Browsing by Author "Saikia, D. J."

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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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    Outflowing material in the CSS quasar 3C48: evidence of jet-cloud interaction?
    (2005-05-01) Gupta, Neeraj; Srianand, R.; Saikia, D. J.
    We report the detection of a zabs=0.3654 associated absorption-line system in the UV spectrum of the CSS quasar 3C48. The absorbing material is blue shifted with respect to the quasar emission-line redshift, zem =0.3700, suggesting an outflow velocity of ∼1000 km s−1. We detect absorption lines over a range of ionization states from Lyβ, Lyγ, C iv, N iv, S vi to O vi and possibly O iv and Ne viii. The kinematical properties of the absorption-line system are similar to the blue-shifted emission line gas seen in [Oiii] λ5007 (Chatzichristou, Vanderriest & Jaffe 1999), which is believed to have interacted with the radio jet. We study the properties of the absorbing material using CLOUDY and find that photoionization models with Solar abundance ratios (with overall metallicity in the range 0.16Z/Z⊙61.3) are enough to explain the observed column densities of all the species except Ne viii , detection of which requires confirmation. Since the cooling and recombination time for the gas is ∼105 yr, the consistency with the photoionization models suggests that any possible interaction of absorbing material with the jet must have taken place before ∼105 yr. The abundance ratio of nitrogen to carbon is close to Solar values, unlike in the case of most quasars, especially at high-redshifts, which have super-Solar values.We observed 3C48 with the Giant Metrewave Radio Telescope (GMRT) to search for redshifted 21cm H i absorption. However, we did not detect any significant feature in our spectra and estimate the 3σ upper limit to the optical depth to be in the range 0.001 to 0.003. However, due to the diffuse nature of the radio source, optical depths as high as 0.1 towards individual knots or compact components cannot be ruled out.