947 research outputs found
High sensitivity low frequency radio observations of cD galaxies
We present the GMRT 235 MHz images of three radio galaxies and 610 MHz images
of two sources belonging to a complete sample of cD galaxies in rich and poor
galaxy clusters. The analysis of the spectral properties confirms the presence
of aged radio emission in two of the presented sources.Comment: 3 pages, 2 figures, To appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching
(Germany
The appearance of a compact jet in the soft-intermediate state of 4U 1543-47
Recent advancements in the understanding of jet-disc coupling in black hole
candidate X-ray binaries (BHXBs) have provided close links between radio jet
emission and X-ray spectral and variability behaviour. In 'soft' X-ray states
the jets are suppressed, but the current picture lacks an understanding of the
X-ray features associated with the quenching or recovering of these jets. Here
we show that a brief, ~4 day infrared (IR) brightening during a predominantly
soft X-ray state of the BHXB 4U 1543-47 is contemporaneous with a strong X-ray
Type B quasi-periodic oscillation (QPO), a slight spectral hardening and an
increase in the rms variability, indicating an excursion to the
soft-intermediate state (SIMS). This IR 'flare' has a spectral index consistent
with optically thin synchrotron emission and most likely originates from the
steady, compact jet. This core jet emitting in the IR is usually only
associated with the hard state, and its appearance during the SIMS places the
'jet line' between the SIMS and the soft state in the hardness-intensity
diagram for this source. IR emission is produced in a small region of the jets
close to where they are launched (~ 0.1 light-seconds), and the timescale of
the IR flare in 4U 1543-47 is far too long to be caused by a single, discrete
ejection. We also present a summary of the evolution of the jet and X-ray
spectral/variability properties throughout the whole outburst, constraining the
jet contribution to the X-ray flux during the decay.Comment: Accepted to MNRAS. 11 pages, 6 figure
Compact steep-spectrum sources from the S4 sample
We present the results of 5-GHz observations with the VLA A-array of a sample
of candidate Compact Steep Spectrum sources (CSSs) selected from the S4 survey.
We also estimate the symmetry parameters of high-luminosity CSSs selected from
different samples of radio sources, and compare these with the larger sources
of similar luminosity to understand their evolution and the consistency of the
CSSs with the unified scheme for radio galaxies and quasars. The majority of
CSSs are likely to be young sources advancing outwards through a dense
asymmetric environment. The radio properties of CSSs are found to be consistent
with the unified scheme, in which the axes of the quasars are observed close to
the line of sight, while radio galaxies are observed close to the plane of the
sky.Comment: accepted for publication in mnras; 8 pages, figure 1 with 21 images,
and two additional figures; 2 table
Dispersionless motion in a driven periodic potential
Recently, dispersionless (coherent) motion of (noninteracting) massive
Brownian particles, at intermediate time scales, was reported in a sinusoidal
potential with a constant tilt. The coherent motion persists for a finite
length of time before the motion becomes diffusive. We show that such coherent
motion can be obtained repeatedly by applying an external zero-mean square-wave
drive of appropriate period and amplitude, instead of a constant tilt. Thus,
the cumulative duration of coherent motion of particles is prolonged. Moreover,
by taking an appropriate combination of periods of the external field, one can
postpone the beginning of the coherent motion and can even have coherent motion
at a lower value of position dispersion than in the constant tilt case.Comment: 4 pages, 4 figure
Jet propagation and the asymmetries of CSS radio sources
As Compact Steep Spectrum radio sources have been shown to be more
asymmetrical than larger sources of similar powers, there is a high probability
that they interact with an asymmetric medium in the central regions of the host
elliptical galaxy. We consider a simple analytical model of the propagation of
radio jets through a reasonable asymmetric environment and show that they can
yield the range of arm-length and luminosity asymmetries that have been
observed. We then generalize this to allow for the effects of orientation, and
quantify the substantial enhancements of the asymmetries that can be produced
in this fashion. We present two-dimensional and three-dimensional simulations
of jets propagating through multi-phase media and note that the results from
the simulations are also broadly consistent with the observations.Comment: 11 pages, 6 figures, 1 table, accepted for publication in A&
Radio detections of IR-selected runaway stellar bow shocks
Massive stars moving at supersonic peculiar velocities through the
interstellar medium (ISM) can create bow shocks, arc-like structures at the
interface between the stellar wind and the ISM. Many such bow shocks have been
detected and catalogued at IR wavelengths, but detections in other wavebands
remain rare. Strikingly, while electrons are expected to be accelerated in the
bow shock and their non-thermal emission may include synchrotron emission at
low frequencies, only two massive runaway stellar bow shocks have to date been
detected in the radio band. Here, we examine a sample of fifty IR-detected bow
shocks from the E-BOSS catalogues in recently released radio images from the
Rapid ASKAP Continuum Survey (RACS). We identify three confident and three
likely counterparts, as well as three inconclusive candidates requiring
confirmation via follow-up observations. These detections significantly
increase the number of known radio massive stellar bow shocks and highlight the
advantage of dedicated searches with current and next-generation radio
telescopes. We investigate the underlying radio emission mechanism for these
radio sources, finding a mix of free-free-dominated and synchrotron-dominated
systems. We also discuss the non-detected targets by putting constraints on
their emission properties and investigating their detectability with future
observations. Finally, we propose several future avenues of research to advance
the study and understanding of bow shocks at radio frequencies.Comment: 17 pages, 12 figures, 3 tables. Resubmitted to MNRAS after final
round of very minor referee comment
A low-frequency study of recently identified double-double radio galaxies
In order to understand the possible mechanisms of recurrent jet activity in
radio galaxies and quasars, which are still unclear, we have identified such
sources with a large range of linear sizes (220 917 kpc), and hence time
scales of episodic activity. Here we present high-sensitivity 607-MHz Giant
Metrewave Radio Telescope (GMRT) images of 21 possible double-double radio
galaxies (DDRGs) identified from the FIRST survey to confirm their episodic
nature. These GMRT observations show that none of the inner compact components
suspected to be hot-spots of the inner doubles are cores having a flat radio
spectrum, confirming the episodic nature of these radio sources. We have
indentified a new DDRG with a candidate quasar, and have estimated the upper
spectral age limits for eight sources which showed marginal evidence of
steepening at higher frequencies. The estimated age limits (11 52 Myr) are
smaller than those of the large-sized ( 1 Mpc) DDRGs.Comment: Accepted for publication in MNRAS. 14 pages, 7 figure
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