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For refcode 2002NewA....7..249B:
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2002NewA....7..249B Evidence for shock acceleration and intergalactic magnetic fields in a large-scale filament of galaxies ZwCl 2341.1+0000 Joydeep Bagchi, Torsten A. Ensslin, Francesco Miniati, C.S. Stalin, M. Singh, Somak Raychaudhury and N.B. Humeshkar Inter-University Centre for Astronomy and Astrophysics (IUCAA), Post Bag 4, Ganeshkhind, Pune 411007, India Max-Planck-Institut fur Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching, Germany State Observatory, Manora Peak, Naini Tal 263129, India School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Department of Physics, Loyola College, University of Madras, Chennai 600034, India Received 8 November 2001; received in revised form 11 March 2002; accepted 4 April 2002 Abstract We report the discovery of large-scale diffuse radio emission from what appears to be a large-scale filamentary network of galaxies in the region of cluster ZwCl 2341.1+0000, and stretching over an area of at least 6 h_50_^-1^ Mpc in diameter. Multicolour CCD observations yield photometric redshifts indicating that a significant fraction of the optical galaxies in this region is at a redshift of z = 0.3. This is supported by spectroscopic measurements of 4 galaxies in the Sloan Digitized Sky Survey (SDSS) survey at a mean z = 0.27. We present VLA images at {lambda} = 20 cm (NVSS) and 90 cm, showing the detailed radio structure of the filaments. Comparison with the high resolution FIRST radio survey shows that the diffuse emission is not due to known individual point sources. The diffuse radio-emission has a spectral index {alpha} <~ -0.5, and is most likely synchrotron emission from relativistic charged particles in an inter-galactic magnetic field. Furthermore, this optical/radio structure is detected in X-rays by the ROSAT all-sky survey. It has a 0.1-2.4 keV luminosity of about 10^44^ erg s^-1^ and shows an extended highly non-relaxed morphology. These observations suggest that ZwCl 2341.1+0000 is possibly a proto-cluster of galaxies in which we are witnessing the process of structure formation. We show that the energetics of accretion shocks generated in forming large-scale structures are sufficient to produce enough high energy cosmic-ray (CR) electrons required to explain the observed radio emission, provided a magnetic field of strength B >~ 0.3 microG is present there. The latter is only a lower limit and the actual magnetic field is likely to be higher depending on the morphology of the emitting region. Finally, we show results from a numerical simulation of large-scale structure formation including acceleration of CR electrons at cosmological shocks and magnetic field evolution. Our results are in accord with the observed radio synchrotron and X-ray thermal bremsstrahlung fluxes. Thus we conclude that the reported radio detection is the first evidence of cosmic-ray particle acceleration taking place at cosmic shocks in a magnetized inter-galactic medium over scales of >~ 5 h_50_^-1^ Mpc. Keywords: Acceleration of particles; Cosmic rays; Cosmology: observations; Galaxies: clusters: general; Large-scale structure of universe; Magnetic fields; Methods: numerical; Radio continuum: general; Shock waves
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