Articles written in Journal of Astrophysics and Astronomy

    • The large-scale distribution of quasars identified with strong radio sources at 5 GHz

      P. Shastri Gopal-Krishna

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      The large-scale angular distribution of quasars from a complete sample of extragalactic radio sources is examined at different redshifts. The sample contains 264 quasars which have been found so far among the complete sample of 518 radio sources stronger than. 1 Jy at 5 GHz. Of these, 19 quasars have redshift z > 2. Dividing the entire sky into three separate declination zones of equal area, the counts of quasars seem to indicate a deficit of high redshift quasars in the northernmost declination zone. On the other hand, the Iow-redshift quasars (z < 1) appear fairly uniformly distributed. We discuss some possible selection effects that might have led to the apparent anomaly at high redshifts and estimate the expected number of high-redshift quasars amongst the radio sources in the sample for which redshifts are presently not available.

    • Extragalactic sources with asymmetric radio structure - I. Observations of 17 sources

      D. J. Saikia P. Shastri R. P. Sinha V. K. Kapahi G. Swarup

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      We present total-intensity and linear-polarization observations with the Very Large Array (VLA) at λ6 and 2 cm of 17 sources, almost all of which were suspected to have extended emission only on one side of the nucleus. Five of them are still one-sided, three appear unresolved, while seven have radio lobes on both sides of the nucleus. The outer components in the double-lobed sources, however, have significantly different surface brightness or are very asymmetrically located with respect to the nucleus.

    • Extragalactic sources with asymmetric radio structure II. further observations of the quasar B2 1320 + 299

      T. J. Cornwell D. J. Saikia P. Shastri L. Feretti G. Giovannini P. Parma C. J. Salter

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      We present VLA A-array observations at λ20, 6 and 2 cm and B-array observations at λ20 and 6 cm of the quasar B2 1320 + 299, which has a very unusual radio structure. In addition to a component, A, coincident with the quasar, there are two lobes of radio emission, B and C, on the same side of A. These are located at distances of -25 and 50 arcsec respectively from A. The present observations show that A has a flat-spectrum component coincident with the quasar and a weak outer component at a distance of-4 arcsec along PA - 100°. The morphology of B resembles a head-tail type of structure with its tail towards the north-east. The magnetic field lines in component B appear to follow the bend in the tail. Component C exhibits some extension towards the north-west. We discuss the possible nature of B2 1320 + 299 and suggest that while A appears to be an independent source, the relation between B and C, if they are associated at all, is unclear. Deep optical observations are essential to help clarify the situation.

    • Extragalactic sources with very asymmetric radio structure: VLA and MERLIN observations

      D. J. Saikia P. Shastri T. J. Cornwell W. Junor T. W. B. Muxlow

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      As part of our study to understand the nature of extragalactic radio sources which are very asymmetric in the surface brightness of the two lobes, often with radio emission on only one side of the nucleus, we have observed a large number of them with high angular resolution and good surface brightness sensitivity at radio frequencies. In this paper we present VLA and MERLIN observations of 15 such sources. We discuss their observed structures and spectra, and possible explanations for their morphologies. We report evidence of a possible correlation between the hot-spot brightness ratio and the degree of core prominence, used as a Statistical measure of source orientation, suggesting that relativistic beaming of the hot-spot emission does play a significant role in the observed brightness asymmetry. To explain the apparently one-sided sources within the relativistic beaming framework, the velocities required are in the range of 0.2 to 0.8c. We discuss the possibility that the lobe which is seen to the south of the jet in 3C273 is the counter-lobe seen in projection. We also draw-attention to a number of one-sided sources with very weak cores, and discuss their possible nature.

    • From Nearby Low Luminosity AGN to High Redshift Radio Galaxies: Science Interests with Square Kilometre Array

      P. Kharb D. V. Lal V. Singh J. Bagchi C. H. Ishwara Chandra A. Hota C. Konar Y. Wadadekar P. Shastri M. Das K. Baliyan B. B. Nath M. Pandey-Pommier

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      We present detailed science cases that a large fraction of the Indian AGN community is interested in pursuing with the upcoming Square Kilometre Array (SKA). These interests range from understanding low luminosity active galactic nuclei in the nearby Universe to powerful radio galaxies at high redshifts. Important unresolved science questions in AGN physics are discussed. Ongoing low-frequency surveys with the SKA pathfinder telescope GMRT, are highlighted.

    • A polarization study of three blazars using uGMRT at $\sim$600 MHz


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      We present results from our radio polarimetric study with the upgraded giant metrewave radio telescope (uGMRT) at band 4 (550–850 MHz) of three blazars: radio-loud quasars 3C390.3, 4C71.07 and BL Lac object 1ES 2344$+$514. The aim of this study was (i) to carry out a feasibility study for band 4 polarization with the uGMRT and (ii) to compare and contrast the kpc-scale polarization properties between the blazarsub-classes. We have detected linear polarization in all the three sources. The degree of linear polarization in the cores of the two quasars is higher than the BL Lac object, consistent with similar differences observed on parsec-scales in blazars. The highest fractional polarization of $\approx$15% is observed in the hotspot region of3C390.3, which also shows extended polarized lobe structures. 1ES 2344$+$514 shows a core–halo structure, whereas 4C71.07 remains unresolved. A rotation of polarization electric vectors along the northern hotspot of 3C390.3 and the core of 1ES 2344$+$514, suggest jet bending. Greater depolarization in the southern lobe of 3C390.3 compared to the northern lobe indicates the presence of Laing–Garrington effect. Multi-frequency uGMRT polarimetric data are underway to study the kpc-scale rotation measures across these sources to find differences in the surrounding media.

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

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