TY - JOUR A1 - Stroman, Thomas A1 - Pohl, Martin A1 - Niemiec, Jacek A1 - Bret, Antoine T1 - Could cosmic rays affect instabilities in the Transition layer of nonrealativistic collisionless shocks? JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - There is an observational correlation between astrophysical shocks and nonthermal particle distributions extending to high energies. As a first step toward investigating the possible feedback of these particles on the shock at the microscopic level, we perform particle-in-cell (PIC) simulations of a simplified environment consisting of uniform, interpenetrating plasmas, both with and without an additional population of cosmic rays. We vary the relative density of the counterstreaming plasmas, the strength of a homogeneous parallel magnetic field, and the energy density in cosmic rays. We compare the early development of the unstable spectrum for selected configurations without cosmic rays to the growth rates predicted from linear theory, for assurance that the system is well represented by the PIC technique. Within the parameter space explored, we do not detect an unambiguous signature of any cosmic-ray-induced effects on the microscopic instabilities that govern the formation of a shock. We demonstrate that an overly coarse distribution of energetic particles can artificially alter the statistical noise that produces the perturbative seeds of instabilities, and that such effects can be mitigated by increasing the density of computational particles. KW - cosmic rays KW - instabilities KW - plasmas KW - shock waves Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/746/1/24 SN - 0004-637X VL - 746 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Telezhinsky, Igor O. A1 - Dwarkadas, Vikram V. A1 - Pohl, Martin T1 - Time-dependent escape of cosmic rays from supernova remnants, and their interaction with dense media JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Supernova remnants (SNRs) are thought to be the main source of Galactic cosmic rays (CRs) up to the "knee" in CR spectrum. During the evolution of a SNR, the bulk of the CRs are confined inside the SNR shell. The highest-energy particles leave the system continuously, while the remaining adiabatically cooled particles are released when the SNR has expanded sufficiently and decelerated so that the magnetic field at the shock is no longer able to confine them. Particles escaping from the parent system may interact with nearby molecular clouds, producing.-rays in the process via pion decay. The soft gamma-ray spectra observed for a number of SNRs interacting with molecular clouds, however, challenge current theories of non-linear particle acceleration that predict harder spectra. Aims. We study how the spectrum of escaped particles depends on the time-dependent acceleration history in both Type Ia and core-collapse SNRs, as well as on different assumptions about the diffusion coefficient in the vicinity of the SNR. Methods. We solve the CR transport equation in a test-particle approach combined with numerical simulations of SNR evolution. Results. We extend our method for calculating the CR acceleration in SNRs to trace the escaped particles in a large volume around SNRs. We calculate the evolution of the spectra of CRs that have escaped from a SNR into a molecular cloud or dense shell for two diffusion models. We find a strong confinement of CRs in a close region around the SNR, and a strong dilution effect for CRs that were able to propagate out as far as a few SNR radii. KW - ISM: supernova remnants KW - ISM: clouds KW - cosmic rays Y1 - 2012 U6 - https://doi.org/10.1051/0004-6361/201118639 SN - 0004-6361 VL - 541 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Arlen, T. A1 - Aune, T. A1 - Beilicke, M. A1 - Benbow, W. A1 - Bouvier, A. A1 - Buckley, J. H. A1 - Bugaev, V. A1 - Byrum, K. A1 - Cannon, A. A1 - Cesarini, A. A1 - Ciupik, L. A1 - Collins-Hughes, E. A1 - Connolly, M. P. A1 - Cui, W. A1 - Dickherber, R. A1 - Dumm, J. A1 - Falcone, A. A1 - Federici, S. A1 - Feng, Q. A1 - Finley, J. P. A1 - Finnegan, G. A1 - Fortson, L. A1 - Furniss, A. A1 - Galante, N. A1 - Gall, D. A1 - Godambe, S. A1 - Griffin, S. A1 - Grube, J. A1 - Gyuk, G. A1 - Holder, J. A1 - Huan, H. A1 - Hughes, G. A1 - Humensky, T. B. A1 - Imran, A. A1 - Kaaret, P. A1 - Karlsson, N. A1 - Kertzman, M. A1 - Khassen, Y. A1 - Kieda, D. A1 - Krawczynski, H. A1 - Krennrich, F. A1 - Lee, K. A1 - Madhavan, A. S. A1 - Maier, G. A1 - Majumdar, P. A1 - McArthur, S. A1 - McCann, A. A1 - Moriarty, P. A1 - Mukherjee, R. A1 - Nelson, T. A1 - de Bhroithe, A. O'Faolain A1 - Ong, R. A. A1 - Orr, M. A1 - Otte, A. N. A1 - Park, N. A1 - Perkins, J. S. A1 - Pohl, Martin A1 - Prokoph, H. A1 - Quinn, J. A1 - Ragan, K. A1 - Reyes, L. C. A1 - Reynolds, P. T. A1 - Roache, E. A1 - Ruppel, J. A1 - Saxon, D. B. A1 - Schroedter, M. A1 - Sembroski, G. H. A1 - Skole, C. A1 - Smith, A. W. A1 - Telezhinsky, Igor O. A1 - Tesic, G. A1 - Theiling, M. A1 - Thibadeau, S. A1 - Tsurusaki, K. A1 - Varlotta, A. A1 - Vivier, M. A1 - Wakely, S. P. A1 - Ward, J. E. A1 - Weinstein, A. A1 - Welsing, R. A1 - Williams, D. A. A1 - Zitzer, B. A1 - Pfrommer, C. A1 - Pinzke, A. T1 - Constraints on cosmic rays, magnetic fields, and dark matter fromgamma-ray observations of the coma cluster of galaxies with veritas and fermi JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Observations of radio halos and relics in galaxy clusters indicate efficient electron acceleration. Protons should likewise be accelerated and, on account of weak energy losses, can accumulate, suggesting that clusters may also be sources of very high energy (VHE; E > 100 GeV) gamma-ray emission. We report here on VHE gamma-ray observations of the Coma galaxy cluster with the VERITAS array of imaging Cerenkov telescopes, with complementing Fermi Large Area Telescope observations at GeV energies. No significant gamma-ray emission from the Coma Cluster was detected. Integral flux upper limits at the 99% confidence level were measured to be on the order of (2-5) x 10(-8) photonsm(-2) s(-1) (VERITAS, >220 GeV) and similar to 2 x 10(-6) photonsm(-2) s(-1) (Fermi, 1-3GeV), respectively. We use the gamma-ray upper limits to constrain cosmic rays (CRs) and magnetic fields in Coma. Using an analytical approach, the CR-to-thermal pressure ratio is constrained to be < 16% from VERITAS data and <1.7% from Fermi data (averaged within the virial radius). These upper limits are starting to constrain the CR physics in self-consistent cosmological cluster simulations and cap the maximum CR acceleration efficiency at structure formation shocks to be <50%. Alternatively, this may argue for non-negligible CR transport processes such as CR streaming and diffusion into the outer cluster regions. Assuming that the radio-emitting electrons of the Coma halo result from hadronic CR interactions, the observations imply a lower limit on the central magnetic field in Coma of similar to(2-5.5) mu G, depending on the radial magnetic field profile and on the gamma-ray spectral index. Since these values are below those inferred by Faraday rotation measurements in Coma (for most of the parameter space), this renders the hadronic model a very plausible explanation of the Coma radio halo. Finally, since galaxy clusters are dark matter (DM) dominated, the VERITAS upper limits have been used to place constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the DM particles, . KW - cosmic rays KW - dark matter KW - galaxies: clusters: general KW - galaxies: clusters: individual (Coma (ACO 1656)) KW - gamma rays: galaxies: clusters KW - magnetic fields Y1 - 2012 U6 - https://doi.org/10.1088/0004-637X/757/2/123 SN - 0004-637X VL - 757 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER -