Attack of the galactic subatomic particles

By Phil Plait | November 2, 2009 10:27 am

hst_m82What is the source of cosmic rays?

Seems like an easy enough question. Cosmic rays are little subatomic particles zipping across the Universe. We’ve known about them for decades, and just about any astronomer who has used a space telescope knows and loathes them; cosmic rays zap our detectors, leaving bright streaks in the images which need to be tediously cleaned out before we can do any real science. I spent a large fraction of my time with Hubble doing just that.

But what’s generating them? They seem to come from all directions in the sky, making it difficult to pin down their source. They’re moving at fantastic speeds, so they must have a huge energy source behind them. For years, astronomers have suspected that they are accelerated to high velocities in supernovae explosions as well as in the fierce solar winds from massive stars. Recent evidence has been making that supposition seem more likely.

And now new results from the VERITAS gamma-ray telescope array have added fuel to that fire. VERITAS stands for Very Energetic Radiation Imaging Telescope Array System, and it’s located atop a mountain in Arizona. When energetic gamma rays (a form of light) hit the Earth’s air, they create a shower of subatomic particle that rains down and can be detected by the telescopes.

If cosmic rays come from supernovae and massive stars, then we should see more of them coming from galaxies that have a lot of stars being born. That’s because massive stars don’t live long. A nearby galaxy vigorously cranking out baby stars will therefore have lots of massive stars making cosmic rays. As a happy by-product, those same massive stars are the ones that blow up as supernovae, giving us a two-fer as far as cosmic ray production goes.

Such a galaxy exists: M82, a weird-looking one located a mere 12 million light years away (it’s close enough to see in binoculars, in fact). The image above is M82 as seen by Hubble, Chandra, and Spitzer. Astronomers trained VERITAS on M82 and took a very, very long exposure. What they found is that is is a weak source of gamma rays, but definitely above the background level. The amount detected is consistent with cosmic rays being generated in the violent environment of the galaxy which then slam into the gas surrounding the stars, generating gamma rays. Another process, called inverse Compton scattering, is probably behind this as well: when a low energy particle of light called a photon hits a cosmic ray, its energy is pumped waaaay up, and it becomes a gamma ray. Think of it like gently throwing a rubber ball in front of a speeding truck on the highway; the ball suddenly and violently finds itself with a lot more energy.

This all may not seem like a big deal, but it is. For one thing, there are a lot of cosmic rays flitting about out there, so knowing what they are and how they formed is clearly a big piece of understanding the high-energy Universe. Also, these cosmic rays may have an effect here on Earth. Scientists have been studying how they may interact with the Earth… and while the effect, if any, is incredibly small (people still argue over whether there is anything to this at all, like cloud formation and such) it’s worth investigating.

And I want to add something that makes me smile. The cosmic rays (which, remember, are subatomic particles) from M82 were detected because while still inside that galaxy they make gamma rays, a form of light, which then travel straight to Earth. But once those gamma rays hit our air they create subatomic particles once again, which are what VERITAS detects. So there are several steps to this process, with cosmic and gamma rays going back and forth until we actually detect their effect. It goes to show that sometimes the key to our understanding the Universe can involve subtle processes piled one atop the other, and it’s up to us to carefully peel back those layers to get to the underlying processes underneath.

CATEGORIZED UNDER: Astronomy, Science

Comments (17)

  1. I swear I remember many years ago seeing a chart of the electromagnetic spectrum, with long wave radio at one end, visible light in the middle, and “cosmic rays” at the other end, as if they were a form of em radiation at a higher energy than gamma rays.

    It’s only in the last few years that I’ve realized that cosmic rays are not em radiation at all. I wish I could find that old chart. Or maybe my memory is playing tricks on me :)

  2. IVAN3MAN AT LARGE

    Phil, at the sixth paragraph, in the fourth line:

    What they found is that is is a weak source of gamma rays…

    I think that should be: What they found is that it is a weak source of gamma rays…

  3. MindDetective

    “So there are several steps to this process, with cosmic and gamma rays going back and forth until we actually detect their effect. It goes to show that sometimes the key to our understanding the Universe can involve subtle processes piled one atop the other, and it’s up to us to carefully peel back those layers to get to the underlying processes underneath.”

    You’ve identified the joy of cognitive science here. It is the basic philosophy taken by many scientists today (called logical positivism) that psychologists borrowed from physicists and chemists. We can’t see some things directly. We can’t even detect some things with instruments directly. So we rely on theory and logical inference to reconstruct the nature of physics (and the mind). It is a wonderful process to see in action!

  4. DrFlimmer

    Talking of cosmic rays. There has been a paper recently that uncovers that one of the key-features of the cosmic ray energy spectrum is not due to any form of dark matter (what has been speculated) but just due to shock-acceleration at Supernova shock fronts:

    adsabs.harvard.edu/abs/2009PhRvL.103f1101B

    Btw: A very nice descripton of the inverse-Compton effect ;)

    Note to add:
    This study here has been led by Martin Pohl from Iowa State University. The thing is, this guy studied in Bochum, Germany… just where I am… the world is small :)

  5. Cheyenne

    Does this article apply to the “Ulta-high cosmic rays” as well (like the one below- Wikipedia source)?

    “Oh-My-God particle (a play on the nickname “God particle” for the Higgs boson) observed on the evening of 15 October 1991 over Dugway Proving Grounds, Utah. Its observation was a shock to astrophysicists, who estimated its energy to be approximately 3 × 1020 electronvolts (50 joules)—in other words, a subatomic particle with macroscopic kinetic energy equal to that of a baseball (142 g or 5 ounces) traveling at 96 km/h (60 mph).”

  6. IVAN3MAN AT LARGE

    @ Carey,

    Your memory is not playing tricks on you; I remember seeing the same (incorrect) chart, as you describe, in old astronomical books I had read in the ’70s.

  7. “So there are several steps to this process, with cosmic and gamma rays going back and forth until we actually detect their effect. It goes to show that sometimes the key to our understanding the Universe can involve subtle processes piled one atop the other, and it’s up to us to carefully peel back those layers to get to the underlying processes underneath.”

    Emphasis mine. Well, there you go! THAT right there is the problem. There is very little left of subtleness in the Amreican psyche. No wonder we are sucking at science…

  8. KC

    Carey – your mind is not paying tricks on you – I too remember seeing EM Spectrum charts that had cosmic rays listed on them. Robert Millikan, who I believe coined the term “Cosmic Rays”, incorrectly though of them as high energy photons.

  9. Yes we have studied CR for decades and they are still fascinating. Many of the first generation of space scientists such as Jim Van Allen and Harry Elliot (Harry died this year and will be commemorated with a symposium at Imperial College, London next week) started off in space to look at cosmic rays above the atmosphere, i.e. to see the primary. It also turns out that the inner Van Allen belt is caused by the albedo of cosmic rays, (CRAND=cosmic ray albedo neutron decay). We seem to have discovered something similar at Saturn with the Cassini spacecraft.
    Thanks for your blog. I hope young people read it and disabuse themselves of the idea that science is a drag and contains beauty.

  10. Tim G

    I recall seeing cosmic rays listed on an electromagnetic spectrum chart as well. I think it was from my 10th grade (1986-1987) chemistry textbook.

  11. llewelly

    That damn electro-magnetic spectrum chart which depicted cosmic rays as if they were the highest energy part of the EM spectrum really got around. I saw it too.

  12. TheBlackCat

    My high school chemistry professor had an EM spectrum poster on his wall that had cosmic rays. It was also in several of my textbooks, and in another poster at a science camp I went to when I was 7 or 8.

  13. I'd rather be fishin'

    I saw it too and mentioned it to my physics teacher. He explained that the chart was in order of increasing energy, not just of the EM spectrum.

  14. When they say weak source of gamma rays they don’t mean weak gamma rays. Weak Cosmic rays would make sense and would mean low to no gamma rays. Gamma rays are the strongest form of EM radiation, they have so much energy (extremely high frequency) that they have a high probability of turning into matter anti matter pairs. But while they are gamma rays they are still definitely an extreme high energy form of light.

  15. Sili

    For those asking about the EM/gamma-ray/cosmic ray mix-up, go read Astroprof – he’s doing a series on just that issue at the moment.

  16. WM

    On a tangential note —- While reading this post, I thought of something which the pop culture geek in you may have also thought of. Cosmic rays, gamma rays, and Marvel. (I seem to remember a What If? where Dr. Banner’s gamma irradiation was used to undo Ben Grimm to mundane form.)

  17. @WM:

    Yeah, the Cosmic Rays that mutated the Fantastic Four (and the Power Cosmic of the Silver Surfer), and gamma rays that mutated Bruce Banner to The Hulk.
    Then, Spider-Man was mutated via a ‘radioactive’ (Gamma?) spider.

    One of the reasons I love reading the occasional Radioactive Man (The Simpsons spinoff)

    J/P=?

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