IUCAA Preprints

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    Discovery of giant 'radio arcs' in cluster Abell 3376 : evidence for shock accleration in a violent cluster merger?
    (2002-10-24) Bagchi, Joydeep
    New multi-wavelength (radio, optical & X-rays) observational evidences are presented which show that the nearby (z = 0.046), rich cluster of galaxies Abell 3376 is experiencing a major event of binary subcluster merger. The key evidence is the discovery of a pair of large, optically unidentified diffuse radio sources (‘arcs’), symmetrically located about 2.6 h−1 50 Mpc apart at the opposite ends of the hot intra-cluster gas mapped by ROSAT in X-rays. It is argued that the gas-dynamical shock-waves, which occur naturally during cluster formation, are accelerating charged particles (cosmic rays) to relativis- tic energies, leading to synchrotron emission from the megaparsec scale radio arcs. If this is so, cluster Abell 3376 would also be a potential source capa- ble of accelerating cosmic ray particles upto ultra-high energies (UHECR) of Emax ∼ 1018−19 eV. Thus this cluster is an excellent test-bed for understand- ing the physics of merger shocks and origin of enigmatic UHECR particles in structure formation process. Hence, Abell 3376 provides unique oppurtu- nities for further multi-wavelength observations with ground and space-borne observatories
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    Multifrequency study of giant radio sources I. Low-frequency Gaint Metrewave Radio Telescope observations of selected sources
    (2007-09-27) Konar, C.; Jamrozy, M.; Saikia, D. J.; et al.
    We present low-frequency observations with the Giant Metrewave Radio Telescope (GMRT) of a sample of giant radio sources (GRSs), and high-frequency observations of three of these sources with the Very Large Array (VLA). From multifrequency observations of the lobes we estimate the magnetic field strengths using three different approaches, and show that these differ at most by a factor of ∼3. For these large radio sources the inverse-Compton losses usually dominate over synchrotron losses when estimates of the classical minimum energy magnetic field are used, consistent with earlier studies. However, this is often not true if the magnetic fields are close to the values estimated using the formalism of Beck & Krause. We also examine the spectral indices of the cores and any evidence of recurrent activity in these sources. We probe the environment using the symmetry parameters of these sources and suggest that their environments are often asymmetric on scales of ∼1 Mpc, consistent with earlier studies.
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    Multifrequency study of giant radio sources II. Spectral ageing analysis of the lobes of selected sources
    (2007-12-02) Jamrozy, M.; Konar, C.; Machalski, J.; et al.
    Multifrequency observations with the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA) are used to determine the spectral breaks in consecutive strips along the lobes of a sample of selected giant radio sources (GRSs) in order to estimate their spectral ages. The maximum spectral ages estimated for the detected radio emission in the lobes of our sources range from ∼6 to 36 Myr with a median value of ∼20 Myr using the classical equipartition fields. Using the magnetic field estimates from the Beck & Krause formalism the spectral ages range from ∼5 to 38 Myr with a median value of ∼22 Myr. These ages are significantly older than smaller sources. In all but one source (J1313+6937) the spectral age gradually increases with distance from the hotspot regions, confirming that acceleration of the particles mainly occurs in the hotspots. Most of the GRSs do not exhibit zero spectral ages in the hotspots, as is the case in earlier studies of smaller sources. This is likely to be largely due to contamination by more extended emission due to relatively modest resolutions. The injection spectral indices range from ∼0.55 to 0.88 with a median value of ∼0.6. We discuss these values in the light of theoretical expectations, and show that the injection spectral index appears to be correlated with luminosity and/or redshift as well as with linear size.