Search

Galaxy clusters are one of the prime sites to search for dark matter (DM) annihilation signals. Depending on the substructure of the DM halo of a galaxy cluster and the cross sections for DM annihilation channels, these signals might be detectable by the latest generation of γ-ray telescopes. Here we use three years of Fermi-Large Area Telescope data, which are the most suitable for searching for very extended emission in the vicinity of the nearby Virgo galaxy cluster. Our analysis reveals statistically significant extended emission which can be well characterized by a uniformly emitting disk profile with a radius of 3 that moreover is offset from the cluster center. We demonstrate that the significance of this extended emission strongly depends on the adopted interstellar emission model (IEM) and is most likely an artifact of our incomplete description of the IEM in this region. We also search for and find new point source candidates in the region. We then derive conservative upper limits on the velocity-averaged DM pair annihilation cross section from Virgo. We take into account the potential γ-ray flux enhancement due to DM sub-halos and its complex morphology as a merging cluster. For DM annihilating into assuming a conservative sub-halo model setup, we find limits that are between 1 and 1.5 orders of magnitude above the expectation from the thermal cross section for mDM ≲ 100 GeV. In a more optimistic scenario, we exclude for mDM ≲ 40 GeV for the same channel. Finally, we derive upper limits on the γ-ray-flux produced by hadronic cosmic-ray interactions in the inter cluster medium. We find that the volume-averaged cosmic-ray-to-thermal pressure ratio is less than ∼6%.

Current theories predict relativistic hadronic particle populations in clusters of galaxies in addition to the already observed relativistic leptons. In these scenarios hadronic interactions give rise to neutral pions which decay into γ rays that are potentially observable with the Large Area Telescope (LAT) on board the Fermi space telescope. We present a joint likelihood analysis searching for spatially extended γ-ray emission at the locations of 50 galaxy clusters in four years of Fermi-LAT data under the assumption of the universal cosmic-ray (CR) model proposed by Pinzke & Pfrommer. We find an excess at a significance of 2.7σ, which upon closer inspection, however, is correlated to individual excess emission toward three galaxy clusters: A400, A1367, and A3112. We discuss these cases in detail and conservatively attribute the emission to unmodeled background systems (for example, radio galaxies within the clusters).Through the combined analysis of 50 clusters, we exclude hadronic injection efficiencies in simple hadronic models above 21% and establish limits on the CR to thermal pressure ratio within the virial radius, R 200, to be below 1.25%-1.4% depending on the morphological classification. In addition, we derive new limits on the γ-ray flux from individual clusters in our sample. © 2014. The American Astronomical Society. All rights reserved.

The discovery of rapidly variable Very High Energy (VHE; E > 100 GeV) γ -ray emission from 4C +21.35 (PKS 1222+216) by MAGIC on 2010 June 17, triggered by the high activity detected by the Fermi Large Area Telescope (LAT) in high energy (HE;E >100 MeV) γ -rays, poses intriguing questions on the location of the γ -ray emitting region in this flat spectrum radio quasar. We present multifrequency data of 4C +21.35 collected from centimeter to VHE during 2010 to investigate the properties of this source and discuss a possible emission model. The first hint of detection at VHE was observed by MAGIC on 2010 May 3, soon after a γ -ray flare detected by Fermi-LAT that peaked on April 29. The same emission mechanism may therefore be responsible for both the HE and VHE emission during the 2010 flaring episodes. Two optical peaks were detected on 2010 April 20 and June 30, close in time but not simultaneous with the two γ -ray peaks, while no clear connection was observed between the X-ray and γ -ray emission. An increasing flux density was observed in radio and mm bands from the beginning of 2009, in accordance with the increasing γ -ray activity observed by Fermi-LAT, and peaking on 2011 January 27 in the mm regime (230 GHz). We model the spectral energy distributions (SEDs) of 4C +21.35 for the two periods of the VHE detection and a quiescent state, using a one-zone model with the emission coming from a very compact region outside the broad line region. The three SEDs can be fit with a combination of synchrotron self-Compton and external Compton emission of seed photons from a dust torus, changing only the electron distribution parameters between the epochs. The fit of the optical/UV part of the spectrum for 2010 April 29 seems to favor an inner disk radius of

Recent precise measurements of primary and secondary cosmic-ray (CR) species in the teravolt rigidity domain have unveiled a bump in their spectra, located between 0.5 and 50 TV. We argue that a local shock may generate such a bump by increasing the rigidity of the preexisting CRs below 50 TV by a mere factor of ∼1.5. Reaccelerated particles below ∼0.5 TV are convected with the interstellar medium flow and do not reach the Sun, thus creating the bump. This single universal process is responsible for the observed spectra of all CR species in the rigidity range below 100 TV. We propose that one viable shock candidate is the Epsilon Eridani star at 3.2 pc from the Sun, which is well aligned with the direction of the local magnetic field. Other shocks, such as old supernova shells, may produce a similar effect. We provide a simple formula, Equation (9), that reproduces the spectra of all CR species with only two nonadjustable shock parameters, uniquely derived from the proton data. We show how our formalism predicts helium and carbon spectra and the B/C ratio. [ABSTRACT FROM AUTHOR]

Since its launch, the Alpha Magnetic Spectrometer—02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species (, e±, and nuclei, 1H–8O, 10Ne, 12Mg, 14Si) which resulted in a number of breakthroughs. One of the latest long-awaited surprises is the spectrum of 26Fe just published by AMS-02. Because of the large fragmentation cross section and large ionization energy losses, most of CR iron at low energies is local and may harbor some features associated with relatively recent supernova (SN) activity in the solar neighborhood. Our analysis of the new AMS-02 results, together with Voyager 1 and ACE-CRIS data, reveals an unexpected bump in the iron spectrum and in the Fe/He, Fe/O, and Fe/Si ratios at 1–2 GV, while a similar feature in the spectra of He, O, and Si and in their ratios is absent, hinting at a local source of low-energy CRs. The found excess extends the recent discoveries of radioactive 60Fe deposits in terrestrial and lunar samples and in CRs. We provide an updated local interstellar spectrum (LIS) of iron in the energy range from 1 MeV nucleon−1 to ∼10 TeV nucleon−1. Our calculations employ the GalProp – HelMod framework, which has proved to be a reliable tool in deriving the LIS of CR , e−, and nuclei Z ≤ 28. [ABSTRACT FROM AUTHOR]

A recently observed bump in the cosmic-ray (CR) spectrum from 0.3 to 30 TV is likely caused by a stellar bow shock that reaccelerates preexisting CRs, which further propagate to the Sun along the magnetic field lines. Along their way, these particles generate an Iroshnikov–Kraichnan (I-K) turbulence that controls their propagation and sustains the bump. Ad hoc fitting of the bump shape requires six adjustable parameters. Our model requires none, merely depending on three physical unknowns that we constrain using the fit. These are the shock Mach number, M, its size, l⊥, and the distance to it, ζobs. Altogether, they define the bump rigidity R0. With M ≈ 1.5–1.6 and R0 ≈ 4.4 TV, the model fits the data with ≈0.08% accuracy. The fit critically requires the I-K spectrum predicted by the model and rules out the alternatives. These attributes of the fit make an accidental agreement highly unlikely. In turn, the R0 and M derived from the fit impose the distance−size relation on the shock: ζobs(pc) ∼. For sufficiently large bow shocks, l⊥ = 10−3−10−2 pc, we find the distance of ζobs = 3–10 pc. Three promising stars in this range are the Scholz's Star at 6.8 pc, Epsilon Indi at 3.6 pc, and Epsilon Eridani at 3.2 pc. Based on their current positions and velocities, we propose that Epsilon Indi and Epsilon Eridani can produce the observed spectral bump. Moreover, Epsilon Eridani's position is only ∼6.°7 off of the magnetic field direction in the solar neighborhood, which also changes the CR arrival direction distribution. Given the proximity of these stars, the bump appearance may change in a relatively short time. [ABSTRACT FROM AUTHOR]

We report the discovery of γ-ray emission from the Circinus galaxy using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. Circinus is a nearby (∼4 Mpc) starburst with a heavily obscured Seyfert-type active nucleus, bipolar radio lobes perpendicular to the spiral disk, and kpc-scale jet-like structures. Our analysis of 0.1-100 GeV events collected during 4 yr of LAT observations reveals a significant (≃ 7.3σ) excess above the background. We find no indications of variability or spatial extension beyond the LAT point-spread function. A power-law model used to describe the 0.1-100 GeV γ-ray spectrum yields a flux of (18.8 ± 5.8) × 10–9 photon cm–2 s–1 and photon index 2.19 ± 0.12, corresponding to an isotropic γ-ray luminosity of 3 × 1040 erg s–1. This observed γ-ray luminosity exceeds the luminosity expected from cosmic-ray interactions in the interstellar medium and inverse Compton radiation from the radio lobes. Thus, the origin of the GeV excess requires further investigation. [ABSTRACT FROM AUTHOR]

Supernova remnants (SNRs), as the major contributors to the galactic cosmic rays (CRs), are believed to maintain an average CR spectrum by diffusive shock acceleration regardless of the way they release CRs into the interstellar medium (ISM). However, the interaction of the CRs with nearby gas clouds crucially depends on the release mechanism. We call into question two aspects of a popular paradigm of the CR injection into the ISM, according to which they passively and isotropically diffuse in the prescribed magnetic fluctuations as test particles. First, we treat the escaping CR and the Alfvén waves excited by them on an equal footing. Second, we adopt field-aligned CR escape outside the source, where the waves become weak. An exact analytic self-similar solution for a CR "cloud" released by a dimmed accelerator strongly deviates from the test-particle result. The normalized CR partial pressure may be approximated as P(p, z, t ) = 2[∣z∣5/3 + z 5/3 dif (p, t )]-3/5 exp[-z2/4DISM(p)t ], where p is the momentum of CR particle, and z is directed along the field. The core of the cloud expands as zdif α √DNL (p) t and decays in time as P α 2z-1 dif (t ). The diffusion coefficient DNL is strongly suppressed compared to its background ISM value DISM: DNL ~ DISM exp (-Π) ⪡ DISM for sufficiently high field-line-integrated CR partial pressure, Π. When Π ⪢ 1, the CRs drive Alfvén waves efficiently enough to build a transport barrier (P ≈ 2/ ∣z∣-"pedestal") that strongly reduces the leakage. The solution has a spectral break at p = pbr, where pbr satisfies the equation DNL (pbr) ≃z²/t. [ABSTRACT FROM AUTHOR]

Using infrared, radio, and γ-ray data, we investigate the propagation characteristics of cosmic-ray (CR) electrons and nuclei in the 30 Doradus (30 Dor) star-forming region in the Large Magellanic Cloud (LMC) using a phenomenological model based on the radio-far-infrared correlation within galaxies. Employing a correlation analysis, we derive an average propagation length of ~100-140 pc for ~3 GeV CR electrons resident in 30 Dor from consideration of the radio and infrared data. Assuming that the observed γ-ray emission toward 30 Dor is associated with the star-forming region, and applying the same methodology to the infrared and γ-ray data, we estimate a ~20 GeV propagation length of 200-320 pc for the CR nuclei. This is approximately twice as large as for ~3 GeV CR electrons, corresponding to a spatial diffusion coefficient that is ~4 times higher, scaling as (R/GV)δ with δ ≈ 0.7-0.8 depending on the smearing kernel used in the correlation analysis. This value is in agreement with the results found by extending the correlation analysis to include ~70 GeV CR nuclei traced by the 3-10 GeV γ-ray data (δ ≈ 0.66 ± 0.23). Using the mean age of the stellar populations in 30 Dor and the results from our correlation analysis, we estimate a diffusion coefficient DR ≈ (0.9-1.0) x 1027 (R/GV)0.7 cm² s-1. We compare the values of the CR electron propagation length and surface brightness for 30 Dor and the LMC as a whole with those of entire disk galaxies. We find that the trend of decreasing average CR propagation distance with increasing disk-averaged star formation activity holds for the LMC, and extends down to single star-forming regions, at least for the case of 30 Dor. [ABSTRACT FROM AUTHOR]

Significant advances have been made in the understanding of the diffuse Galactic hard X-ray continuum emission using data from the INTEGRAL observatory. The diffuse hard power-law component seen with the Spectrometer on INTEGRAL (SPI) has been identified with inverse-Compton emission from relativistic (GeV) electrons on the cosmic microwave background and Galactic interstellar radiation field. In the present analysis, SPI data from 2003 to 2009, with a total exposure time of ~108 s, are used to derive the Galactic ridge hard X-ray spatial distribution and spectrum between 20 keV and 2.4 MeV. Both are consistent with predictions from the GALPROP code. The good agreement between measured and predicted emission from keV to GeV energies suggests that the correct production mechanisms have been identified. We discuss the potential of the SPI data to provide an indirect probe of the interstellar cosmic-ray electron distribution, in particular for energies below a few GeV. [ABSTRACT FROM AUTHOR]

Local interstellar spectra (LIS) of secondary cosmic-ray (CR) nuclei, lithium, beryllium, boron, and partially secondary nitrogen, are derived in the rigidity range from 10 MV to ∼200 TV using the most recent experimental results combined with state-of-the-art models for CR propagation in the Galaxy and in the heliosphere. The lithium spectrum appears somewhat flatter at high energies compared to other secondary species, which may imply a primary lithium component. Two propagation packages, GALPROP and HelMod, are combined to provide a single framework that is run to reproduce direct measurements of CR species at different modulation levels, and at both polarities of the solar magnetic field. An iterative maximum-likelihood method is developed that uses GALPROP-predicted LIS as input to HelMod, which provides the modulated spectra for specific time periods of the selected experiments for the model-data comparison. The proposed LIS accommodates the low-energy interstellar spectra measured by Voyager 1, the High Energy Astrophysics Observatory-3 (HEAO-3), and the Cosmic Ray Isotope Spectrometer on board of the Advanced Composition Explorer (ACE/CRIS), as well as the high-energy observations by the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA), Alpha Magnetic Spectrometer-02 (AMS-02), and earlier experiments that are made deep in the heliosphere. The interstellar and heliospheric propagation parameters derived in this study are consistent with our earlier results for propagation of CR protons, helium, carbon, oxygen, antiprotons, and electrons. [ABSTRACT FROM AUTHOR]

Cosmic rays (CRs) in the Galaxy are an important dynamical component of the interstellar medium (ISM) that interact with the other major components (interstellar gas and magnetic and radiation fields) to produce broadband interstellar emissions that span the electromagnetic spectrum. The standard modeling of CR propagation and production of the associated emissions is based on a steady-state assumption, where the CR source spatial density is described using a smoothly varying function of position that does not evolve with time. While this is a convenient approximation, reality is otherwise, where primary CRs are produced in and about highly localized regions, e.g., supernova remnants, which have finite lifetimes. In this paper, we use the latest version of the GALPROP CR propagation code to model time-dependent CR injection and propagation through the ISM from a realistic 3D discretized CR source density distribution, together with full 3D models for the other major ISM components, and make predictions of the associated broadband nonthermal emissions. We compare the predictions for the discretized and equivalent steady-state model, finding that the former predicts novel features in the broadband nonthermal emissions that are absent for the steady-state case. Some of the features predicted by the discretized model may be observable in all-sky observations made by WMAP and Planck, the recently launched eROSITA, the Fermi-LAT, and ground-based observations by HESS, HAWC, and the forthcoming CTA. The nonthermal emissions predicted by the discretized model may also provide explanations of puzzling anomalies in high-energy γ-ray data, such as the Fermi-LAT north/south asymmetry and residuals like the so-called "Fermi bubbles." [ABSTRACT FROM AUTHOR]

The Andromeda galaxy is the closest spiral galaxy to us and has been the subject of numerous studies. It harbors a massive dark matter halo, which may span up to ∼600 kpc across and comprises ∼90% of the galaxy’s total mass. This halo size translates into a large diameter of 42° on the sky, for an M31–Milky Way (MW) distance of 785 kpc, but its presumably low surface brightness makes it challenging to detect with γ-ray telescopes. Using 7.6 yr of Fermi Large Area Telescope (Fermi–LAT) observations, we make a detailed study of the γ-ray emission between 1–100 GeV toward M31's outer halo, with a total field radius of 60° centered at M31, and perform an in-depth analysis of the systematic uncertainties related to the observations. We use the cosmic-ray propagation code GALPROP to construct specialized interstellar emission models to characterize the foreground γ-ray emission from the MW, including a self-consistent determination of the isotropic component. We find evidence for an extended excess that appears to be distinct from the conventional MW foreground, having a total radial extension upward of ∼120–200 kpc from the center of M31. We discuss plausible interpretations of the excess emission, but emphasize that uncertainties in the MW foreground—and in particular, modeling of the H i-related components—have not been fully explored and may impact the results. [ABSTRACT FROM AUTHOR]

The High Altitude Water Cherenkov (HAWC) telescope recently observed extended emission around the Geminga and PSR B0656+14 pulsar wind nebulae (PWNe). These observations have been used to estimate cosmic-ray (CR) diffusion coefficients near the PWNe that appear to be more than two orders of magnitude smaller than the diffusion coefficients typically derived for the interstellar medium from measured abundances of secondary species in CRs. Two-zone diffusion models have been proposed as a solution to this discrepancy, where the slower diffusion zone (SDZ) is confined to a small region around the PWN. Such models are shown to successfully reproduce the HAWC observations of the Geminga PWN while retaining consistency with other CR data. It is found that the size of the SDZ influences the predicted positron flux and the spectral shape of the extended γ-ray emission at lower energies that can be observed with the Fermi Large Area Telescope. If the two observed PWNe are not unique, then it is likely that there are similar pockets of slow diffusion around many CR sources elsewhere in the Milky Way. The consequences of such a picture for Galactic CR propagation is explored. [ABSTRACT FROM AUTHOR]

Local interstellar spectra (LIS) of primary cosmic ray (CR) nuclei, such as helium, oxygen, and mostly primary carbon are derived for the rigidity range from 10 MV to ∼200 TV using the most recent experimental results combined with the state-of-the-art models for CR propagation in the Galaxy and in the heliosphere. Two propagation packages, GALPROP and HelMod, are combined into a single framework that is used to reproduce direct measurements of CR species at different modulation levels, and at both polarities of the solar magnetic field. The developed iterative maximum-likelihood method uses GALPROP-predicted LIS as input to HelMod, which provides the modulated spectra for specific time periods of the selected experiments for model–data comparison. The interstellar and heliospheric propagation parameters derived in this study are consistent with our prior analyses using the same methodology for propagation of CR protons, helium, antiprotons, and electrons. The resulting LIS accommodate a variety of measurements made in the local interstellar space (Voyager 1) and deep inside the heliosphere at low (ACE/CRIS, HEAO-3) and high energies (PAMELA, AMS-02). [ABSTRACT FROM AUTHOR]

Direct measurements of cosmic ray (CR) species combined with observations of their associated γ-ray emissions can be used to constrain models of CR propagation, trace the structure of the Galaxy, and search for signatures of new physics. The spatial density distribution of interstellar gas is a vital element for all these studies. So far, models have employed the 2D cylindrically symmetric geometry, but their accuracy is well behind that of the available data. In this paper, 3D spatial density models for neutral and molecular hydrogen are constructed based on empirical model fitting to gas line-survey data. The developed density models incorporate spiral arms and account for the warping of the disk, and the increasing gas scale height with radial distance from the Galactic center. They are employed together with the GALPROP CR propagation code to investigate how the new 3D gas models affect calculations of CR propagation and high-energy γ-ray intensity maps. The calculations reveal non-trivial features that are directly related to the new gas models. The best-fit values for propagation model parameters employing 3D gas models are presented and they differ significantly from those derived with the 2D gas density models that have been widely used. The combination of 3D CR and gas density models provide a more realistic basis for the interpretation of non-thermal emissions from the Galaxy. [ABSTRACT FROM AUTHOR]

The local interstellar spectrum (LIS) of cosmic-ray (CR) electrons for the energy range 1 MeV to 1 TeV is derived using the most recent experimental results combined with the state-of-the-art models for CR propagation in the Galaxy and in the heliosphere. Two propagation packages, GALPROP and HelMod, are combined to provide a single framework that is run to reproduce direct measurements of CR species at different modulation levels, and at both polarities of the solar magnetic field. An iterative maximum-likelihood method is developed that uses GALPROP-predicted LIS as input to HelMod, which provides the modulated spectra for specific time periods of the selected experiments for model-data comparison. The optimized HelMod parameters are then used to adjust GALPROP parameters to predict a refined LIS with the procedure repeated subject to a convergence criterion. The parameter optimization uses an extensive data set of proton spectra from 1997 to 2015. The proposed CR electron LIS accommodates both the low-energy interstellar spectra measured by Voyager 1 as well as the high-energy observations by PAMELA and AMS-02 that are made deep in the heliosphere; it also accounts for Ulysses counting rate features measured out of the ecliptic plane. The interstellar and heliospheric propagation parameters derived in this study agree well with our earlier results for CR protons, helium nuclei, and anti-protons propagation and LIS obtained in the same framework. [ABSTRACT FROM AUTHOR]

High-energy γ-rays of interstellar origin are produced by the interaction of cosmic-ray (CR) particles with the diffuse gas and radiation fields in the Galaxy. The main features of this emission are well understood and are reproduced by existing CR propagation models employing 2D galactocentric cylindrically symmetrical geometry. However, the high-quality data from instruments like the Fermi Large Area Telescope reveal significant deviations from the model predictions on few to tens of degrees scales, indicating the need to include the details of the Galactic spiral structure and thus requiring 3D spatial modeling. In this paper, the high-energy interstellar emissions from the Galaxy are calculated using the new release of the GALPROP code employing 3D spatial models for the CR source and interstellar radiation field (ISRF) densities. Three models for the spatial distribution of CR sources are used that are differentiated by their relative proportion of input luminosity attributed to the smooth disk or spiral arms. Two ISRF models are developed based on stellar and dust spatial density distributions taken from the literature that reproduce local near- to far-infrared observations. The interstellar emission models that include arms and bulges for the CR source and ISRF densities provide plausible physical interpretations for features found in the residual maps from high-energy γ-ray data analysis. The 3D models for CR and ISRF densities provide a more realistic basis that can be used for the interpretation of the nonthermal interstellar emissions from the Galaxy. [ABSTRACT FROM AUTHOR]

Local interstellar spectra (LIS) for protons, helium, and antiprotons are built using the most recent experimental results combined with state-of-the-art models for propagation in the Galaxy and heliosphere. Two propagation packages, GALPROP and HelMod, are combined to provide a single framework that is run to reproduce direct measurements of cosmic-ray (CR) species at different modulation levels and at both polarities of the solar magnetic field. To do so in a self-consistent way, an iterative procedure was developed, where the GALPROP LIS output is fed into HelMod, providing modulated spectra for specific time periods of selected experiments to compare with the data; the HelMod parameter optimization is performed at this stage and looped back to adjust the LIS using the new GALPROP run. The parameters were tuned with the maximum likelihood procedure using an extensive data set of proton spectra from 1997 to 2015. The proposed LIS accommodate both the low-energy interstellar CR spectra measured by Voyager 1 and the high-energy observations by BESS, Pamela, AMS-01, and AMS-02 made from the balloons and near-Earth payloads; it also accounts for Ulysses counting rate features measured out of the ecliptic plane. The found solution is in a good agreement with proton, helium, and antiproton data by AMS-02, BESS, and PAMELA in the whole energy range. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present the results of the most complete scan of the parameter space for cosmic ray (CR) injection and propagation. We perform a Bayesian search of the main GALPROP parameters, using the MultiNest nested sampling algorithm, augmented by the BAMBI neural network machine-learning package. This is the first study to separate out low-mass isotopes (p, , and He) from the usual light elements (Be, B, C, N, and O). We find that the propagation parameters that best-fit , and He data are significantly different from those that fit light elements, including the B/C and 10Be/9Be secondary-to-primary ratios normally used to calibrate propagation parameters. This suggests that each set of species is probing a very different interstellar medium, and that the standard approach of calibrating propagation parameters using B/C can lead to incorrect results. We present posterior distributions and best-fit parameters for propagation of both sets of nuclei, as well as for the injection abundances of elements from H to Si. The input GALDEF files with these new parameters will be included in an upcoming public GALPROP update. [ABSTRACT FROM AUTHOR]

It is widely accepted that cosmic rays (CRs) up to at least PeV energies are Galactic in origin. Accelerated particles are injected into the interstellar medium where they propagate to the farthest reaches of the Milky Way, including a surrounding halo. The composition of CRs coming to the solar system can be measured directly and has been used to infer the details of CR propagation that are extrapolated to the whole Galaxy. In contrast, indirect methods, such as observations of γ-ray emission from CR interactions with interstellar gas, have been employed to directly probe the CR densities in distant locations throughout the Galactic plane. In this article we use 73 months of data from the Fermi Large Area Telescope in the energy range between 300 MeV and 10 GeV to search for γ-ray emission produced by CR interactions in several high- and intermediate-velocity clouds (IVCs) located at up to ∼7 kpc above the Galactic plane. We achieve the first detection of IVCs in γ rays and set upper limits on the emission from the remaining targets, thereby tracing the distribution of CR nuclei in the halo for the first time. We find that the γ-ray emissivity per H atom decreases with increasing distance from the plane at 97.5% confidence level. This corroborates the notion that CRs at the relevant energies originate in the Galactic disk. The emissivity of the upper intermediate-velocity Arch hints at a 50% decline of CR densities within 2 kpc from the plane. We compare our results to predictions of CR propagation models. [ABSTRACT FROM AUTHOR]

A dramatic increase in the accuracy and statistics of space-borne cosmic ray (CR) measurements has yielded several breakthroughs over the last several years. The most puzzling is the rise in the positron fraction above ∼10 GeV over the predictions of the propagation models assuming pure secondary production. The accuracy of the antiproton production cross section is critical for astrophysical applications and searches for new physics since antiprotons in CRs seem to hold the keys to many puzzles including the origin of those excess positrons. However, model calculations of antiproton production in CR interactions with interstellar gas are often employing parameterizations that are out of date or are using outdated physical concepts. This may lead to an incorrect interpretation of antiproton data which could have broad consequences for other areas of astrophysics. In this work, we calculate antiproton production in pp-, pA-, and AA-interactions using EPOS-LHC and QGSJET-II-04, two of the most advanced Monte Carlo (MC) generators tuned to numerous accelerator data including those from the Large Hadron Collider (LHC). We show that the antiproton yields obtained with these MC generators differ by up to an order of magnitude from yields of parameterizations commonly used in astrophysics. [ABSTRACT FROM AUTHOR]

The concept of the nuclear enhancement factor has been used since the beginning of γ-ray astronomy. It provides a simple and convenient way to account for the contribution of nuclei (A > 1) in cosmic rays (CRs) and in the interstellar medium (ISM) to the diffuse γ-ray emission. An accurate treatment of the dominant emission process, such as hadronic interactions of CRs with the ISM, enables one to study CR acceleration processes and CR propagation in the ISM, and provides a reliable background model for searches of new phenomena. The Fermi Large Area Telescope launched in 2008 provides excellent quality data in a wide energy range 30 MeV-1 TeV where the diffuse emission accounts for the majority of photons. Exploiting its data to the fullest requires a new study of the processes of γ-ray production in hadronic interactions. In this paper we point out that several commonly used studies of the nuclear enhancement factor fail to account for the spectrally averaged energy loss fraction which ensures that the energy fraction transferred to photons is averaged properly with the spectra of CR species. We present a new calculation of the spectrally averaged energy loss fraction and the nuclear enhancement factor using the QGSJET-II-04 and EPOS-LHC interaction models. [ABSTRACT FROM AUTHOR]

Recent accurate measurements of cosmic-ray (CR) protons and nuclei by ATIC-2, CREAM, and PAMELA reveal (1) unexpected spectral hardening in the spectra of CR species above a few hundred GeV per nucleon, (2) a harder spectrum of He compared to protons, and (3) softening of the CR spectra just below the break energy. These newly discovered features may offer a clue to the origin of the observed high-energy Galactic CRs. We discuss possible interpretations of these spectral features and make predictions for the secondary CR fluxes and secondary-to-primary ratios, anisotropy of CRs, and diffuse Galactic γ-ray emission in different phenomenological scenarios. Our predictions can be tested by currently running or near-future high-energy astrophysics experiments. [ABSTRACT FROM AUTHOR]