12 research outputs found
Influence of the jet opening angle on the derived kinematical parameters of blazar jets having uniform and stratified bulk motion
We present analytical modelling of conical relativistic jets, in order to
evaluate the role of the jet opening angle on certain key parameters that are
inferred from VLBI radio observations of blazar nuclear jets. The key
parameters evaluated are the orientation angle (i.e., the viewing angle) of the
jet and the apparent speed and Doppler factor of the radio knots on parsec
scales. Quantitative comparisons are made of the influence of the jet opening
angle on the above parameters of the radio knots, as would be estimated for two
widely discussed variants of relativistic nuclear jets, namely, those having
uniform bulk speed and those in which the bulk Lorentz factor of the flow
decreases with distance from the jet axis (a `spine--sheath' flow). Our
analysis shows that for both types of jet velocity distributions the
expectation value of the jet orientation angle at first falls dramatically with
increases in the (central) jet Lorentz factor, but it levels off at a fraction
of the opening angle for extremely relativistic jets. We also find that the
effective values of the apparent speeds and Doppler factors of the knots always
decline substantially with increasing jet opening angle, but that this effect
is strongest for ultra-relativistic jets with uniform bulk speed. We suggest
that the paucity of highly superluminal parsec-scale radio components in TeV
blazars can be understood if their jets are highly relativistic and, being
intrinsically weaker, somewhat less well collimated, in comparison to the jets
in other blazars.Comment: 8 pages, 4 figures, official version appeared in MNRA
Kinematical diagrams for conical relativistic jet
We present diagrams depicting the expected inter-dependences of two key kinematical parameters of radio knots in the parsec-scale jets of blazars, deduced from VLBI observations. The two parameters are the apparent speed (υapp = cβapp) and the effective Doppler boosting factor (δeff) of the relativistically moving radio knot. A novel aspect of these analytical computations of β-δ diagrams is that they are made for parsecscale jets having a conical shape, with modest opening angles (ω up to 10°), in accord with the VLBI observations of the nuclei of the nearest radio galaxies. Another motivating factor is the recent finding that consideration of a conical geometry can have important implications for the interpretation of a variety of radio observations of blazar jets. In addition to uniform jet flows (i.e., those having a uniform bulk Lorentz factor, Γ), computational results are also presented for stratified jets where an ultra-relativistic central spine along the jet axis is surrounded by a slower moving sheath, possibly arising from a velocity shear
Bulk Motion of Ultrarelativistic Conical Blazar Jets
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 be much larger than the values usually inferred
by combining their flux variability and proper motion measurements. This is in
accord with our earlier finding that such ultrarelativistic (Lorentz factor,
Gamma > 30) conical jets can reconcile the relatively slow apparent motions of
VLBI knots in TeV blazars with the extremely fast flows implied by their rapid
gamma-ray variability. This jet geometry also implies that de-projected jet
opening angles will typically be significantly underestimated from VLBI
measurements. In addition, de-projected jet lengths will be considerably
overestimated if high Lorentz factors and significant opening angles are not
taken into account.Comment: 7 pages, 3 figures; accepted for publication in MNRA
Do the mildly superluminal VLBI knots exclude ultrarelativistic blazar jets?
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 these quantities across the widths of jets with
finite opening angles but constant velocities. For high bulk Lorentz factors
(Gamma > 10) variations across the jet can be quite large if the opening angle,
omega, is even a few degrees on sub-parsec scales. The resulting apparent
speeds are often much lower than those obtained from the usual analyses that
ignore the finite jet opening angles. We can thus reconcile the usually
observed subluminal or mildly superluminal speeds with the very high (>~ 20)
Gamma factors, required by the inverse Compton origin and rapid variability of
TeV fluxes, as well as by intraday radio variability. Thus it is possible to
associate the VLBI radio knots directly with shocks in the ultra-relativistic
main jet flow, without invoking very rapid jet deceleration on parsec scales,
or extremely unlikely viewing angles.Comment: 10 pages, 1 figure, to appear in ApJ Letters, Nov. 10 2004 issu
`Zwicky's Nonet': a compact merging ensemble of nine galaxies and 4C 35.06, a peculiar radio galaxy with dancing radio jets
We report the results of our radio, optical and infra-red studies of a
peculiar radio source 4C~35.06, an extended radio-loud AGN at the center of
galaxy cluster Abell 407 (). The central region of this cluster hosts
a remarkably tight ensemble of nine galaxies, the spectra of which resemble
those of passive red ellipticals, embedded within a diffuse stellar halo of
1~arcmin size. This system (named the `Zwicky's Nonet') provides unique
and compelling evidence for a multiple-nucleus cD galaxy precursor.
Multifrequency radio observations of 4C~35.06 with the Giant Meterwave Radio
Telescope (GMRT) at 610, 235 and 150 MHz reveal a system of 400~kpc scale
helically twisted and kinked radio jets and outer diffuse lobes. The outer
extremities of jets contain extremely steep spectrum (spectral index -1.7 to
-2.5) relic/fossil radio plasma with a spectral age of a few yr. Such ultra-steep spectrum relic radio lobes without definitive
hot-spots are rare, and they provide an opportunity to understand the
life-cycle of relativistic jets and physics of black hole mergers in dense
environments. We interpret our observations of this radio source in the context
of the growth of its central black hole, triggering of its AGN activity and jet
precession, all possibly caused by galaxy mergers in this dense galactic
system. A slow conical precession of the jet axis due to gravitational
perturbation between interacting black holes is invoked to explain the unusual
jet morphology.Comment: Published in MNRAS | No. of pages 12, 10 figures and 4 tables.
Comments are welcom
IUCAA: genesis of a unique research centre
The Inter-University Centre for Astronomy and Astrophysics (IUCAA) is the second Inter-University Centre established by the Government of India for promotion of astronomy and astrophysical research. In this article, the historical development, as well as the motivation, for establishing IUCAA has been discussed which comprises of the period 1988–1993, i.e. the first 5 years. A glimpse of research work in pre- and post-colonial era in India has also been presented to have a holistic view of the genesis