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Cosmic Rays Reveal A Mysterious Galactic Neighbour? November 19, 2008

Posted by Sarah in science.
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The Earth’s atmosphere is continuously pelted by highly energetic cosmic rays from an unknown source local to the Solar system, scientists from Louisiana Stat University announced yesterday. Using a NASA-funded balloon-borne insrument called the Advanced Thin Ionization Calorimeter, or ATIC, the team found an excess of cosmic radiation at very high energies that can only have originated within the Galaxy, in the relative vicinity of our Sun.

I’m a bit confused, cosmic rays aren’t my thing, but I’ll try to make sense of it anyway as it sounds pretty exciting….

Produced in a wide variety of astrophysical processes both inside and beyond our Galaxy, cosmic rays, a collective term for protons, alpha particles, electrons and other charged particles, are ubiquitous in the Universe.  Their energies range from around one billiion electron volts (1 GeV) to a staggering 10^21 electron volts (10^12 GeV) – that’s 10 million times more energetic than can be produced in the largest particle accelerators on Eearth. The vast majority of them are blocked by the Earth’s atmosphere, so they do us no harm here on Earth; they are however a real threat to the health of astronauts. They’ve also intriguingly been suggested to play a role in global warming – this is an ongoing hot potato (or here).

Cosmic rays produced inside our Galaxy tend to have low enough energies to be transported along the local magnetic fields of the Sun, Earth and of the Galaxy itself. This scrambles their direction making it impossible to trace back where they originated. But using the ATIC instrument, which takes its measurements from a balloon high in the Earth’s atmosphere, scientists have identified an excess of particles at the high-energy end of the galactic cosmic ray energy spectrum, around 300-800 GeV, that defy current models of cosmic ray origins within our Galaxy.

The ATIC team, led by John Wefel of Louisiana State University, have therefore suggested in their paper, published in tomorrow’s edition of Nature, that these particles originated at a mysterious and highly energetic source inside our own Galaxy, at a maximum distance of 3000 light years from the Sun – just a fraction of the size of the Milky Way Galaxy. Huh, that’s pretty close to us. Possible sources may be an intermediate mass black hole, a pulsar, a “mini-quasar” or a supernova remnant.  What’s a mini-quasar?! An intermediate mass black hole with an accretion disk? Remember also, intermediate black holes have never been detected.

Alternatively, they suggest the particles may be the result of the annihilation of some exotic type of particles suggested for an explanation of dark matter. Dark matter has of course never been detected either and is one of modern astronomy’s Holy Grails.

This sounds like a very interesting result. I’m going to dig up the paper in Nature tomorrow and give an update…. If anyone would like to pitch in please leave me a comment.



1. andyxl - November 20, 2008

Sarah – as often with astro press releases, its kinda hard to tell what they have really done until we see the real paper. Is this just an excess in the CR N(E) ? Excess compared to what ? Hasn’t N(E) been well measured for years ? And are there really no candidates within 3kpc ?? The Crab is about 6kpc, so maybe not. Oh well, guess we’d better read the paper !

2. Sarah - November 22, 2008

Hi Andy – you’re right, the press release was actually pretty unhelpful. realised after I’d written all this that I should just wait for the paper but thought I’d throw it out there anyway. I just read the paper and will add a little update. It’s not my field though – Kaluza-Klein particles….?… my head hurts….

3. Sarah - November 22, 2008

And for a study into into possible astrophysical sources of the electron excess the paper refers to Kobayashi et al, ApJ 601, 340-351 (2004). More useful plots in the supplementary information online at Nature. They looked at the Vela SNR, Geminga neutron star, the Monogem SNR and Loop 1.

4. SarahAskew » The cosmic ray signature of dark matter? - October 14, 2010

[…] interesting paper was published in last week’s issue of Nature – I blogged about it before after reading the NASA press release. It’s wasn’t all that helpful without reading the […]

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