Observation of a Large-scale Anisotropy in the Arrival Directions of Cosmic Rays above 8×1018 eV

Cosmic rays are atomic nuclei arriving from outer space that reach the highest energies observed in nature. Clues to their origin come from studying the distribution of their arrival directions. Using 3×104 cosmic rays above 8×1018 electron volts, recorded with the Pierre Auger Observatory from a total exposure of 76,800 square kilometers steradian year, we report an anisotropy in the arrival directions. The anisotropy, detected at more than the 5.2σ level of significance, can be described by a dipole with an amplitude of 6.5+1.3 −0.9% towards right ascension αd = 100± 10 degrees and declination δd = −24+12 −13 degrees. That direction indicates an extragalactic origin for these ultra-high energy particles. Particles with energies ranging from below 109 eV up to beyond 1020 eV, known as cosmic rays, constantly hit the Earth’s atmosphere. The flux of these particles steeply decreases as their energy increases; for energies above 10 EeV (1 EeV ≡ 1018 eV), the flux is about one particle per km2 per year. The existence of cosmic rays with such ultra-high energies has been known for more than 50 years [1, 2], but the sites and mechanisms of their production remain a mystery. Information about their origin can be obtained from the study of the energy spectrum and the mass composition of cosmic rays. However, the most direct evidence of the location of the progenitors is expected to come from studies of the distribution of their arrival directions. Indications of possible hot spots in arrival directions for cosmic rays with energy above 50 EeV have been reported by the Pierre Auger and Telescope Array Collaborations [3, 4], but the statistical significance of these results is low. We report the observation, significant at a level of more than 5.2σ, of a large-scale anisotropy in arrival directions of cosmic rays above 8 EeV. Above 1014 eV, cosmic rays entering the atmosphere create cascades of particles (called extensive air-showers) that are sufficiently large to reach the ground. At 10 EeV, an extensive airshower (hereafter shower) contains ∼1010 particles spread over an area of ∼20 km2 in a thin disc moving close to the speed of light. The showers contain an electromagnetic component (electrons, positrons and photons) and a muonic component that can be sampled using arrays of particle detectors. Charged particles in the shower also excite nitrogen molecules in the air, producing fluorescence light that can be observed with telescopes during clear nights. The Pierre Auger Observatory, located near the city of Malargüe, Argentina, at latitude 35.2◦S, is designed to detect showers produced by primary cosmic rays above 0.1 EeV. It is a hybrid system, a combination of an array of particle detectors and a set of telescopes used to detect the fluorescence light. Our analysis is based on data gathered from 1600 water-Cherenkov detectors deployed over an area of 3000 km2 on a hexagonal grid with 1500-m spacing. Each detector contains 12 tonnes of ultrapure water in a cylindrical container, 1.2 m deep and 10 m2 in area, viewed by three 9-inch photomultipliers. A full description of the observatory, together with details of the methods used to reconstruct the arrival directions and energies of events, has been published [5]. *correspondence to: auger [email protected] †The authors with their affiliations appear at the end of this article. 1 ar X iv :1 70 9. 07 32 1v 1 [ as tr oph .H E ] 2 1 Se p 20 17 It is difficult to locate the sources of cosmic rays, as they are charged particles and thus interact with the magnetic fields in our galaxy and the intergalactic medium that lies between the sources and Earth. They undergo angular deflections with amplitude proportional to their atomic number Z, to the integral along the trajectory of the magnetic field (orthogonal to the direction of propagation), and to the inverse of their energy E. At E ≈ 10 EeV, the best estimates for the mass of the particles [6] lead to a mean value for Z between 1.7 and 5. The exact number derived is dependent on extrapolations of hadronic physics, which are poorly understood because they lie well beyond the observations made at the Large Hadron Collider. Magnetic fields are not well constrained by data, but if we adopt recent models of the Galactic magnetic field [7,8], typical values of the deflections of particles crossing the Galaxy are a few tens of degrees for E/Z = 10 EeV, depending on the direction considered [9]. Extragalactic magnetic fields may also be relevant for cosmic rays propagating through intergalactic space [10]. However, even if particles from individual sources are strongly deflected, it remains possible that anisotropies in the distribution of their arrival directions will be detectable on large angular scales, provided the sources have a nonuniform spatial distribution or, in the case of a single dominant source, if the cosmic-ray propagation is diffusive [11–14]. Searches for large-scale anisotropies are conventionally made by looking for nonuniformities in the distribution of events in right ascension [15,16] because, for arrays of detectors that operate with close to 100% efficiency, the total exposure as a function of this angle is almost constant. The nonuniformity of the detected cosmic-ray flux in declination (fig. S1) imprints a characteristic nonuniformity in the distribution of azimuth angles in the local coordinate system of the array. From this distribution it becomes possible to obtain information on the three components of a dipolar model. Event observations, selection, and calibration We analyzed data recorded at the Pierre Auger Observatory between 1 January 2004 and 31 August 2016, from a total exposure of about 76,800 km2 sr year. The 1.2-m depth of the waterCherenkov detectors enabled us to record events at a useful rate out to large values of the zenith angle, θ. We selected events with θ < 80◦ enabling the declination range −90◦ < δ < 45◦ to be explored, thus covering 85% of the sky. We adopted 4 EeV as the threshold for selection; above that energy, showers falling anywhere on the array are detected with 100% efficiency [17]. The arrival directions of cosmic rays were determined from the relative arrival times of the shower front at each of the triggered detectors; the angular resolution was better than 1◦ at the energies considered here [5]. Two methods of reconstruction have been used for showers with zenith angles above and below 60◦ [17, 18]. These have to account for the effects of the geomagnetic field [17, 19] and, in the case of showers with θ < 60◦, also for atmospheric effects [20] because systematic modulations to the rates could otherwise be induced (see supplementary materials). The energy estimators for both data sets were calibrated using events detected simultaneously by the water-Cherenkov detectors and the fluorescence telescopes, with a quasi-calorimetric determination of the energy coming from the fluorescence measurements. The statistical uncertainty in the energy determination is 16% above 4 EeV and 12% above 10 EeV, whereas the systematic uncertainty on the absolute energy scale, common to both data sets, is 14% [21]. Evidence that the analyses of the events with θ < 60◦ and of those with 60◦ < θ < 80◦ are consistent with each other comes from the energy spectra determined for the two angular bands. The spectra agree within the statistical uncertainties over the energy range of interest [22]. We consider events in two energy ranges, 4 EeV < E < 8 EeV and E ≥ 8 EeV, as adopted in previous analyses (e.g., [23–25]). The bin limits follow those chosen previously in [26, 27]. The median energies for these bins are 5.0 EeV and 11.5 EeV, respectively. In earlier work [23–25], the event selection required that the station with the highest signal be surrounded by six operational detectors—a demanding condition. The number of triggered stations is greater than four for 99.2% of all events above 4 EeV and for 99.9% of events above 8 EeV, making it possible to use