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Active Galactic Nuclei: Sources for Ultra High Energy Cosmic Rays!

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Active Galactic Nuclei: Sources for Ultra High Energy Cosmic Rays! ACTIVE GALACTIC NUCLEI: SOURCES FOR ULTRA HIGH ENERGY COSMIC RAYS! Peter L. Biermann1,2,3,4, Laurent¸iu Caramete1,5, Alex Curut¸iu1, Ioana Dut¸an1, Ioana C. Mari¸s6, Oana Ta¸sc˘au7, Julia Becker8,9, Ralph Engel6, Heino Falcke10, Karl-Heinz Kampert7, & Todor Stanev11 1 MPI for Radioastronomy, Bonn, Germany 2 Dept. of Phys. & Astron., Univ. of Bonn, Germany 3 Dept. of Phys. & Astr., Univ. of Alabama, Tuscaloosa, AL, USA 4 Dept. of Phys., Univ. of Alabama at Huntsville, AL, USA 1 5 Institute for Space Studies, Bucharest, Romania 6 FZ Karlsruhe, and Phys. Dept., Univ. Karlsruhe, Germany 7 Phys. Dept., Univ. Wuppertal, Germany 8 Institution f¨orFysik, G¨oteborgs Univ., Sweden 9 Dept. of Phys., Univ. Dortmund, Dortmund, Germany 10 Dept. of Astrophys., IMAP, Radboud Univ., Nijmegen, Netherlands 11 Bartol Research Inst., Univ. of Delaware, Newark, DE, USA www.mpifr-bonn.mpg.de/div/theory 2 Abstract: Ultra high energy cosmic rays were discovered over forty years ago (Linsley 1963). The first prediction has been that due to in- teraction with the microwave background their spectrum should show a turn-down near 5 1019 eV (Greisen 1966, Zatsepin & Kuzmin 1966): this has now been confirmed by both HiRes and Auger (ICRC Mexico 2007). While many sites of origin have been proposed, only one argument demonstrated that protons of order 1021 eV are required in a source region to ex- plain observations, specifically the ubiquitous turnoff in the optical synchrotron spectrum near 3 1014 Hz (Biermann & Strittmatter 1987); this argument led to the second prediction that radio galaxies are sources of ultra high en- ergy cosmic rays, one rare and specific class of active galactic nuclei (AGN). Some other classes of AGN have been discounted already 3 (Becker et al. 2007). However, since radio galaxies are rare, the near isotropy of arrival directions on the sky shows that the particles suffer occasionally substantial scattering along their path; in order to avoid a substantial in- crease in the path-length to us, this scattering has to happen near to our Galaxy: presumably all disk galaxies have a magnetic halo-wind, as supported by dynamo arguments and radio ob- servations (Hanasz et al. 2004 +; Chy˙zy et al., 2000 +; Kulsrud & Zweibel 2007); our Galaxy has a wind (Westmeier et al 2005); and quasar absorption lines show absorption by clouds in a halo-wind to very large distances from galaxies (Williger et al. 2006, and Weymann et al.). In a magneto-hydrodynamic wind (Parker 1958) the dominant magnetic field is Bφ ∼ 1/r, al- lowing large bending; this wind is probably highly irregular. Therefore a third prediction 4 is that ultra high energy particles show a dis- tribution function of their bending angle from source to us, which is a highly pointed func- tion, with a core of a few degrees (from the hot disk of the Galaxy), and steep tails for large bending from the wind. The jet-disk symbiosis picture (Falcke et al. 1995 +) or the spin-down model (Blandford & Znajek 1977) allow to pre- dict maximum particle energy and maximum particle flux from such radio galaxies, and this yields a fourth prediction. The origin of ultra high energy cosmic rays has now been traced to active galactic nuclei (Auger-Coll. 2007). This is consistent with the second prediction. The third and fourth predictions remain to be tested. 5 Discovery and Prediction 1: The GZK-cutoff ] 1 -1 -1 sr 10 -1 s -2 -2 10 -3 10 -4 GeV cm 10 [ p -5 10 /dE -6 p 10 -7 ´dN 2 10 p E -8 10 -9 10 2 3 4 5 6 7 8 9 10 11 12 log10(Ep/GeV) Figure 1 All-particle cosmic ray spectrum from many earlier experiments. Filled cir- cles at the highest energies are recent results from Auger (ICRC 2007), clearly showing the Greisen-Zatsepin-Kuzmin cutoff, which is due to the interaction with the cosmic mi- crowave background. Clearly the particle energies require revision and rescaling. This is the spectrum to explain, and the strongest radio galaxies can provide an explanation. 6 • Linsley (1963): Detection of first event > 20 ∼ 10 eV - uncontainable in magnetic field of Galaxy • Prediction (1966) of GZK-turnoff near 5 × 1019 eV due to in- teraction with the cosmic microwave back- ground (MWBG: Greisen, Zatsepin & Kuzmin; sometimes called GZK-cutoff). • Turnoff seen by HiRes (2005) and AUGER (ICRC 2007, Mexico), and so prediction confirmed • Now many events near and beyond 5×1019 eV – nearly isotropic (Stanev et al. Phys. Rev. Letters 1995) and yet many correlations with BL Lac type active galactic nuclei claimed = low power radio galaxies, aiming their relativistic jet at us 7 Basics of active galactic nuclei Figure 2 A sketch of the cylindrically symmetric AGN. The cut shows the r - z-plane, both axes logarithmically scaled to 1 pc, with the black hole at the center providing the symmetry. The basic constituents are the central BH with a surrounding accretion disk, the jet perpendicular to the disk and the torus encircling this configuration. The dark patches in the torus indicate the clouds, made up by stellar winds in the concept proposed (Zier & Biermann 2001, 2002). 8 Figure 3 This figure illustrates the change of the direction of the spin of the BH, induced by the merger of 2 massive BHs, and consequently the change of the direction of the jet. Basically the orbital spin wins over the two intrinsic spins. The left panel shows the situation before the merger, when the jet is aligned with the individual spin of the primary black hole of the binary system (Zier & Biermann 2001, 2002). 9 Figure 4 The spin-flip phenomenon in supermassive black hole binary mergers. These three steps show the envisaged temporal evolution of the final stages of the merger. L.A. Gergely, P.L. Biermann: [arXiv: 0704.1968] 10 Physics of active galactic nuclei • Almost all galaxies have a central super- massive black hole • Super-massive black hole mass distribution 6 9 from about 10 M to about 3 10 M , with 8 break of slope near 10 M • Activity defined as non-stellar: All super- massive black holes appear to have activity, very few have strong activity: observable in radio emission, and/or emission lines • This activity appears to be always visible in a weak compact radio jet, and in broad emission lines (sometimes only in polarized light, so scattered photons) • Two color diagram of the total fluxes of galaxy plus AGN at 2 keV, 60 micron, and 5 GHz: 11 • Seyfert galaxies, quasars, radio galaxies, and normal and starburst galaxies are clearly separated (Chini et al. 1989): • with Seyfert galaxies and quasars showing a lot more X-ray emission, and radio galaxies showing additionally a lot more radio emis- sion, than normal and starburst galaxies • Activity episodic, driven by minor (merging with another galaxy without a central black hole) and major mergers (merging with an- other galaxy, also with a central black hole) • Merging activity began early, far beyond redshift 6.4 (800 million years in the con- cordance cosmology): already then black 9 holes with > 10 M ; after redshift 1.5 - 2 merging activity rapidly decreases • Luminosities up to about 1047 erg/s 12 • Highest photon energy observed about 10 TeV • Shortest variability time scale minutes • Highest deduced Lorentz factor for the rel- ativistic jet: 40 • Luminosity function of activity similar shape to mass function, a powerlaw steepening to a steeper slope at some characteristic power • Such behavior typical for multiple merger or agglomeration process, shallow power- law until a maximum, and a much steeper law beyond that (Press & Schechter, Silk & Takahashi, ...); the break is then a function of time • Question on final spin of merged supermas- sive black hole 13 90o 360o 0o -90o > 7 Figure 5 The sky in black holes, ∼ 10 M : Aitoff projection in galactic coordinates of 5,978 candidate sources in the case of a complete sub sample (the Galactic plane remains obscured). The choice was made from a complete sample of 10,284 candidate brighter than 0.03 Jy at 2 micron, and selected at z< 0.025; this uses the 2 micron all sky survey, limited in a 20 degree band in the Galactic plane. Normal and starburst galaxies were counter-selected using color and FIR/radio ratio (Biermann & Fricke 1977, Kronberg et al. 1985, Chini et al. 1989, and other work). These candidate sources are probably all black holes, with masses near to or above 107 solar masses; the black hole mass was determined with the black hole versus mass spheroidal stellar population correlation, and tested. The color code is Black, Blue, Green, Orange, Red corresponding to redshifts betwen 0, 0.005, 0.01, 0.015, 0.02, 0.025: Caramete et al. 2008 14 • Stellar analogon: microquasars or Gamma Ray Bursts, black holes with relativistic jets; black hole masses from 3 - 10 solar masses • Most noticeable activity from accretion disk, broad line region, and relativistic jet; often shading by torus covering more than half of 4π, if it exists • X-ray emission probably from Inverse Comp- ton emission near foot of jet • Observations strongly influenced by rela- tivistic boosting, and so aspect angle • Gamma ray emission visible best from boosted sources, when the compact jet dominates the overall emission 15 • Eddington limit (radiation limited infall, or instability in atmosphere): radiation in ultra- violet 46 8 LEdd ' 10 (MBH/10 M ) erg/s • Disk can be thought of as similar to atmo- spheres of upper main sequence stars, so with radiation driven instabilities: Broad line clouds? The disk emission obeys the Eddington limit • In mergers three spins are involved, two in- trinsic spins, and orbital spin, orbital spin will win, lead to a spin-flip • Phase
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