Low-energy Cosmic Rays Research Articles

GAPS: A balloon-borne cosmic-ray antimatter experiment

Theories beyond the Standard Model predict dark matter candidates that could provide a significant enhancement of the antideuteron and antiproton flux, in particular at low energies. The General Antiparticle Spectrometer (GAPS) experiment is the first antimatter search experiment designed specifically for low-energy cosmic ray antideuterons and antiprotons. GAPS identifies antideuterons and antiprotons using a technique based on exotic atoms. This novel detection technique allows GAPS to have unprecedented sensitivity in the low energy range (< 0.25 GeV/n) for antiprotons and antideuterons. The apparatus consists of 10 planes of lithium-drifted Si (Si(Li)) detectors, surrounded on all sides by a plastic scintillator time-of-flight. GAPS is designed to carry out its science program using long-duration balloon flights in Antarctica and is currently scheduled by NASA for its first flight in late 2020. This presentation will describe the design, status, and discovery potential of the GAPS scientific program.

The EUSO-SPB2 mission

EUSO-SPB2 is a second generation Extreme Universe Space Observatory (EUSO) on a Super-Pressure Balloon (SPB). The mission broadens the scientific objectives of the JEM-EUSO program and constitutes the first step toward the study of neutrino signals from the high atmosphere and space.The EUSO-SPB2 science payload will be equipped with two detectors designed for a long duration mission. One is a fluorescence telescope developed to detect Ultra High Energy Cosmic Rays via the UV fluorescence emission of the particle showers generated in the atmosphere. The other telescope will measure Cherenkov light emission from showers of lower energy cosmic rays to study and measure the background contribution for detecting cosmogenic neutrinos. These specific techniques and detection methods are performed in light of the realization of the Probe of Extreme Multi Messenger Astronomy (POEMMA), a space mission, currently under NASA-funded conceptual design studies. The EUSO-SPB2 mission has been approved by NASA and foreseen to be launched in 2022.In this paper we will give a description of the payload, including details on the detection techniques and the telescopes design.

Possible Evidence for Cosmic-Ray Acceleration in the Type Ia SNR RCW 86: Spatial Correlation between TeV Gamma-Rays and Interstellar Atomic Protons

Abstract We present a detailed morphological study of TeV gamma-rays, synchrotron radiation, and interstellar gas in the young Type Ia supernova remnant (SNR) RCW 86. We find that the interstellar atomic gas shows good spatial correlation with the gamma-rays, indicating that the TeV gamma-rays from RCW 86 are likely predominantly of hadronic origin. In contrast, the spatial correlation between the interstellar molecular cloud and the TeV gamma-rays is poor in the southeastern shell of the SNR. We argue that this poor correlation can be attributed to the low-energy cosmic rays (∼1 TeV) not penetrating into the dense molecular cloud due to an enhancement of the turbulent magnetic field around the dense cloud of ∼10–100 μG. We also find that the southwestern shell, which is bright in both synchrotron X-ray and radio continuum radiation, shows a significant gamma-ray excess compared with the interstellar proton column density, suggesting that leptonic gamma-rays via inverse Compton scattering possibly contribute alongside the hadronic gamma-rays. The total cosmic-ray energies of the young TeV gamma-ray SNRs—RX J1713.7−3946, Vela Jr, HESS J1731−347, and RCW 86—are roughly similar, which indicates that cosmic rays can be accelerated in both the core-collapse and Type Ia supernovae. The total energy of cosmic rays derived using the gas density, ∼1048–1049 erg, gives a safe lower limit due mainly to the low filling factor of interstellar gas within the shell.

Laboratory investigations aimed at building a database for the interpretation of JWST spectra

AbstractThe James Webb Space Telescope (JWST) is expected to be launched in 2021. The JWST’s science instruments will provide high quality spectra acquired in the line of sight to young stellar objects whose interpretation will require a robust database of laboratory data. With this in mind, an experimental work is in progress in the Laboratory for Experimental Astrophysics in Catania to study the profile (shape, width, and peak position) of the main infrared bands of molecular species expected to be present in icy grain mantles. Our study also takes into account the modifications induced on icy samples by low-energy cosmic ray bombardment and by thermal processing. Here we present some recent results on deuterium hydrogen monoxide (HDO), N-bearing species, and carbon dioxide (CO2).

Can a cosmic ray carrot explain the ionization level in diffuse molecular clouds?

Low energy cosmic rays are the major ionization agents of molecular clouds. However, it has been shown that, if the cosmic ray spectrum measured by Voyager 1 is representative of the whole Galaxy, the predicted ionization rate in diffuse clouds fails to reproduce data by 1-2 orders of magnitude, implying that an additional source of ionization must exist. One of the solutions proposed to explain this discrepancy is based on the existence of an unknown low energy (in the range 1 keV-1 MeV, not probed by Voyager) cosmic ray component, called carrot when first hypothesized by Reeves and collaborators in the seventies. Here we investigate the energetic required by such scenario. We show that the power needed to maintain such low energy component is comparable of even larger than that needed to explain the entire observed cosmic ray spectrum. Moreover, if the interstellar turbulent magnetic field has to sustain a carrot, through second-order Fermi acceleration, the required turbulence level would be definitely too large compared to the one expected at the scale resonant with such low energy particles. Our study basically rules out all the plausible sources of a cosmic ray carrot, thus making such hidden component unlikely to be an appealing and viable source of ionization in molecular clouds.

Measurement of Low-Energy Cosmic Rays via the Neutral Iron Line

There has been little information of Galactic low-energy cosmic rays (LECRs) so far because observations of LECRs in the solar system are affected by solar modulation and there is no effective way of indirect measurement. When LECRs in the MeV energy band collide with neutral iron atoms in the interstellar medium, uorescent X-rays at 6.4 keV are produced via inner-shell ionization of neutral iron atoms. We investigate the 6.4 keV line in supernova remnants (SNRs) as a prime candidate for the Galactic cosmic-ray origin. The line emission is discovered from 11 SNRs. The spectra and morphologies suggest that the 6.4 keV line is produced by LECRs interacting with cold gas. The proton energy density is estimated to be 10–100 eV cm−3. Furthermore, we measure the distribution of the 6.4 keV line emission near the Galactic center. The intensity profile is very similar to that of molecular clouds. The most plausible origin of the enhancement is the LECR proton bombardment. The energy density of MeV protons is estimated to be ∼ 80 eV cm−3. Since the diffusion length of MeV protons is short, they should be produced in situ. Surprisingly, there is no SNR in the vicinity, and thus another mechanism such as the stochastic acceleration would possibly work.

Red Dwarfs as Sources of Cosmic Rays and First Detection of TeV Gamma-rays from these stars

Conventional sources of Galactic cosmic rays are believed to be supernovae and supernova remnants that are powerful enough to accelerate particles up to 1017 eV. Meanwhile, an interpretation of recent experimental data obtained by PAMELA, Fermi-LAT, and AMS-02 spectrometers hints at a possibility that some cosmic-ray sources may be located in a close proximity of the solar system, at the distances less than 1 kpc. Presence of such local sources could explain the unexpected raise of the positron fraction in cosmic rays, complex shapes of the proton and helium spectra, and even anomalous low-energy cosmic rays. We consider active dwarf stars as possible sources of Galactic cosmic rays in the energy range up to 1014 eV. These stars are producing powerful stellar flares. Meanwhile, generation of high-energy cosmic rays should be accompanied by the high-energy γ-ray emission, which may be observed. Here we present the results of the SHALON long-term observations aimed to search for γ-ray emission above 800 GeV from the active red dwarf stars.

New scenario of plasma evolution in IC 443

Abstract Most young and middle-aged supernova remnants (SNRs) exhibit an ionizing plasma (IP), an ionizing process following a shock-heated SNR gas. On the other hand, significant fractions of SNRs exhibit a recombining plasma (RP). The origin and the mechanisms of the RP, however, are not yet well understood. This paper proposes a new model that the RP follows after the IP process taken at the first epoch of the SNR evolution. Using the high-quality and wide-band (0.6–10 keV) spectrum of IC 443, we fitted it with a model of two RPs (two-RP model) plus a power law (PL) with an Fe i Kα line component. The ionization temperature in one RP monotonically increases from Ne–Ca, while that in the other RP shows a drastic increase from Cr–Ni. The origin and mechanism of the two-RP and PL with an Fe i Kα line components are possibly due to different evolution of two plasmas and ionization by low-energy cosmic rays.

The Limadou-HEPD Segmented Calorimeter

The core of the High-Energy Particle Detector (HEPD) on board of the China Seismo-Electromagnetic Satellite (CSES) is a segmented calorimeter, which is composed with an upper tower of plastic scintillator counters and a bottom array of LYSO large crystals. Electrons with energy below 100MeV, protons and light nuclei, below few hundreds ofMeV/nucleon are fully contained within this calorimeter. Mainly the LYSO array (density 7.3 g/cm3, thickness around 29.2 g/cm2) extends the HEPD energy range, allowing those measurements (solar energetic particles, low-energy cosmic rays) which are more related to astroparticle physics topics. Two identical copies of HEPD, and then of its calorimeter, exist: the Flight (FM) and the Qualification (QM) models. While the FM has achieved the orbit on board of the CSES satellite in February 2018, the Qualification Model, is used, at ground, for tests and calibrations. A report on the characterization of this compact particle space detector and on preliminary studies and results, will be given.

Search for correlations of high-energy neutrinos and ultrahigh- energy cosmic rays

The IceCube Neutrino Observatory has recently found compelling evidence for a particular blazar producing high-energy neutrinos and PeV cosmic rays, however the sources of cosmic rays above several EeV remain unidentified. It is believed that the same environments that accelerate ultra-high-energy cosmic rays (UHECRs) also produce high-energy neutrinos via hadronic interactions of lower-energy cosmic rays. Two out of three joint analyses of the Ice- Cube Neutrino Observatory, the Pierre Auger Observatory and the Telescope Array yielded hints for a possible directional correlation of high-energy neutrinos and UHECRs. These hints however became less significant with more data. Recently, an improved analysis with an approach complementary to the other analyses has been developed. This analysis searches for neutrino point sources in the vicinity of UHECRs with search windows estimated from deflections by galactic magnetic fields. We present this new analysis method for searching common hadronic sources, additionally including neutrino data measured by ANTARES in order to increase the sensitivity to possible correlations in the Southern Hemisphere.

Revisit cosmic ray propagation by using 1H, 2H, 3He and 4He

The secondary-to-primary ratios are unique tools to investigate cosmic ray propagation mechanisms. In this work, we use the latest data of deuteron-to-helium 4 ratio and helium 3-to-helium 4 ratio measured by PAMELA combined with other Z≤2 primary fluxes measured by PAMELA and Voyager-1, to constrain the cosmic ray acceleration and propagation models. The analysis is performed by interfacing statistical tools with the GALPROP propagation package. To better fit both the modulated and unmodulated low energy cosmic ray data, we find that a time-, charge- and rigidity-dependent solar modulation model is better than the force-field approximation. Among all the studied cosmic ray propagation models, the diffusion–reacceleration–convection model is strongly supported by the derived Bayesian evidence. The robustness of the estimated diffusion slope δ is cross-checked by another low-mass secondary-to-primary ratio, i.e. the antiproton-to-proton ratio. It is shown that the diffusion–reacceleration–convection model can reconcile well with the high energy antiproton-to-proton ratio. This indicates that the estimated value of δ is reliable. The well constraint δ from the “best” model is found to be close to 1/3, inferring a Kolmogorov-type interstellar magnetic turbulence.

Molecular Ionization Rates and Ultracompact Dark Matter Minihalos.

Molecular ionization in the central molecular zone of our Galaxy is enhanced over the typical galactic value by an order of magnitude or more. This cannot be easily explained for dense Galactic center molecular complexes in the absence of embedded sources of low energy cosmic rays. We provide such a source in the form of ultracompact minihalos of self-annihilating dark matter for a variety of annihilation channels that depend on the particle mass and model. Such sources are motivated for plausible inflationary power spectrum parameters, and while possibly subdominant in terms of the total dark matter mass within the Galactic bulge, might also account for, or at least not be in tension with, the Fermi Galactic center γ-ray excess.

Gamma-Ray Emission from Molecular Clouds Generated by Penetrating Cosmic Rays

Abstract We analyze the processes governing cosmic-ray (CR) penetration into molecular clouds and the resulting generation of gamma-ray emission. The density of CRs inside a cloud is depleted at lower energies due to the self-excited MHD turbulence. The depletion depends on the effective gas column density (“size”) of the cloud. We consider two different environments where the depletion effect is expected to be observed. For the central molecular zone, the expected range of CR energy depletion is E ≲ 10 GeV, leading to the depletion of gamma-ray flux below E γ ≈ 2 GeV. This effect can be important for the interpretation of the GeV gamma-ray excess in the Galactic Center, which has been revealed from the standard model of CR propagation (assuming the CR spectrum inside a cloud to be equal to the interstellar spectrum). Furthermore, recent observations of some local molecular clouds suggest the depletion of the gamma-ray emission, indicating possible self-modulation of the penetrating low-energy CRs.

The Cosmic Ray Energy Spectrum between 2 PeV and 2 EeV Observed with the TALE Detector in Monocular Mode

Abstract We report on a measurement of the cosmic ray energy spectrum by the Telescope Array Low-Energy Extension (TALE) air fluorescence detector (FD). The TALE air FD is also sensitive to the Cherenkov light produced by shower particles. Low-energy cosmic rays, in the PeV energy range, are detectable by TALE as Cherenkov events. Using these events, we measure the energy spectrum from a low energy of ∼2 PeV to an energy greater than 100 PeV. Above 100 PeV, TALE can detect cosmic rays using air fluorescence. This allows for the extension of the measurement to energies greater than a few EeV. In this paper, we describe the detector, explain the technique, and present results from a measurement of the spectrum using ∼1000 hr of observation. The observed spectrum shows a clear steepening near 1017.1 eV, along with an ankle-like structure at 1016.2 eV. These features present important constraints on the origin of galactic cosmic rays and on propagation models. The feature at 1017.1 eV may also mark the end of the galactic cosmic ray flux and the start of the transition to extragalactic sources.

What causes the ionization rates observed in diffuse molecular clouds? The role of cosmic ray protons and electrons

Cosmic rays are usually assumed to be the main ionization agent for the interior of molecular clouds where UV and X-ray photons cannot penetrate. Here we test this hypothesis by limiting ourselves to the case of diffuse clouds and assuming that the average cosmic ray spectrum inside the Galaxy is equal to the one at the position of the Sun as measured by Voyager 1 and AMS-02. To calculate the cosmic ray spectrum inside the clouds, we solve the one-dimensional transport equation taking into account advection, diffusion and energy losses. While outside the cloud particles diffuse, in its interior they are assumed to gyrate along magnetic field lines because ion-neutral friction is effective in damping all the magnetic turbulence. We show that ionization losses effectively reduce the CR flux in the cloud interior for energies below $\approx 100$ MeV, especially for electrons, in such a way that the ionization rate decreases by roughly 2 order of magnitude with respect to the case where losses are neglected. As a consequence, the predicted ionization rate is more than 10 times smaller than the one inferred from the detection of molecular lines. We discuss the implication of our finding in terms of spatial fluctuation of the Galactic cosmic ray spectra and possible additional sources of low energy cosmic rays.

Cosmic ray heating of intergalactic medium: patchy or uniform?

We study the heating of the intergalactic medium (IGM) surrounding high redshift star forming galaxies due to cosmic rays (CR). We take into account the diffusion of low energy cosmic rays and study the patchiness of the resulting heating. We discuss the case of IGM heating around a high redshift minihalo ($z\sim 10\hbox{--}20$, M$\sim 10^5\hbox{--}10^7$ M$_\odot$),and put an upper limit on the diffusion coefficient $D\le 1\times 10^{26}$ cm$^2$ s$^{-1}$ for the heating to be inhomogeneous at $z\sim 10$ and $D\le 5\hbox{--}6 \times 10^{26}$ cm$^2$ s$^{-1}$ at $z\sim 20$. For typical values of $D$, our results suggest uniform heating by CR at high redshift, although there are uncertainties in magnetic field and other CR parameters. We also discuss two cases with continuous star formation, one in which the star formation rate (SFR) of a galaxy is high enough to make the IGM in the vicinity photoionized, and another in which the SFR is low enough to keep it neutral but high enough to cause significant heating by cosmic ray protons. In the neutral case (low SFR), we find that the resulting heating can make the gas hotter than the cosmic microwave background (CMB) radiation for $D < 10^{30}$ cm$^2$ s$^{-1}$, within a few kpc of the galaxy, and unlikely to be probed by near future radio observations. In the case of photoionized IGM (high SFR), the resulting heating of the gas in the vicinity of high redshift ($z\sim 4)$ galaxies of mass $\ge 10^{12}$ M$_\odot$ can suppress gas infall into the galaxy. At lower redshifts ($z\sim 0$), an SFR of $\sim 1$ M$_\odot$ yr$^{-1}$ can suppress the infall into galaxies of mass $\le 10^{10}$ M$_\odot$.

Cosmic Rays near Proxima Centauri b

The discovery of a terrestrial planet orbiting Proxima Centauri has led to a lot of papers discussing the possible conditions on this planet. Since the main factors determining space weather in the Solar System are the solar wind and cosmic rays (CRs), it seems important to understand what the parameters of the stellar wind, Galactic and stellar CRs near exoplanets are. Based on the available data, we present our estimates of the stellar wind velocity and density, the possible CR fluxes and fluences near Proxima b. We have found that there are virtually no Galactic CRs near the orbit of Proxima b up to particle energies ~1 TeV due to their modulation by the stellar wind. Nevertheless, more powerful and frequent flares on Proxima Centauri than those on the Sun can accelerate particles to maximum energies ~3150αβ GeV (α, β < 1). Therefore, the intensity of stellar CRs in the astrosphere may turn out to be comparable to the intensity of low-energy CRs in the heliosphere.

Outflowing OH+ in Markarian 231: The Ionization Rate of the Molecular Gas

Abstract The oxygen-bearing molecular ions OH+, H2O+, and H3O+ are key species that probe the ionization rate of (partially) molecular gas that is ionized by X-rays and cosmic-rays permeating the interstellar medium. We report Herschel far-infrared and submillimeter spectroscopic observations of OH+ in Mrk 231, showing both ground-state P-Cygni profiles, and excited line profiles with blueshifted absorption wings extending up to ≈1000 km s−1. In addition, OH+ probes an excited component peaking at central velocities, likely arising from the torus probed by the OH centimeter-wave megamaser. Four lines of H2O+ are also detected at systemic velocities, but H3O+ is undetected. Based on our earlier OH studies, we estimate an abundance ratio of for the outflowing components and ≈20 for the torus, and an OH+ abundance relative to H nuclei of ≳10−7. We also find high OH+/H2O+ and OH+/H3O+ ratios; both are ≳4 in the torus and ≳10–20 in the outflowing gas components. Chemical models indicate that these high OH+ abundances relative to OH, H2O+, and H3O+ are characteristic of gas with a high ionization rate per unit density, cm3 s−1 and ∼(1–2) × 10−16 cm3 s−1 for the above components, respectively, an ionization rate of ζ ∼ (0.5–2) × 10−12 s−1, and a low molecular fraction, . X-rays appear to be unable to explain the inferred ionization rate, and thus we suggest that low-energy (10–400 MeV) cosmic-rays are primarily responsible for the ionization, with M ⊙ yr−1 and erg s−1; the latter corresponds to ∼1% of the luminosity of the active galactic nucleus and is similar to the energetics of the molecular outflow. We suggest that cosmic-rays accelerated in the forward shock associated with the molecular outflow are responsible for the ionization, as they diffuse through the outflowing molecular phase downstream.

X-ray Observations of Supernova Remnants – environment study of acceleration sites

X-ray observations are one of the strong tools to study acceleration sites of cosmic rays, not only with synchrotron emission but also with thermal emission from heated plasma. Thermal X-rays with emission lines give us plenty of information on temperature, density, plasma time scale and so on, for heated interstellar medium and ejecta of progenitors, which are critical to understand the background plasma of the acceleration sites. It is debated for long time about which environment strongly accelerate particles, thin environment with faster shocks or thick environment with amplified magnetic field. It is also important tp known in which environment and when accelerated particles can escape to be cosmic rays, which is totally open issue to be solved. Observations of low energy cosmic rays in the MeV range is also crucial to understand the injection of accelerated particles. In this paper, we will show recent results with X-ray observations, as hints of environments of acceleration and escape sites on shocks of supernova remnants.

Diffusive shock acceleration of cosmic rays from two stationary shocks

Diffusive shock acceleration (Fermi acceleration) of cosmic rays in a system containing two shock waves is investigated using test particle simulations and analysis of the diffusion convection equation. We assume that the cosmic ray acceleration timescale is much less than the approaching timescale of the two shocks. Low-energy cosmic rays are primarily accelerated as they interact with one of the two shocks. However, as they are energized, and their mean free path becomes comparable to the separation distance of the two shocks, they start to accelerate as they interact with both shocks. As a result, a double power-law-type spectrum for the cosmic ray distribution is produced. The numerical result is well explained by a model based on the diffusion convection equation, which accounts for the dependence of the diffusion coefficient on the cosmic ray energy. The break point of the spectrum can be estimated by considering transport of cosmic rays from one shock to the other.