Sources Of Galactic Cosmic Rays Research Articles

Magnetar SGR 1900+14 as a Potential Source of Galactic Cosmic Rays with Energies Above 1020 eV

The analysis of the influence of the Galactic magnetic field on the trajectory of cosmic rays of extremely high energies (EHECR, E > 1020 eV) detected by Auger and Telescope Array (TA) detectors shows that the magnetar SGR 1900+14 is a potential Galactic source of the EHECR triplet – three events in a circle of radius 2° in the vicinity of the Galactic Center (l = 35°, b = –4°). Magnetar SGR 1900+14 – a neutron star with magnetic field of the order of 1015 G – was formed as a result of the Supernova outburst 1000…6000 years ago. In our work we investigate possible manifestations of cosmic rays accelerated by the Supernova remnant’s shock, due to their interaction with the molecular clouds of the interstellar medium in the vicinity of Supernova. In particular, the possibility of such a gamma-ray emission model of the newly discovered unidentified TeV gamma-ray source 2HWC J1907+084 is analyzed.

TAIGA Gamma Observatory: Status and Prospects

Over the past few years, the TAIGA (Tunka Advanced Instrument for cosmic ray physics and Gamma-ray Astronomy) observatory has been being deployed in the Tunka Valley, Republic of Buryatia. It is designed for studying gamma rays of energy above 30 TeV and performing searches for sources of galactic cosmic rays with energies in the vicinity of 1 PeV, which is an energy region around the classic knee in the cosmic-ray energy spectrum. The first phase of the observatory will be situated at a distance of about 50 km from Lake Baikal at the site of the Tunka-133 array. The TAIGA gamma observatory will include a network of 500 wide-angle (0.6 sr) Cherenkov detectors (TAIGA-HiSCORE array) and up to 16 atmospheric Cherenkov telescopes (ACT) designed for analyzing the EAS images (imaging atmospheric Cherenkov telescopes, or IACT) and positioned within an area of 5 km2. The observatory will also include muon detectors of total area 2000 m2 distributed over an area of 1 km2. Within the next three years, it is planned to enhance the area of the TAIGA-HiSCORE array by a factor of four—from 0.25 km2 to 1 km2; to supplement the existing IACT with two new ones; and to deploy new muon detectors with a total coverage of 200 m2. The structure of the new observatory is described along with the data analysis techniques used. The most interesting physical results are presented, and the research program for the future is discussed.

Magnetar SGR 1900+14 as a potential source of galactic cosmic rays with energies above 10E20 eV

Magnetar SGR 1900+14 as a potential source of galactic cosmic rays with energies above 10E20 eV

On the search for Galactic supernova remnant PeVatrons with current TeV instruments

The supernova remnant hypothesis for the origin of Galactic cosmic rays has passed several tests, but the firm identification of a supernova remnant pevatron, considered to be a decisive step to prove the hypothesis, is still missing. While a lot of hope has been placed in next-generation instruments operating in the multi-TeV range, it is possible that current gamma-ray instruments, operating in the TeV range, could pinpoint these objects or, most likely, identify a number of promising targets for instruments of next generation. Starting from the assumption that supernova remnants are indeed the sources of Galactic cosmic rays, and therefore must be pevatrons for some fraction of their lifetime, we investigate the ability of current instruments to detect such objects, or to identify the most promising candidates.

Supernova remnants in the very–high–energy gamma-ray domain: the role of the Cherenkov telescope array

Supernova remnants are often presented as the most probable sources of Galactic cosmic rays. This idea is supported by the accumulation of evidence that particle acceleration is happening at supernova remnant shocks. Observations in the TeV range have especially contributed to increase the understanding of the mechanisms, but many aspects of the particle acceleration at supernova remnant shocks are still debated. The Cherenkov telescope array is expected to lead to the detection of many new supernova remnants in the TeV and multi-TeV range. In addition to the individual study of each, the study of these objects as a population can help constrain the parameters describing the acceleration of particles and increase our understanding of the mechanisms involved.

Supernovae in compact star clusters as sources of high-energy cosmic rays and neutrinos

We discuss a specific population of galactic PeVatrons which may be the source of the galactic CR component well above PeV energies. Supernovae in compact clusters of massive stars are proposed as powerful sources of CRs, neutrinos, and gamma-ray emission. Numerical simulations of non-linear CR acceleration at converging shock flows have revealed that these accelerators can provide very hard spectra of protons up to 1016–1017eV which is well above the “knee” in the all-particle CR spectrum at about 3×1015eV. We suggest that known supernova remnants interacting with stellar winds in the compact clusters of young massive stars Westerlund I and Cl*1806-20 can be associated with the sources of the TeV gamma-emission detected by H.E.S.S. and may be responsible for a fraction of the high-energy neutrinos detected with the IceCube observatory. A recent cosmic ray (CR) composition measurement with the LOFAR array has revealed a light-mass component possibly dominating the all-particle spectrum at energies around 1017eV. Such a strong light component (mainly protons and helium) may require specific galactic CR sources such as supernovae interacting with compact clusters of massive stars in addition to isolated supernova remnants.

Supernova remnant W44: a case of cosmic-ray reacceleration

Supernova remnants (SNRs) are thought to be the primary sources of Galactic Cosmic Rays (CRs). In the last few years, the wealth of gamma-ray data collected by GeV and TeV instruments has provided important information about particle energisation in these astrophysical sources, allowing us to make progress in assessing their role as CR accelerators. In particular, the spectrum of the gamma-ray emission detected by AGILE and Fermi-LAT from the two middle aged Supernova Remnants (SNRs) W44 and IC443, has been proposed as a proof of CR acceleration in SNRs. Here we discuss the possibility that the radio and gamma-ray spectra from W44 may be explained in terms of re-acceleration and compression of Galactic CRs. The recent measurement of the interstellar CR flux by Voyager I has been instrumental for our work, in that the result of the reprocessing of CRs by the shock in W44 depends on the CR spectrum at energies that are precluded to terrestrial measurement due to solar modulation. We introduce both CR protons and helium nuclei in our calculations, and secondary electrons produced in situ are compared with the flux of Galactic CR electrons reprocessed by the slow shock of this SNR.We find that the multi-wavelength spectrum of W44 can be explained by reaccelerated particles with no need of imposing any break on their distribution, but just a high energy cut-off at the maximum energy the accelerator can provide.We also find that a model including both re-acceleration and a very small fraction of freshly accelerated particles may be more satisfactory on physical grounds

GALACTIC COSMIC RAYS IN THE LOCAL INTERSTELLAR MEDIUM: VOYAGER 1 OBSERVATIONS AND MODEL RESULTS.

Since 2012 August Voyager 1 has been observing the local interstellar energy spectra of Galactic cosmic-ray nuclei down to 3 MeV nuc-1 and electrons down to 2.7 MeV. The H and He spectra have the same energy dependence between 3 and 346 MeV nuc-1, with a broad maximum in the 10-50 MeV nuc-1 range and a H/He ratio of 12.2 ± 0.9. The peak H intensity is ~15 times that observed at 1 AU, and the observed local interstellar gradient of 3-346 MeV H is -0.009 ± 0.055% AU-1, consistent with models having no local interstellar gradient. The energy spectrum of electrons (e - + e +) with 2.7-74 MeV is consistent with E -1.30±0.05 and exceeds the H intensity at energies below ~50 MeV. Propagation model fits to the observed spectra indicate that the energy density of cosmic-ray nuclei with >3 MeV nuc-1 and electrons with >3 MeV is 0.83-1.02 eV cm-3 and the ionization rate of atomic H is in the range of 1.51-1.64 × 10-17 s-1. This rate is a factor >10 lower than the ionization rate in diffuse interstellar clouds, suggesting significant spatial inhomogeneity in low-energy cosmic rays or the presence of a suprathermal tail on the energy spectrum at much lower energies. The propagation model fits also provide improved estimates of the elemental abundances in the source of Galactic cosmic rays.

Transport of magnetic turbulence in supernova remnants

Context. Supernova remnants are known as sources of galactic cosmic rays for their non-thermal emission of radio waves, X-rays, and gamma-rays. However, the observed soft broken power-law spectra are hard to reproduce within standard acceleration theory based on the assumption of Bohm diffusion and steady-state calculations. Aims. We point out that a time-dependent treatment of the acceleration process together with a self-consistent treatment of the scattering turbulence amplification is necessary. Methods. We numerically solve the coupled system of transport equations for cosmic rays and isotropic Alfvenic turbulence. The equations are coupled through the growth rate of turbulence determined by the cosmic-ray gradient and the spatial diffusion coefficient of cosmic rays determined by the energy density of the turbulence. The system is solved on a co-moving expanding grid extending upstream for dozens of shock radii, allowing for the self-consistent study of cosmic-ray diffusion in the vicinity of their acceleration site. The transport equation for cosmic rays is solved in a test-particle approach. Results. We demonstrate that the system is typically not in a steady state. In fact, even after several thousand years of evolution, no equilibrium situation is reached. The resulting time-dependent particle spectra strongly differ from those derived assuming a steady state and Bohm diffusion. Our results indicate that proper accounting for the evolution of the scattering turbulence and hence the particle diffusion coefficient is crucial for the formation of the observed soft spectra. In any case, the need to continuously develop magnetic turbulence upstream of the shock introduces non-linearity in addition to that imposed by cosmic-ray feedback.

Non-linear diffusion of cosmic rays escaping from supernova remnants – I. The effect of neutrals

Supernova remnants are believed to be the main sources of galactic cosmic rays (CR). Within this framework, particles are accelerated at supernova remnant shocks and then released in the interstellar medium. The mechanism through which CRs are released and the way in which they propagate still remain open issues. The main difficulty is the high non-linearity of the problem: CRs themselves excite the magnetic turbulence that confines them close to their sources. We solve numerically the coupled differential equations describing the evolution in space and time of the escaping particles and of the waves generated through the CR streaming instability. The warm ionized and warm neutral phases of the interstellar medium are considered. These phases occupy the largest fraction of the disc volume, where most supernovae explode, and are characterized by the significant presence of neutral particles. The friction between those neutrals and ions results in a very effective wave damping mechanism. It is found that streaming instability affects the propagation of CRs even in the presence of ion-neutral friction. The diffusion coefficient can be suppressed by more than a factor of ∼2 over a region of few tens of pc around the remnant. The suppression increases for smaller distances. The propagation of ≈10 GeV particles is affected for several tens of kiloyears after escape, while ≈1 TeV particles are affected for few kiloyears. This might have a great impact on the interpretation of gamma-ray observations of molecular clouds located in the vicinity of supernova remnants.

Acceleration of petaelectronvolt protons in the Galactic Centre.

Galactic cosmic rays reach energies of at least a few petaelectronvolts (of the order of 10(15) electronvolts). This implies that our Galaxy contains petaelectronvolt accelerators ('PeVatrons'), but all proposed models of Galactic cosmic-ray accelerators encounter difficulties at exactly these energies. Dozens of Galactic accelerators capable of accelerating particles to energies of tens of teraelectronvolts (of the order of 10(13) electronvolts) were inferred from recent γ-ray observations. However, none of the currently known accelerators--not even the handful of shell-type supernova remnants commonly believed to supply most Galactic cosmic rays--has shown the characteristic tracers of petaelectronvolt particles, namely, power-law spectra of γ-rays extending without a cut-off or a spectral break to tens of teraelectronvolts. Here we report deep γ-ray observations with arcminute angular resolution of the region surrounding the Galactic Centre, which show the expected tracer of the presence of petaelectronvolt protons within the central 10 parsecs of the Galaxy. We propose that the supermassive black hole Sagittarius A* is linked to this PeVatron. Sagittarius A* went through active phases in the past, as demonstrated by X-ray outburstsand an outflow from the Galactic Centre. Although its current rate of particle acceleration is not sufficient to provide a substantial contribution to Galactic cosmic rays, Sagittarius A* could have plausibly been more active over the last 10(6)-10(7) years, and therefore should be considered as a viable alternative to supernova remnants as a source of petaelectronvolt Galactic cosmic rays.

REVEALING W51C AS A COSMIC RAY SOURCE USING FERMI-LAT DATA

ABSTRACT Supernova remnants (SNRs) are commonly believed to be the primary sources of Galactic cosmic rays. Despite intensive study of the non-thermal emission of many SNRs the identification of the accelerated particle type relies heavily on assumptions of ambient-medium parameters that are only loosely constrained. Compelling evidence of hadronic acceleration can be provided by detecting a strong roll-off in the secondary γ-ray spectrum below the π 0 ?> production threshold energy of about 135 MeV, the so called “pion bump.” Here we use five years of Fermi-Large Area Telescope data to study the spectrum above 60 MeV of the middle-aged SNR W51C. A clear break in the power-law γ-ray spectrum at E break = 290 ± 20 MeV ?> is detected with 9 &sgr; ?> significance and we show that this break is most likely associated with the energy production threshold of π 0 ?> mesons. A high-energy break in the γ-ray spectrum at about 2.7 GeV is found with 7.5 &sgr; ?> significance. The spectral index at energies beyond this second break is Γ 2 = 2.52 − 0.07 + 0.06 ?> and closely matches the spectral index derived by the MAGIC Collaboration above 75 GeV. Therefore our analysis provides strong evidence to explain the γ-ray spectrum of W51C by a single particle population of protons with a momentum spectrum best described by a broken power law with break momentum p break ∼ 80 G e V / c . ?> W51C is the third middle-aged SNR that displays compelling evidence for cosmic-ray acceleration and thus strengthens the case of SNRs as the main source of Galactic cosmic rays.

Gamma-ray emission from young supernova remnants: Hadronic or leptonic?

The debate on the nature of the gamma-ray emission from young supernova remnants is still open. Ascribing such emission to hadronic rather than leptonic processes would provide an evidence for the acceleration of protons and nuclei, and this fact would fit with the very popular (but not proven) paradigm that supernova remnants are the sources of Galactic cosmic rays. Here, we discuss this issue with a particular focus on the best studied gamma-ray-bright supernova remnant: RX~J1713.7-3946.

ORIGIN OF THE PROTON-TO-HELIUM RATIO ANOMALY IN COSMIC RAYS

Recent data on Galactic cosmic rays (CRs) revealed that the helium energy spectrum is harder than the proton spectrum. The AMS experiment has now reported that the proton-to-helium ratio as function of rigidity $R$ (momentum-to-charge ratio) falls off steadily as p/He $\sim R^\Delta$, with $\Delta\approx$-0.08 between $R\sim$40 GV and $R\sim$2 TV. Besides, the single spectra of proton and helium are found to progressively harden at $R\gtrsim$100 GV. The p/He anomaly is generally ascribed to particle-dependent acceleration mechanisms occurring in Galactic CR sources. However, this explanation poses a challenge to the known mechanisms of particle acceleration since they are believed to be "universal", composition blind, rigidity mechanisms. Using the new AMS data, we show that the p/He anomaly can be simply explained in terms of a two-component scenario where the GeV-TeV flux is ascribed to a hydrogen-rich source, possibly a nearby supernova remnant, characterized by a soft acceleration spectrum. This simple idea provides a common interpretation for the p/He ratio and for the single spectra of proton and helium: both anomalies are explained by a flux transition between two components. The "universality" of particle acceleration in sources is not violated in this model. A distinctive signature of our scenario is the high-energy flattening of the p/He ratio at multi-TeV energies, which is hinted at by existing data and will be resolutely tested by new space experiments ISS-CREAM and CALET.

Computational Assessment of Naturally Occurring Neutron and Photon Background Radiation Produced by Extraterrestrial Sources

A computational assessment of the variation in terrestrial neutron and photon background from extraterrestrial sources is presented. The motivation of this assessment is to evaluate the practicality of developing a tool or database to estimate background in real time (or near–real time) during an experimental measurement or to even predict the background for future measurements. The extraterrestrial source focused on during this assessment is naturally occurring galactic cosmic rays (GCRs). The MCNP6 transport code was used to perform the computational assessment. However, the GCR source available in MCNP6 was not used. Rather, models developed and maintained by NASA were used to generate the GCR sources. The largest variation in both neutron and photon background spectra was found to be caused by changes in elevation on Earth's surface, which can be as large as an order of magnitude. All other perturbations produced background variations on the order of a factor of 3 or less. The most interesting finding was that ~80% and 50% of terrestrial background neutrons and photons, respectively, are generated by interactions in Earth's surface and other naturally occurring and man-made objects near a detector of particles from extraterrestrial sources and their progeny created in Earth's atmosphere. This assessment shows that it will be difficult to estimate the terrestrial background from extraterrestrial sources without a good understanding of a detector's surroundings. Therefore, estimating or predicting background during a measurement environment like a mobile random search will be difficult.

Neutron yields and effective doses produced by Galactic Cosmic Ray interactions in shielded environments in space

In order to define the ranges of relevant neutron energies for the purposes of measurement and dosimetry in space, we have performed a series of Monte Carlo transport model calculations that predict the neutron field created by Galactic Cosmic Ray interactions inside a variety of simple shielding configurations. These predictions indicate that a significant fraction of the neutron fluence and neutron effective dose lies in the region above 20 MeV up to several hundred MeV. These results are consistent over thicknesses of shielding that range from very thin (2.7 g/cm2) to thick (54 g/cm2), and over both shielding materials considered (aluminum and water). In addition to these results, we have also investigated whether simplified Galactic Cosmic Ray source terms can yield predictions that are equivalent to simulations run with a full GCR source term. We found that a source using a GCR proton and helium spectrum together with a scaled oxygen spectrum yielded nearly identical results to a full GCR spectrum, and that the scaling factor used for the oxygen spectrum was independent of shielding material and thickness. Good results were also obtained using a GCR proton spectrum together with a scaled helium spectrum, with the helium scaling factor also independent of shielding material and thickness. Using a proton spectrum alone was unable to reproduce the full GCR results.

Cosmic ray proton plus helium energy spectrum measured by the ARGO-YBJ experiment in the energy range 3–300 TeV

The ARGO–YBJ experiment is a full-coverage air shower detector located at the Yangbajing Cosmic Ray Observatory (Tibet, People’s Republic of China, 4300 m a.s.l.). The high altitude, combined with the full-coverage technique, allows the detection of extensive air showers in a wide energy range and offer the possibility of measuring the cosmic ray proton plus helium spectrum down to the TeV region, where direct balloon/space-borne measurements are available. The detector has been in stable data taking in its full configuration from November 2007 to February 2013. In this paper the measurement of the cosmic ray proton plus helium energy spectrum is presented in the region 3–300 TeV by analyzing the full collected data sample. The resulting spectral index is γ=−2.64±0.01, the error is dominated by systematic uncertainties. The accurate measurement of the spectrum of light elements with a ground based air shower detector demonstrates the possibility of extending these measurements at larger energies, where galactic cosmic ray sources should run out of power in accelerating light elements.Received 24 March 2015DOI:https://doi.org/10.1103/PhysRevD.91.112017© 2015 American Physical Society

Diffusion of cosmic rays in a multiphase interstellar medium swept-up by a supernova remnant blast wave

Supernova remnants (SNRs) are one of the most energetic astrophysical events and are thought to be the dominant source of Galactic cosmic rays (CRs). A recent report on observations from the Fermi satellite has shown a signature of pion decay in the gamma-ray spectra of SNRs. This provides strong evidence that high-energy protons are accelerated in SNRs. The actual gamma-ray emission from pion decay should depend on the diffusion of CRs in the interstellar medium. In order to quantitatively analyse the diffusion of high-energy CRs from acceleration sites, we have performed test particle numerical simulations of CR protons using a three-dimensional magnetohydrodynamics (MHD) simulation of an interstellar medium swept-up by a blast wave. We analyse the diffusion of CRs at a length scale of order a few pc in our simulated SNR, and find the diffusion of CRs is precisely described by a Bohm diffusion, which is required for efficient acceleration at least for particles with energies above 30 TeV for a realistic interstellar medium. Although we find the possibility of a superdiffusive process (travel distance ∝ t0.75) in our simulations, its effect on CR diffusion at the length scale of the turbulence in the SNR is limited.

Propagation in 3D spiral-arm cosmic-ray source distribution models and secondary particle production using Picard

We study the impact of possible spiral-arm distributions of Galactic cosmic-ray sources on the flux of various cosmic-ray nuclei throughout our Galaxy. We investigate model cosmic-ray spectra at the nominal position of the sun and at different positions within the Galaxy. The modelling is performed using the recently introduced numerical cosmic ray propagation code Picard. Assuming non-axisymmetric cosmic-ray source distributions yields new insights on the behaviour of primary versus secondary nuclei.We find that primary cosmic rays are more strongly confined to the vicinity of the sources, while the distribution of secondary cosmic rays is much more homogeneous compared to the primaries. This leads to stronger spatial variation in secondary to primary ratios when compared to axisymmetric source distribution models. A good fit to the cosmic-ray data at Earth can be accomplished in different spiral-arm models, although leading to decisively different spatial distributions of the cosmic-ray flux. These lead to different cosmic ray anisotropies, where even reproducing the data becomes possible. Consequently, we advocate directions to seek best fit propagation parameters that take into account the higher complexity introduced by the spiral-arm structure on the cosmic-ray distribution. We specifically investigate whether the flux at Earth is representative for a large fraction of the Galaxy. The variance among possible spiral-arm models allows us to quantify the spatial variation of the cosmic-ray flux within the Galaxy in presence of non-axisymmetric source distributions.

On the cosmic ray spectrum from type II supernovae expanding in their red giant presupernova wind

While from the energetic point of view supernova remnants are viable sources of Galactic cosmic rays (CRs), the issue of whether they can accelerate protons up to a few PeV remains unsolved. Here we discuss particle acceleration at the forward shock of supernovae, and discuss the possibility that the current of escaping particles may excite a non-resonant instability that in turn leads to the formation of resonant modes that confine particles close to the shock, thereby increasing the maximum energy. This mechanism is at work throughout the expansion of the supernova explosion, from the ejecta dominated (ED) phase to the Sedov–Taylor (ST) phase. The transition from one stage to the other reflects in a break in the spectrum of injected particles. Because of their higher explosion rate, we focus our work on type II SNe expanding in the slow, dense wind, produced by the red super-giant progenitor stars. When the explosion occurs in such winds, the transition between the ED and the ST phase is likely to take place within a few tens of years. The highest energies are reached at even earlier times, when, however, a small fraction of the mass of ejecta has been processed. As a result, the spectrum of accelerated particles shows a break in the slope, at an energy that is the maximum energy (EM) achieved at the beginning of the ST phase. Above this characteristic energy, the spectrum becomes steeper but remains a power law rather than developing an exponential cutoff. An exponential cut is eventually present at much higher energies but it does not have a phenomenological relevance. We show that for parameters typical of type II supernovae, EM for protons can easily reach values in the PeV range, confirming that type II SNRs are the best candidate sources for CRs at the knee.From the point of view of implications of this scenario on the measured particle spectra, we have tried to fit KASCADE-Grande, ARGO -YBJ and YAC1-Tibet Array data with our model but we could not find any combination of the parameters that could explain all data sets. Indeed the recent measurement of the proton and helium spectra in the knee region, with the ARGO-YBJ and YAC1-Tibet Array, has made the situation very confused. These measurements suggest that the knee in the light component is at ∼650TeV, appreciably below the knee in the overall spectrum. On one hand this finding would resolve the problem of reaching very high energies in supernovae, but on the other it would open a critical issue in the transition region between Galactic and extragalactic CRs.