Publications (116)
Ultra-High Energy Cosmic Rays from Centaurus A: Jet Interaction with Gaseous Shells
Gopal-Krishna, Peter L. Biermann, Vitor de Souza +1
Ultra high energy cosmic rays (UHECRs), with energies above ~6 x 10^19 eV, seem to show a weak correlation with the distribution of matter relatively near to us in the universe. It…
Origin of the double-peaked narrow emission-lines in the optical spectra of X-shaped Radio Galaxies
Ghosh Prajnadipt, Ravi Joshi, Xiaolong Yang +13
We investigate X-shaped radio galaxies (XRGs) with optical double-peaked narrow emission lines (DPNELs) as potential hosts of dual or binary supermassive black holes (SMBHs). Using…
A giant radio jet ejected by an ultramassive black hole in a single-lobed radio galaxy
Joydeep Bagchi, Gopal-Krishna, Marita Krause +1
We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be trace…
Intranight variability of UV emission from powerful blazars
Krishan Chand, Gopal-Krishna, Amitesh Omar +4
We report the first study to characterise intranight variability of the blazar class from the perspective of (rest-frame) UV emission. For this, we carried out intranight optical m…
Search for HI emission from superdisk candidates associated with radio galaxies
Abhijeet Anand, Nirupam Roy, Gopal-Krishna
Giant gaseous layers (termed "superdisk") have been hypothesized in the past to account for the strip-like radio emission gap (or straight-edged central brightness depression) obse…
Are Cluster Radio Relics Revived Fossil Radio Cocoons?
Torsten A. Ensslin, Gopal-Krishna
A new model for the, so called, `cluster radio relics' is presented (Ensslin & Gopal-Krishna 2000). Fossil radio cocoons, resulting from the former activity of radio galaxies, shou…
X-shaped radio galaxy 3C 223.1: A 'double boomerang' with an anomalous spectral gradient
Gopal-Krishna, Pratik Dabhade
A comparison of the recent LOFAR 144 MHz map of the radio source 3C 223.1 (J094124.028+394441.95) with the VLA maps at 4.9 GHz and 8.3 GHz that we built based on archival data, est…
Polarimetric and spectroscopic study of radio-quiet weak emission line quasars
Parveen Kumar, H. Chand, R. Srianand +3
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…
Extragalactic radio sources with hybrid morphology: implications for the Fanaroff-Riley dichotomy
Gopal-Krishna, Paul J. Wiita
We provide observational and theoretical perspectives on the currently much debated issue of the Fanaroff-Riley (FR) morphological dichotomy of extragalactic radio sources. In this…
Effects of Jet Opening Angle and Velocity Structure on Blazar Parameters
Paul J. Wiita, Gopal-Krishna, Samir Dhurde +1
We had earlier shown that for a constant velocity jet the discrepancy between the low speeds indicated by VLBI knot motions and the high Doppler factors inferred from emission of T…
Clues on the nature of the quasi-periodic optical outbursts of the blazar OJ 287
Gopal-Krishna
The fascination with the blazar OJ 287 stems not only from its status as a prominent candidate for a close supermassive black hole (SMBH) binary, but also because of the thermal br…
Optical variability properties of high luminosity AGN classes
C. S. Stalin, Gopal-Krishna, Ram Sagar +1
We present the results of a comparative study of the intra-night optical variability (INOV) characteristics of radio-loud and radio-quiet quasars, which involves a systematic intra…
Constraints on supermassive black hole spins from observations of active galaxy jets
E. Kun, P. J. Wiita, L. Ã. Gergely +3
We discuss the origin of the low-energy cutoff, or LEC, seen in the radio spectra of many extragalactic jets and relate this to the spin of the supermassive black holes that presum…
Clear Evidence for Intranight Optical Variability in Radio-quiet Quasars
Gopal-Krishna, C. S. Stalin, Ram Sagar +1
We present new clues to the problem of the radio loudness dichotomy arising from an extensive search for intranight optical variability in seven sets of optically luminous radio-qu…
Multi-wavelength variability study of the classical BL Lac object PKS 0735+178 on timescales ranging from decades to minutes
Arti Goyal, Lukasz Stawarz, Michal Ostrowski +6
We present the results of our power spectral analysis for the BL Lac object PKS 0735+178 utilizing the Fermi-LAT survey at high-energy -rays, several ground-based optical teles…
On the photometric error calibration for the `differential light curves' of point-like Active Galactic Nuclei
Arti Goyal, Mukul Mhaskey, Gopal-Krishna +3
It is important to quantify the underestimation of rms photometric errors returned by the commonly used algorithm in the software, in the context of dif…
Stellar Disruption and the Quasar Radio Dichotomy
Gopal-Krishna, A. Mangalam, Paul J. Wiita
The origin of the dichotomy of radio loudness among quasars can be explained using recent findings that the mass of the central supermassive black hole (SMBH) in extended radio-lou…
Mega parsec relativistic jets launched from an accreting supermassive blackhole in an extreme spiral galaxy
Joydeep Bagchi, Vivek M., Vinu Vikram +6
Radio galaxy phenomenon is directly connected to mass accreting, spinning supermassive black holes found in the active galactic nuclei (AGN). It is still unclear how the collimated…
Radio emission and the optical isophotal twist of radio-loud ellipticals
Gopal-Krishna, Amit R. Dhakulkar, Paul J. Wiita +1
Using the surface photometric analysis data for a well defined sample of 79 nearby radio galaxies (z < 0.12), it is shown that the level of radio emission associated with an ellipt…
GPS in the cosmos
Gopal-Krishna
This note chronicles the early steps and incidences from over four decades ago, that gave a kickstart to research on the now prominent subclass of radio galaxies, called 'Gigahertz…
The Linear Sizes of Quasars and Radio Galaxies in the Unified Scheme
Gopal-Krishna, Vasant Kulkarni, Paul J. Wiita
A key argument in favor of orientation based unification schemes is the finding that among the most powerful 3CRR radio sources the (apparent) median linear size of quasars is smal…
Incidence of Mg II absorption systems towards flat-spectrum radio quasars
Hum Chand, Gopal-Krishna
The conventional wisdom that the rate of incidence of Mg II absorption systems, dN/dz (excluding `associated systems' having velocity beta*c relative to the AGN of less than ~5000…
Diffuse bubble-like radio-halo emission in MRC 0116+111: Imprint of AGN feedback in a distant cluster of galaxies
Joydeep Bagchi, Joe Jacob, Gopal-Krishna +4
We report the discovery of a luminous, mini radio halo of ~240 kpc dimension at the center of a distant cluster of galaxies at redshift z = 0.131. Our optical and multi-wavelength…
Catching butterflies in the sky: Extended catalog of winged or X-shaped radio sources from the latest FIRST data release
Xiaolong Yang, Ravi Joshi, Gopal-Krishna +8
We present a catalog of 290 "winged" or X-shaped radio galaxies (XRGs) extracted from the latest (2014 December 17) data release of the "Very Large Array Faint Images of the Radio…
A Search for Blazar Activity in Broad-Absorption-Line Quasars
Sapna Mishra, Gopal-Krishna, Hum Chand +3
Our recently reported lack of Intra-Night Optical Variability (INOV) among Broad-Absorption-Line (BAL) quasars exhibiting some blazar-like radio properties, either questions polar…
Intra-night Optical Variability of BL Lacs, Radio-Quiet Quasars and Radio-Loud Quasars
C. S. Stalin, A. C. Gupta, Gopal-Krishna +2
We report optical monitoring observations of 20 high-luminosity AGN, 12 of which are radio-quiet quasars (RQQs). Intra-night optical variability (INOV) was detected for 13 of the 2…
Cosmic ray contributions from rapidly rotating stellar mass black holes: Cosmic Ray GeV to EeV proton and anti-proton sources
M. L. Allen, P. L. Biermann, A. Chieffi +12
In Radio Super Novae (RSNe) a magnetic field of is observed; these are the same numbers for Blue Supe…
The Twin Radio galaxy TRG J104454+354055
Gopal-Krishna, Ravi Joshi, Dusmanta Patra +4
We report observations of a bright dumb-bell system of galaxies () with the upgraded Giant Metrewave Radio Telescope (GMRT), which show that each member of this gravit…
Discovery of X-shaped Morphology of the Giant Radio Galaxy GRG 0503-286
Pratik Dabhade, Gopal-Krishna
The high surface-brightness sensitivity of the GLEAM survey image of the giant radio galaxy GRG 0503-28 at 70-230 MHz has revealed an inversion-symmetric bending of its two lobes,…
Was the Cosmic Web of Protogalactic Material Permeated by Lobes of Radio Galaxies During the Quasar Era?
Gopal-Krishna, Paul J. Wiita
Evidence for extended active lifetimes (> 10^8 yr) for radio galaxies implies that many large radio lobes were produced during the `quasar era', 1.5 < z < 3, when the comoving dens…
In-situ acceleration of radio-emitting particles in the lobes of radio galaxies: Evolving observational perspective and recent clues
Gopal-Krishna, Paul Wiita
The issue of radiation mechanisms had triggered in 1950-60s the first applications of plasma physics to understand the nature of radio galaxies. This interplay has steadily intensi…
On the origin of X-shaped radio galaxies
Gopal-Krishna, Peter L. Biermann, László Ã. Gergely +1
After a brief, critical review of the leading explanations proposed for the small but important subset of radio galaxies showing an X-shaped morphology (XRGs) we propose a generali…
Spectral index variation across X-shaped radio galaxies
Dusmanta Patra, Ravi Joshi, Gopal-Krishna
The formation mechanism of the enigmatic subclass of radio galaxies, called 'X-shaped radio galaxies' (XRGs), or 'winged' radio galaxies, which account for of the radio…
Intra-night optical variability of ray detected narrow-line Seyfert 1 galaxies
Vineet Ojha, Hum Chand, Gopal-Krishna
We report the first attempt to systematically characterise intra-night optical variability (INOV) of the rare and enigmatic subset of Narrow-Line Seyfert1 galaxies (NLSy1s), which…
Incidence of Strong MgII Absorbers Towards Different Types of Quasars
Ravi Joshi, Hum Chand, Gopal-Krishna
We report the first comparative study of strong MgII absorbers seen towards radio-loud quasars of core-dominated (CDQs) and lobe-dominated (LDQs) types and normal QSOs. The MgII as…
The changing ISM of massive elliptical galaxies and cosmic evolution of radio galaxies and quasars
A. Mangalam, Gopal-Krishna, Paul J. Wiita
The recently discovered apparent dramatic expansion in the effective radii of massive elliptical galaxies from to has been interpreted in terms of eithe…
Intranight optical variability of low-mass Active Galactic Nuclei: A Pointer to blazar-like activity
Gopal-Krishna, Krishan Chand, Hum Chand +4
This study aims to characterise, for the first time, intranight optical variability (INOV) of low-mass active galactic nuclei (LMAGN) which host a black hole (BH) of mass $M_{BH} \…
Unusual optical quiescence of the classical BL Lac object PKS 0735+178 on intranight time scale
Arti Goyal, Gopal-Krishna, G. C. Anupama +6
We present the result of our extensive intranight optical monitoring of the well known low-energy peaked BL Lac (LBL) object PKS 0735+178. This long-term follow-up consists of -…
GMRT search for 150 MHz radio emission from the transiting extrasolar planets HD189733b and HD209458b
A. Lecavelier des Etangs, S. K. Sirothia, Gopal-Krishna +1
We report a sensitive search for meter-wavelength emission at 150 MHz from two prominent transiting extrasolar planets, HD189733b and HD209458b. To distinguish any planetary emissi…
Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars-II
Hum Chand, Parveen Kumar, Gopal-Krishna
This is continuation of our search for the elusive radio-quiet blazars, by carrying out a systematic programme to detect intranight optical variability (INOV) in a subset of `Weak-…
GMRT radio observations of the transiting extrasolar planet HD189733b at 244 and 614 MHz
A. Lecavelier des Etangs, S. K. Sirothia, Gopal-Krishna +1
We report a sensitive search for meter-wavelength emission at 244 and 614 MHz from HD189733b, the nearest known extrasolar transiting planet of `hot-Jupiter' type. To discriminate…
Radio Galaxies and the Magnetization of the IGM
Gopal-Krishna, Paul J. Wiita
Observed radio galaxies had a much higher comoving density during the `quasar era', at z ~ 2-3, but these sources are only detectable for small fractions of their active lifetimes…
Tracing Ghost Cavities with Low Frequency Radio Observations
Tracy Clarke, Elizabeth Blanton, Craig Sarazin +5
We present X-ray and multi-frequency radio observations of the central radio sources in several X-ray cavity systems. We show that targeted radio observations are key to determinin…
Unified schemes for active galaxies: a clue from the missing Fanaroff-Riley type I quasar population
Heino Falcke, Gopal-Krishna, Peter L. Biermann
We link the lack of FR I type structure among quasars to the void of radio loud quasars below a critical disk luminosity of ~10^46 erg/sec in the PG sample. We argue that the openi…
On the incidence of MgII absorbers along the blazar sightlines
S. Mishra, H. Chand, Gopal-Krishna +3
It is widely believed that the cool gas clouds traced by MgII absorption, within a velocity offset of 5000 km/s relative to the background quasar are mostly associated with the qua…
Intranight optical variability of radio-quiet and radio lobe dominated quasars
C. S. Stalin, Gopal-Krishna, Ram Sagar +1
We present results of a programme of multi-epoch, intra-night optical monitoring of a sample of non-blazar type AGN, which includes seven radio-quiet QSOs (RQQs) and an equal numbe…
Rapid optical variability of TeV blazars
Gopal-Krishna, Arti Goyal, S. Joshi +5
In this first systematic attempt to characterise the intranight optical variability (INOV) of TeV detected blazars, we have monitored a well defined set of 9 TeV blazars on total 2…
Do the mildly superluminal VLBI knots exclude ultrarelativistic blazar jets?
Gopal-Krishna, Samir Dhurde, Paul J. Wiita
We compute the effective values of apparent transverse velocity and flux boosting factors for the VLBI radio knots of blazar jets, by integrating over the angular distributions of…
Intranight Optical Variability of Blazars and Radio-quiet Quasars using the ZTF Survey
Vibhore Negi, Gopal-Krishna, Ravi Joshi +4
We explore the potential of the ongoing Zwicky-Transient-Facility (ZTF) survey for studying Intra-Night Optical Variability (INOV) of active galactic nuclei (AGN), in particular fo…
The 'spectral index-flux density relation' for extragalactic radio sources selected at metre and decametre wavelengths
Pratik Dabhade, Gopal-Krishna
We use the recent releases of sensitive VLA/LOFAR large-area surveys at 340 MHz and 54 MHz, in conjunction with the 1.4 GHz NVSS, to accurately determine the `spectral index - flux…
Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars
Gopal-Krishna, Ravi Joshi, Hum Chand
Based on a recently started programme, we report the first search for intranight optical variability among radio-quiet weak-line-quasars (RQWLQs). Eight members of this class were…
Radio galaxies and the star formation history of the universe
Gopal-Krishna, Paul J. Wiita, M. Angela Osterman
Multi-wavelength observations made in the last decade suggest that the universe underwent an intense phase of star formation in the past (z > 1). This intensive activity is commonl…
Hint of 150 MHz radio emission from the Neptune-mass extrasolar transiting planet HAT-P-11b
A. Lecavelier des Etangs, S. K. Sirothia, Gopal-Krishna +1
Since the radio-frequency emission from planets is expected to be strongly influenced by their interaction with the magnetic field and corona of the host star, the physics of this…
Superdisks in Radio Galaxies: Jet-Wind Interactions
Gopal-Krishna, Paul J. Wiita, Santosh Joshi
Taking a clue from their sharp-edged (strip-like) morphology observed in several cases, a new mechanism is proposed for the formation of the emission gaps seen between the radio lo…
Tracing Multiple Generations of AGN Feedback in the Core of Abell 262
T. E. Clarke, E. L. Blanton, C. L. Sarazin +4
We present new radio and X-ray observations of Abell 262. The X-ray residual image provides the first evidence of an X-ray tunnel in this system while the radio data reveal that th…
The radio source in Abell 980: A Detached-Double-Double Radio Galaxy?
Gopal-Krishna, Surajit Paul, Sameer Salunkhe +1
It is argued that the new morphological and spectral information gleaned from the recently published LoFAR Two meter Sky Survey data release 2 (LoTSS-2 at 144 MHz) observations of…
The alternating 'changing-look' blazar OQ 334 (B2 1420+32): New observational clues to the blazar state transitions
Krishan Chand, Gopal-Krishna
The high-luminosity blazar OQ 334 is a leading exponent of the intriguing rare phenomenon of alternating between a flat-spectrum radio quasar (FSRQ) and a BL Lac (BLL) states. Its…
X-shaped Radio Galaxies: Optical Properties, Large-scale Environment and Relationship to Radio Structure
Ravi Joshi, Gopal-Krishna, Xiaolong Yang +9
In order to find clues to the origin of the "winged" or "X-shaped" radio galaxies (XRGs) we investigate here the parent galaxies of a large sample of 106 XRGs for optical-radio axe…
Weak-emission-line quasars: A new clue from their optical variability
Ritish Kumar, Gopal-Krishna, Hum Chand +1
Weak-emission-line QSOs (WLQs) are an enigmatic subclass of the QSO population, as their optical/UV spectra are marked by abnormally weak (or absent) emission lines. To obtain much…
A physically-motivated perspective of the Fanaroff-Riley classification of radio galaxies
Gopal-Krishna, Paul J. Wiita, Ravi Joshi +1
A small subset of extragalactic double radio sources, termed HYMORS (HYbrid MOrpholgy Radio Sources), is distinguished by a very unusual, hybrid morphology in terms of the Fanaroff…
GMRT detection of a new wide-angle tail (WAT) radio source associated with the galaxy PGC 1519010
N. G. Kantharia, M. Das, Gopal-Krishna
We report the serendipitous detection of a Wide-Angle-Tail (WAT) radio galaxy at 240 and 610 MHz, using the Giant Metrewave Radio Telescope (GMRT). This WAT is hosted by a cD galax…
Asymmetries in Powerful Extragalactic Radio Sources
Gopal-Krishna, Paul J. Wiita
This review provides the background and current perspective of some of the striking asymmetries exhibited by extragalactic double-lobed radio sources, paying special attention to t…
Discovery of inverse-Compton X-ray emission and estimate of the volume-averaged magnetic field in a galaxy group
F. Mernier, N. Werner, J. Bagchi +6
Observed in a significant fraction of clusters and groups of galaxies, diffuse radio synchrotron emission reveals the presence of relativistic electrons and magnetic fields permeat…
Precession-induced Variability in AGN Jets and OJ 287
Silke Britzen, Michal ZajaÄek, Gopal-Krishna +5
The combined study of the flaring of Active Galactic Nuclei (AGN) at radio wavelengths and pc-scale jet kinematics with Very Long Baseline Interferometry (VLBI) has led to the view…
Pathway to Devasthal Astronomical Observatory, ARIES
Ram Sagar, Gopal-Krishna
Devasthal observatory, established over a time span of about 5 decades, is located in central Himalayan region of Devabhumi in Nainital district of Uttarakhand state, India. Operat…
Further evidence for intra-night optical variability of radio-quiet quasars
A. Goyal, Gopal-Krishna, Ram Sagar +2
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 sin…
On the Origin of the Fanaroff-Riley Dichotomy
Gopal-Krishna, Paul J. Wiita
A small fraction of double radio sources show a peculiar and striking hybrid morphology; they have a distinctly FR I structure on one side of the nucleus, and a FR II structure on…
Radio continuum emission and HI gas accretion in the NGC 5903/5898 compact group of early-type galaxies
Gopal-Krishna, Mukul Mhaskey, Paul J. Wiita +3
We discuss the nature of the multi-component radio continuum and HI emission associated with the nearby galaxy group comprised of two dominant ellipticals, NGC 5898 and NGC 5903, a…
Are there broad absorption-line blazars?
Sapna Mishra, Gopal-Krishna, Hum Chand +2
We report the first systematic search for blazars among broad-absorption-line (BAL) quasars. This is based on our intranight optical monitoring of a well-defined sample of 10 candi…
Comparative intra-night optical variability of X-ray and -ray detected narrow-line Seyfert 1 galaxies
Ojha Vineet, Chand Hum, Gopal-Krishna +2
In a systematic program to characterise the intra-night optical variability (INOV) of different classes of narrow-line Seyfert 1 galaxies (NLSy1s), we report here the first compara…
Astrophysical Black holes: An Explanation for the Galaxy Quenching
Jay Verma Trivedi, Pankaj S. Joshi, Gopal-Krishna +1
In light of increasing observational evidence supporting the existence of ultra-compact objects, we adopt the term astrophysical black hole to refer to any object having a huge mas…
A diffuse bubble-like radio-halo source MRC 0116+111: imprint of AGN feedback in a low-mass cluster of galaxies
Joydeep Bagchi, Joe Jacob, Gopal-Krishna +4
We present detailed observations of MRC 0116+111, revealing a luminous, mini radio-halo of ~240 kpc diameter located at the centre of a cluster of galaxies at redshift z = 0.131. O…
Intranight optical variability of Blazars
Ram Sagar, C. S. Stalin, Gopal-Krishna +1
We present results of a multi-epoch intra-night optical monitoring of eleven blazars consisting of six BL Lac objects and five radio core-dominated quasars (CDQs). These densely sa…
Intranight optical variability of TeV blazars with parsec-scale jets dominated by slow-moving radio knots
Vibhore Negi, Gopal-Krishna, Hum Chand +1
BL Lac objects detected at TeV energies preferentially belong to the subclass called 'high-frequency-peaked' BL Lacs (HBLs). Parsec-scale radio jets in these TeV-HBLs often show do…
Evidence for in-situ acceleration of relativistic particles in the wings of X-shaped radio galaxies
Dusmanta Patra, Gopal-Krishna, Ravi Joshi
We report evidence for in-situ acceleration/re-acceleration of relativistic particles in 11 radio wings out of a total of 68 wings sufficiently well-resolved for spectral mapping,…
Intra-Night Optical monitoring of three γ-ray detected Narrow-line Seyfert 1 galaxies
Vineet Ojha, Gopal-Krishna, Hum Chand
For 3 radio-loud -ray detected Narrow-Line Seyfert 1 (-ray NLSy1) galaxies, we report optical variability on intra-night and/or week-like time scales, based on five …
Extragalactic radio sources with sharply inverted spectrum at metre wavelengths
Gopal-Krishna, S. K. Sirothia, Mukul Mhaskey +5
We present the first results of a systematic search for the rare extragalactic radio sources showing an inverted (integrated) spectrum, with spectral index , a previous…
Supermassive black hole mergers as dual sources for electromagnetic flares in the jet emission and gravitational waves
M. Tápai, L. Ã. Gergely, Z. Keresztes +3
We present a new type of observation relating the gravitational wave emission of supermassive black hole mergers to their electromagnetic counterparts. This dual emission involves…
Reviving Fossil Radio Plasma in Clusters of Galaxies by Adiabatic Compression in Environmental Shock Waves
Torsten A. Ensslin, Gopal-Krishna
We give for a plasma with a history of several expansion and contraction phases an analytical model of the evolution of a contained relativistic electron population under synchrotr…
Low-velocity precessing jets can explain observed morphologies in the Twin Radio Galaxy TRG J104454+354055
Santanu Mondal, Gourab Giri, Ravi Joshi +3
Our understanding of large-scale radio jets in merger systems has been drastically improved in the era of VLA, VLBA/EVN, uGMRT, and MeerKAT. Twin Radio Galaxies (TRGs) are the rare…
Multi-epoch intra-night optical monitoring of 8 radio-quiet BL Lac candidates
P. Kumar, Gopal-Krishna, C. S. Stalin +3
For a new sample of 8 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 ti…
Multiband optical monitoring of the blazars S5 0716+714 and BL Lacertae
C. S. Stalin, Gopal-Krishna, Ram Sagar +3
We report results of multiband optical monitoring of two well known blazars, S5 0716+714 and BL Lacertae, carried out in 1996 and 2000-01 with an aim to study optical variations on…
Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars-III
Parveen Kumar, Gopal-Krishna, Hum Chand
This is continuation of our programme to search for the elusive radio-quiet BL Lacs, by carrying out a systematic search for intranight optical variability (INOV) in a subset of `w…
Expansion of radio galaxies in a cosmologically evolving medium: Possible implications for the cosmic star-formation history
Paramita Barai, Gopal-Krishna, M. Angela Osterman +1
We compare earlier estimates of the volumes filled by lobes of radio galaxies during the quasar era based upon non-evolving ambient media with new ones assuming a strong cosmologic…
Persistence of blazar state in flat-spectrum radio quasars
Krishan Chand, Gopal-Krishna
Flat-spectrum radio quasars (FSRQs) whose brightness is dominated by a relativistically beamed core, are frequently found in 'blazar state' commonly inferred from a high optical po…
Stochastic modeling of multiwavelength variability of the classical BL Lac object OJ 287 on timescales ranging from decades to hours
A. Goyal, L. Stawarz, S. Zola +109
We present the results of our power spectral density analysis for the BL Lac object OJ\,287, utilizing the {\it Fermi}-LAT survey at high-energy -rays, {\it Swift}-XRT in X-ray…
Intra-night optical variability of core dominated radio quasars: the role of optical polarization
Arti Goyal, Gopal-Krishna, Paul J. Wiita +4
{Abridged} 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 exte…
The transience and persistence of high optical polarization state in beamed radio quasars
Krishan Chand, Gopal-Krishna, Amitesh Omar +2
We examine the long-term stability (on decade-like time scales) of optical `high polarization' (HP) state with , which commonly occurs in flat-spectrum (i.e., beam…
Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars-IV
Parveen Kumar, Hum Chand, Gopal-Krishna
We report an extension of our program to search for radio-quiet BL Lac candidates using intra-night optical variability (INOV) as a probe. The present INOV observations cover a wel…
Bulk Motion of Ultrarelativistic Conical Blazar Jets
Gopal-Krishna, Paul J. Wiita, Samir Dhurde
Allowing for the conical shape of ultrarelativistic blazar jets with opening angles of a few degrees on parsec-scales we show that their bulk Lorentz factors and viewing angles can…
On the injection spectrum of relativistic electrons in high-redshift radio galaxies
Gopal-Krishna, Mukul Mhaskey, A. Mangalam
We point out that the remarkable linearity of the ultra-steep radio spectra of high redshift radio galaxies reflects a previously reported general trend for powerful radio galaxies…
GMRT observations of extragalactic radio sources with steeply inverted spectra
Mukul Mhaskey, Gopal-Krishna, Pratik Dabhade +3
We report quasi-simultaneous GMRT observations of seven extragalactic radio sources at 150, 325, 610 and 1400 MHz, in an attempt to accurately define their radio continuum spectra,…
Towards building a first northern-sky sample of 'Extremely Inverted Spectrum Extragalactic Radio Sources (EISERS)'
Mukul Mhaskey, Gopal-Krishna, Surajit Paul
We present here an extension of our search for EISERS (Extremely Inverted Spectrum Extragalactic Radio Sources) to the northern hemisphere. With an inverted radio spectrum of slope…
Do the central engines of quasars evolve by accretion ?
R. Srianand, Gopal-Krishna
According to a currently popular paradigm, nuclear activity in quasars is sustained via accretion of material onto super-massive black holes located at the quasar nuclei. A useful…
Optical variability of radio-intermediate quasars
Arti Goyal, Gopal-Krishna, S. Joshi +4
We report the results of our intensive intranight optical monitoring of 8 `radio-intermediate quasars' (RIQs) having flat or inverted radio spectra. The monitoring was carried out…
Discovery of the first giant double radio relic in a galaxy cluster found in the PLANCK Sunyaev-Zel'dovich cluster survey: PLCK G287.0+32.9
Joydeep Bagchi, S. K. Sirothia, Norbert Werner +6
We report the discovery of large scale diffuse non-thermal radio emission in PLCK G287.0+32.9, an exceptionally hot (T ~ 13 keV), massive and luminous galaxy cluster, strongly dete…
Improved characterisation of intra-night optical variability of prominent AGN classes
Arti Goyal, Gopal-Krishna, Paul J. Wiita +2
The incidence of intra-night optical variability (INOV) is known to to differ significantly among different classes of powerful active galactic nuclei (AGN). A number of statistica…
Understanding BL Lac objects Structural & kinematic mode changes in the BL Lac object PKS 0735+178
S. Britzen, A. Witzel, B. P. Gong +6
Context. We present evidence that parsec-scale jets in BL Lac objects may be significantly distinct in kinematics from their counterparts in quasars. We argued this previously for…
The Origin of X-shaped Radio Galaxies: Clues from the Z-symmetric Secondary Lobes
Gopal-Krishna, Peter L. Biermann, Paul J. Wiita
Existing radio images of a few X-shaped radio galaxies reveal Z-symmetric morphologies in their weaker secondary lobes which cannot be naturally explained by either the galactic me…
The Fanaroff-Riley Transition and the Optical Luminosity of the Host Elliptical Galaxy
Gopal-Krishna, Paul J. Wiita
We show that a model for radio source dynamics we had earlier proposed can readily reproduce the relationship between the radio power division separating the two Fanaroff-Riley cla…