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  • 1. Boschini, M. J.
    et al.
    Della Torre, S.
    Gervasi, M.
    Grandi, D.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Kachelriess, M.
    La Vacca, G.
    Masi, N.
    Moskalenko, I. V.
    Orlando, E.
    Ostapchenko, S. S.
    Pensotti, S.
    Porter, T. A.
    Quadrani, L.
    Rancoita, P. G.
    Rozza, D.
    Tacconi, M.
    Solution of Heliospheric Propagation: Unveiling the Local Interstellar Spectra of Cosmic-ray Species2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 840, no 2, article id 115Article in journal (Refereed)
    Abstract [en]

    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.

  • 2. Boschini, M. J.
    et al.
    Della Torre, S.
    Gervasi, M.
    Grandi, D.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    La Vacca, G.
    Masi, N.
    Moskalenko, I. V.
    Pensotti, S.
    Porter, T. A.
    Quadrani, L.
    Rancoita, P. G.
    Rozza, D.
    Tacconi, M.
    Deciphering the Local Interstellar Spectra of Primary Cosmic-Ray Species with HELMOD2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 858, no 1, article id 61Article in journal (Refereed)
    Abstract [en]

    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 similar 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).

  • 3. Boschini, M. J.
    et al.
    Della Torre, S.
    Gervasi, M.
    Grandi, D.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    La Vacca, G.
    Masi, N.
    Moskalenko, I. V.
    Pensotti, S.
    Porter, T. A.
    Quadrani, L.
    Rancoita, P. G.
    Rozza, D.
    Tacconi, M.
    HelMod in the Works: From Direct Observations to the Local Interstellar Spectrum of Cosmic-Ray Electrons2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 854, no 2, article id 94Article in journal (Refereed)
    Abstract [en]

    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.

  • 4.
    Jóhannesson, Guðlaugur
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Porter, Troy A.
    Moskalenko, Igor V.
    The Three-dimensional Spatial Distribution of Interstellar Gas in the Milky Way: Implications for Cosmic Rays and High-energy Gamma-ray Emissions2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 856, no 1, article id 45Article in journal (Refereed)
    Abstract [en]

    Direct measurements of cosmic ray (CR) species combined with observations of their associated gamma-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 gamma-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.

  • 5.
    Jóhannesson, Guđlaugur
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Björnsson, Gunnlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Low-energy Electrons in Gamma-Ray Burst Afterglow Models2018In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 859, no 1, article id LllArticle in journal (Refereed)
    Abstract [en]

    Observations of gamma-ray burst (GRB) afterglows have long provided the most detailed information about the origin of this spectacular phenomenon. The model that is most commonly used to extract physical properties of the event from the observations is the relativistic fireball model, where ejected material moving at relativistic speeds creates a shock wave when it interacts with the surrounding medium. Electrons are accelerated in the shock wave, generating the observed synchrotron emission through interactions with the magnetic field in the downstream medium. It is usually assumed that the accelerated electrons follow a simple power-law distribution in energy between specific energy boundaries, and that no electron exists outside these boundaries. This Letter explores the consequences of adding a low-energy power-law segment to the electron distribution with energy that contributes insignificantly to the total energy budget of the distribution. The low-energy electrons have a significant impact on the radio emission, providing synchrotron absorption and emission at these long wavelengths. Shorter wavelengths are affected through the normalization of the distribution. The new model is used to analyze the light curves of GRB 990510, and the resulting parameters are compared to a model without the extra electrons. The quality of the fit and the best-fit parameters are significantly affected by the additional model component. The new component is in one case found to strongly affect the X-ray light curves, showing how changes to the model at radio frequencies can affect light curves at other frequencies through changes in best-fit model parameters.

  • 6. Porter, T. A.
    et al.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Moskalenko, I. V.
    High-energy Gamma Rays from the Milky Way: Three-dimensional Spatial Models for the Cosmic-Ray and Radiation Field Densities in the Interstellar Medium2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 846, no 1, article id 67Article in journal (Refereed)
    Abstract [en]

    High-energy gamma-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 gamma-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.

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