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Browsing by Author "Gopal-Krishna"

Browsing by Author "Gopal-Krishna"

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  • The changing interstellar medium of massive elliptical galaxies and cosmic evolution of radio galaxies and quasars 
    Mangalam, A; Gopal-Krishna; Wiita, Paul J (Blackwell Publishing, 2009-06-03)
    The recently discovered apparent dramatic expansion in the effective radii of massive elliptical galaxies from z≃ 2 to ≃0.1 has been interpreted in terms of either galaxy mergers or the rapid loss of cold gas due to ...
  • A comparative study of the intra-night optical variability of powerful AGNs 
    Stalin, C. S; Gopal-Krishna; Sagar, R; Wiita, P. J (Astronomical Society of India, 2002-09)
    In order to address a currently much debated issue, we are carrying out a systematic search for intra-night optical micro-variability in major classes of powerful (M'II ~ -23.0 mag) active galactic nuclei (AGNs). Our ...
  • Expansion of radio galaxies in a cosmologically evolving medium: possible implications for the cosmic star-formation history 
    Barai, P; Gopal-Krishna; Osterman, M. A; Wiita, P. J (Astronomical Society of India, 2004-12)
    We compare earlier estimates of the volumes filed by lobes of radio galaxies during the quasar era based upon non-evolving ambient media with new ones assuming a strong cosmological evolution of the ambient medium. If the ...
  • Further evidence for intra-night optical variability of radio-quiet quasars 
    Goyal, Arti; Gopal-Krishna; Sagar, R; Anupama, G. C; Sahu, D. K (Astronomical Society of India, 2007-06)
    Although well established for BL Lac objects and radio-loud quasars, the occurrence of intra-night optical variability (INOV) in radio-quiet quasars is still debated, primarily since only a handful of INOV events with good ...
  • Improved characterization of intranight optical variability of prominent AGN classes 
    Goyal, A; Gopal-Krishna; Wiita, P. J; Stalin, C. S; Sagar, R (Oxford University Press on behalf of the Royal Astronomical Society, 2013-10-21)
    The incidence of intranight optical variability (INOV) is known to differ significantly among different classes of powerful active galactic nuclei (AGN). A number of statistical methods have been employed in the literature ...
  • Intra-night optical variability of core dominated radio quasars: the role of optical polarization 
    Goyal, A; Gopal-Krishna; Wiita, P. J; Anupama, G. C; Sahu, D. K; Sagar, R; Joshi, S (EDP Sciences, 2012-08)
    Context. Rapid variations in optical flux are seen in many quasars and all blazars. The amount of variability in different classes of active galactic nuclei has been studied extensively but many questions remain unanswered. ...
  • Intranight optical monitoring of optically selected bright quasars 
    Sagar, R; Gopal-Krishna; Wiita, P. J (Royal Astronomical Society, 1996-08)
    As part of an ongoing programme to search for intranight optical variations, we report results for six radio-quiet and one radio-moderate, but all optically bright and luminous, quasi-stellar objects (QSOs), using the ...
  • Intranight optical variability in optically selected QSOs 
    Gopal-Krishna; Sagar, R; Wiita, J. Paul (Blackwell Publishing, 1995-06)
    We report results from a continuing programme to search for intranight optical variability in one radio-moderate and six radio-quiet (but all optically bright and luminous) quasi-stellar objects, using the 2.34-m Vainu ...
  • Intranight optical variability of blazars and radio-quiet quasars using the ZTF survey 
    Negi, Vibhore; Gopal-Krishna; Joshi, Ravi; Chand, Hum; Wiita, Paul J; Navaneeth, P. K; Singh, Ravi S (Oxford University Press on behalf of Royal Astronomical Society, 2023-07)
    We explore the potential of the ongoing Zwicky Transient Facility (ZTF) survey for studying intranight optical variability (INOV) of active galactic nuclei (AGNs), in particular for picking rare events of large INOV ...
  • Multi-colour optical monitoring of the intra-day variable blazar S5 0716+71 
    Sagar, R; Gopal-Krishna; Mohan, V; Pandey, A. K; Bhatt, B. C; Wagner, S. J (The European Southern Observatory, 1999-02)
    We report multi-colour optical monitoring of the intra-day variable blazar S5 0716+71 with a dense temporal coverage over a total duration of 4 weeks in February/March 1994. Four filters (BV Johnson and RI Cousins) were ...
  • Multi-epoch intranight optical monitoring of eight radio-quiet BL Lac candidates 
    Kumar, P; Gopal-Krishna; Stalin, C. S; Chand, H; Srianand, R; Petitjean, P (Oxford University Press on behalf of the Royal Astronomical Society, 2017-10)
    For a new sample of eight weak-line quasars (WLQs) we report a sensitive search in 20 intranight monitoring sessions, for blazar-like optical flux variations on hour-like and longer time-scale (day/month/year-like). The ...
  • Near-infrared and optical imaging of Q2345+007: The largest gravitationally lensed QSO system? 
    Gopal-Krishna; Yates, M; Wiita, P. J; Smette, A; Pati, A. K; Altieri, B (The European Southern Observatory, 1993-12)
    We have taken K, R, V, and B band images of Q 2345+007, which has been claimed to be the gravitationally lensed quasar with the widest separation between the two images (approximately equal 7 arcsec), although the lens ...
  • On the injection spectrum of relativistic electrons in high-redshift radio galaxies 
    Gopal-Krishna; Mhaskey, Mukul; Mangalam, A (IOP Publishing, 2012-01-01)
    We point out that the remarkable linearity of the ultra-steep radio spectra of high redshift radio galaxies re ects a previously reported general trend for powerful radio galaxies, according to which the spectral curvature ...
  • On the Photometric Error Calibration for the Differential Light Curves of Point-like Active Galactic Nuclei 
    Goyal, Arti; Mhaskey, Mukul; Gopal-Krishna; Witta, P. J; Stalin, C. S; Sagar, R (Indian Academy of Sciences, 2013-09)
    It is important to quantify the underestimation of rms photometric errors returned by the commonly used APPHOT algorithm in the IRAF software, in the context of differential photometry of point-like AGN, because of the ...
  • Optical microvariability in radio quiet quasars 
    Wiita, P. J; Gopal-Krishna; Sagar, R (Kluwer Academic Publishers, 1994)
    We have observed 11 radio quiet QSOs (RQQSOs) to see if they exhibit intranight variability in the optical. The detection of such microvariability would support models in which flutuations on accretion disks are dominant, ...
  • Optical variability of radio-intermediate quasars 
    Goyal, Arti; Gopal-Krishna; Joshi, S; Sagar, R; Wiita, Paul J; Anupama, G. C; Sahu, D. K (Blackwell Publishing, 2010-02)
    We report the results of our intensive intranight optical monitoring of eight optically bright `radio-intermediate quasars' (RIQs) having flat or inverted radio spectra. The monitoring was carried out in R band on 25 nights ...
  • A physically-motivated perspective of Fanaroff–Riley classification of radio galaxies 
    Gopal-Krishna; Wiita, Paul J; Joshi, Ravi; Patra, Dusmanta (Indian Academy of Sciences, 2023-06)
    A small subset of extragalactic double radio sources, termed HYbrid MOrpholgy Radio Sources (HYMORS), is distinguished by a very unusual, hybrid morphology in terms of Fanaroff–Riley (FR) classification. In HYMORS, one ...
  • Polarimetric and spectroscopic study of radio quiet weak emission line quasars 
    Kumar, P; Chand, H; Srianand, R; Stalin, C. S; Petitjean, P; Gopal-Krishna (Oxford University Press on behalf of the Royal Astronomical Society, 2018-10)
    A small subset of optically selected radio-quiet quasars showing weak or no emission lines may turn out to be the elusive radio-quiet BL Lac objects, or simply be radio-quiet QSOs with a still-forming/shielded broad line ...
  • Prolonged intranight optical quiescence of the classical BL Lac object PKS 0735+178 
    Goyal, A; Gopal-Krishna; Anupama, G. C; Sahu, D. K; Sagar, R; Britzen, S; Karazous, M; Aller, M. F; Aller, H. D (Astronomical Society of the Pacific, 2009-09)
    We report our extensive intranight optical monitoring of the well known low-energy peaked (LBL) BL Lac object PKS 0735+178. This long-term follow-up consists of R-band monitoring for a minimum duration of ˜ 4 hours, on 17 ...
  • The Radio Source Projected Near the Absorption-Line QSO PHL 5200 
    Gopal-Krishna (Astronomical Society of India, 1978-12)
    Some recently discovered properties of the radio source 4C - 05.93 are summarised. Their relevance to the observed absorption-line spectrum of the QSO PHL 5200, which is located onl 1 arcmin away from the radio source, is ...

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