Low Statistics of EHECRs

Astronomy & Astrophysics, 2013

The Greisen-Zatsepin-Kuzmin (GZK) effect, i.e. the interaction of ultra-high-energy cosmic ray (UHECR) protons and nuclei with the intergalactic photon background, results in a drastic reduction of the number of sources contributing to the observed flux above ∼60 EeV. We study quantitatively the source statistics as a function of energy for a range of models compatible with the current data, varying source composition, injection spectrum, source density, and luminosity distribution. We also explore various realizations of the source distribution. We find that, in typical cases, the brightest source in the sky contributes more than one-fifth of the total flux above 80 EeV and about one-third of the total flux at 100 EeV. We show that typically between two and five sources contribute more than half of the UHECR flux at 100 EeV. With such low source numbers, the isolation of the few brightest sources in the sky may be possible for experiments collecting sufficient statistics at the highest energies, even in the event of relatively large particle deflections.

The Tenth Marcel Grossmann Meeting, 2006

Journal of Physics: Conference Series, 2008

The year 2007 has furnished us with outstanding results about the origin of the most energetic cosmic rays: a flux suppression as expected from the GZK-effect has been observed in the data of the HiRes and Auger experiments and correlations between the positions of nearby AGN and the arrival directions of trans-GZK events have been observed by the Pierre Auger Observatory. The latter finding marks the beginning of ultra high-energy cosmic ray astronomy and is considered a major breakthrough starting to shed first light onto the sources of the most extreme particles in nature. This report summarizes those observations and includes other major advances of the field, mostly presented at the 30 th International Cosmic Ray Conference held in Mérida, Mexico, in July 2007. With increasing statistics becoming available from current and even terminated experiments, systematic differences amongst different experiments and techniques can be studied in detail which is hoped to improve our understanding of experimental techniques and their limitations.

Brazilian Journal of Physics, 2013

Observations of cosmic rays have been improved at all energies, both in terms of higher statistics and reduced systematics. As a result, the all particle cosmic ray energy spectrum starts to exhibit more structures than could be seen previously. Most importantly, a second knee in the cosmic ray spectrum-dominated by heavy primaries-is reported just below 10 17 eV. The light component, on the other hand, exhibits an ankle like feature above 10 17 eV and starts to dominate the flux at the ankle. The key question at the highest energies is about the origin of the flux suppression observed at energies above 5 • 10 19 eV. Is this the long awaited GZK-effect or the exhaustion of sources? The key to answering this question is again given by the still largely unknown mass composition at the highest energies. Data from different observatories don't quite agree and common efforts have been started to settle that question. The high level of isotropy observed even at the highest energies starts to challenge a proton dominated composition if extragalactic (EG) magnetic fields are on the order of a few nG or more. We shall discuss the experimental and theoretical progress in the field and the prospects for the next decade.

Astroparticle Physics, 2004

We calculate the number density and luminosity of the sources of ultra-high energy cosmic rays (UHECRs), using the information about the small scale anisotropies and the observed spectra. We find that the number of doublets and triplets observed by AGASA can be best reproduced for a source density of $10 À5 Mpc À3 , with large uncertainties. The spectrum of UHECRs implies an energy input of $6 · 10 44 erg yr À1 Mpc À3 above 10 19 eV and an injection spectrum / E À2:6 . A flatter injection spectrum, E À2:4 , can be adopted if the sources have luminosity evolution / ð1 þ zÞ 4 . The combination of these two pieces of information suggests that the single sources should on average have a cosmic ray luminosity above 10 19 eV of L source % 2 Â 10 42 erg s À1 , weakly dependent upon the injection spectrum. Unfortunately, with the limited statistics of events available at present, there are approximately one-two orders of magnitude uncertainty in the source density provided above. We make predictions on the expected performances of the Auger and EUSO experiments, with particular attention for the expected improvements in our understanding of the nature of the sources of UHECRs. We find that a critical experimental exposure R c exists, such that experiments with exposure larger than R c can detect at least one event from each source at energies above 10 20 eV. This represents a unique opportunity to directly count and identify the sources of UHECRs.

arXiv: Astrophysics, 2007

We present a method to constrain the injection spectrum of ultrahigh energy cosmic rays (UHECRs) from supposedly identified extragalactic sources, which can be applied even when only one or two events per source are observed, and is more efficient than a simple fit of the UHECR energy spectrum including only the contribution of all identified sources. The method is based on the analysis of the probability for a given source to populate different energy bins, depending on the actual CR injection spectral index. In particular, we show that for a typical source density of 4*10^{-5}/{Mpc}^{3}, a data set of 100 events above 60 EeV allows one in 97 % of the cases to distinguish a source spectrum dN/dE ~1/E^{1.1} from one with 1/E^{2.7} at 95% confidence level.

JETP Letters, 2009

We discuss the GZK horizon of protons and present a method to constrain the injection spectrum of ultrahigh energy cosmic rays (UHECRs) from supposedly identified extragalactic sources. This method can be applied even when only one or two events per source are observed and is based on the analysis of the probability for a given source to populate different energy bins, depending on the actual CR injection spectral index. In particular, we show that for a typical source density of 4 × 10 −5 Mpc −3 , a data set of 100 events above 6 × 10 19 eV allows one in 97% of all cases to distinguish a source spectrum dN/dE ∝ E −1.1 from one with E −2.7 at 95% confidence level.

2013

We discuss the GZK horizon of protons and present a method to constrain the injection spectrum of ultrahigh energy cosmic rays (UHECRs) from supposedly identified extragalactic sources. This method can be applied even when only one or two events per source are observed and is based on the analysis of the probability for a given source to populate different energy bins, depending on the actual CR injection spectral index. In particular, we show that for a typical source density of 4 × 10 −5 Mpc −3, a data set of 100 events above 6 × 10 19 eV allows one in 97 % of all cases to distinguish a source spectrum dN/dE ∝ E −1.1 from one with E −2.7 at 95 % confidence level. PACS: 98.70.Sa Introduction—One of the main obstacles to fast progress in cosmic ray (CR) physics has been the impossibility to identify individual sources. However, there are two pieces of evidence indicating that we are at the dawn of “charged particle astronomy. ” First,

Astroparticle Physics

Pin down the shower energy observable to use μ as a composition-only Reduce hadronic uncertainties interaction model E threshold (TBD) Other messengers Galactic * μ / em separation † * depending on analysis progress † depending on detector configuration Figure 1: Diagram summarizing the strong connections of UHECRs with particle physics and astrophysics, the fundamental objectives of the field (in orange) for the next two decades, and the complementarity of current and next-generation experiments in addressing them. observatories offer a unique probe of the dark matter mass spectrum near the scale of grand unified theories (GUTs). The origin of super-heavy dark matter (SHDM) particles can be connected to inflationary cosmologies and their decay to instanton-induced processes, which would produce a cosmic flux of ultra-high-energy (UHE) neutrinos and photons. While their non-observation sets restrictive constraints on the gauge couplings of the DM models, the unambiguous detection of a single UHE photon or neutrino would be a game changer in the quest to identify the DM properties. UHECR experiments could be also sensitive to interactions induced by macroscopic DM or nuclearites in the atmosphere, offering further windows to identify the nature of DM. Astrophysics at the Energy Frontier The ability to precisely measure both energy and mass composition on an event-by-event basis simultaneously is critical as together they would give access to each primary particle's rigidity as a new observable. Given the natural relationship between rigidity and magnetic deflection, rigidity-based measurements will facilitate revealing the nature and origin(s) of UHECRs and enable charged-particle astronomy, the ability to study individual (classes of) sources with UHECRs. At the highest energies, the classic approach of maximizing exposure and achieving good energy resolution and moderate mass discrimination may well be sufficient if the composition is pure or is bimodal comprising a mix of only protons and Fe nuclei, for example. We already know however that this is not the case at energies below the flux suppression. Thus, a purposely-built observatory combining excellent energy resolution and mass discrimination will be complementary to instruments with possibly larger exposure. It is also clear that both approaches will benefit from the reduction of systematic uncertainties between hadronic interaction models. UHECRs also have an important role to play in multi-messenger astrophysics, not only as cosmic messengers themselves but also as the source of UHE photons and neutrinos. v • Full-sky coverage with low cross-hemisphere systematic uncertainties is critical for astrophysical studies. To this end, next generation experiments should be space-based or multi-site. Common sites between experiments are encouraged. • Based on the productive results from inter-collaboration and inter-disciplinary work, we recommend the continued progress/formation of joint analyses between experiments and with other intersecting fields of research (e.g., magnetic fields). • The UHECR community should continue its efforts to advance diversity, equity, inclusion, and accessibility. It also needs to take steps to reduce its environmental impacts and improve open access to its data to reduce the scientific gap between countries.

The Astrophysical Journal, 2003

We predict the arrival distribution of UHECRs above 4 × 10 19 eV with the event number expected by future experiments in the next few years. We perform event simulations with the source model which is adopted in our recent study and can explain the current AGASA observation. At first, we calculate the harmonic amplitude and the two point correlation function for the simulated event sets. We find that significant anisotropy on large angle scale will be observed when ∼ 10 3 cosmic rays above 4 × 10 19 eV are detected by future experiments. The Auger array will detect cosmic rays with this event number in a few years after its operation. The statistics of the two point correlation function will also increase. The angle scale at which the events have strong correlation with each other corresponds to deflection angle of UHECR in propagating in the EGMF, which in turn can be determined by the future observations. We further investigate the relation between the number of events clustered at a direction and the distance of their sources. Despite the limited amount of data, we find that the C2 triplet events observed by the AGASA may originate from the source within 100 Mpc from us at 2σ confidence level. Merger galaxy Arp 299 (NGC 3690 + IC 694) is the best candidate for their source. If data accumulate, the UHECR sources within ∼ 100 Mpc can be identified from observed event clusterings significantly. This will provide some kinds of information about poorly known parameters which influence the propagation of UHECRs, such as extragalactic and galactic magnetic field, chemical composition of observed cosmic rays. Also, we will reveal their origin with our method to identify the sources of UHECR. Finally, we predict the arrival distribution of UHECRs above 10 20 eV, which is expected to be observed if the current HiRes spectrum is correct, and discuss their statistical features and implications.