• D. J. Saikia

      Articles written in Journal of Astrophysics and Astronomy

    • Compact radio cores and the relation between the radio and optical axes of elliptical galaxies

      V. K. Kapahi D. J. Saikia

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      We have reinvestigated the reported tendency for the extended radio structures associated with bright elliptical galaxies to be oriented preferentially along the optical minor axes. It is found that such a tendency exists only for those galaxies in which the compact radio cores coincident with their nuclei are quite prominent. If the galaxies are divided into two groups according to whether their cores account for less than or greater than 10 per cent of the total flux density at 2.7 GHz, the angle Φ (between the radio axis and the optical minor axis) appears to be uniformly distributed between 0‡ and 90‡ for the former, but is nearly always < 30‡ for the latter group. One possible explanation is that the radio emission from compact cores suffers thermal absorption by ionized gas that is distributed differently in the two groups.

    • Relativistic beaming in the central components of double radio quasars

      V. K. Kapahi D. J. Saikia

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      Using a large sample of 78 well-observed double quasars, we have investigated several consequences of the relativistic beaming model. In this model the ratio of the strengths of the central component and outer lobes of a double source depends on whether the jet axis lies close to or away from the line of sight, If this is the actual situation, the fraction of emission from the core,fc, may be used as a statistical measure of the orientation of the source and should be correlated with other source parameters which also depend on the inclination of the jet axis to the line of sight.

      We findfc to be anticorrelated with the overall projected linear size of the extended emission but to exhibit a positive correlation with both the observed degree of misalignment from a collinear double structure, and the ratio of separations of the outer hotspots from the central component. As might be expected from these relationships, we also find sources of smaller projected linear sizes to appear more misaligned and the degree of misalignment to be correlated with the ratio of separations of the outer hotspots. All these correlations are consistent with the predictions of the relativistic beaming model.

    • 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.

    • Radio observations of the BL Lae object 1400+ 162

      D. J. Saikia G. Swarup R. P. Sinha

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      We present total-intensity and linear-polarization observations made with the VLA at 5 GHz of 1400+ 162, a BL Lac object in a group of galaxies. It has a misaligned triple structure with a prominent radio jet towards the east. There is evidence of a weak counter-jet towards the western component, which also has the more prominent warm-spot.

      We discuss possible explanations for some of the observed features of this source. Although interaction with the cluster medium is possibly partly responsible for the observed distortion, we suggest that the large observed misalignment could also be due to amplification of a smaller misalignment by projection effects. In the relativistic beaming model, where BL Lac objects arise when the relativistic jets are seen end-on, we suggest that 1400+ 162 is more oblique to the line of sight than most members of this class.

    • 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.

    • A Multifrequency Study of Five Large Radio Galaxies

      A. Pirya S. Nandi D. J. Saikia C. Konar M. Singh

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      We present the results of GMRT and VLA observations of five large radio sources over a wide frequency range to investigate their structural and spectral asymmetries. The hot-spot brightness ratios suggest intrinsic source asymmetries, while the spectral indices show evidence of re-acceleration of particles.

    • The Dynamics of Radio Galaxies and Double–Double Radio Galaxies

      C. Konar M. Jamrozy M. J. Hardcastle J. H. Croston S. Nandi D. J. Saikia J. Machalski

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      Relativistic and magnetized plasma ejected by radio loud AGNs through jets form the diffuse lobes of radio galaxies. The radiating particles (electron/electron–positron) in lobes emit in radio via the synchrotron process and X-ray via inverse-Compton scattering of cosmic microwave background photons. The thermal environment around radio galaxies emits X-rays via the thermal bremsstrahlung process. By combining information from these processes we can measure physical conditions in and around the radio lobes and thus study the dynamics of radio galaxies, including double–double radio galaxies.

    • The Double–Double Radio Galaxy 3C293

      S. A. Joshi S. Nandi D. J. Saikia C. H. Ishwara-Chandra C. Konar

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      We present the results of radio continuum observations at frequencies ranging from ∼ 150–5000 MHz of the misaligned double–double radio galaxy (DDRG) 3C293 (J1352+3126) using the GMRT and the VLA, and estimate the time-scale of interruption of jet activity to be less than ∼ 0.1 Myr.

    • Interaction of the WAT Source in A3395 with the Intracluster Medium

      Kiran Lakhchaura K. P. Singh D. J. Saikia R. W. Hunstead

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      Using X-ray observations from Chandra and XMM-Newton and radio observations from the Australia Telescope Compact Array (ATCA), we have examined the merging environment of the bimodal cluster Abell 3395. From X-ray data we have produced thermodynamic maps of the cluster. The Wide Angle Tail (WAT) galaxy seen in the radio is slightly offset from the X-ray emission peak of the southern part of the cluster. The unsharp masked Chandra image of the cluster does not show any deficit in the X-ray flux near the location of the source possibly because the thermal plasma has leaked into the cavities.

    • ATLAS, and Wide-Angle Tail Galaxies in ATLAS

      Minnie Y. Mao Rob Sharp D. J. Saikia Ray P. Norris Melanie Johnston-Hollitt Enno Middelberg Jim E. J. Lovell

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      Using the Australia Telescope Compact Array (ATCA), ATLAS (Australia Telescope Large Area Survey) is imaging two fields totalling 7 square degrees down to 10 𝜇Jy beam-1 at 1.4 GHz. We have found 6 wide-angle tail galaxies (WATs), 4 of which have sufficient data to identify associated galaxy overdensities. The largest WAT, at a red-shift of 0.22, appears to be associated with an overdensity of galaxies that is spread over an unusually large extent of 12Mpc, with a velocity range of 4500 km s-1. Here we present the WATs in ATLAS and discuss the implications of these observations for future large-scale radio surveys such as ASKAP-EMU.

    • Jets in radio galaxies and quasars: an observational perspective

      D. J. SAIKIA

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      This article gives a brief historical introduction and reviews our current understanding of jets in radio galaxies and quasars from an observational perspective, with an emphasis on observations at radio wavelengths. Recent results on the Fanaroff–Riley (FR) classification scheme, and the nature of radio structures and jets in the FR classes as well as in high-excitation and low-excitation radio galaxies are summarized. The collimation and propagation of jets from nuclear sub-pc to hundreds of kpc scales from both observational and theoretical works have been discussed. The jets exhibit evidence of interaction with a clumpy interstellar medium, especially inyoung radio sources, and could trigger both star formation as well as suppress star formation depending on the physical conditions. Observational evidence for such interactions and jet feedback which have profound implications in our understanding of galaxy evolution have been presented. Recurrent jet activity which has been seen over a wide range of projected linear size and time scales has been discussed. This review article concludes with a brief discussion of unresolved questions on jets which new telescopes should help address.

    • Decoding the giant extragalactic radio sources


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      Giant radio sources (GRSs) defined to be >0.7 Mpc are the largest single objects in the Universe and can be associated with both galaxies (GRGs) and quasars (GRQs). They are important for understanding the evolution of radio galaxies and quasars whose sizes range from pc to Mpc scales and are also valuable probes of their environment. These radio-loud active galactic nuclei (RLAGN) interact with the interstellar medium of the host galaxy on small scales and the large-scale intracluster or intergalactic medium for the GRSs. Withseveral new and sensitive surveys over the last few years, the number of known GRSs has increased many fold, which has led a resurgence of interest in the field. This review article summarises our current understanding ofthese sources based on nearly five decades of research, and discusses the importance of the Square Kilometer Array (SKA) in addressing some of the outstanding questions.

  • 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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