- Email: [email protected]
Neutrino signatures of the origins of cosmic rays
Available online at www.sciencedirect.com
Nuclear Physics B (Proc. Suppl.) 256–257 (2014) 264–266 www.elsevier.com/locate/npbps
Neutrino signatures of the origins of cosmic rays Karl Mannheim a a
Institute for Theoretical Physics and Astrophysics, Emil-Fischer-Str. 31, D-97074 Würzburg, Germany
Abstract The intensity of extraterrestrial neutrinos discovered by IceCube [1] is in reasonable agreement with predictions of neutrinos from the jets of active galactic nuclei due to pion production by accelerated protons [2]. The observed deficit of Glashow-resonance events at 6.3 PeV could result from the suppression of events with energies larger than PeV due to the presence of a strong “big blue bump” radiation field in flat-spectrum radio quasars. The total neutrino spectrum could exhibit a two-component structure in which the sub-PeV component is dominated by the jets from AGN with high accretion rates and another component peaking at EeV energies due to those with low accretion rates. Each component of the neutrino spectrum should carry the energy flux that corresponds to its relative contribution to the extragalactic gamma ray background. The arrival directions should correlate with known sources, and a simple test shows that the PeV events can indeed be explained by known blazars with prominent radio jets. If a Galactic component of cosmic rays with energies per nucleon above knee energies exists, as air shower array data seem to indicate, the neutrinos due to pion production from these sources are also detectable, pinpointing them an energies where gamma-ray observations are not yet possible.
1. Introduction In spite of major breakthroughs at GeV and TeV energies, where hadronic gamma-ray emitters have been identified based on their gamma-ray spectra, the origin of the sources of cosmic rays at knee and higher energies still remains elusive. Imaging gamma-ray observatories covering large enough effective areas are still lacking. Owing to the steeply falling atmospheric background, observations of high-energy neutrinos using underwater or -ice Cherenkov telescopes promise to remedy the situation, and this has been the main motivation to build the neutrino telescopes such as IceCube, ANTARES or BAIKAL [3]. The sensitivity reached by the IceCube detector has now become sufficient to probe down to the level of a putative extragalactic component of cosmic ray protons [1,4]. Other experiments (e.g., KM3NeT) will soon
http://dx.doi.org/10.1016/j.nuclphysbps.2014.10.031 0920-5632/© 2014 Published by Elsevier B.V.
follow provided the necessary funding can be established. Neutrino flux predictions for a broad range of putative sources have been obtained in the past [5], which can now be confronted with measurements. Their reliability mainly rests upon the validity of the assumptions about how the flux prediction was normalized. The commonly adopted approach is to first consider pion decay kinematics to estimate the photonto-neutrino flux ration at the time of production, and then to account for the effect of electromagnetic cascades on the photon-to-neutrino ratio. The cascades conserve the energy released by pion decay, but shift the photon flux to lower energies, so that eventually the bolometric photon and neutrino fluxes are related according to pion decay kinematics, while the photonto-neutrino ratio at a fixed energy can be very different from the one expected from pion decay. In blazars, the cascade photon flux is released at energies between MeV and TeV, depending on the
K. Mannheim / Nuclear Physics B (Proc. Suppl.) 256–257 (2014) 264–266
prevailing radiation fields. Likewise, the proton maximum energy may be limited by photo-pion production to a wide range of energies. Extragalactic background light (EBL) can additionally modify the photon-to-neutrino ratio [6]. Obviously, large differences in the threshold energy for photomeson production should exist between AGN with a high and low accretion rate, since the accretion disk constitutes a strong radiation source in addition to the synchrotron-emitting electrons within the jets [7] In “model A” of [2], the neutrino intensity prediction for jets in AGN with low-accretion rate sources (BL Lacertae objects and Fanaroff-Riley type I radio galaxies) was normalized to the extragalactic gammaray background (EGB) above 100 MeV, and in “model B” it was normalized for jets in AGN (flat-spectrum radio quasars, narrow-line Seyfert 1 galaxies, and Fanaroff-Riley type II radio galaxies) with high accretion rates to the gamma-ray background above 1 MeV. Although “model B” of [2] bolometrically accounts for sources with strong radiation fields shifting the cascade power down to the MeV-to-GeV range, the impact of the stronger internal radiation field due to the accretion disk on the neutrino spectrum has not been taken into account properly. This simplifying assumption is now challenged by the IceCube measurements. The “model A” flux prediction of [2] for the active galactic nuclei (AGN) with jets is in reasonable agreement with the IceCube measurement, but overpredicts Glashow-resonance events at energies of 6.3 PeV. The interactions of accelerated protons with the accretion disk photons can suppress the multi-PeV neutrino flux. This mechanism has formerly been proposed to also explain the steepness of the gammaray spectrum of FSRQ-type blazars such as 3C273 [8]. An improved model for the total neutrino intensity from AGN jets should therefore start from decomposing the spectrum into one component peaking in the PeV range [9,10] and another component peaking in the EeV range. The normalization should start from the EGB > 1 MeV and use weight factors to account for the relative contribution of the two types of blazars to the total EGB according to the results of Fermi-LAT. This should lead to rather precise numbers, since the EGB has been almost entirely resolved into the cumulative flux of the sources in the 3rd Fermi-LAT catalogue. 2. Extragalactic jet sources coincident with PeV neutrino events In their seminal work, Krauß et al. [11] have characterized the six most powerful blazars whose
265
positions are consistent with the arrival directions of the Ernie & Bert PeV neutrino events in IceCube [12]. Assuming that the bolometric photon flux determined from their simultaneous spectral energy distributions equals the bolometric neutrino power, the expected neutrino fluence of a spectrum centered abou 1 PeV is in agreement with the observed two events in two years of IceCube data. Both assumptions, the equal bolometric photon and neutrino fluxes and the neutrino spectrum centering about 1 PeV deserve some explanations. In astrophysical radiation fields, the photo-meson production yields closely resemble isospin symmetry in which case LJ ~ LQ [7]. The dominance of the 'resonance is washed out by the integration over the broad bandwidth of the target photon distribution. Full flavour-mixing can be assumed owing to the cosmological distances of AGN, unless new physics would lead to modifications of the mixing matrix. One should generally be open-minded about such a possibility, given the unknown nature of dark matter and dark energy and possible new types of interaction. Working out the expected neutrino fluence from the six blazars, the observed two events are consistent with electron-neutrino cascade events for which the effective area of the IceCube contained cascade event search is largest. The neutrino spectrum of AGN jets where proton cooling takes place in the radiation field of the accretion disk peaking at PeV energies can be understood as follows. In the frame co-moving with the jet (primed quantities), the disk photons are redshifted according to H’ = H*, if they originate from the base of the jet. If the disk photons scatter off free electrons in the broad emission line region, they instead appear blueshifted according to H’ *H. Photo-production of pions starts above the threshold energy Ep,th = 2(H’/30 eV)-1 PeV. The neutrinos carry away 0.05Ep,th, implying a neutrino energy of EQ = 0.1 * (H’/30eV)-1 PeV in the observer’s frame. For generic values H = 30 eV and * = 10, the neutrino spectrum therefore covers the energy range from 100 TeV to 10 PeV. If cooling is very efficient, the acceleration of protons could be limited by photomeson production in which case those sources would not produce neutrinos at higher energies. The event at 2 PeV coined “Big Bird” is positionally coincident with the nearby radio galaxy Cen A where also an excess of UHE cosmic ray events was detected by the Pierre Auger Observatory. There are additional candidate radio and gamma-ray emitting AGN consistent within the errors which are currently under investigation.
266
K. Mannheim / Nuclear Physics B (Proc. Suppl.) 256–257 (2014) 264–266
3. Conclusions The sensitivity of the IceCube telescope has reached the level where most probably the sources of extragalactic cosmic rays have been detected. The observed total intensity is consistent with the intensity predicted for extragalactic jets in blazars and radio galaxies based on their calorimetric output in gamma rays and ultrahigh-energy cosmic rays. No other known source population can readily produce the observed intensity. Galactic sources of cosmic ray protons above the knee should also be detectable [5]. Galactic sources of cosmic rays with energies larger than tens of PeV per nucleon seem to be implied by recent analyses of the composition of cosmic rays discussed controversially at this conference. Also, the Sun has been predicted as a source of high-energy neutrinos due to cosmic-ray irradiation of the solar atmosphere on the rear side producing a flatter spectrum than in Earth’s atmosphere [13]), albeit only up to energies of tens of TeV. Acknowledgments I deeply indepted to the valuable contributions of my collaborators Felicitas Krauß and Matthias Kadler in
trying to uncover the astrophysical origin of highenergy neutrinos, Francis Halzen for valuable correspondence, and to Omar Tibolla and Luke Drury for the most inspiring workshop on the origins of cosmic rays. Support by the Helmholtz-Alliance for Astroparticle Physics is gratefully acknowledged. References [1] M. G. Aartsen et al. (IceCube Collaboration), Science 342 (2013)1242856. [2] K. Mannheim, Astropart. Phys. 3 (1995) 295. [3] E. Waxman, K. Mannheim, Europhysics News 32 (2001) 216. [4] E. Waxman, PhRvL. 75 (1995) 386. [5] J.G. Learned, K.Mannheim K., Annual Review of Nuclear and Particle Science 50 (2000) 679. [6] T.M. Kneiske, T. Bretz, K. Mannheim, D. Hartmann, Astron. & Astrophys. 413 (2004) 807. [7] K. Mannheim, P.L. Biermann, Astron. Astrophys. 221 (1989) 211. [8] K. Mannheim, Phys. Rev. D 48 (1993) 2408. [9] C.D. Dermer, K. Murase, Y. Inoue, (2014) arxiv1406.2633v1. [10] I. Saba, J. Tjus, F. Halzen, Astropart. Phys. 48 (2014) 30. [11] F. Krauß, M. Kadler, K. Mannheim, et al., Astron. & Astrophys. (2014) accepted for publication, arXiv:1406.0645. [12] M.G. Aartsen, R. Abbasi, Y. Abdou, et al., Phys. Rev. Lett. 111 (2013) 021103. [13] C. Hettlage, K. Mannheim, J.G. Learned, Astropar. Physik 13 (2000) 45.
Nuclear Physics B (Proc. Suppl.) 256–257 (2014) 264–266 www.elsevier.com/locate/npbps
Neutrino signatures of the origins of cosmic rays Karl Mannheim a a
Institute for Theoretical Physics and Astrophysics, Emil-Fischer-Str. 31, D-97074 Würzburg, Germany
Abstract The intensity of extraterrestrial neutrinos discovered by IceCube [1] is in reasonable agreement with predictions of neutrinos from the jets of active galactic nuclei due to pion production by accelerated protons [2]. The observed deficit of Glashow-resonance events at 6.3 PeV could result from the suppression of events with energies larger than PeV due to the presence of a strong “big blue bump” radiation field in flat-spectrum radio quasars. The total neutrino spectrum could exhibit a two-component structure in which the sub-PeV component is dominated by the jets from AGN with high accretion rates and another component peaking at EeV energies due to those with low accretion rates. Each component of the neutrino spectrum should carry the energy flux that corresponds to its relative contribution to the extragalactic gamma ray background. The arrival directions should correlate with known sources, and a simple test shows that the PeV events can indeed be explained by known blazars with prominent radio jets. If a Galactic component of cosmic rays with energies per nucleon above knee energies exists, as air shower array data seem to indicate, the neutrinos due to pion production from these sources are also detectable, pinpointing them an energies where gamma-ray observations are not yet possible.
1. Introduction In spite of major breakthroughs at GeV and TeV energies, where hadronic gamma-ray emitters have been identified based on their gamma-ray spectra, the origin of the sources of cosmic rays at knee and higher energies still remains elusive. Imaging gamma-ray observatories covering large enough effective areas are still lacking. Owing to the steeply falling atmospheric background, observations of high-energy neutrinos using underwater or -ice Cherenkov telescopes promise to remedy the situation, and this has been the main motivation to build the neutrino telescopes such as IceCube, ANTARES or BAIKAL [3]. The sensitivity reached by the IceCube detector has now become sufficient to probe down to the level of a putative extragalactic component of cosmic ray protons [1,4]. Other experiments (e.g., KM3NeT) will soon
http://dx.doi.org/10.1016/j.nuclphysbps.2014.10.031 0920-5632/© 2014 Published by Elsevier B.V.
follow provided the necessary funding can be established. Neutrino flux predictions for a broad range of putative sources have been obtained in the past [5], which can now be confronted with measurements. Their reliability mainly rests upon the validity of the assumptions about how the flux prediction was normalized. The commonly adopted approach is to first consider pion decay kinematics to estimate the photonto-neutrino flux ration at the time of production, and then to account for the effect of electromagnetic cascades on the photon-to-neutrino ratio. The cascades conserve the energy released by pion decay, but shift the photon flux to lower energies, so that eventually the bolometric photon and neutrino fluxes are related according to pion decay kinematics, while the photonto-neutrino ratio at a fixed energy can be very different from the one expected from pion decay. In blazars, the cascade photon flux is released at energies between MeV and TeV, depending on the
K. Mannheim / Nuclear Physics B (Proc. Suppl.) 256–257 (2014) 264–266
prevailing radiation fields. Likewise, the proton maximum energy may be limited by photo-pion production to a wide range of energies. Extragalactic background light (EBL) can additionally modify the photon-to-neutrino ratio [6]. Obviously, large differences in the threshold energy for photomeson production should exist between AGN with a high and low accretion rate, since the accretion disk constitutes a strong radiation source in addition to the synchrotron-emitting electrons within the jets [7] In “model A” of [2], the neutrino intensity prediction for jets in AGN with low-accretion rate sources (BL Lacertae objects and Fanaroff-Riley type I radio galaxies) was normalized to the extragalactic gammaray background (EGB) above 100 MeV, and in “model B” it was normalized for jets in AGN (flat-spectrum radio quasars, narrow-line Seyfert 1 galaxies, and Fanaroff-Riley type II radio galaxies) with high accretion rates to the gamma-ray background above 1 MeV. Although “model B” of [2] bolometrically accounts for sources with strong radiation fields shifting the cascade power down to the MeV-to-GeV range, the impact of the stronger internal radiation field due to the accretion disk on the neutrino spectrum has not been taken into account properly. This simplifying assumption is now challenged by the IceCube measurements. The “model A” flux prediction of [2] for the active galactic nuclei (AGN) with jets is in reasonable agreement with the IceCube measurement, but overpredicts Glashow-resonance events at energies of 6.3 PeV. The interactions of accelerated protons with the accretion disk photons can suppress the multi-PeV neutrino flux. This mechanism has formerly been proposed to also explain the steepness of the gammaray spectrum of FSRQ-type blazars such as 3C273 [8]. An improved model for the total neutrino intensity from AGN jets should therefore start from decomposing the spectrum into one component peaking in the PeV range [9,10] and another component peaking in the EeV range. The normalization should start from the EGB > 1 MeV and use weight factors to account for the relative contribution of the two types of blazars to the total EGB according to the results of Fermi-LAT. This should lead to rather precise numbers, since the EGB has been almost entirely resolved into the cumulative flux of the sources in the 3rd Fermi-LAT catalogue. 2. Extragalactic jet sources coincident with PeV neutrino events In their seminal work, Krauß et al. [11] have characterized the six most powerful blazars whose
265
positions are consistent with the arrival directions of the Ernie & Bert PeV neutrino events in IceCube [12]. Assuming that the bolometric photon flux determined from their simultaneous spectral energy distributions equals the bolometric neutrino power, the expected neutrino fluence of a spectrum centered abou 1 PeV is in agreement with the observed two events in two years of IceCube data. Both assumptions, the equal bolometric photon and neutrino fluxes and the neutrino spectrum centering about 1 PeV deserve some explanations. In astrophysical radiation fields, the photo-meson production yields closely resemble isospin symmetry in which case LJ ~ LQ [7]. The dominance of the 'resonance is washed out by the integration over the broad bandwidth of the target photon distribution. Full flavour-mixing can be assumed owing to the cosmological distances of AGN, unless new physics would lead to modifications of the mixing matrix. One should generally be open-minded about such a possibility, given the unknown nature of dark matter and dark energy and possible new types of interaction. Working out the expected neutrino fluence from the six blazars, the observed two events are consistent with electron-neutrino cascade events for which the effective area of the IceCube contained cascade event search is largest. The neutrino spectrum of AGN jets where proton cooling takes place in the radiation field of the accretion disk peaking at PeV energies can be understood as follows. In the frame co-moving with the jet (primed quantities), the disk photons are redshifted according to H’ = H*, if they originate from the base of the jet. If the disk photons scatter off free electrons in the broad emission line region, they instead appear blueshifted according to H’ *H. Photo-production of pions starts above the threshold energy Ep,th = 2(H’/30 eV)-1 PeV. The neutrinos carry away 0.05Ep,th, implying a neutrino energy of EQ = 0.1 * (H’/30eV)-1 PeV in the observer’s frame. For generic values H = 30 eV and * = 10, the neutrino spectrum therefore covers the energy range from 100 TeV to 10 PeV. If cooling is very efficient, the acceleration of protons could be limited by photomeson production in which case those sources would not produce neutrinos at higher energies. The event at 2 PeV coined “Big Bird” is positionally coincident with the nearby radio galaxy Cen A where also an excess of UHE cosmic ray events was detected by the Pierre Auger Observatory. There are additional candidate radio and gamma-ray emitting AGN consistent within the errors which are currently under investigation.
266
K. Mannheim / Nuclear Physics B (Proc. Suppl.) 256–257 (2014) 264–266
3. Conclusions The sensitivity of the IceCube telescope has reached the level where most probably the sources of extragalactic cosmic rays have been detected. The observed total intensity is consistent with the intensity predicted for extragalactic jets in blazars and radio galaxies based on their calorimetric output in gamma rays and ultrahigh-energy cosmic rays. No other known source population can readily produce the observed intensity. Galactic sources of cosmic ray protons above the knee should also be detectable [5]. Galactic sources of cosmic rays with energies larger than tens of PeV per nucleon seem to be implied by recent analyses of the composition of cosmic rays discussed controversially at this conference. Also, the Sun has been predicted as a source of high-energy neutrinos due to cosmic-ray irradiation of the solar atmosphere on the rear side producing a flatter spectrum than in Earth’s atmosphere [13]), albeit only up to energies of tens of TeV. Acknowledgments I deeply indepted to the valuable contributions of my collaborators Felicitas Krauß and Matthias Kadler in
trying to uncover the astrophysical origin of highenergy neutrinos, Francis Halzen for valuable correspondence, and to Omar Tibolla and Luke Drury for the most inspiring workshop on the origins of cosmic rays. Support by the Helmholtz-Alliance for Astroparticle Physics is gratefully acknowledged. References [1] M. G. Aartsen et al. (IceCube Collaboration), Science 342 (2013)1242856. [2] K. Mannheim, Astropart. Phys. 3 (1995) 295. [3] E. Waxman, K. Mannheim, Europhysics News 32 (2001) 216. [4] E. Waxman, PhRvL. 75 (1995) 386. [5] J.G. Learned, K.Mannheim K., Annual Review of Nuclear and Particle Science 50 (2000) 679. [6] T.M. Kneiske, T. Bretz, K. Mannheim, D. Hartmann, Astron. & Astrophys. 413 (2004) 807. [7] K. Mannheim, P.L. Biermann, Astron. Astrophys. 221 (1989) 211. [8] K. Mannheim, Phys. Rev. D 48 (1993) 2408. [9] C.D. Dermer, K. Murase, Y. Inoue, (2014) arxiv1406.2633v1. [10] I. Saba, J. Tjus, F. Halzen, Astropart. Phys. 48 (2014) 30. [11] F. Krauß, M. Kadler, K. Mannheim, et al., Astron. & Astrophys. (2014) accepted for publication, arXiv:1406.0645. [12] M.G. Aartsen, R. Abbasi, Y. Abdou, et al., Phys. Rev. Lett. 111 (2013) 021103. [13] C. Hettlage, K. Mannheim, J.G. Learned, Astropar. Physik 13 (2000) 45.