H. Takami

Propagation of Ultra-High-Energy Cosmic Rays above 1019 eV in a Structured Extragalactic Magnetic Field and Galactic Magnetic Field

We present numerical simulations of the propagation of ultra-high-energy cosmic rays (UHECRs) above 1019 eV in a structured extragalactic magnetic field (EGMF) and simulate their arrival distributions at the Earth. We use the IRAS PSCz catalog in order to construct a model of the EGMF and source models of UHECRs, both of which reproduce the local structures observed around the Milky Way. We also consider modifications of UHECR arrival directions by the Galactic magnetic field. We follow an inverse process of their propagation from the Earth and record the trajectories. This enables us to calculate only trajectories of UHECRs arriving at the Earth, which saves CPU time. From these trajectories and our source models, we construct arrival distributions of UHECRs and calculate their harmonic amplitudes and two-point correlation functions. We estimate the number density of sources that best reproduces the Akeno Ground Air Shower Array (AGASA) observation. As a result, we find that the most appropriate number density of the sources is ~5 × 10-6 Mpc-3. This constrains the source candidates of UHECRs. We also demonstrate sky maps of their arrival distribution with the event number expected by future experiments and examine how the EGMF affects their arrival distribution. A main result is the diffusion of clustering events, which are obtained from calculations in the absence of the EGMF. This tendency allows us to reproduce the observed two-point correlation function better.

Cosmogenic neutrinos as a probe of the transition from Galactic to extragalactic cosmic rays

Abstract There are two promising scenarios that explain the ankle, which is a dip in the spectrum of cosmic rays at ∼ 10 19 xa0eV. A scenario interprets the ankle as the transition from Galactic to extragalactic cosmic rays (ankle-transition scenario), while the other is that the dip is caused by pair production on the cosmic microwave background radiation (proton-dip scenario). In this paper, we considered whether cosmogenic neutrinos can be a clue to judge which scenario is favored. We calculated the fluxes of cosmogenic neutrinos following these scenarios with plausible physical parameter sets, and found several important features as follows. First of all, the neutrino flux at ∼ 10 20 xa0eV becomes much higher in the ankle-transition scenario as long as the maximum energy of the cosmic rays at sources is sufficiently high. On the other hand, the neutrino spectrum has a characteristic peak at ∼ 10 16 xa0eV in the proton-dip scenario on the condition that extragalactic protons significantly contribute to the observed cosmic rays down to 10 17 xa0eV. Thus, we concluded cosmogenic neutrinos should give us a clue to judge which scenario is favored, unless these features are masked by the neutrino background coming from possible, powerful neutrino sources such as active galactic nuclei and γ-ray bursts. We also found an interesting feature that the neutrino flux at ∼ 10 18 xa0eV depends only on the cosmological evolution of the cosmic ray sources. That means cosmogenic neutrinos with the energy bring us information on the cosmological evolution of the sources of ultra-high energy cosmic rays. Finally, we compared the fluxes of cosmogenic neutrinos with the expected sensitivity curves of several neutrino detectors, and conclude the detection of cosmogenic neutrinos in the near future is promising.

IMPLICATIONS OF ULTRA-HIGH-ENERGY COSMIC RAYS FOR TRANSIENT SOURCES IN THE AUGER ERA

We study ultra-high-energy cosmic rays (UHECRs) from transient sources, propagating in the Galactic and intergalactic space. Based on recent observational results, we also estimate upper and lower bounds on the rate of transient UHECR sources and the required isotropic cosmic-ray energy input per burst as 0.1 Gpc–3 yr–1 ρ0 103.5 Gpc–3 yr–1 and by constraining the apparent burst duration, i.e., dispersion in arrival times of UHECRs. Based on these bounds, we discuss implications for proposed candidates such as gamma-ray bursts and active galactic nuclei.

Distortion of Ultra-High-Energy Sky by Galactic Magnetic Field

We investigate the deflections of UHE protons by Galactic magnetic field (GMF) using four conventional GMF models in order to discuss the positional correlation between the arrival distribution of UHECRs and their sources. UHE protons coming from the direction around the Galactic center are highly deflected above 8° by the dipole magnetic field during their propagation in Galactic space. However, in bisymmetric spiral field models, there are directions in which the deflection angle is below 1°. One of these directions is toward Centaurus A, the nearest radio-loud active galactic nucleus that is a possible UHECR source candidate. On the other hand, UHE protons arriving from the direction of the Galactic anticenter are generally less deflected, especially in bisymmetric spiral field models. Thus, the Northern Hemisphere, not including the Galactic center, is suitable for studies of correlation with sources. The dependence on model parameters is also investigated. The deflection angles of UHE protons are dependent on the pitch angle of the spiral field. We also investigate distortion of the supergalactic plane by the GMF. Since the distortion in the direction around the Galactic center strongly depends on the GMF model, we can obtain information on the GMF around the Galactic center if Pierre Auger Observatory finds significant positional correlation around the supergalactic plane.

Implications to sources of ultra-high-energy cosmic rays from their arrival distribution

Abstract We estimate the local number density of sources of ultra-high-energy cosmic rays (UHECRs) based on the statistical features of their arrival direction distribution. We calculate the arrival distributions of protons above 1019xa0eV taking into account their propagation process in the Galactic magnetic field and a structured intergalactic magnetic field, and statistically compare those with the observational results of the Pierre Auger Observatory. The anisotropy in the arrival distribution at the highest energies enables us to estimate the number density of UHECR sources as ∼10−4xa0Mpc−3 assuming the persistent activity of UHECR sources. We compare the estimated number density of UHECR sources with the number densities of known astrophysical objects. This estimated number density is consistent with the number density of Fanaroff–Reily I galaxies. We also discuss the reproducibility of the observed isotropy in the arrival distribution above 1019xa0eV. We find that the estimated source model cannot reproduce the observed isotropy. However, the observed isotropy can be reproduced with the number density of 10 - 2 – 10 - 3 Mpc - 3 . This fact indicates the existence of UHECR sources with a maximum acceleration energy of ∼1019xa0eV whose number density is an order of magnitude more than that injecting the highest energy cosmic rays.

Cross-correlation between UHECR arrival distribution and large-scale structure

We investigate correlation between the arrival directions of ultra-high-energy cosmic rays (UHECRs) and the large-scale structure (LSS) of the Universe by using statistical quantities which can find the angular scale of the correlation. The Infrared Astronomical Satellite Point Source Redshift Survey (IRAS PSCz) catalog of galaxies is adopted for LSS. We find a positive correlation of the highest energy events detected by the Pierre Auger Observatory (PAO) with the IRAS galaxies inside z = 0.018 within the angular scale of ~ 15°. This positive correlation observed in the southern sky implies that a significant fraction of the highest energy events comes from nearby extragalactic objects. We also analyze the data of the Akeno Giant Air Shower Array (AGASA) which observed the northern hemisphere, but the obvious signals of positive correlation with the galaxy distribution are not found. Since the exposure of the AGASA is smaller than the PAO, the cross-correlation in the northern sky should be tested using a larger number of events detected in the future. We also discuss the correlation using the all-sky combined data sets of both the PAO and AGASA, and find a significant correlation within ~ 8°. These angular scales can constrain several models of intergalactic magnetic field. These cross-correlation signals can be well reproduced by a source model in which the distribution of UHECR sources is related to the IRAS galaxies.

Toward Unravelling the Structural Distribution of Ultra-High-Energy Cosmic Ray Sources

We investigate the possibility that observations of ultra-high-energy cosmic rays (UHECRs) in the near future may be able to unveil their local source distribution, which will reflect observed local structures if their origins are astrophysical objects. In order to discuss this possibility, we calculate the arrival distribution of UHE protons taking into account their propagation process in intergalactic space, i.e., energy losses and deflections by the extragalactic magnetic field (EGMF). For a realistic simulation, we construct and adopt a model of a structured EGMF and UHECR source distribution, which reproduces the local structures actually observed around the Milky Way. The arrival distribution is compared statistically to the source distribution using a correlation coefficient. We find that UHECRs above 1019.8 eV are the best indicators for deciphering the source distribution within 100 Mpc, and the detection of about 500 events on all the sky would allow us to unveil the local structure of the UHE universe for plausible EGMF strength and source number density. This number of events could be detected by 5 years observation by the Pierre Auger Observatory.

Estimation prospects of the source number density of ultra-high-energy cosmic rays

Abstract We discuss the possibility of accurately estimating the source number density of ultra-high-energy cosmic rays (UHECRs) using small-scale anisotropy in their arrival distribution. The arrival distribution has information on their source and source distribution. We calculate the propagation of UHE protons in a structured extragalactic magnetic field (EGMF) and simulate their arrival distribution at the Earth using our previously developed method. The source number density that can best reproduce observational results by Akeno Giant Air Shower Array is estimated at about 10 −5 xa0Mpc −3 in a simple source model. Despite having large uncertainties of about one order of magnitude, due to small number of observed events in current status, we find that more detection of UHECRs in the Auger era can sufficiently decrease this so that the source number density can be more robustly estimated. Two hundred event observation above 4xa0×xa010 19 xa0eV in a hemisphere can discriminate between 10 −5 and 10 −6 xa0Mpc −3 . Number of events to discriminate between 10 −4 and 10 −5 xa0Mpc −3 is dependent on EGMF strength. We also discuss the same in another source model in this paper.

Towards unravelling the structural distribution of ultra‐high‐energy cosmic ray sources

We investigate the possibility that observations of ultra‐high‐energy cosmic‐rays (UHE‐CRs) in the near future may be able to unveil their local source distribution. In order to discuss this possibility, we simulate the arrival distribution of UHE protons taking into account their propagation process in intergalactic space with a realistic model of extragalactic magnetic field (EGMF) and investigate the similarity of the spatial patterns of the arrival distribution and the source distribution. We find that UHE protons above 1019.8u2009eV are the best indicators for unravelling the source distribution within 100 Mpc, and the detection of about 500 events on the whole sky would allow us to unveil the local structure of the UHE universe for plausible EGMF strength and source number density. This number of events could be detected by 5 years observation by the Pierre Auger Observatory (PAO). We also discuss the deflection of the trajectories of UHE protons by Galactic magnetic field. On the GMF models which are p...

Non-thermal neutrinos from supernovae leaving a magnetar

Under the fossil field hypothesis of the origin of magnetar magnetic fields, the magnetar inherits its magnetic field from its progenitor. We show that during the supernova of such a progenitor, protons may be accelerated to ∼10^4 GeV as the supernova shock propagates in the stellar envelope. Inelastic nuclear collisions of these protons produce a flash of high-energy neutrinos arriving a few hours after thermal (10 MeV) neutrinos. The neutrino flash is characterized by energies up to O(100) GeV and durations seconds to hours, depending on the progenitor: those from smaller Type Ibc progenitors are typically shorter in duration and reach higher energies compared to those from larger Type II progenitors. A Galactic Type Ib supernova leaving behind a magnetar remnant will yield up to ∼160 neutrino-induced muon events in Super-Kamiokande, and up to ∼7000 in a km^3 class detector such as IceCube, providing a means of probing supernova models and the presence of strong magnetic fields in the stellar envelope.