Thursday, November 08, 2007

News from AUGER

The AUGER collaboration has released a new analysis of their data of the ultra high energetic cosmic rays (UHECR). As reported in Science (Science 9 November 2007: 896-897) They find correlations between the events of highest energies and active galactic nuclei (AGN), and are able to reject the hypothesis of an isotropic distribution of these cosmic rays at a confidence level of 99%. This reliably rules out speculations about the origin of these UHECRs in local, galactic sources. Though it has been expected, until now there was no experimental confirmation that they originate outside our galaxy.

What is really cute about their analysis is that this correlation with AGNs up to a distance of ~100 Mpc is present for UHECR with energies higher than a certain threshold, but not for those with lower energies: the correlation increases abruptly at the energy of about 5.7 x 1019 eV, which coincides with the point on the energy spectrum recently reported from the observatory at which the flux is reduced by ~50%. To understand this feature, recall that the GZK-cutoff (for an introduction see here) predicts that the mean free path of protons drops dramatically with increasing energy after a threshold is crossed, and the proton's energy is high enough to scatter at CMB photons to produce pions. In the energy range above ~8 x 1019 eV, protons that come from farther away than ~90 Mpc should be scattered, loose energy, and could never reach the earth. The vanishing of the correlation with AGNs around this threshold is thus is an independent confirmation of the GZK-cutoff, on which we also reported in July.

Aaron Chou from the AUGER collaboration gave a talk on the recent results today at our previously mentioned workshop. The recording and the slides should be available at the PI website soon.

More Info at the AUGER websites.

Update Nov 9th: The talk is now online, see PIRSA 07110054.

14 comments:

  1. Dear Bee,


    ah, that's very interesting news! And thanks for the pointer to the talk - I will have a look as soon as it will appear online there!

    BTW, did they say anything about magnetic fields? I mean, cosmic rays with "lower" energies would be deflected more than higher-energetic rays by the intergalactic magnetic fields, which may destroy correlations for these lower-energetic particles?

    Have a great time at the workshop,
    best,

    Stefan

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  2. Hi Bee,

    Thanks for the update!

    Of course the UHECR are thought to be produced by plasma jets from the AGN. Some jets are huge, radiating orders of magnitude more energy than the AGN host galaxy. It's also clear they have huge magnetic fields.

    If protons or heavier nuclei are leaving the AGN region as UHECR, wouldn't that leave behind a surplus negative charge? As described here, maybe the resulting charge imbalance plays a significant role in the magnetic fields and dynamics of AGN jets and their host galaxies.

    Anyone seen this idea discussed elsewhere?

    Best wishes,

    Kris

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  3. Hi Kris,

    There's a difference between 'are of course thought to be' and experimental confirmation. This difference is the reason why it's called science.

    Sure if the source is neutral and emits positive charges only, there should be negative charge left behind. Just why would that be a significant effect? Best,

    B.

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  4. KEWL

    Active galactic nuclei continue to be interesting.

    ReplyDelete
  5. So is it safe to say the OhmyGod particles can be chalked up to experimental error?

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  6. I've just seen, there is an "Authors' Summary: Correlation of the Highest-Energy Cosmic Rays with Nearby Extragalactic Objects" to the Science paper, which is available without subscription. It has also a map of the UHECR and AGN distributions.

    Best, Stefan

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  7. Hi Bee,

    You're right to leave open the possibility that the UHECR are not coming from jets. Is there observational evidence of UHECR from AGN where jets are absent?

    Briefly, in an environment with excess negative charge, accretion to an AGN could lead to charge concentration there. As described in the paper I linked to, a rapidly spinning charged AGN (not a black hole) could develop a very large magnetic field.

    The high collimation and large scale of jets from AGN (often much longer than galaxies) appears to demand very great magnetic fields, which I think are not provided by current models.

    The paper describes the resulting dynamics in this scheme. One nice feature is that it explains why the nuclei of old galaxies remain the same proportion of galactic mass as in young ones.

    Kris

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  8. Hi Kris,

    I think their present statistic is too low to say anything about the type of AGN.

    Best,

    B.

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  9. If the sources of the UHECRs are AGN, then there must be something happening that we don't understand. I don't see how they they wouldn't interact with the CMB to fall below the GZK limit or why they wouldn't be deflected by the galactic magnetic field if cosmic ray primary is charged. The primary would have to be either neutral or very heavy.

    Eric

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  10. UHECRs being protons eliminates one of the better standard physics explanations for the unusual paths seen in nuclear emulsions taken up in baloons.

    Probably the best paper for a review of weird stuff in cosmic ray emulsion film is this one which postulated that they might be signs of the quark gluon plasma, but nevertheless gives a nice collection of the experimental results in one place.

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  11. Thanks very much for the news. Great post, as always!

    Best,

    Siddharth

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  12. The thing I missed earlier is that the AGN sources that they match up with the UHECRs are within 75 MPc, so that there is no GZK suppression. So, if the correlation holds up then this solves the mystery. However, I personally would like to see more statistics before I'm convinced that a correlation exists.

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  13. Hi Eric,

    Regarding statistics, I can really recommend the talk by Aaron. He has very carefully explained the data analysis they have made. If I look at the image, it doesn't seem to convincing to me, but one gets easily fooled by points on maps. The chances that the observed UHECRs coincidentally fall into the regions around AGNs if they were isotropically distributed is extremely small. One should also point out that what they have found is a correlation with the AGNs. That doesn't necessarily mean, these are the sources. It could just be that the sources of UHECRs are also correlated with AGNs. Subtle but important difference. Best,

    B.

    ReplyDelete

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