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Context.The concept of a new space very long baseline interferometry (SVLBI) system named the Event Horizon Imager (EHI) has been proposed to dramatically improve black hole imaging and provide precise tests of the theory of general relativity. Aims.This paper presents imaging simulations for the EHI. We investigate the ability to make high-resolution movies of the black hole shadow and jet launching region around the supermassive black hole M87* and other black hole jets with a three-satellite EHI configuration. We aim to identify orbital configurations to optimize theuυ-coverage to image variable sources. Methods.Observations of general relativistic magnetohydrodynamics (GRMHD) models were simulated for the configuration, consisting of three satellites in circular medium earth orbits with an orbital plane perpendicular to the line of sight. The expected noise was based on preliminary system parameters. Movie frames, for which a part of theuυ-coverage may be excessively sparse, were reconstructed with algorithms that recover missing information from other frames. Averaging visibilities accumulated over multiple epochs of observations with an appropriate orbital configuration then improves the image quality. With an enhanced signal-to-noise ratio, timescales of observed variability were decreased. Results.Our simulations show that the EHI with standard system parameters is capable of imaging the variability in the M87* environment on event horizon scales with approximately a month-long temporal resolution. The EHI with more optimistic noise parameters (enhancing the signal-to-noise ratio about 100-fold) would allow for imaging of the variability on gravitational timescales. Observations with an EHI setup at lower frequencies are capable of imaging the variability in extended jets. Conclusions.Our study shows that the EHI concept can be used to image the variability in a black hole environment and extended jets, allowing for stronger tests of gravity theories and models of black hole accretion, plasma dynamics, and jet launching. 

The imaging fidelity of the Event Horizon Telescope (EHT) is currently determined by its sparse baseline coverage. In particular, EHT coverage is dominated by long baselines, and is highly sensitive to atmospheric conditions and loss of sites between experiments. The limited short/mid-range baselines especially affect the imaging process, hindering the recovery of more extended features in the image. We present an algorithmic contingency for the absence of well-constrained short baselines in the imaging of compact sources, such as the supermassive black holes observed with the EHT. This technique enforces a specific second moment on the reconstructed image in the form of a size constraint, which corresponds to the curvature of the measured visibility function at zero baseline. The method enables the recovery of information lost in gaps of the baseline coverage on short baselines and enables corrections of any systematic amplitude offsets for the stations giving short-baseline measurements present in the observation. The regularization can use historical source size measurements to constrain the second moment of the reconstructed image to match the observed size. We additionally show that a characteristic size can be derived from available short-baseline measurements, extrapolated from other wavelengths, or estimated without complementary size constraints with parameter searches. We demonstrate the capabilities of this method for both static and movie reconstructions of variable sources. 

Abstract Data collected so far by the Pierre Auger Observatory have enabled major advances in ultra-high energy cosmic ray physics and demonstrated that improved determination of masses of primary cosmic-ray particles, preferably on an event-by-event basis, is necessary for understanding their origin and nature. Improvement in primary mass measurements was the main motivation for the upgrade of the Pierre Auger Observatory, called AugerPrime. As part of this upgrade, scintillator detectors are added to the existing water-Cherenkov surface detector stations. By making use of the differences in detector response to the electromagnetic particles and muons between scintillator and water-Cherenkov detectors, the electromagnetic and muonic components of cosmic-ray air showers can be disentangled. Since the muonic component is sensitive to the primary mass, such combination of detectors provides a powerful way to improve primary mass composition measurements over the original Auger surface detector design. In this paper, the so-called Scintillator Surface Detectors are discussed, including their design characteristics, production process, testing procedure and deployment in the field. 

Abstract The modulation of low-energy galactic cosmic rays reflects interplanetary magnetic field variations and can provide useful information on solar activity. An array of ground-surface detectors can reveal the secondary particles, which originate from the interaction of cosmic rays with the atmosphere. In this work, we present an investigation of the low-threshold rate (scaler) time series recorded in 16 yr of operation by the Pierre Auger Observatory surface detectors in Malargüe, Argentina. Through an advanced spectral analysis, we detected highly statistically significant variations in the time series with periods ranging from the decadal to the daily scale. We investigate their origin, revealing a direct connection with solar variability. Thanks to their intrinsic very low noise level, the Auger scalers allow a thorough and detailed investigation of the galactic cosmic-ray flux variations in the heliosphere at different timescales and can, therefore, be considered a new proxy of solar variability. 

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 km²sr 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. 

Abstract Diffuse photons of energy above 0.1 PeV, produced through the interactions between cosmic rays and either interstellar matter or background radiation fields, are powerful tracers of the distribution of cosmic rays in the Galaxy. Furthermore, the measurement of a diffuse photon flux would be an important probe to test models of super-heavy dark matter decaying into gamma-rays. In this work, we search for a diffuse photon flux in the energy range between 50 PeV and 200 PeV using data from the Pierre Auger Observatory. For the first time, we combine the air-shower measurements from a 2 km²surface array consisting of 19 water-Cherenkov surface detectors, spaced at 433 m, with the muon measurements from an array of buried scintillators placed in the same area. Using 15 months of data, collected while the array was still under construction, we derive upper limits to the integral photon flux ranging from 13.3 to 13.8 km^-2sr^-1yr^-1above tens of PeV. We extend the Pierre Auger Observatory photon search program towards lower energies, covering more than three decades of cosmic-ray energy. This work lays the foundation for future diffuse photon searches: with the data from the next 10 years of operation of the Observatory, this limit is expected to improve by a factor of ∼20. 

A dedicated search for upward-going air showers at zenith angles exceeding 110° and energies $E>0.1\mathrm{EeV}$has been performed using the Fluorescence Detector of the Pierre Auger Observatory. The search is motivated by two “anomalous” radio pulses observed by the ANITA flights I and III that appear inconsistent with the standard model of particle physics. Using simulations of both regular cosmic-ray showers and upward-going events, a selection procedure has been defined to separate potential upward-going candidate events and the corresponding exposure has been calculated in the energy range [0.1–33] EeV. One event has been found in the search period between January 1, 2004, and December 31, 2018, consistent with an expected background of $0.27\pm 0.12$events from misreconstructed cosmic-ray showers. This translates to an upper bound on the integral flux of $(7.2\pm 0.2)\times {10}^{-21}{\mathrm{cm}}^{-2}{\mathrm{sr}}^{-1}{\mathrm{y}}^{-1}$and $(3.6\pm 0.2)\times {10}^{-20}{\mathrm{cm}}^{-2}{\mathrm{sr}}^{-1}{\mathrm{y}}^{-1}$for an $E^{-1}$and $E^{-2}$spectrum, respectively. An upward-going flux of showers normalized to the ANITA observations is shown to predict over 34 events for an $E^{-3}$spectrum and over 8.1 events for a conservative $E^{-5}$spectrum, in strong disagreement with the interpretation of the anomalous events as upward-going showers. Published by the American Physical Society2025