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Abstract: Ultrahigh-energy cosmic rays are known to be mainly of extragalactic origin, and their propagation is limited by energy losses, so their arrival directions are expected to correlate with the large-scale structure of the local Universe. In this work, we investigate the possible presence of intermediate-scale excesses in the flux of the most energetic cosmic rays from the direction of the supergalactic plane region using events with energies above 20 EeV recorded with the surface detector array of the Pierre Auger Observatory up to 2022 December 31, with a total exposure of 135,000 sq. km sr yr. The strongest indication for an excess that we find, with a posttrial significance of 3.1σ, is in the Centaurus region, as in our previous reports, and it extends down to lower energies than previously studied. We do not find any strong hints of excesses from any other region of the supergalactic plane at the same angular scale. In particular, our results do not confirm the reports by the Telescope Array Collaboration of excesses from two regions in the Northern Hemisphere at the edge of the field of view of the Pierre Auger Observatory. With a comparable integrated exposure over these regions, our results there are in good agreement with the expectations from an isotropic distribution.
Keywords: ultra-high-energy cosmic rays, UHECR propagation, large-scale structure, UHECR energy losses, UHECR deflections, supergalactic plane region, Centaurus region, Pierre Auger Observatory, Auger surface detector array
Published in RUNG: 06.05.2025; Views: 511; Downloads: 2
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Abstract: We use a new method to estimate the injected mass composition of ultrahigh cosmic rays (UHECRs) at energies higher than 10 EeV. The method is based on comparison of the energy-dependent distribution of cosmic ray arrival directions as measured by the Telescope Array (TA) experiment with that calculated in a given putative model of UHECR under the assumption that sources trace the large-scale structure (LSS) of the Universe. As we report in the companion Letter, the TA data show large deflections with respect to the LSS which can be explained, assuming small extragalactic magnetic fields (EGMF), by an intermediate composition changing to a heavy one (iron) in the highest energy bin. Here we show that these results are robust to uncertainties in UHECR injection spectra, the energy scale of the experiment and galactic magnetic fields. The assumption of weak EGMF, however, strongly affects this interpretation at all but the highest energies E > 100 EeV, where the remarkable isotropy of the data implies a heavy injected composition even in the case of strong EGMF. This result also holds if UHECR sources are as rare as 2 × 10[sup]−5 Mpc[sup]−3, that is the conservative lower limit for the source number density.
Keywords: ultrahigh energy cosmic rays, large-scale structure, extragalactic magnetic fields, UHECR propagation, Telescope Array, UHECR mass composition, UHECR arrival directions
Published in RUNG: 23.04.2025; Views: 396; Downloads: 2
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Abstract: The Pierre Auger Observatory has measured the spectrum of ultra-high-energy cosmic rays with unprecedented precision, as well as the distribution of the depths of the maximum of the shower development in the atmosphere, which provide a reliable estimator of the mass composition. The measurements above 10[sup]17.8 eV can be interpreted assuming two populations of uniformly distributed sources, one with a soft spectrum dominating the flux below few EeV, and another one with a very hard spectrum dominating above that energy. When considering the presence of intense extragalactic magnetic fields between our Galaxy and the closest sources and a high-energy population with low spatial density, a magnetic horizon appears, suppressing the cosmic ray's flux at low-energies, which could explain the very hard spectrum observed at Earth. The distribution of arrival directions, which at energies above 32 EeV shows indications of a correlation with a population of starburst galaxies or the radio galaxy Centaurus A (Cen A), are also important to constrain the sources. It is shown that adding a fractional contribution from these sources of about 20% on top of an homogeneous background leads to an improvement of the model likelihood.
Keywords: ultra-high-energy cosmic rays, UHECR energy spectrum, UHECR mass composition, UHECR anisotropies, UHECR propagation, UHECR data interpretation, extragalactic magnetic fields, starburst galaxies, Centaurus A, Pierre Auger Observatory
Published in RUNG: 24.03.2025; Views: 619; Downloads: 7
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Abstract: The flux of ultra-high energy cosmic rays reaching Earth above the ankle energy (5 EeV) can be described as a mixture of nuclei injected by extragalactic sources with very hard spectra and a low rigidity cutoff. Extragalactic magnetic fields existing between the Earth and the closest sources can affect the observed CR spectrum by reducing the flux of low-rigidity particles reaching Earth. We perform a combined fit of the spectrum and distributions of depth of shower maximum measured with the Pierre Auger Observatory including the effect of this magnetic horizon in the propagation of UHECRs in the intergalactic space. We find that, within a specific range of the various experimental and phenomenological systematics, the magnetic horizon effect can be relevant for turbulent magnetic field strengths in the local neighbourhood in which the closest sources lie of order Brms ≃ (50–100) nG (20 Mpc/ds)( 100 kpc/Lcoh)1/2, with ds the typical intersource separation and Lcoh the magnetic field coherence length. When this is the case, the inferred slope of the source spectrum becomes softer and can be closer to the expectations of diffusive shock acceleration, i.e., ∝ E-2. An additional cosmic-ray population with higher source density and softer spectra, presumably also extragalactic and dominating the cosmic-ray flux at EeV energies, is also required to reproduce the overall spectrum and composition results for all energies down to 0.6 EeV.
Keywords: ultra high energy cosmic rays, UHECR propagation, magnetic horizon effect, Pierre Auger Observatory
Published in RUNG: 24.09.2024; Views: 1172; Downloads: 3
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Abstract: The ISS-based Calorimetric Electron Telescope (CALET) is directly measuring the energy spectrum and direction distribution of electron+positron cosmic-rays up to 20 TeV. A main goal of CALET is to identify a signature of a nearby supernova remnant (SNR) in electron+positron cosmic-rays. The Vela SNR has the highest potential to cause a spectral feature in the TeV region and/or a detectable anisotropy. Using the numerical cosmic-ray propagation code DRAGON, the spectrum and expected anisotropy of the Vela SNR together with background from more distant SNR was calculated depending on injection and propagation conditions. The results of these calculations were used to simulate CALET event sky-maps on which several analysis methods were employed to estimate the CALET sensitivity. Assuming that there is no anisotropy, the expected limits on the dipole amplitude from an all-sky search were calculated as a function of the selected energy range and the shape of the predicted spectra. However for the detection of a dipole anisotropy, the direction towards Vela is predetermined, and sensitivity is strongly boosted by a directed search. It is shown that with this method, CALET has a significant probability to identify an anisotropy signature from Vela. As it may disturb the Vela signature, the contribution to the local cosmic-ray anisotropy from several other nearby SNR and pulsars, as well as from the general source distribution in the galaxy was studied. It was found that Vela is expected to dominate and have a detectable signature, though there is some influence from other sources on direction and strength of the anisotropy. Furthermore, the implications of detecting an dipole anisotropy directed towards Vela for the local propagation parameters, such as the diffusion coefficient, are explained.
Keywords: cosmic-rays, CALET, cosmic-ray propagation
Published in RUNG: 05.10.2023; Views: 2063; Downloads: 7
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