We investigate the gravitational wave (GW) signals emitted by planetary-mass primordial black holes (PBHs) passing nearby or traversing neutron stars (NSs). While previous studies mainly focused on the detailed waveforms of the signals, we estimate the rate of PBH-NS gravitational-wave events originating from the Galactic Center and compute the probability of detecting a signal over 10 years of LIGO-VirgoKAGRA observations. We examine in detail the case of PBHs bound to NSs, focusing on eccentric orbits that give rise to repeated GW bursts emitted in correlated series, each burst corresponding to a periastron passage. Despite the enhancement from the large number of bursts produced by a single PBH-NS pair, the total number of signals produced in this way remains subdominant to those due to random unbound encounters of PBHs with NSs. We also find that both types of signals have a very small probability P less than or similar to 10-8 to be detected in a 10 year period.

We study the deflections of ultra-high-energy cosmic rays in several widely used models of the coherent Galactic magnetic field (GMF), including PT11 (Pshirkov et al. [1]), JF12 (Jansson and Farrar [2]), UF23 (Unger and Farrar [3]) and KST24 (Korochkin, Semikoz, and Tinyakov [4]). We propagate particles with rigidities of 5, 10, and 20 EV and analyze the differences in deflection predictions across these GMF models. We identify the GMF components responsible for deflections in various regions of the sky and discuss the uncertainties in modeling these components, as well as potential future improvements.

The ultra-high-energy cosmic ray (UHECR) spectra measured by the Pierre Auger Observatory (Auger) and the Telescope Array (TA) agree very well below 10^19.5 eV but differ significantly at higher energies. We show that these differences can be explained by a single nearby source superimposed on a nearly isotropic background. Taking into account deflections in Galactic and extragalactic magnetic fields, such a source can account for the excess in the TA spectrum without producing excessive anisotropy. The required hard spectrum of the source and intermediate-mass composition are consistent with previous fits of the Auger-only spectrum and composition. This scenario offers several additional advantages: (i) the source produces a broad excess partially overlapping the TA hotspots, suggesting their possible explanation; (ii) without additional tuning, it reproduces the ∼90^∘ shift in dipole direction observed between the Auger-only and combined Auger-TA analyses; and (iii) the best-fit position of the source lies near M82, the brightest nearby starburst galaxy, making it a plausible source of the UHECR.

We study the deflections of ultrahigh-energy cosmic rays in several widely used models of the coherent particles with rigidities of 5, 10, and 20 EV and analyze the differences in deflection predictions across these GMF models. We identify the GMF components responsible for deflections in various regions of the sky and discuss the uncertainties in modeling these components, as well as potential future improvements.

Context. A recent catalogue of the Faraday rotation measures (RMs) of extragalactic sources, together with the synchrotron polarisation data from WMAP and Planck, provide us with a wealth of information on the magnetic fields of the Galaxy. However, the integral character of these observables, together with our position inside the Galaxy, make the inference of the coherent Galactic magnetic field (GMF) complicated and ambiguous. Aims. We combine several phenomenological components of the GMF - the spiral arms, the toroidal halo, the X-shaped field, and the field of the Local Bubble - to construct a new model of the regular GMF outside the thin disc. Methods. We use the binned chi(2 )approach to fit the parameters of the model to the data. To have control over the relative contributions of the RM and polarisation data to the fit, we pay special attention to the estimation of errors in data bins. To this end, we developed a systematic method that is uniformly applicable to different data sets. This method takes into account individual measurement errors, the variance in the bin, and fluctuations in the data at angular scales that are larger than the bin size. This leads to a decrease in the errors and, as a result, to better sensitivity of the data to the model content. We cross checked the stability of our method with the new LOFAR data, which have very small errors on the measurements of individual sources. Results. We find that the four components listed above are sufficient to fit both the RM and polarisation data over the whole sky with only a small fraction masked out. Moreover, we have achieved several important improvements compared to previous approaches. Due to our location inside of the Local Bubble, our model does not require the introduction of striated fields. For the first time, we show that the Fan region can be modelled as a Galactic-scale feature. The pitch angle of the magnetic field in our fit converges to a value of around 20 degrees. Interestingly, this value is very close to the direction of the spiral arms inferred recently from Gaia data on upper-main sequence stars.

Primordial black holes (PBHs) in the asteroid-mass range remain a viable and, until now, unconstrained dark matter (DM) candidate. If such PBHs exist, they could be captured by stars in DM-dominated environments with low velocity dispersion, such as ultra-faint dwarf galaxies (UFDs). The capture probability increases with the stellar mass, and captured PBHs would rapidly destroy their host stars. As a result, the presence of PBHs in UFDs would alter their stellar mass functions. Using photometric observations of three UFDs from the Hubble Space Telescope, we show that it is unlikely that their mass functions have been significantly modified by PBHs, and we place constraints on the PBH abundance. In the UFD Triangulum II, PBHs around 10^19,g are excluded at the $2σ$ ($3σ$) level from constituting more than ∼55% (∼78%) of the DM, while the possibility that PBHs represent the entirety of the DM is excluded at the $3.7σ$ level.

We report an estimation of the injected mass composition of ultrahigh energy cosmic rays (UHECRs) at energies higher than 10 EeV. The composition is inferred from an energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector by comparing it to the Large Scale Structure of the local Universe. In the case of negligible extragalactic magnetic fields (EGMFs), the results are consistent with a relatively heavy injected composition at E similar to 10 EeV that becomes lighter up to E similar to 100 EeV, while the composition at E > 100 EeV is very heavy. The latter is true even in the presence of highest experimentally allowed extragalactic magnetic fields, while the composition at lower energies can be light if a strong EGMF is present. The effect of the uncertainty in the galactic magnetic field on these results is subdominant.

We report an estimation of the injected mass composition of ultrahigh energy cosmic rays (UHECRs) at energies higher than 10 EeV. The composition is inferred from an energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector by comparing it to the Large Scale Structure of the local Universe. In the case of negligible extragalactic magnetic fields (EGMFs), the results are consistent with a relatively heavy injected composition at E∼10  EeV that becomes lighter up to E∼100  EeV, while the composition at E>100  EeV is very heavy. The latter is true even in the presence of highest experimentally allowed extragalactic magnetic fields, while the composition at lower energies can be light if a strong EGMF is present. The effect of the uncertainty in the galactic magnetic field on these results is subdominant.

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 x 10(-5) Mpc(-3), that is the conservative lower limit for the source number density.

On 11 September 2021, two small thunderstorms developed over the Telescope Array Surface Detector (TASD) that produced an unprecedented number of six downward terrestrial gamma ray flashes (TGFs) within one-hour timeframe. The TGFs occurred during the initial stage of negative cloud-to-ground flashes whose return strokes had increasingly large peak currents up to -223 kA, 147 GeV energy deposit in up to 25 1.2 km-spaced surface detectors, and intermittent bursts of gamma-rays with total durations up to 717 mu s. The analyses are based on observations recorded by the TASD network, complemented by data from a 3D lightning mapping array, broadband VHF interferometer, fast electric field change sensor, high-speed video camera, and the National Lightning Detection Network. The TGFs of the final two flashes had gamma fluences of similar or equal to 2 x 10(14) and 8x10(14), logarithmically bridging the gap between previous TASD and satellite-based detections. The observations further emphasize the similarity between upward and downward TGF varieties, suggesting a common mechanism for their production. Plain Language Summary Terrestrial Gamma-Ray Flashes (TGFs) are high-energy lightning-produced radiation events that have primarily been detected by satellites, but are increasingly detected by ground-based instrumentation. While the upward satellite-detected events exhibit extremely high numbers of gamma photons, the downward ground-based versions observed by the Telescope Array Surface Detector have, until now, displayed much weaker numbers by orders of magnitude. In this paper, we report observations of downward TGFs obtained at the large-area cosmic ray detector Telescope Array facility in west-central Utah, showing strengths midway between previously detected downward TGFs and upward satellite-detected events. The results indicate that TGFs span a wide range of fluences, a finding further supported by recent airborne observations over storm tops.

If primordial black holes constitute the dark matter, stars forming in dark-matter dominated environments with low velocity dispersions, such as ultra-faint dwarf galaxies, may capture a black hole at birth. The capture probability is non-negligible for primordial black holes of masses around $10^{20}$g, and increases with stellar mass. Moreover, infected stars are turned into virtually invisible black holes on cosmologically short timescales. Hence, the number of observed massive main-sequence stars in ultra-faint dwarfs should be suppressed if the dark matter was made of asteroid-mass primordial black holes. This would impact the measured mass distribution of stars, making it top-light (i.e. depleted in the high-mass range). Using simulated data that mimic the present-day observational power of telescopes, we show that already existing measurements of the mass function of stars in local ultra-faint dwarfs could be used to constrain the fraction of dark matter composed of primordial black holes in the -- currently unconstrained -- mass range of $10^{19}-10^{21}$g.

Optical emissions associated with Terrestrial Gamma-ray Flashes (TGFs) have recently become important subjects in space-based and ground-based observations since these emissions play crucial roles in understanding the generation of TGFs during thunderstorms. In this paper, we present the first time-resolved leader spectra of the optical component associated with a downward TGF. The TGF was observed at the Telescope Array Surface Detector (TASD) simultaneously with other lightning detectors, including a Lightning Mapping Array (LMA), an INTerFerometer (INTF), a Fast Antenna (FA), and a spectroscopic system. The spectroscopic system recorded leader spectra at 29,900 frames per second (33.44 $\mu$s time resolution), covering a spectral range from 400 nm to 900 nm, with 2.1 nm per pixel. The recordings of the leader spectra began 11.7 ms before the $-18$ kA return stroke and at a height of 2.37 km above the ground. These spectra reveal that optical emissions of singly-ionized nitrogen and oxygen occur between 167 $\mu$s before and 267 $\mu$s after the TGF detection, while optical emissions of neutrals (H I, 656 nm; N I, 744 nm, and O I, 777 nm) occur right at the moment of the detection. The time-dependent spectra reveal differences in the optical emissions of lightning leaders with and without downward TGFs.

We report on an observation of the difference between northern and southern skies of the ultrahigh energy cosmic ray energy spectrum with a significance of ∼8σ. We use measurements from the two largest experimentsx2014the Telescope Array observing the northern hemisphere and the Pierre Auger Observatory viewing the southern hemisphere. Since the comparison of two measurements from different observatories introduces the issue of possible systematic differences between detectors and analyses, we validate the methodology of the comparison by examining the region of the sky where the apertures of the two observatories overlap. Although the spectra differ in this region, we find that there is only a 1.8σ difference between the spectrum measurements when anisotropic regions are removed and a fiducial cut in the aperture is applied.

Primordial black holes (PBHs) are an attractive dark matter candidate, particularly if they can explain the totality of it. At PBH masses below ∼ 10^17g and above ∼ 10^23g this possibility is excluded from the variety of arguments and with different confidence. The range in between, often referred to as the "asteroid mass window", currently remains unconstrained. The most promising, in our view, way to probe this mass range is to use stars as the PBH detectors. If a star captures even a single PBH it starts being accreted onto it and eventually gets destroyed – converted into a sub-solar mass black hole. This process may have a variety of signatures form a mere star disappearance to supernova-type explosions of a new kind. The viability of this approach depends crucially on the probability of PBH capture by stars. In this chapter we summarize the existing capture mechanisms and discuss their implications for constraining the abundance of (or perhaps discovering) PBHs in the asteroid mass window.

After over 60 years, the powerful engines that accelerate ultra-high-energy cosmic rays (UHECRs) to the formidable energies at which we observe them from Earth remain mysterious. Assuming standard physics, we expect UHECR sources to lie within the local Universe (up to a few hundred~Mpc). The distribution of matter in the local Universe is anisotropic, and we expect this anisotropy to be imprinted on the distribution of UHECR arrival directions. Even though intervening intergalactic and Galactic magnetic fields deflect charged UHECRs and can distort these anisotropies, some amount of information on the distribution of the sources is preserved. In this proceedings contribution, we present the results of the joint Pierre Auger Observatory and Telescope Array searches for (a) the largest-scale anisotropies (the harmonic dipole and quadrupole) and (b) correlations with a sample of nearby starburst galaxies and the 2MRS catalogue tracing stellar mass within~250~Mpc. This analysis updates our previous results with the most recent available data, notably with the addition of 3~years of new Telescope Array data. The main finding is a correlation between the arrival directions of $12.1\%_{-3.1\%}^{+4.5\%}$~of UHECRs detected with $E \geq 38$~EeV by~Auger or with~$E \gtrsim 49$~EeV by~TA and the positions of nearby starburst galaxies on a ${15.1\text{deg}}_{-3.0\text{deg}}^{+4.6\text{deg}}$~angular scale, with a $4.7\sigma$~post-trial significance, up from $4.2\sigma$ obtained in our previous study.

The present white paper is submitted as part of the "Snowmass" process to help inform the long-term plans of the United States Department of Energy and the National Science Foundation for high-energy physics. It summarizes the science questions driving the Ultra-High-Energy Cosmic-Ray (UHECR) community and provides recommendations on the strategy to answer them in the next two decades.

Complementing the overview contribution about the whitepaper on ultra-high-energy cosmic rays (UHECR) prepared for the Snowmass community survey in the U.S. [Astroparticle Physics 149 (2023) 102819 - arXiv:2205.05845], this contribution focuses on Chapter 6, the ‘Instrumentation Roadmap’ for UHECR physics in the next decades. In addition to an increase in statistics, a higher measurement accuracy of cosmic-ray air showers is needed to answer open questions regarding the astrophysics and particle physics related to UHECR. The needed boost in exposure can be provided by space-borne fluorescence detectors with POEMMA or by huge ground arrays using a single cost-effective technique, such as the giant radio arrays envisioned with GRAND. These observatories maximizing the exposure need to be complemented by ground arrays featuring an event-by-event resolution of the rigidity of the primary particle, which is the essence of GCOS. The required high mass resolution demands the simultaneous measurement of the electromagnetic (energy and $X_\mathrm{max}$) and muonic shower components, possibly by combining layered water-Cherenkov with radio detectors and next-generation fluorescence telescopes, together with novel analysis techniques, such as neural networks. The higher accuracy for air-shower measurements is also important for UHECR particle physics because it will enable stricter tests of hadronic interaction models and will help to identify ultra-high-energy photons or neutrinos. This contribution will give an overview of the instrumentation needed for the future of UHECR physics in the context of the next generation experiments discussed in the whitepaper.

The origin of ultra-high-energy cosmic rays (UHECRs), particles from outer space with energies~$E \geq 1\;$EeV, is still unknown, though the near-isotropy of their arrival direction distribution excludes a dominant Galactic contribution, and interactions with background photons prevent them from travelling cosmologically large distances. This suggests that their sources must be searched for in nearby galaxy groups and clusters. Deflections by intergalactic and Galactic magnetic fields are expected to hinder such searches but not preclude them altogether. So far, the only anisotropy detected with statistical significance $\geq 5\sigma$ is a modulation in right ascension in the data from the Pierre Auger Observatory at~$E \geq 8\;$EeV interpretable as a 7% dipole moment. Various hints for higher-energy, smaller-scale anisotropies have been reported. UHECR arrival direction data from both the Pierre Auger Observatory and the Telescope Array experiment have been searched for anisotropies by a working group with members from both collaborations; combining the two datasets requires a cross-calibration procedure due to the different systematic uncertainties on energy measurements but allows us to perform analyses that are less model-dependent than what can be done with partial sky coverage. We report a significant dipole pointing away from the Galactic Center and a ${\sim}4.6\sigma$ anisotropy found when comparing the directions of UHECRs with a catalog of starburst galaxies.

We study the energy-dependent distribution of ultra-high energy cosmic ray arrival directions with respect to luminous matter in the local Universe. We use a specially designed test statistic (TS) that is robust to uncertainties of the galactic magnetic field. We generate realistic mock UHECR sets assuming various injected compositions, and different strengths of the extragalactic magnetic field (EGMF). Applying the TS to both mock sets and the Telescope Array Surface Detector data we constrain, for a given EGMF strength, the UHECR injected mass composition at energies above 10 EeV. At highest energies, higher than 100 EeV, only heavy composition - iron or at least silicon - is compatible with the data, irrespective of the EGMF strength. We then compare the obtained results with the direct Telescope Array fluorescence measurements of the UHECR mass composition. Requiring that the TA composition measurements are compatible with the arrival direction distribution allows us to constrain the parameters of the EGMF. It appears that light composition, measured by TA at around 10 EeV, is compatible with the arrival directions if the EGMF has strength of order 1 nG.