Origin Of Galactic Cosmic Rays Research Articles

The Cryogenic Anticoincidence Detector for the NewAthena X-IFU Instrument: A Program Overview

Athena (advanced telescope for high-energy astrophysics) is an ESA large-class mission, at present under a re-definition “design-to-cost” phase, planned for a prospective launch at L1 orbit in the second half of the 2030s. It will be an observatory alternatively focusing on two complementary instruments: the X-IFU (X-ray Integral Field Unit), a TES (TransitionEdge Sensor)-based kilo-pixel array which is able to perform simultaneous high-grade energy spectroscopy (~3 eV@7 keV) and imaging over 4′ FoV (field of view), and the WFI (Wide Field Imager), which has good energy spectral resolution (~170 eV@7 keV) and imaging on wide 40′ × 40′ FoV. Athena will be a truly transformational observatory, operating in conjunction with other large observatories across the electromagnetic spectrum available in the 2030s like ALMA, ELT, JWST, SKA, CTA, etc., and in multi-messenger synergies with facilities like LIGO A+, Advanced Virgo+, LISA, IceCube and KM3NeT. The Italian team is involved in both instruments. It has the co-PIship of the cryogenic instrument for which it has to deliver the TES-based Cryogenic AntiCoincidence detector (CryoAC) necessary to guarantee the X-IFU sensitivity, degraded by a primary particle background of both solar and galactic cosmic ray (GCR) origins, and by secondary electrons produced by primaries interacting with the materials surrounding the main detector. The outcome of Geant4 studies shows the necessity for adopting both active and passive techniques to guarantee the residual particle background at 5 × 10−3 cts cm−2 s−1 keV−1 level in 2–10 keV scientific bandwidth. The CryoAC is a four-pixel detector made of Si-suspended absorbers sensed by Ir/Au TESes placed at <1 mm below the main detector. After a brief overview of the Athena mission, we will report on the particle background reduction techniques highlighting the impact of the Geant4 simulation on the X-IFU focal plane assembly design, then hold a broader discussion on the CryoAC program in terms of detection chain system requirements, test, design concept against trade-off studies and programmatic.

High-energy cosmic rays and gamma-rays from star clusters: the case of Cygnus OB2

ABSTRACT We investigate the acceleration of cosmic rays at the termination shock that results from the interaction of the collective wind of star clusters with the surrounding interstellar medium. The solution of the transport equation of accelerated particles in the wind-excavated cavity, including energy losses due to CR interactions with neutral gas in the bubble, shows several interesting properties that are discussed in detail. The issue of the maximum energy of the accelerated particles is discussed with special care, because of its implications for the origin of Galactic cosmic rays. Gamma-ray emission is produced in the cavity due to inelastic pp scattering, while accelerated particles are advected downstream of the termination shock and diffuse at the same time. Both the spectrum and the morphology of such emission are discussed, with a comparison of our results with the observations of gamma-ray emission from the Cygnus OB2 region.

Multiwavelength analysis of Galactic Supernova Remnants

The origin of Galactic Cosmic Rays (CRs) and the possibility of Supernova Remnants (SNRs) being potential CR accelerators is still an open debate. The charged CRs can be detected indirectly by the γ-ray observatories through the π 0 production and consequent decay, leading to the generation of high-energy γ-rays. The goal of the study is to identify qualitative and quantitative trends in favour of hadronic scenario and search for SNRs which could be potential accelerators up to PeV energies (PeVatrons).We have performed a Multiwavelength (MWL) study using different radiative models to evaluate the hadronic contribution. The spectral energy distributions (SEDs) of selected SNRs are modeled using the Naima [1] package. Two different radiative scenarios are considered, pure leptonic and lepto-hadronic scenarios and different methods are used to evaluate their importance.This study shows that the lepto-hadronic scenario is favored for most SNRs. Two particular indicators of hadronic contribution come from the data around the π 0 production threshold and the data above a few TeV. The hard rise at the π 0 production threshold cannot be explained by leptonic processes. More data in this region would be valuable for these studies. For some SNRs, an important hadronic contribution is observed up to a few TeV, thus making them promising PeVatron candidates. In this high-energy region where the leptonic processes are expected to be suppressed, more data is required to help distinguish between the leptonic and hadronic origin of γ-ray emission. In the future, we intend to use the obtained model parameters to simulate data for CTA and assess its capability to identify PeVatrons.

Explaining the Hardening Structures of Helium Spectrum and Boron to Carbon Ratio through Different Propagation Models

Recently, a series of high-precision measurements by various experiments show that cosmic ray nuclei spectra begin to harden at ∼200 GV and the boron-to-carbon (B/C) ratio has a similar trend around the same energy. These anomalous structures possibly result from the journey of cosmic rays (CRs) from their sources to our solar system, which has important implications for our understanding of the origin and propagation of Galactic cosmic rays (GCRs). In this work, we investigate several propagation models and attempt to explain these anomalous observations. We have verified that an extension of the traditional propagation model taking into account spatially dependent propagation and secondary particle acceleration provides a more accurate description of the latest B/C ratio and the Helium flux data measured by DAMPE, CALET, and AMS-02.

Understanding the TeV γ-ray emission surrounding the young massive star cluster Westerlund 1

Context. Young massive star clusters (YMCs) have increasingly become the focus of discussions on the origin of galactic cosmic rays (CRs). The proposition that CRs are accelerated inside superbubbles (SBs) blown by the strong winds of these clusters avoids issues faced by the standard paradigm of acceleration at supernova remnant shocks. Aims. We provide an interpretation of the latest TeV γ-ray observations of the region around the YMC Westerlund 1 taken with the High Energy Stereoscopic System (H.E.S.S.) in terms of diffusive shock acceleration at the cluster wind termination shock, taking the spectrum and morphology of the emission into account. As Westerlund 1 is a prototypical example of a YMC, this study is relevant to the general question about the role of YMCs for the Galactic CR population. Methods. We generated model γ-ray spectra, characterised particle propagation inside the SB based on the advection, diffusion, and cooling timescales, and constrained key parameters of the system. We considered hadronic emission from proton-proton interaction and subsequent pion decay and leptonic emission from inverse Compton scattering on all relevant photon fields, including the cosmic microwave background, diffuse and dust-scattered starlight, and the photon field of Westerlund 1 itself. The effect of the magnetic field on cooling and propagation is discussed. Klein-Nishina effects are found to be important in determining the spectral evolution of the electron population. Results. A leptonic origin of the bulk of the observed γ-rays is preferable. The model is energetically plausible, consistent with the presence of a strong shock, and allows for the observed energy-independent morphology. The hadronic model faces two main issues: confinement of particles to the emission region, and an unrealistic energy requirement.

Measurement of Cosmic-ray Energy Spectrum with the TALE Detector in Hybrid Mode

The TA Low-energy Extension (TALE) experiment extends the energy range observed by the TA experiment to below 1016 eV. We aim to study the transition from galactic to extragalactic cosmic rays. The TALE detector is a hybrid apparatus composed of fluorescence telescopes and surface detectors. The surface detectors are arranged to be suitable for hybrid energy spectrum measurements in the low-energy region. We measured the energy spectrum with the 392 hours observation data of the TALE hybrid detector. This energy spectrum measurement will play an important role in understanding the transition from cosmic rays of galactic origin to those of extragalactic origin.

Exploring the high energy frontiers of the Milky Way with ground-based gamma-ray astronomy: PeVatrons and the quest for the origin of Galactic cosmic-rays

Cosmic rays (CRs) are charged particles that arrive at Earth isotropically from all directions and interact with the atmosphere. The presence of a spectral knee feature seen in the CR spectrum at 3 PeV energies is an evidence that astrophysical objects within our Galaxy, which are known as 'Galactic PeVatrons', are capable of accelerating particles to PeV energies. Scientists have been trying to identify the origin of Galactic CRs and have been looking for signatures of Galactic PeVatrons through neutral messengers. Recent advancements in ground-based γ-ray astronomy have led to the discovery of 12 Galactic sources emitting above 100 TeV energies, and even the first time detection of PeV photons from the direction of Crab Nebula. These groundbreaking discoveries have opened up the field of ultrahigh-energy (UHE, E>100 TeV) γ-ray astronomy, which can help us explore the high-energy frontiers of our Galaxy, hunt for PeVatron sources, and shed light on the century-old problem of the origin of CRs. This review article provides an overview of the current state of the art and potential future directions for the search for Galactic PeVatrons using ground-based γ-ray observations.

The origin of galactic cosmic rays

The origin of galactic cosmic rays

Editorial

Editorial

Ultrahigh-energy diffuse gamma-ray emission from cosmic-ray interactions with the medium surrounding acceleration sources

The diffuse $\gamma$-ray spectrum at sub-PeV energy region has been measured for the first time by the Tibet-AS$\gamma$ experiment. It will shed new light on the understanding of origin and propagation of Galactic cosmic rays at very high energies. It has been pointed out that the traditional cosmic ray propagation model based on low energy measurements undershoot the new data, and modifications of the model with new ingredients or alternative propagation framework is required. In this work, we propose that the hadronic interactions between freshly accelerated cosmic rays and the medium surrounding the sources, which was neglected in the traditional model, can naturally account for the Tibet-AS$\gamma$ diffuse emission. We show that this scenario gives a consistent description of other secondary species such as the positron spectrum, the boron-to-carbon ratio, and the antiproton-to-proton ratio. As a result, the electron spectrum above 10 TeV will have a hardening due to this secondary component, which may be tested by future measurements.

Sir Arnold Whittaker Wolfendale. 25 June 1927—21 December 2020

For over 50 years Arnold Wolfendale was an international leader in the fields of cosmic ray and gamma ray astronomy, making many seminal contributions. His extensive studies of the muon particle culminated in 1965 when, using an installation in the Kolar Gold Mine in India, he played a major role in the first detection of the neutrinos associated with muons produced in the atmosphere. His interests in the origin of high-energy cosmic rays were extensive and required the development of a better understanding of particle physics at energies beyond those accessible at accelerators. Recognizing that high-energy gamma rays can arise from cosmic ray interactions with the interstellar gas, he used early satellite data to argue for the galactic origin of intermediate-energy cosmic rays and for studies of the distribution of molecular hydrogen. His interests in astronomy, which he firmly held to be a branch of physics, drove him to develop a world-class activity in this area at Durham University. This achievement, in part, led to him being appointed Astronomer Royal in 1991. He used this position, and his roles as president of the Royal Astronomical Society, the Institute of Physics and the European Physical Society, to lobby tirelessly for more governmental support for science. He was an early advocate for improvements in the public understanding of science, leading by example. In his later years Arnold's interests extended to cosmology and horology, and he argued against a possible connection between cosmic rays and global warming. A brilliant communicator, Arnold gave a huge number of lectures each year to general audiences, almost to the end of his life.

The origin of Galactic cosmic rays as revealed by their composition

ABSTRACT Galactic cosmic rays (GCRs) are thought to be accelerated in strong shocks induced by massive star winds and supernova explosions sweeping across the interstellar medium. But the phase of the interstellar medium from which the CRs are extracted has remained elusive until now. Here, we study in detail the GCR source composition deduced from recent measurements by the AMS-02, Voyager 1, and SuperTIGER experiments to obtain information on the composition, ionization state, and dust content of the GCR source reservoirs. We show that the volatile elements of the CR material are mainly accelerated from a plasma of temperature ≳ 2 MK, which is typical of the hot medium found in Galactic superbubbles energized by the activity of massive star winds and supernova explosions. Another GCR component, which is responsible for the overabundance of 22Ne, most likely arises from acceleration of massive star winds in their termination shocks. From the CR-related gamma-ray luminosity of the Milky Way, we estimate that the ion acceleration efficiency in both supernova shocks and wind termination shocks is of the order of 10−5. The GCR source composition also shows evidence for a preferential acceleration of refractory elements contained in interstellar dust. We suggest that the GCR refractories are also produced in superbubbles, from shock acceleration and subsequent sputtering of dust grains continuously incorporated into the hot plasma through thermal evaporation of embedded molecular clouds. Our model explains well the measured abundances of all primary and mostly primary CRs from H to Zr, including the overabundance of 22Ne.

The Hunt for Pevatrons: The Case of Supernova Remnants

The search for Galactic pevatrons is now a well-identified key science project of all instruments operating in the very-high-energy domain. Indeed, in this energy range, the detection of gamma rays clearly indicates that efficient particle acceleration is taking place, and observations can thus help identify which astrophysical sources can energize particles up to the ~PeV range, thus being pevatrons. In the search for the origin of Galactic cosmic rays (CRs), the PeV range is an important milestone, since the sources of Galactic CRs are expected to accelerate PeV particles. This is how the central scientific goal that is ’solving the mystery of the origin of CRs’ has often been distorted into ’finding (a) pevatron(s)’. Since supernova remnants (SNRs) are often cited as the most likely candidates for the origin of CRs, ’finding (a) pevatron(s)’ has often become ’confirming that SNRs are pevatrons’. Pleasingly, the first detection(s) of pevatron(s) were not associated to SNRs. Moreover, all clearly detected SNRs have yet revealed to not be pevatrons, and the detection from VHE gamma rays from regions unassociated with SNRs, are reminding us that other astrophysical sites might well be pevatrons. This short review aims at highlighting a few important results on the search for Galactic pevatrons.

Cosmic-ray production from neutron escape in microquasar jets

Context. The origin of Galactic cosmic rays remains a matter of debate, but supernova remnants are commonly considered to be the main place where high-energy cosmic rays are accelerated. Nevertheless, current models predict cosmic-ray spectra that do not match observations and the efficiency of the acceleration mechanism is still undetermined. On the other hand, the contribution of other kinds of sources to the Galactic cosmic-ray population is still unclear, and merits investigation. Aims. In this work we explore a novel mechanism through which microquasars might produce cosmic rays. In this scenario, microquasar jets generate relativistic neutrons, which escape and decay outside the system; protons and electrons, created when these neutrons decay, escape to the interstellar medium as cosmic rays. Methods. We introduce the relativistic neutron component through a coupling term in the transport equation that governs the jet proton population. We compute the escape rate and decay distribution of these neutrons, and follow the propagation of the decay products until they escape the system and become cosmic rays. We then compute the spectra of these cosmic rays. Results. Neutrons can drain only a small fraction of the jet power as cosmic rays. The most promising scenarios arise in extremely luminous systems (Ljet ∼ 1040 erg s−1), in which the fraction of jet power deposited in cosmic rays can reach ∼0.001. Slow jets (Γ ≲ 2, where Γ is the bulk Lorentz factor) favour neutron production. The resulting cosmic-ray spectrum is similar for protons and electrons, which share the power in the ratio given by neutron decay. The spectrum peaks at roughly half the minimum energy of the relativistic protons in the jet; it is soft (spectral index ∼3) above this energy, and almost flat below. Conclusions. The proposed mechanism produces more energetic cosmic rays from microquasars than those presented by previous works in which the particles escape through the jet terminal shock. Values of spectral index steeper than 2 are possible for cosmic rays in our model and these indeed agree with those required to explain the spectral signatures of Galactic cosmic rays, although only the most extreme microquasars provide power comparable to that of a typical supernova remnant. The mechanism explored in this work may provide stronger and softer cosmic-ray sources in the early Universe, and therefore contribute to the heating and reionisation of the intergalactic medium.

High Latitude Zones of GeV Heavy Ions at the Inner Edge of Jupiter's Relativistic Electron Belt

AbstractJuno has discovered signatures of a high energy heavy ion population (>100 MeV/nucleon) within the inner edge of Jupiter's relativistic electron belt. The particles were detected in narrow zones in the high‐latitude lobes of the synchrotron emission region, a location never explored by prior spacecraft (∼31°–46° magnetic latitude; radial distances 1.12–1.41 Jovian radii, M‐shells 1.5–2.37). The population was revealed by extremely high signal ionization signatures seen in images collected by Juno's Stellar Reference Unit star camera (equivalent to 0.2–0.7 MeV of deposited energy per pixel). Signature morphology indicates a population of GeV ions (Z > 1) with atomic mass as high as sulfur, although species with 2 ≤ Z ≤ 8 potentially account for all of the signatures. The detections have occurred while the spacecraft is magnetically connected to Jupiter's halo and gossamer rings, but not while it is connected to the main ring; similar to equatorial observations of GeV carbon and sulfur ions by the Galileo Probe. We find that the particles reside on magnetic drift shells that suggest they are stably trapped within Jupiter's magnetosphere. Because the cosmic ray albedo neutron decay mechanism can only produce protons and electrons, it cannot be the source of this trapped Z > 1 ion population; other galactic cosmic ray origins are suspected. Inward radial transport of tens of keV heavy ions from the outer magnetosphere is another potential source.

On particle acceleration and transport in plasmas in the Galaxy: theory and observations

Accelerated particles are ubiquitous in the Cosmos and play a fundamental role in many processes governing the evolution of the Universe at all scales, from the sub-AU scale relevant for the formation and evolution of stars and planets to the Mpc scale involved in Galaxy assembly. We reveal the presence of energetic particles in many classes of astrophysical sources thanks to their production of non-thermal radiation, and we detect them directly at the Earth as cosmic rays. In the last two decades both direct and indirect observations have provided us a wealth of new, high-quality data about cosmic rays and their interactions both in sources and during propagation, in the Galaxy and in the Solar System. Some of the new data have confirmed existing theories about particle acceleration and propagation and their interplay with the environment in which they occur. Some others have brought about interesting surprises, whose interpretation is not straightforward within the standard framework and may require a change of paradigm in terms of our ideas about the origin of cosmic rays of different species or in different energy ranges. In this article, we focus on cosmic rays of galactic origin, namely with energies below a few petaelectronvolts, where a steepening is observed in the spectrum of energetic particles detected at the Earth. We review the recent observational findings and the current status of the theory about the origin and propagation of galactic cosmic rays.

The Cryogenic AntiCoincidence Detector for ATHENA X-IFU: The Project Status

The ATHENA observatory is the second large class ESA mission to be launched on 2031 at L2 orbit. One of the two onboard instruments is X-IFU, a TES-based kilo-pixel array able to perform simultaneous high-grade energy spectroscopy (FWHM 2.5 eV@7 keV) and imaging over the 5′ field of view. The X-IFU sensitivity is degraded by primary particle background of both solar and galactic cosmic ray (GCR) origins, and by secondary electrons produced by primaries, interacting with the materials surrounding the detector: These particles cannot be distinguished by the scientific photons, thus degrading the instrument performance. Results from studies regarding the GCR component performed by Geant4 simulations address the necessity to use background reduction techniques to enable the study of several key science topics. This is feasible by combining an active Cryogenic AntiCoincidence detector (CryoAC) and a passive electron shielding to reach the required residual particle background of 0.005 cts/cm2/s/keV inside the 2–10 keV scientific energy band. The CryoAC is a four-pixel detector made of Si-suspended absorbers sensed by a network of IrAu TESes and placed at a distance < 1 mm below the TES array. Here we will provide an overview of the CryoAC program, starting with some details on the background assessment having impacts on the CryoAC design; then, we continue with its design concept including electronics and the Demonstration Model results, to conclude with programmatic aspects.

Measurement of the knees of proton and H&He spectra below 1 PeV

Galactic Cosmic ray (CR) origin is still a mystery. Measuring the knees of the CR spectra for individual species is a very important approach to solve the problem. ARGO-YBJ and LHAASO-WFCTA[1] combined experiment made the first step by measuring the spectrum of hydrogen plus helium nuclei and finding the knee around 0.7 PeV[2]. A significant boost is expected by using LHAASO experiment[3] to measure the spectra and their knees for pure proton and other species in few years. The key is to separate the specific species from all CR samples. In this paper, a multi variate analysis (MVA) approach for the CR composition analysis in LHAASO experiment is discussed. Preliminary results of the analysis and expectations are presented.

Observation of Very-High-Energy Cosmic Rays with the Tibet Air Shower Experiment—Quest for the Galactic Cosmic Ray Origin—

Observation of Very-High-Energy Cosmic Rays with the Tibet Air Shower Experiment—Quest for the Galactic Cosmic Ray Origin—

Anisotropies of different mass compositions of cosmic rays

The spectral hardenings of cosmic ray nuclei above ∼ 200 GV followed by softenings around 10 TV, the knee of the all-particle spectrum around PeV energies, as well as the pattern change of the amplitude and phase of the large-scale anisotropies around 100 TeV indicate the complexities of the origin and transportation of Galactic cosmic rays. It has been shown that nearby source(s) are most likely to be the cause of such spectral features of both the spectra and the anisotropies. In this work, we study the anisotropy features of different mass composition (or mass groups) of cosmic rays in this nearby source model. We show that even if the spectral features from the nearby source component is less distinctive compared with the background component from e.g., the population of distant sources, the anisotropy features are more remarkable to be identified. Measurements of the anisotropies of each mass composition (group) of cosmic rays by the space experiments such as DAMPE and HERD and the ground-based experiments such as LHAASO in the near future are expected to be able to critically test this scenario.