Ultra High Energy Cosmic Rays: Strangers Shrouded In Mystery Scott Fleming High Energy Series 24 Feb. 2005

Questions What are they? We don’t know. Where do they come from? We don’t know. How do they obtain such huge speeds? We don’t know. Are they isotropic or anisotropic on the sky? We don’t know. Are they galactic or extragalactic in origin? We don’t know. What is the average air speed velocity of an unladen swallow? I don’t know.

Definition Any particle or nuclei with energies > 1E+19 eV Known as Ultra High Energy Cosmic Rays (UHECR’s). (Some authors have two classes, UHECR’s and “Extremely High Energy Cosmic Rays” EHECR’s, > 1E+20 eV) The particles are likely protons that have (somehow) been accelerated to such high energies, though author’s also examine the possibility they are Oxygen or Iron nuclei, etc.

Slow down there... UHECR’s are known to travel at speeds up to c!!

Air Showers and Detectors Whipple Air Cerenkov Observatory Next-Gen = AGASA, HiRes, OWL, EUSO, et al. See:

Spectra of UHECR’s: Knees and Ankles? Slide from M. Teshima

The GZK Cutoff Named after Greisen, Zatsepin and Kuzmin. (Thankfully) now referred to as simply GZK Cutoff or GZK Feature Is a sharp decrease in expected flux of sources > 7E+19 eV (or so) caused by the interaction of these UHECR’s with photons from the CMB, which produces pions. Is an important feature to measure as it constrains how far these cosmic rays could have originated (“loss length”). E.g., for E ~ 4E+19 eV, loss length = ~1000 Mpc. Given angular resolution of current detectors is a few degrees, the number of possible sources is huge. For E ~ few E+20 eV, loss length = Mpc, and on average there is only one galaxy in an area of 2° about 20 Mpc from the Milky Way.

The GZK Cutoff De Marco et al., 2003 “We find that the very low statistics of the presently available data hinders any statistically significant claim for either detection or the lack of the GZK feature.”

The Galactic B-field And Associated Uncertainties (B-fields are such pains...) UNLIKE gamma ray showers, particles with energies > 1E12 eV are ionized. THEREFORE, determining the origins of these particles MUST include the scourge and bane of all of astronomy and physics....magnetic fields (in particular, the Galactic B-field) found in Prouza and Smida (2003)

Hot Off The Press: Cao et al. (yesterday) Model of Galactic B-field as a function of r (along direction of Sun) and as a function of z (height above galactic disk). Are UHECR’s correlated with BL Lac objects as previously suggested in literature? “This indicates that the correlation analysis method might be too sensitive to the model, especially at the stage of that there is not enough statistics for the UHECR samples. To draw the conclusion about the conclusion on the correlation between the UHECRs and the BL Lacs may need to enlarge the UHECR event samples.”

Conclusions and References Ultra High Energy Cosmic Rays remain great mysteries: Many are probably protons, but all we can really say is that they are positively charged particles. We just don’t know. Are UHECR’s galactic or extragalactic? We think the highest-energy cosmic rays are extragalactic, but we really just don’t know. Pinning down a distance for the sources requires a knowledge of the GZK cutoff (if it exists). Where that precise cutoff lies and even if it exists at all, we just don’t know. The magnetic field of our galaxy would play an important role in determining the direction individual cosmic rays originate from, but it is extremely model-dependent given current data sets, so we just don’t know. And the average air speed velocity depends upon the continent of origin of the swallow, so I just can’t answer!

Conclusions and References Slides from a lecture by M. Teshima (AGASA Project): Arnos, J., 2003, ApJ, 589, 871. Blasi, P. and De Marco, D., 2004, APh, 20, 559. Cao, Z., Dai, B.Z., Yang, J.P., Zhang, L., astro-ph/ De Marco, D., Blasi, P., Olinto, A.,2003, APh, 20, 53. Prouza, M., Smida, R.,2003, A&A, 410, 1.