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Detecting the end of the cosmic-ray (CR) electron spectrum would provide important new insights. While we know that Milky Way sources can accelerate electrons up to at least $\sim$1PeV, the observed CR electron spectrum at Earth extends only up to 5TeV (possibly 20TeV), a large discrepancy. The question of the end of the CR electron spectrum has received relatively little attention, despite its importance. We take a comprehensive approach, showing that there are multiple steps at which the observed CR electron spectrum could be cut off. At the highest energies, the accelerators may not have sufficient luminosity, or the sources may not allow sufficient escape, or propagation to Earth may not be sufficiently effective, or present detectors may not have sufficient sensitivity. For each step, we calculate a rough range of possibilities. Although all of the inputs are uncertain, a clear vista of exciting opportunities emerges. We outline strategies for progress based on CR electron observations and auxiliary multi-messenger observations. In addition to advancing our understanding of CRs in the Milky Way, progress will also sharpen sensitivity to dark matter annihilation or decay., Comment: Main text 10 pages, 7 figures more...

Identifying the Milky Way's very high energy hadronic cosmic-ray accelerators -- the PeVatrons -- is a critical problem. While gamma-ray observations reveal promising candidate sources, neutrino detection is needed for certainty, and this has not yet been successful. Why not? There are several possibilities, as we delineated in a recent paper [T. Sudoh and J. F. Beacom, Phys. Rev. D 107, 043002 (2023)]. Here we further explore the possibility that the challenges arise because PeVatrons have a large angular extent, either due to cosmic-ray propagation effects or due to clusters of sources. We show that while extended neutrino sources could be missed in the commonly used muon-track channel, they could be discovered in the all-flavor shower channel, which has a lower atmospheric-neutrino background flux per solid angle. Intrinsically, showers are quite directional and would appear so in water-based detectors like the future KM3NeT, even though they are presently badly smeared by light scattering in ice-based detectors like IceCube. Our results motivate new shower-based searches as part of the comprehensive approach to identifying the Milky Way's hadronic PeVatrons., Comment: Main text 9 pages, 5 figures. Accepted by PRD more...

Observations of the Milky Way at TeV-PeV energies reveal a bright diffuse flux of hadronic cosmic rays and also bright point sources of gamma rays. If the gamma-ray sources are hadronic cosmic-ray accelerators, then they must also be neutrino sources. However, no neutrino sources have been detected. Where are they? We introduce a new population-based approach to probe Milky Way hadronic PeVatrons, demanding consistency between diffuse and point-source PeV-range data on cosmic rays, gamma rays, and neutrinos. For the PeVatrons, two extreme scenarios are allowed: (1) the hadronic cosmic-ray accelerators and the gamma-ray sources are the same objects, so that bright neutrino sources exist and improved telescopes can detect them, versus (2) the hadronic cosmic-ray accelerators and the gamma-ray sources are distinct, so that there are no detectable neutrino sources. The latter case is possible if hadronic accelerators have sufficiently thin column densities. We quantify present constraints and future prospects, showing how to reveal the nature of the hadronic PeVatrons., Comment: Main text 16 pages, 8 figures. Accepted by PRD more...

The X-ray binary SS 433, embedded in the W50 nebula (or supernova remnant W50), shows bipolar jets that are ejected with mildly relativistic velocities, and extend toward the east and west out to scales of tens of parsecs. Previous X-ray observations revealed twin lobes along the jet precession axis that contain compact bright knots dominated by synchrotron radiation, which provide evidence of electron acceleration in this system. Particle acceleration in this system is substantiated by the recently detected gamma rays with energies up to at least 25 TeV. To further elucidate the origin of the knots and particle acceleration sites in SS 433/W50, we report here on detailed, spatially resolved X-ray spectroscopy of its western lobe with Chandra. We detect synchrotron emission along the jet precession axis, as well as optically thin thermal emission that is more spatially extended. Between the two previously known knots, w1 and w2, we discover another synchrotron knot, which we call w1.5. We find no significant synchrotron emission between SS 433 and the innermost X-ray knot (w1), suggesting that electrons only begin to be accelerated at w1. The X-ray spectra become gradually steeper from w1 to w2, and then rapidly so immediately outside of w2. Comparing with a model taking into account electron transport and cooling along the jet, this result indicates that the magnetic field in w2 is substantially enhanced, which also explains its brightness. We discuss possible origins of the enhanced magnetic field of w2 as well as scenarios to explain the other two knots., Comment: 14 pages, 11 figures, Accepted to PASJ more...

We propose magnetically arrested disks (MADs) in quiescent black-hole (BH) binaries as the origin of the multiwavelength emission, and argue that this class of sources can dominate the cosmic-ray spectrum around the knee. X-ray luminosities of Galactic BH binaries in the quiescent state are far below the Eddington luminosity, and thus, radiatively inefficient accretion flows (RIAFs) are formed in the inner region. Strong thermal and turbulent pressures in RIAFs produce outflows, which can create large-scale poloidal magnetic fields. These fields are carried to the vicinity of the BH by the rapid inflow motion, forming a MAD. Inside the MAD, non-thermal protons and electrons are naturally accelerated by magnetic reconnections or stochastic acceleration by turbulence. Both thermal and non-thermal electrons emit broadband photons via synchrotron emission, which are broadly consistent with the optical and X-ray data of the quiescent BH X-ray binaries. Moreover, protons are accelerated up to PeV energies and diffusively escape from these MADs, which can account for the cosmic-ray intensity around the knee energy., Comment: 16 pages, 8 figures, 1 table, accepted for publication in ApJ, minor changes more...

Prospects of the Cherenkov Telescope Array (CTA) for the study of very high energy gamma-ray emission from nearby star-forming galaxies are investigated. In the previous work, we constructed a model to calculate luminosity and energy spectrum of pion-decay gamma-ray emission produced by cosmic-ray interaction with the interstellar medium (ISM), from four physical quantities of galaxies [star formation rate (SFR), gas mass, stellar mass, and effective radius]. The model is in good agreement with the observed GeV--TeV emission of several nearby galaxies. Applying this model to nearby galaxies that are not yet detected in TeV (mainly from the KINGFISH catalog), their hadronic gamma-ray luminosities and spectra are predicted. We identify galaxies of the highest chance of detection by CTA, including NGC 5236, M33, NGC 6946, and IC 342. Concerning gamma-ray spectra, NGC 1482 is particularly interesting because our model predicts that this galaxy is close to the calorimetric limit and its gamma-ray spectral index in GeV--TeV is close to that of cosmic-ray protons injected into ISM. Therefore this galaxy may be detectable by CTA even though its GeV flux is below the {\it Fermi} Large Area Telescope sensitivity limit. In the TeV regime, most galaxies are not in the calorimetric limit, and the predicted TeV flux is lower than that assuming a simple relation between the TeV luminosity and SFR of M82 and NGC 253, typically by a factor of 15. This means that a more sophisticated model beyond the calorimetric limit assumption is necessary to study TeV emission from star-forming galaxies., Comment: Accepted for publication in MNRAS more...

The HAWC Collaboration has observed gamma rays at energies above 56 TeV from a collection of nine sources. It has been suggested that this emission could be hadronic in nature, requiring that these systems accelerate cosmic-ray protons or nuclei up to PeV-scale energies. In this paper, we instead show that the spectra of these objects favor a leptonic (inverse Compton) origin for their emission. More specifically, the gamma-ray emission from these objects can be straightforwardly accommodated within a model in which $\sim \mathcal{O}(10\%)$ of the host pulsar's spindown power is transferred into the acceleration of electrons and positrons with a power-law spectrum that extends to several hundred TeV or higher. The spectral break that is observed among these sources is naturally explained within the context of this simple model, and occurs at the energy where the timescale for energy losses matches the age of the pulsar. In contrast, this spectral feature cannot be straightforwardly accommodated in hadronic scenarios. Furthermore, hadronic models predict that these sources should produce more emission at GeV-scale energies than is observed. In light of these considerations, we conclude that HAWC's highest energy sources should be interpreted as TeV halos or pulsar wind nebulae, which produce their emission through inverse Compton scattering, and are powered by the rotational kinetic energy of their host pulsar., Comment: Main text 17 pages, 6 figures. Matches published version more...

The non-thermal emission from the kiloparsec-scale jet of Centaurus A exhibits two notable features, bright diffuse emission and many compact knots, which have been intensively studied in X-ray and radio observations. H.E.S.S. recently reported that the very-high-energy gamma-ray emission from this object is extended along the jet direction beyond a kiloparsec from the core. Here, we combine these observations to constrain the physical conditions of the kpc-jet and study the origin of the non-thermal emission. We show that the diffuse jet is weakly magnetized ($\eta_B\sim10^{-2}$) and energetically dominated by thermal particles. We also show that knots are the sites of both amplified magnetic field and particle (re-)acceleration. To keep sufficient energy in thermal particles, the magnetic and non-thermal particle energy in the knot regions are tightly constrained. The most plausible condition is an energy equipartition between them, $\eta_B\sim\eta_e\sim0.1$. Such weak magnetic energy implies that particles in the knots are in the slow cooling regime. We suggest that the entire kpc-scale diffuse emission could be powered by particles that are accelerated at and escaped from knots., Comment: Main text 6 pages, 2 figures, accepted for publication in ApJ Letters more...

The observed correlation between the far-infrared and radio luminosities of galaxies illustrates the close connection between star formation and cosmic-ray production. Intriguingly, recent gamma-ray observations indicate that recycled/millisecond pulsars (MSPs), which do not trace recent star formation, may also efficiently accelerate cosmic-ray electrons. We study the contribution of MSPs to the galactic non-thermal radio emission, finding that they can dominate the emission from massive quiescent galaxies. This model can explain recent LOFAR observations that found a peculiar radio excess in galaxies with high stellar masses and low star-formation rates. We show that MSP-based models provide a significantly improved fit to LOFAR data. We discuss the implications for the radio-FIR correlation, the observation of radio excesses in nearby galaxies, and local electron and positron observations., Comment: Main text 14 pages, 4 figures. Accepted in PRD more...

SS433 is a Galactic microquasar with powerful jets, where very-high-energy particles are produced. We study particle acceleration in the jets of SS433 in the light of the recent multi-wavelength data from radio to TeV gamma ray. We first present a general framework for the particle acceleration, cooling, and transport in relativistic jets. We then apply this to two X-ray knots in the jets of SS433, focusing on leptonic emission. Our detailed treatment of particle transport and evolution produces substantially different predictions from previous papers. For both regions, our model can account for the multi-wavelength data except for the GeV data. This suggests that GeV emission is mostly from different regions and/or mechanisms. We find that the acceleration process should be efficient, which could be realized by diffusive shock acceleration close to the Bohm limit. Provided that protons are accelerated at the same efficiency as electrons, our results imply that SS433 is a PeVatron, i.e., can accelerate protons beyond a PeV. Future hard X-ray and MeV gamma-ray observations can critically test our models by detecting the spectral turnover or cutoff., Comment: Main text 11 pages, 3 figures. To appear in ApJ more...

Milagro and HAWC have detected extended TeV gamma-ray emission around nearby pulsar wind nebulae (PWNe). Building on these discoveries, Linden et al. [1] identified a new source class -- TeV halos -- powered by the interactions of high-energy electrons and positrons that have escaped from the PWN, but which remain trapped in a larger region where diffusion is inhibited compared to the interstellar medium. Many theoretical properties of TeV halos remain mysterious, but empirical arguments suggest that they are ubiquitous. The key to progress is finding more halos. We outline prospects for new discoveries and calculate their expectations and uncertainties. We predict, using models normalized to current data, that future HAWC and CTA observations will detect in total $\sim$50--240 TeV halos, though we note that multiple systematic uncertainties still exist. Further, the existing HESS source catalog could contain $\sim$10--50 TeV halos that are presently classified as unidentified sources or PWN candidates. We quantify the importance of these detections for new probes of the evolution of TeV halos, pulsar properties, and the sources of high-energy gamma rays and cosmic rays., Comment: Main text 14 pages, 10 figures, updated to match published version more...

The observed value $\Lambda_{\rm obs}$ of the cosmological constant $\Lambda$ is extremely smaller than theoretical expectations, and the anthropic argument has been proposed as a solution to this problem because galaxies do not form when $\Lambda \gg \Lambda_{\rm obs}$. However, the contemporary galaxy formation theory predicts that stars form even with a high value of $\Lambda / \Lambda_{\rm obs} \sim$ 50, which makes the anthropic argument less persuasive. Here we calculate the probability distribution of $\Lambda$ using a model of cosmological galaxy formation, considering extinction of observers caused by radiation from nearby supernovae. The life survival probability decreases in a large $\Lambda$ universe because of higher stellar density. Using a reasonable rate of lethal supernovae, we find that the mean expectation value of $\Lambda$ can be close to $\Lambda_{\rm obs}$, and hence this effect may be essential to understand the small but nonzero value of $\Lambda$. It is predicted that we are located on the edge of habitable regions about stellar density in the Galaxy, which may be tested by future exoplanet studies., Comment: 13 pages, 2 figures. the version accepted to Astrobiology more...

We present a new theoretical modeling to predict luminosity and spectrum of gamma-ray and neutrino emission of a star-forming galaxy, from star formation rate ($\psi$), gas mass ($M_{\rm gas}$), stellar mass, and disk size, taking into account production, propagation and interactions of cosmic rays. The model reproduces the observed gamma-ray luminosities of nearby galaxies detected by {\it Fermi} better than the simple power-law models as a function of $\psi$ or $\psi M_{\rm gas}$. Then this model is used to predict the cosmic background flux of gamma-ray and neutrinos from star-forming galaxies, by using a semi-analytical model of cosmological galaxy formation that reproduces many observed quantities of local and high-redshift galaxies. Calibration of the model using gamma-ray luminosities of nearby galaxies allows us to make a more reliable prediction than previous studies. In our baseline model star-forming galaxies produce about 20% of isotropic gamma-ray background unresolved by {\it Fermi}, and only 0.5% of IceCube neutrinos. Even with an extreme model assuming a hard injection cosmic-ray spectral index of 2.0 for all galaxies, at most 22% of IceCube neutrinos can be accounted for. These results indicate that it is difficult to explain most of IceCube neutrinos by star-forming galaxies, without violating the gamma-ray constraints from nearby galaxies., Comment: 9 pages, 5 figures, submitted to PASJ more...

The anthropic principle is one of the possible explanations for the cosmological constant ($\Lambda$) problem. In previous studies, a dark halo mass threshold comparable with our Galaxy must be assumed in galaxy formation to get a reasonably large probability of finding the observed small value, $P(<$$\Lambda_{\rm obs})$, though stars are found in much smaller galaxies as well. Here we examine the anthropic argument by using a semi-analytic model of cosmological galaxy formation, which can reproduce many observations such as galaxy luminosity functions. We calculate the probability distribution of $\Lambda$ by running the model code for a wide range of $\Lambda$, while other cosmological parameters and model parameters for baryonic processes of galaxy formation are kept constant. Assuming that the prior probability distribution is flat per unit $\Lambda$, and that the number of observers is proportional to stellar mass, we find $P(<$$\Lambda_{\rm obs}) = 6.7 \%$ without introducing any galaxy mass threshold. We also investigate the effect of metallicity; we find $P(<$$\Lambda_{\rm obs}) = 9.0 \%$ if observers exist only in galaxies whose metallicity is higher than the solar abundance. If the number of observers is proportional to metallicity, we find $P(<$$\Lambda_{\rm obs}) = 9.7 \%$. Since these probabilities are not extremely small, we conclude that the anthropic argument is a viable explanation, if the value of $\Lambda$ observed in our universe is determined by a probability distribution., Comment: 6 pages, 4 figures, accepted for publication in MNRAS more...