SWIFT and GLAST can detect these rays and possibly locate their source. Sources of many cosmic rays are still unidentified. They could be the explosions of tiny primordial Black Holes, or possibly something else. We are only begiining to learn about cosmic gamma rays.

CERN claims to have detected a fountain of antimatter erupting from the core of our Milky Way galaxy. Their "antimatter flash" video is above. More news in the new Carnival of Space!

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Blogger nige said...

Louise, thank you for a very interesting post on a fascinating subject! Cosmic rays are amazing. Apparently 90% that hit the Earth's atmosphere are protons from the sun, 9% are alpha particles (helium nuclei) and 1% are electrons.

Of course the protons don't make it through the Earth's atmosphere (equivalent to a radiation shield of 10 metres of water, which is quite adequate to shield the core of a critical water-moderated nuclear reactor!!).

When the high-energy protons hit air nuclei, you get some secondary radiation being created like pions which decay into muons and then electrons.

A lot of the electrons get trapped into spiralling around the Earth's magnetic field lines at high altitudes, in space, forming the Van Allen radiation belts.

Where the magnetic field lines dip at the poles, they all come together, and so the electron density increases at the poles. At some point this negative electric charge density is sufficiently large to "reflect" most incoming electrons back, and that spot is called the "mirror point".

Hence the captured electrons are trapped into spiralling around magnetic field lines, to-and-fro between mirror points in the Northern and Southern hemispheres.

There are also of course occasional irregular gamma ray flashes from gamma ray bursters, heavy particles, etc.

It's not clear what the actual radiation levels involved are: obviously the radiation level from cosmic radiation on Earth's surface is known. It's highest at the poles where incoming radiation runs down parallel to magnetic field lines (without being captured), hence the "aurora" around the polar regions where cosmic rays leak into the atmosphere in large concentrations.

It's also high in the Van Allen belts of trapped electrons.

It's not quite as bad in space well away from the Earth. Apparently, the cosmic radiation level on the Moon's surface is approximately 1 milliRoentgens/hour (10 micro Sieverts/hour), about 100 times the level on the Earth's surface. If that's true, then presumably the Earth's atmosphere (and the Earth's magnetic field) is shielding 99% of the cosmic radiation exposure rate.

All satellites have to have radiation-hardened solar cells and electronics, in order to survive the enhanced cosmic radiation exposure rate in space.

In the original version of Hawking's "A Brief History of Time" he has a graph showing the gamma ray energy spectrum of cosmic radiation in outer space, with another curve showing the gamma ray output from black holes via Hawking radiation. Unfortunately, the gamma background radiation intensity at all frequencies in the spectrum is way higher than the predicted gamma ray output from massive black holes (which is tiny), so there is too much "noise" to identify this Hawking radiation.

2:15 AM  
Blogger L. Riofrio said...

ncc1701Thank you nige. Cpomic rays are an endlessly fascinating subject on wehich we could write much more. The graph in the original "Brief History of Time" is interesting, but many things have changed in 30 years. I hope we conclusively find Hawking radiation sometime soon.

6:14 AM  
Anonymous Anonymous said...

There is no possibility of these ultra-high energy cosmic rays coming from ordinary black hole accretion disks. The radiation would necessarily be distant from such a source, else it would be instantly Compton- and Thomson- scattered into ordinary low-energy radiation. The larger a black hole is, the less dense, and the less violent the accretion disk. This puts an upper limit on how much bang you get for your mass buck from a BH accretion disk. It isn't even close to being able to produce UHECRs.

Since theorists are completely lost, the black hole is the catch-all explanation of everything astrophysical - well, almost everything - the remainder is assigned to (extremely improbable) collisions.

In fact, I believe UHE CRs are the sign of new physics of gravitation and electromagnetism, only manifest under extremely dense conditions. GR and EM exist in a given ratio (energy per unit volume) and this ratio goes up as matter becomes more dense. When EM starts dominating, we have conditions that can directly produce these CRs, without concomitant scattering off an enormous accretion disk. The place to look is in AGNs and associated quasars. Wait, the astronomers don't believe their own instruments, and deny such associations. Oh well!


1:15 AM  

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