- Email: [email protected]
Cosmological Large Scale Anisotropies in the high-energy cosmic rays and gamma-ray sky
Nuclear Physics B (Proc. Suppl.) 168 (2007) 280–282 www.elsevierphysics.com
Cosmological Large Scale Anisotropies in the high-energy cosmic rays and gamma-ray sky A. Cuocoa∗ a
Dipartimento di Scienze Fisiche, Universit` a di Napoli “Federico II”, and INFN-Sezione di Napoli, Complesso Universitario di Monte Sant’Angelo, Via Cintia, I-80126 Napoli, Italy The interactions that characterize the propagation of γ photons in the TeV energy range and of the Ultra High-Energy cosmic rays at energies > 1019 eV both introduce a cosmological horizon at the distance of few hundreds Mpc, implying a correlation with the local Large Scale Structures. We provide detailed predictions of the expected anisotropies based on the map of the local universe from the PSCz astronomical catalogue. We discuss the chances to detect the predicted signal with the forthcoming observatories like Auger for the UHECRs or the satellite Glast for γ-rays.
1. Gamma and UHECRs Astronomy The 0.1–10 TeV range represents one of the “last” photonic windows yet to be explored at large distances. Besides single sources, wide field of view instruments like the extensive air showers detectors (EAS) Milagro, Argo and the planned HAWC and satellite-based observatories like GLAST are sensitive to diffuse γ-ray emissions. A particularly interesting emission is the extragalactic diffuse γ-ray background (in the following, cosmic gamma background, or CGB), whose detection has been clearly reported by the EGRET mission [1]. In this work, we study in particular the large scale anisotropy expected in the CGB in the energy range 0.1-10 TeV. The lower part of this range will be probed by the GLAST telescope [2], while the energy window above the TeV is in principle accessible to EAS detectors. Starting from an energy of about 100 GeV (the very-high energy regime, VHE), the absorption of high energy photons onto the extragalactic background light (EBL) via pair-production introduces an energy-loss horizon of the order of a few hundreds Mpc or smaller, well below the size of the observable Universe. Distinctive features in the anisotropy pattern of the CGB are then expected due the anisotropic distribution of matter in the local universe. ∗ electronic
address: [email protected]
0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2007.02.087
The situation is less established for Cosmic Rays above a few ×1019 eV, the ultra-high energy (UHE) regime. The bulk of CRs at these energies is constituted of protons propagating through the Galaxy and the Inter-Galactic Medium. Provided that the extragalactic magnetic field (EGMF) is negligible, protons retain most of their initial direction therefore allowing us to probe into the nature and properties of their cosmic sources and perform UHE Astronomy. A few hundreds Mpc horizon is expected also for UHECRS at 19 due to the photo-meson energies E > ∼ 5 × 10 interaction processes which takes place on cosmic microwave background (CMB) photons (the Greisen-Zatsepin-Kuzmin (GZK) effect). Due to this cutoff, correlations of the UHECRs arrival directions with the local Large Scale Structures (LSS) is expected similarly to the case of the CGB. We shall then use the redshift Point Sources Catalogue (PSCz) [3] as tracer of the real structures in the nearby universe, thus producing maps of the VHE gamma sky and the UHECR sky. However, both the issues of UHECRs chemical composition and the intensity and structure of the EGMF, although well justified and supported by some observational evidence, are still problems far from a complete understanding. The signatures predicted in the anisotropy pattern of UHECRs can be employed as a powerful mean to disentangle the above scenario from more exotic
A. Cuoco / Nuclear Physics B (Proc. Suppl.) 168 (2007) 280–282
Figure 1. Equatorial density γ sky maps from the PSCz catalogue for Ecut = 100 GeV and 1 TeV. The color scale is linear and the average flux outside the mask of the PSCz is normalized to 1 so that to represent adimensional maps. The mask of the PSCz survey is indicated by the thick grey contour. physics possibilities. Actually, with the present world statistics of about 100 events collected from all the up to date experiment a first hint of large scale anisotropies could have been already detected [4]. A final answer is expected by the high statistics that the Auger observatory and the forthcoming Telescope Array will collect during the next years. For a more complete and detailed discussion of the present issues we refer the reader to the papers [5,6] and [7].
281
due to the mask the pattern is quite isotropic, with some hot spots like e.g. from the Virgo and Perseus Clusters. Other structures which appear are the Shapley concentration and the Columba cluster (for a key of the local cosmological structures see [5]). Given the limited statistics of GLAST at high energies, the TeV map is of interest especially for the EAS gamma detectors like MILAGRO. We see in this case that the nearest structures, forming the Super-Galactic Plane, dominate. In Fig. 2 is shown the analogous contour map in galactic coordinates of the expected integrated flux of UHECRs above the energy threshold Ecut = 5 × 1019 . Due to the GZK-effect, as it was expected, the nearest structures, and in particular the Super-Galactic Plane, are also the most prominent features in the map. The pattern, in general, is quite similar to the 1 TeV γ map as expected given that the relative gamma and UHECRs cutoff distances at these two energies are almost the same. To obtain the maps the effects of the propagation of the particles through the relevant Infrared/Optical and Microwave backgrounds have been properly taken into account. Further details can be found in the papers [5,6] and [7]. What are the real chances to detect these anisotropies with the data from the forthcoming experiments? To answer the question in the case of the CGB we have performed an harmonic deˆ ˆ = alm Ylm (Ω) composition of the maps f (Ω) lm and then assessed the predicted shot noise errors on the alm due to the finite statistics collected by
2. Sky maps and forecast In Fig. 1 we plot the resulting γ maps from the PSCz catalogue in equatorial coordinates for Ecut = 100 GeV and 1 TeV. For the case of the map with Ecut = 100 GeV, modulo the “hole”
Figure 2. Galactic UHECRs sky map for Ecut = 5 × 1019 eV from the PSCz catalogue. The contours enclose 95%, 68%, 38%, 20% of the corresponding distribution.
282
A. Cuoco / Nuclear Physics B (Proc. Suppl.) 168 (2007) 280–282
3.0 2.5
s = 2.0 N = 1000 Isotropic
2.0 LSS
1.5 1.0 0.5 0.0
Figure 3. The coefficients alm up to lmax = 10 calculated from the PSCz gamma maps of Fig. 1. The shaded band shows the 1-σ shot noise error; in the bottom panel the inner shaded region refers to HAWC, the outer one to MILAGRO. a given experiment. In Fig. 3 we report the coefficients alm ’s up to lmax = 10 calculated from the PSCz gamma maps of Fig.1, with the relative errors for a 4 year exposure of the GLAST mission and 10 years for the EAS Milagro and HAWC. GLAST should be able to detect some structures above 100 GeV at the 2σ level, while, on the contrary, instruments like MILAGRO may hardly find hints of structures at 1 TeV (gray band in the bottom panel of Fig. 3). For a realistic detection of the features in the VHE sky one would need the improvement in effective area planned to be reached by instruments like HAWC (green band in the bottom panel of Fig. 3). An instrument like ARGO is expected to have performances in between MILAGRO and HAWC, and may have some chance of detection. Given the poor statistics expected, for the UHECRs case we have performed a complete likelihood analysis on the map to determine the minimum statistics required for a statistically significant detection of the LSS pattern. In particular we multiplied the map by the proper exposure function to refer the predictions to the Auger experiment. The results are shown in Fig. 4, where the χ2 distributions for the null hypothesis of simple isotropic distribution and for the LSS model are calculated through a large number of Montecarlo simulations. It could be read that
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Figure 4. The χ2 distributions of the uniform and LSS model in UHECRs arrival directions for the case N = 1000 events. a statistics of about 1000 UHECRs events (with Ecut > 5 × 1019 ) should suffice to reliably distinguish the two models. Likely, this statistic is expected to be collected in few years of operation of the Auger observatory [8], hopefully providing a decisive step toward the solution of the mystery of UHECRs origin. REFERENCES 1. P. Sreekumar et al. [EGRET Collaboration], Astrophys. J. 494 (1998) 523, astroph/9709257. 2. J. E. McEnery, I. V. Moskalenko and J. F. Ormes, astro-ph/0406250. 3. W. Saunders et al., Mon. Not. Roy. Astron. Soc. 317 (2000) 55, astro-ph/0001117. 4. M. Kachelriess and D. V. Semikoz, Astropart. Phys. 26 (2006) 10, astro-ph/0512498. 5. A. Cuoco, R. D. Abrusco, G. Longo, G. Miele and P. D. Serpico, JCAP 0601 (2006) 009, astro-ph/0510765. 6. A. Cuoco, G. Miele and P. D. Serpico, Phys. Rev. D 74 (2006) 123008, astro-ph/0610374. 7. A. Cuoco, S. Hannestad, T. Haugbølle, G. Miele, P. D. Serpico, H. Tu, astroph/0612559. 8. X. Bertou [Pierre Auger Collaboration], astro-ph/0508466, in the Proceedings of the 29th ICRC, (2005) Pune India.
Cosmological Large Scale Anisotropies in the high-energy cosmic rays and gamma-ray sky A. Cuocoa∗ a
Dipartimento di Scienze Fisiche, Universit` a di Napoli “Federico II”, and INFN-Sezione di Napoli, Complesso Universitario di Monte Sant’Angelo, Via Cintia, I-80126 Napoli, Italy The interactions that characterize the propagation of γ photons in the TeV energy range and of the Ultra High-Energy cosmic rays at energies > 1019 eV both introduce a cosmological horizon at the distance of few hundreds Mpc, implying a correlation with the local Large Scale Structures. We provide detailed predictions of the expected anisotropies based on the map of the local universe from the PSCz astronomical catalogue. We discuss the chances to detect the predicted signal with the forthcoming observatories like Auger for the UHECRs or the satellite Glast for γ-rays.
1. Gamma and UHECRs Astronomy The 0.1–10 TeV range represents one of the “last” photonic windows yet to be explored at large distances. Besides single sources, wide field of view instruments like the extensive air showers detectors (EAS) Milagro, Argo and the planned HAWC and satellite-based observatories like GLAST are sensitive to diffuse γ-ray emissions. A particularly interesting emission is the extragalactic diffuse γ-ray background (in the following, cosmic gamma background, or CGB), whose detection has been clearly reported by the EGRET mission [1]. In this work, we study in particular the large scale anisotropy expected in the CGB in the energy range 0.1-10 TeV. The lower part of this range will be probed by the GLAST telescope [2], while the energy window above the TeV is in principle accessible to EAS detectors. Starting from an energy of about 100 GeV (the very-high energy regime, VHE), the absorption of high energy photons onto the extragalactic background light (EBL) via pair-production introduces an energy-loss horizon of the order of a few hundreds Mpc or smaller, well below the size of the observable Universe. Distinctive features in the anisotropy pattern of the CGB are then expected due the anisotropic distribution of matter in the local universe. ∗ electronic
address: [email protected]
0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2007.02.087
The situation is less established for Cosmic Rays above a few ×1019 eV, the ultra-high energy (UHE) regime. The bulk of CRs at these energies is constituted of protons propagating through the Galaxy and the Inter-Galactic Medium. Provided that the extragalactic magnetic field (EGMF) is negligible, protons retain most of their initial direction therefore allowing us to probe into the nature and properties of their cosmic sources and perform UHE Astronomy. A few hundreds Mpc horizon is expected also for UHECRS at 19 due to the photo-meson energies E > ∼ 5 × 10 interaction processes which takes place on cosmic microwave background (CMB) photons (the Greisen-Zatsepin-Kuzmin (GZK) effect). Due to this cutoff, correlations of the UHECRs arrival directions with the local Large Scale Structures (LSS) is expected similarly to the case of the CGB. We shall then use the redshift Point Sources Catalogue (PSCz) [3] as tracer of the real structures in the nearby universe, thus producing maps of the VHE gamma sky and the UHECR sky. However, both the issues of UHECRs chemical composition and the intensity and structure of the EGMF, although well justified and supported by some observational evidence, are still problems far from a complete understanding. The signatures predicted in the anisotropy pattern of UHECRs can be employed as a powerful mean to disentangle the above scenario from more exotic
A. Cuoco / Nuclear Physics B (Proc. Suppl.) 168 (2007) 280–282
Figure 1. Equatorial density γ sky maps from the PSCz catalogue for Ecut = 100 GeV and 1 TeV. The color scale is linear and the average flux outside the mask of the PSCz is normalized to 1 so that to represent adimensional maps. The mask of the PSCz survey is indicated by the thick grey contour. physics possibilities. Actually, with the present world statistics of about 100 events collected from all the up to date experiment a first hint of large scale anisotropies could have been already detected [4]. A final answer is expected by the high statistics that the Auger observatory and the forthcoming Telescope Array will collect during the next years. For a more complete and detailed discussion of the present issues we refer the reader to the papers [5,6] and [7].
281
due to the mask the pattern is quite isotropic, with some hot spots like e.g. from the Virgo and Perseus Clusters. Other structures which appear are the Shapley concentration and the Columba cluster (for a key of the local cosmological structures see [5]). Given the limited statistics of GLAST at high energies, the TeV map is of interest especially for the EAS gamma detectors like MILAGRO. We see in this case that the nearest structures, forming the Super-Galactic Plane, dominate. In Fig. 2 is shown the analogous contour map in galactic coordinates of the expected integrated flux of UHECRs above the energy threshold Ecut = 5 × 1019 . Due to the GZK-effect, as it was expected, the nearest structures, and in particular the Super-Galactic Plane, are also the most prominent features in the map. The pattern, in general, is quite similar to the 1 TeV γ map as expected given that the relative gamma and UHECRs cutoff distances at these two energies are almost the same. To obtain the maps the effects of the propagation of the particles through the relevant Infrared/Optical and Microwave backgrounds have been properly taken into account. Further details can be found in the papers [5,6] and [7]. What are the real chances to detect these anisotropies with the data from the forthcoming experiments? To answer the question in the case of the CGB we have performed an harmonic deˆ ˆ = alm Ylm (Ω) composition of the maps f (Ω) lm and then assessed the predicted shot noise errors on the alm due to the finite statistics collected by
2. Sky maps and forecast In Fig. 1 we plot the resulting γ maps from the PSCz catalogue in equatorial coordinates for Ecut = 100 GeV and 1 TeV. For the case of the map with Ecut = 100 GeV, modulo the “hole”
Figure 2. Galactic UHECRs sky map for Ecut = 5 × 1019 eV from the PSCz catalogue. The contours enclose 95%, 68%, 38%, 20% of the corresponding distribution.
282
A. Cuoco / Nuclear Physics B (Proc. Suppl.) 168 (2007) 280–282
3.0 2.5
s = 2.0 N = 1000 Isotropic
2.0 LSS
1.5 1.0 0.5 0.0
Figure 3. The coefficients alm up to lmax = 10 calculated from the PSCz gamma maps of Fig. 1. The shaded band shows the 1-σ shot noise error; in the bottom panel the inner shaded region refers to HAWC, the outer one to MILAGRO. a given experiment. In Fig. 3 we report the coefficients alm ’s up to lmax = 10 calculated from the PSCz gamma maps of Fig.1, with the relative errors for a 4 year exposure of the GLAST mission and 10 years for the EAS Milagro and HAWC. GLAST should be able to detect some structures above 100 GeV at the 2σ level, while, on the contrary, instruments like MILAGRO may hardly find hints of structures at 1 TeV (gray band in the bottom panel of Fig. 3). For a realistic detection of the features in the VHE sky one would need the improvement in effective area planned to be reached by instruments like HAWC (green band in the bottom panel of Fig. 3). An instrument like ARGO is expected to have performances in between MILAGRO and HAWC, and may have some chance of detection. Given the poor statistics expected, for the UHECRs case we have performed a complete likelihood analysis on the map to determine the minimum statistics required for a statistically significant detection of the LSS pattern. In particular we multiplied the map by the proper exposure function to refer the predictions to the Auger experiment. The results are shown in Fig. 4, where the χ2 distributions for the null hypothesis of simple isotropic distribution and for the LSS model are calculated through a large number of Montecarlo simulations. It could be read that
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Figure 4. The χ2 distributions of the uniform and LSS model in UHECRs arrival directions for the case N = 1000 events. a statistics of about 1000 UHECRs events (with Ecut > 5 × 1019 ) should suffice to reliably distinguish the two models. Likely, this statistic is expected to be collected in few years of operation of the Auger observatory [8], hopefully providing a decisive step toward the solution of the mystery of UHECRs origin. REFERENCES 1. P. Sreekumar et al. [EGRET Collaboration], Astrophys. J. 494 (1998) 523, astroph/9709257. 2. J. E. McEnery, I. V. Moskalenko and J. F. Ormes, astro-ph/0406250. 3. W. Saunders et al., Mon. Not. Roy. Astron. Soc. 317 (2000) 55, astro-ph/0001117. 4. M. Kachelriess and D. V. Semikoz, Astropart. Phys. 26 (2006) 10, astro-ph/0512498. 5. A. Cuoco, R. D. Abrusco, G. Longo, G. Miele and P. D. Serpico, JCAP 0601 (2006) 009, astro-ph/0510765. 6. A. Cuoco, G. Miele and P. D. Serpico, Phys. Rev. D 74 (2006) 123008, astro-ph/0610374. 7. A. Cuoco, S. Hannestad, T. Haugbølle, G. Miele, P. D. Serpico, H. Tu, astroph/0612559. 8. X. Bertou [Pierre Auger Collaboration], astro-ph/0508466, in the Proceedings of the 29th ICRC, (2005) Pune India.