Origin Of Cosmic Rays Research Articles

Electron and ion acceleration by relativistic shocks: particle-in-cell simulations

The problem of particle acceleration by collisionless astrophysical shocks is of fundamental importance for the cosmic ray origin problem and for the high energy astrophysics as a whole. Fast magnetohydrodynamical shocks produced by relativistic jets are considered as very likely sites of cosmic ray acceleration. While cosmic ray acceleration by both non-relativistic and ultra relativistic shocks was studied in some details, the shocks of moderate Lorentz factors have attracted much less attention so far, while they are important for modeling gamma-ray burst afterglows and a number of other interesting objects. In this paper we present simulations of relativistic shock waves in electron-proton plasmas obtained with an implicit particle-in-cell code for the initial study the particle injection efficiencies.

The Propagation of Cosmic Rays from the Galactic Wind Termination Shock: Back to the Galaxy?

Abstract Although several theories exist for the origin of cosmic rays (CRs) in the region between the spectral “knee” and “ankle,” this problem is still unsolved. A variety of observations suggest that the transition from Galactic to extragalactic sources occurs in this energy range. In this work, we examine whether a Galactic wind that eventually forms a termination shock far outside the Galactic plane can contribute as a possible source to the observed flux in the region of interest. Previous work by Bustard et al. estimated that particles can be accelerated to energies above the “knee” up to R max = 1016 eV for parameters drawn from a model of a Milky Way wind. A remaining question is whether the accelerated CRs can propagate back into the Galaxy. To answer this crucial question, we simulate the propagation of the CRs using the low-energy extension of the CRPropa framework, based on the solution of the transport equation via stochastic differential equations. The setup includes all relevant processes, including three-dimensional anisotropic spatial diffusion, advection, and corresponding adiabatic cooling. We find that, assuming realistic parameters for the shock evolution, a possible Galactic termination shock can contribute significantly to the energy budget in the “knee” region and above. We estimate the resulting produced neutrino fluxes and find them to be below measurements from IceCube and limits by KM3NeT.

Ultrahigh energy cosmic rays from nearby starburst galaxies

Ultrahigh energy cosmic rays are the most energetic of any subatomic particles ever observed in nature. The quest for their mysterious origin is currently a major scientific challenge. Here we explore the possibility that these particles originate from nearby starburst galaxies, a scenario that matches the recent observation by the Telescope Array experiment of a cosmic-ray hotspot above 57~EeV not far from the direction of the starburst galaxy M82. Specifically, we study the stochastic propagation in space of ultrahigh energy cosmic rays through the state-of-the-art simulation framework CRPropa~3, taking into account all relevant particle interactions as well as deflections by the intervening magnetic fields. To ensure a comprehensive understanding of this model, we consider the energy spectrum, the cosmogenic neutrinos and gamma rays, and the distribution of arrival directions. The starburst galaxy scenario reproduces well observations from both the Telescope Array and Pierre Auger Observatories, making it very attractive for explaining the origin of cosmic rays at the highest energies.

Observing the energetic universe at very high energies with the VERITAS gamma ray observatory

Very high energy gamma-ray observations offer indirect methods for studying the highest energy cosmic rays in our Universe. The origin of cosmic rays at energies greater than 1018 eV remains a mystery, and many questions in particle astrophysics exist. The VERITAS observatory in southern Arizona, USA, carries out an extensive observation program of the gamma-ray sky at energies above 85 GeV. Observations of Galactic and extragalactic sources in the TeV band provide clues to the highly energetic processes occurring in these objects, and could provide indirect evidence for the origin of cosmic rays and the sites of particle acceleration in the Universe. VERITAS has now been operational for ten years with the complete array of four atmospheric Cherenkov telescopes. In this review, we present the status of VERITAS, and give few results from three of its key scientific programs: extragalactic science, Galactic physics, and study of fundamental physics and cosmology.

New prospects for detecting high-energy neutrinos from nearby supernovae

Neutrinos from supernovae (SNe) are crucial probes of explosive phenomena at the deaths of massive stars and neutrino physics. High-energy neutrinos are produced through hadronic processes by cosmic rays, which are accelerated during interaction between the supernova (SN) ejecta and circumstellar material (CSM). Recent observations of extragalactic SNe have revealed that a dense CSM is commonly expelled by the progenitor star. We provide new quantitative predictions of time-dependent high-energy neutrino emission from diverse types of SNe. We show that IceCube and KM3Net can detect about 1000 events from a SN II-P (and about 300000 events from a SN IIn) at a distance of 10 kpc. The new model also enables us to critically optimize the time window for dedicated searches for nearby SNe. A successful detection will give us a multienergy neutrino view of SN physics and new opportunities to study neutrino properties, as well as clues to the cosmic-ray origin. GeV-TeV neutrinos may also be seen by KM3Net, Hyper-Kamiokande, and PINGU.

Cosmic Ray Origin – Beyond the Standard Models

Given the success of the first meeting of “Cosmic Ray Origin – Beyond the Standard Models” (CRBTSM 2014), it was decided to hold a second meeting of this international conference. In these introductory remarks, we rehearse the motivation for reconsidering the origin(s) of cosmic rays (CR). We argue that the standard model, in which the majority of Galactic cosmic rays are produced through Diffusive Shock Acceleration (DSA) in SuperNova Remnants (SNR), is insufficient to account for recent observations. Some alternative scenarios are introduced and examined.

Ancient Pulsar Wind Nebulae as a natural explanation for unidentified gamma-ray sources

A large part of the Galactic sources emitting very high energy (VHE; > 1011 eV) gamma-rays are currently still unidentified. The evolution of Pulsar Wind Nebulae (PWNe) plays a crucial role in interpreting these sources. The time-dependent modeling of PWNe has been tested on a sample of well-known young and intermediate age PWNe; and it is currently applied to the full-sample of unidentified VHE Galactic sources. The consequences of this interpretation go far beyond the interpretation of “dark sources” (i.e. VHE gamma-ray sources without lower energies, radio or X-ray, counterparts): e.g. there could be strong implication in the origin of cosmic rays and (when considering a leptonic origin of the gamma-ray signal) they can be important for reinterpreting the detection of starburst galaxies in the TeV gamma-ray band. Moreover, the number of Galactic VHE sources is currently increasing with further observation by Imaging Atmospheric Cherenkov Telescopes (IACTs) and by the advent of more sensitive water Cherenkov telescopes such as HAWC (High-Altitude Water Cherenkov Observatory); therefore the physical interpretation of unidentified sources becomes more and more crucial.

What can we learn if we try to account for the all-sky UHECR data?

I examine the question of the origin of the Ultrahigh Energy Cosmic Rays (UHECRs) in the light of the data available at the highest energies.

Cosmic Ray Origin(s) revisited

The motivations for reconsidering the origin(s) of cosmic rays remain as valid as they were two years ago. While we have a standard model, or better perhaps a standard folklore, the reasons to be somewhat sceptical have if anything increased. In this talk I will survey the arguments for and against the standard SNR origin for the Galactic component of the cosmic rays. I will also briefly review the evidence for a distinct component coming from the Galactic centre.

On the origin of ultra-high rigidity cosmic rays

This paper argues in a first part that the observation of anisotropies at energies close to the GZK cut-off strongly argues in favor of the existence of protons at these energies. It then discusses the possible source of such protons, providing first general bounds on the luminosity of the source and on the amount of energy to be released in the ultra-high energy range. The necessary conditions point towards relativistic shock acceleration as an acceleration agent. Then, it discusses the physics of relativistic shock acceleration from a more microphysical point of view.

The supernova remnant W49B as seen with H.E.S.S. and Fermi-LAT

The supernova remnant (SNR) W49B originated from a core-collapse supernova that occurred between one and four thousand years ago, and subsequently evolved into a mixed-morphology remnant, which is interacting with molecular clouds (MC). Gamma-ray observations of SNR-MC associations are a powerful tool to constrain the origin of Galactic cosmic rays, as they can probe the acceleration of hadrons through their interaction with the surrounding medium and subsequent emission of non-thermal photons. We report the detection of a γ-ray source coincident with W49B at very high energies (VHE; E > 100 GeV) with the H.E.S.S. Cherenkov telescopes together with a study of the source with five years of Fermi-LAT high-energy γ-ray (0.06–300 GeV) data. The smoothly connected, combined source spectrum, measured from 60 MeV to multi-TeV energies, shows two significant spectral breaks at 304 ± 20 MeV and 8.4−2.5+2.2 GeV; the latter is constrained by the joint fit from the two instruments. The detected spectral features are similar to those observed in several other SNR-MC associations and are found to be indicative of γ-ray emission produced through neutral-pion decay.

Fermi-LAT highlights on Supernova Remnants

Supernova Remnants (SNRs) are a very well studied class of objects in our Galaxy and are closely related to the origin of Cosmic Rays (CRs), being candidates to host the acceleration process of Galactic CRs. Accelerated particles in SNRs can produce γ-rays through interactions with gas (nucleon-nucleon interactions or e± bremsstrahlung) or low-energy photons (inverse Compton scattering by electrons). In more than eight years of data taking, the Large Area Telescope (LAT) onboard the Fermi satellite, has detected more than 30 SNRs in the γ-ray energy band. The energy range in which Fermi-LAT is sensitive, namely from less than 100 MeV up to a few hundred GeV, is crucial to provide information on the physical processes occurring at the source and disentangle between lepton-based and hadron-based interpretation models. The Fermi-LAT collaboration has recently performed a systematic study of the known SNRs, producing the first Supernova Remnant Catalog in the GeV energy range. The spatial and spectral information obtained allow a systematic study of the SNR characteristics, which, together with multi-wavelength information, provide more general constraints on the CR population of our Galaxy. We present here the latest results from the observations of Galactic SNRs by Fermi-LAT, with a particular focus on the recent results obtained in the first SNR catalog.

Non-linear Cosmic Ray Propagation

The description of the transport of cosmic rays in magnetized media is central to both acceleration and propagation of these particles in our Galaxy and outside. The investigation of the process of particle acceleration, especially at shock waves, has already emphasized that non-linear effects such as self-generation of waves and dynamical reaction of cosmic rays on the background plasmas, are crucial if to achieve a physical understanding of the origin of cosmic rays. Here we discuss how similar non-linear effects on Galactic scales may affect the propagation of cosmic rays, not only through the excitation of plasma waves important for particle scattering, but also by inducing the motion of the interstellar medium in the direction opposite to the gravitational pull exerted by matter in the Galaxy, thereby resulting in the launching of a wind. The recent discovery of several unexpected features in cosmic ray spectra (discrepant hardening, spectral breaks in the H and He spectra, rising positron fraction and unexpectedly hard antiproton spectrum) raises the question of whether at least some of these effects may be attributed to poorly understood aspects of cosmic ray transport.

An apparatus to measure water optical attenuation length for LHAASO-MD

The large high altitude air shower observatory (LHAASO) is being constructed at 4400 m a.s.l. in Daocheng, Sichuan Province, aiming to reveal the secrets of cosmic rays origin. And it has the largest surface muon detector array in the world. Due to the needs of calibration and construction of muon detector, we developed a water optical attenuation measurement device using an 8 m long water tank. The results are presented for filtered water at wavelength of 405 nm, which proves this apparatus can reach an accuracy of about 20% at 100 m. This apparatus has not only a high precision measurement of water attenuation length up to 100 m but is also very convenient to be used, which is crucial for water optical properties study during LHAASO detector construction.

Study of the γ/p discrimination at ∼100 TeV energy range with LHAASO experiment

The observation of high energy γ-rays is essential to unveil the long-standing enigma of the origin and acceleration of Galactic Cosmic Rays (CRs). Given its powerful capability of distinguishing between protons and γ-rays owing to its very large area of underground muon detectors, the LHAASO observatory will be the most sensitive ground-based detectors for γ-rays at 100 TeV with a CRs background rejection rate better than 10−5. To evaluate the very small rejection rate with sufficient precision at energies above 100 TeV, one needs a large number of Monte Carlo events which is time consuming and challenging. As only the μ-poor events are interesting in the calculation of the rejection rate and take up a tiny fraction of the all CRs events, we modify the popular air shower simulation package, CORSIKA, by outputting only the μ-poor events for the following full detector simulation. As a result, our method is fully consistent with the evaluation made with the official CORSIKA at lower energy. Particularly, our improvement significantly escalate the calculation efficiency above 100 TeV, where it can be at least 50 times faster than using all events in simulation. By virtue of this new method, the γ/p discrimination of the LHAASO experiment at energies above 100 TeV is obtained for the first time, which indicates that LHAASO can reject CR backgrounds at a level of 10−5 and 10−9 at 100 TeV and 1 PeV respectively.

Is the Kerr black hole a super accelerator?

A number of long-standing puzzles, such as the origin of ultrahigh-energy cosmic rays, could perhaps be solved if we found a mechanism for effectively transferring energy from black holes to particles and, correspondingly, accelerating the latter to (unboundedly, as long as we neglect the back reaction) large velocities. As of today the only such candidate mechanism in the case of the non-extreme Kerr black hole is colliding a particle that freely falls from infinity with a particle whose trajectory is subject to some special requirements to fulfil which it has to be suitably corrected by auxiliary collisions. In the present paper we prove that---at least when the relevant particles move in the equatorial plane and experience a single correcting collision---this mechanism does not work too. The energy of the final collision becomes unboundedly high only when the energies of the incoming particles do.

Design of the LHAASO detectors

The Large High Altitude Air Shower Observatory plans to build a hybrid extensive air shower array with an area of about 1 km $$^2$$ at an altitude of 4,410 m a.s.l. in Sichuan province, China, to explore the origin of high-energy cosmic rays. LHAASO-KM2A will detect gamma ray sources with a sensitivity of about 1% Crab Unit at 100 TeV. It covers an area of 1 km $$^2$$ with a total of 5195 scintillation detectors. Its angular resolution reaches about 0.3 degrees, and the energy resolution is better than 25%. With the help of 1171 muon detectors, cosmic nuclei background will be rejected to a level of 10-4 at 50 TeV. The design and performances of the scintillation detectors and muon detectors are described in detail. LHAASO-WCDA focuses on surveying the northern sky for steady and transient sources from 100 GeV to 20 TeV, with a very high background rejection power and a good angular resolution. The WCDA consists of three water ponds with a total area of 78,000 m $$^2$$ , and the effective water depth is 4 m. Each water pond is divided into 5m $$\times $$ 5m cells partitioned by black plastic curtains to prevent penetration of the light from neighboring cells. An 8-inch PMT sits at the bottom center of each cell, looking upward to collect Cherenkov light generated by shower secondary particles in water. LHAASO-WFCTA is composed of 12 wide-field-of-view Cherenkov/fluorescence telescopes. Each telescope consists of a spherical light collector of about 4.7 m $$^2$$ and focal plane camera of 32 $$\times $$ 32 pixels with a pixel size of 0.5 degree. A prototype array about 1% of LHAASO has been constructed at Yangbajing Cosmic Ray Observatory and has been in operation for more than 2 years. Its performance fully meets the design requirements. The LHAASO detectors are designed to fulfill the physical goals in gamma ray astronomy and cosmic ray physics. One-fourth of LHAASO will be constructed and put into operation to produce physical data by the end of 2018. The whole array will be finished in the beginning of 2021.

Cosmic Ray Production in Supernovae

We give a brief review of the origin and acceleration of cosmic rays (CRs), emphasizing the production of CRs at different stages of supernova evolution by the first-order Fermi shock acceleration mechanism. We suggest that supernovae with trans-relativistic outflows, despite being rather rare, may accelerate CRs to energies above $10^{18}\mbox{ eV}$ over the first year of their evolution. Supernovae in young compact clusters of massive stars, and interaction powered superluminous supernovae, may accelerate CRs well above the PeV regime. We discuss the acceleration of the bulk of the galactic CRs in isolated supernova remnants and re-acceleration of escaped CRs by the multiple shocks present in superbubbles produced by associations of OB stars. The effects of magnetic field amplification by CR driven instabilities, as well as superdiffusive CR transport, are discussed for nonthermal radiation produced by nonlinear shocks of all speeds including trans-relativistic ones.

Linking high-energy cosmic particles by black-hole jets embedded in large-scale structures

The origin of ultrahigh-energy cosmic rays (UHECRs) is a half-century old enigma (Linsley 1963). The mystery has been deepened by an intriguing coincidence: over ten orders of magnitude in energy, the energy generation rates of UHECRs, PeV neutrinos, and isotropic sub-TeV gamma rays are comparable, which hints at a grand-unified picture (Murase and Waxman 2016). Here we report that powerful black hole jets in aggregates of galaxies can supply the common origin of all of these phenomena. Once accelerated by a jet, low-energy cosmic rays confined in the radio lobe are adiabatically cooled; higher-energy cosmic rays leaving the source interact with the magnetized cluster environment and produce neutrinos and gamma rays; the highest-energy particles escape from the host cluster and contribute to the observed cosmic rays above 100 PeV. The model is consistent with the spectrum, composition, and isotropy of the observed UHECRs, and also explains the IceCube neutrinos and the non-blazar component of the Fermi gamma-ray background, assuming a reasonable energy output from black hole jets in clusters.

The Pierre Auger Observatory: latest results and future perspectives

The Pierre Auger Observatory is the largest ultrahigh-energy cosmic ray observatory in the world. The huge amount of high quality data collected since 2004 up to now led to great improvements in our knowledge of the ultra-energetic cosmic rays. The suppression of the cosmic-ray flux at highest energies was clearly established, and the extra-galactic origin of these particles was confirmed. On the other hand, measurements of the depth of shower maximum indicate a puzzling trend in the mass composition of cosmic rays at energy around the ankle up to the highest energy. The just started upgrade of the Observatory, dubbed AugerPrime, will improve the identification of the mass of primaries allowing us to disentangle models of origin and propagation of cosmic rays.