3,777 research outputs found
The cluster environments of radio loud quasars
We have carried out multi-colour imaging of the fields of a statistically
complete sample of low-frequency selected radio loud quasars at 0.6<z<1.1, in
order to determine the characteristics of their environments. The largest radio
sources are located in the field, and smaller steep-spectrum sources are more
likely to be found in richer environments, from compact groups through to
clusters. This radio-based selection (including source size) of high redshift
groups and clusters is a highly efficient method of detecting rich environments
at these redshifts. Although our single filter clustering measures agree with
those of other workers, we show that these statistics cannot be used reliably
on fields individually, colour information is required for this.Comment: 5 pages, 3 figures, contribution to "Tracing Cosmic Evolution with
Galaxy Clusters" (Sesto 2001), ASP Conference Serie
Dynamic PRA: an Overview of New Algorithms to Generate, Analyze and Visualize Data
State of the art PRA methods, i.e. Dynamic PRA
(DPRA) methodologies, largely employ system
simulator codes to accurately model system dynamics.
Typically, these system simulator codes (e.g., RELAP5 )
are coupled with other codes (e.g., ADAPT,
RAVEN that monitor and control the simulation. The
latter codes, in particular, introduce both deterministic
(e.g., system control logic, operating procedures) and
stochastic (e.g., component failures, variable uncertainties)
elements into the simulation. A typical DPRA analysis is
performed by:
1. Sampling values of a set of parameters from the
uncertainty space of interest
2. Simulating the system behavior for that specific set of
parameter values
3. Analyzing the set of simulation runs
4. Visualizing the correlations between parameter values
and simulation outcome
Step 1 is typically performed by randomly sampling
from a given distribution (i.e., Monte-Carlo) or selecting
such parameter values as inputs from the user (i.e.,
Dynamic Event Tre
The heating mechanism for the warm/cool dust in powerful, radio-loud AGN
The uncertainty surrounding the nature of the heating mechanism for the dust
that emits at mid- to far-IR (MFIR) wavelengths in active galaxies limits our
understanding of the links between active galactic nuclei (AGN) and galaxy
evolution, as well as our ability to interpret the prodigious infrared and
sub-mm emission of some of the most distant galaxies in the Universe. Here we
report deep Spitzer observations of a complete sample of powerful, intermediate
redshift (0.05 < z < 0.7) radio galaxies and quasars. We show that AGN power,
as traced by [OIII]5007 emission, is strongly correlated with both the mid-IR
(24 micron) and the far-IR (70 micron) luminosities, however, with increased
scatter in the 70 micron correlation. A major cause of this increased scatter
is a group of objects that falls above the main correlation and displays
evidence for prodigious recent star formation activity at optical wavelengths,
along with relatively cool MFIR colours. These results provide evidence that
illumination by the AGN is the primary heating mechanism for the dust emitting
at both 24 and 70 microns, with starbursts dominating the heating of the cool
dust in only 20 -- 30% of objects. This implies that powerful AGN are not
always accompanied by the type of luminous starbursts that are characteristic
of the peak of activity in major gas-rich mergers.Comment: 13 pages, 3 figures. Accepted for publication in astrophysical
journal letter
The cluster environments of radio-loud quasars at 0.6 < z < 1.1
We have carried out multicolour imaging of a complete sample of radio-loud
quasars at 0.6 < z < 1.1 and find groups or clusters of galaxies in the fields
of at least 8 and possibly 13 of the 21 sources. There is no evidence for an
evolution in the richness of the environments of radio-loud quasars from other
low-redshift studies to z >~ 0.9. The quasars associated with groups and
clusters in our sample do not necessarily reside in the centre of the galaxy
distribution which rarely displays a spherical geometry. Clustering is
preferentially associated with small or asymmetric steep-spectrum radio
sources. The quasars with the largest projected angular size are, in nearly all
cases, found in non-clustered environments. Radio-based selection (including
source size) of high-redshift groups and clusters can be a very efficient
method of detecting rich environments at these redshifts.
We find that in optical searches for galaxy overdensities above z ~ 0.6
multiple filters must be used. If the single-filter counting statistics used by
groups at lower redshift are applied to our data, uncertainties are too large
to make accurate quantifications of cluster richness. This means that genuine
clustering of galaxies about quasars will be missed and, in ~10% of cases,
putative clusters turn out to be false detections. The statistics are further
diluted by the fact that galaxy overdensities are generally not centred on the
quasar.Comment: 21 pages, 26 figures, accepted for publication in MNRA
The 1.4 GHz light curve of GRB 970508
We report on Westerbork 1.4 GHz radio observations of the radio counterpart
to -ray burst GRB~970508, between 0.80 and 138 days after this event.
The 1.4 GHz light curve shows a transition from optically thick to thin
emission between 39 and 54 days after the event. We derive the slope of the
spectrum of injected electrons () in two
independent ways which yield values very close to . This is in agreement
with a relativistic dynamically near-adiabatic blast wave model whose emission
is dominated by synchrotron radiation and in which a significant fraction of
the electrons cool fast.Comment: Paper I. Accepted for publication in the Astrophysical Journal
Letter
ISO observations of a sample of Compact Steep Spectrum and GHz Peaked Spectrum Radio Galaxies
We present results from observations obtained with ISOPHOT, on board the ISO
satellite, of a representative sample of seventeen CSS/GPS radio galaxies and
of a control sample of sixteen extended radio galaxies spanning similar ranges
in redshift (0.2 = 10^26 W/Hz).
The observations have been performed at lambda = 60, 90, 174 and 200 microns.
Seven of the CSS/GPS sources have detections >= 3 sigma at one or more
wavelengths, one of which is detected at >= 5 sigma. By co-adding the data we
have obtained average flux densities at the four wavelengths. We found no
evidence that the FIR luminosities of the CSS/GPS sources are significantly
different from those of the extended objects and therefore there is not any
support for CSS/GPS sources being objects "frustrated" by an abnormally dense
ambient medium. The two samples were then combined, providing FIR information
on a new sample of radio galaxies at intermediate redshifts. We compare this
information with what previously known from IRAS and discuss the average
properties of radio galaxies in the redshift range 0.2 - 0.8. The FIR emission
cannot be accounted for by extrapolation of the synchrotron radio spectrum and
we attribute it to thermal dust emission. The average FIR luminosity is >=
6*10^11 L_sun. Over the observed frequency range the infrared spectrum can be
described by a power law with spectral index alpha >~1.0 +/- 0.2. Assuming the
emission to be due to dust, a range of temperatures is required, from >=80 K to
\~25 K. The dust masses required to explain the FIR emission range from 5*10^5
M_sun for the hotter component up to 2*10^8 M_sun for the colder one.
(abridged)Comment: Astronomy & Astrophysics, in press, 16 pages, 2 Figure
PKS2250-41: a case study for triggering
We present the results of a multiwavelength study of the z = 0.31 radio
source PKS2250-41. Integral field unit and long-slit spectroscopy obtained
using VIMOS and FORS1 on the VLT, and archival HST optical imaging observations
are used to study the morphology, kinematics and ionisation state of the
extended emission line region (EELR) surrounding this source, and also a
companion galaxy at a similar redshift. Near-infrared imaging observations
obtained using the NTT are used to analyse the underlying galaxy morphologies.
The EELR displays a complex variety of different gas kinematics and ionization
states, consistent with a mixture of radio source shocks and AGN
photoionization. The radio galaxy is likely to lie within a group environment,
and is plausibly undergoing interactions with one or more other objects. The
disk-like galaxy to the northeast of the radio source lies at a similar
redshift to the radio galaxy itself, and has its major axis position angle
aligned with the filamentary continuum and line emission extending outwards
from the radio galaxy. This filamentary structure is most plausibly interpreted
as a tidal structure associated with an interaction involving the radio source
host galaxy and the aligned companion galaxy to the north-east; this encounter
may have potentially triggered the current epoch of radio source activity.
Overall, PKS2250-41 displays some of the best evidence that radio source
activity can be triggered in this manner. [abridged]Comment: 16 pages, 13 figures (some colour). Accepted for publication in
MNRAS. Abstract abridge
Optical and Radio observations of the bright GRB010222 afterglow: evidence for rapid synchrotron cooling?
We report photometric observations of the optical afterglow of GRB010222 in
V,R and I passbands carried out at UPSO, Naini Tal between 22-27 Feb 2001. We
determine CCD Johnson BV and Cousins RI photometric magnitudes for 31 stars in
the field of GRB010222 and use them to calibrate our measurements as well as
other published BVRI photometric magnitudes of GRB010222 afterglow. We
construct the light curve in V,R,I passbands and from a broken power-law fit
determine the decay indices of 0.74+/-0.05 and 1.35+/-0.04 before and after the
break at 0.7 days. Using reported X-ray flux measurements at 0.35 and 9.13 days
after the burst we determine X-ray to opt/IR spectral index of 0.61+/-0.02 and
0.75+/-0.02 on these two days. We also report upper limits to the radio flux
obtained from the RATAN-600 telescope and the GMRT, and millimeter-wave upper
limits obtained from the Plateau de Bure Millimeter interferometer. We argue
that the synchrotron cooling frequency is below the optical band for most of
the observing period. We also estimate an initial jet opening angle of about
2.0n^(1/8) degrees, where n is the number density of the ambient medium.Comment: 16 pages, 4 postscript figures, minor revisions according to
referee's comments, millimeter upper limit added, accepted for publication in
Bulletin of the Astronomical Society of Indi
The magnetic field structure in CTA 102 from high-resolution mm-VLBI observations during the flaring state in 2016-2017
CONTEXT: Investigating the magnetic field structure in the innermost regions of relativistic jets is fundamental to understanding the crucial physical processes giving rise to jet formation, as well as to their extraordinary radiation output up to γ-ray energies.
AIMS: We study the magnetic field structure of the quasar CTA 102 with 3 and 7 mm VLBI polarimetric observations, reaching an unprecedented resolution (∼50 μas). We also investigate the variability and physical processes occurring in the source during the observing period, which coincides with a very active state of the source over the entire electromagnetic spectrum.
METHODS: We perform the Faraday rotation analysis using 3 and 7 mm data and we compare the obtained rotation measure (RM) map with the polarization evolution in 7 mm VLBA images. We study the kinematics and variability at 7 mm and infer the physical parameters associated with variability. From the analysis of γ-ray and X-ray data, we compute a minimum Doppler factor value required to explain the observed high-energy emission.
RESULTS: Faraday rotation analysis shows a gradient in RM with a maximum value of ∼6 × 104⁴ rad m⁻² and intrinsic electric vector position angles (EVPAs) oriented around the centroid of the core, suggesting the presence of large-scale helical magnetic fields. Such a magnetic field structure is also visible in 7 mm images when a new superluminal component is crossing the core region. The 7 mm EVPA orientation is different when the component is exiting the core or crossing a stationary feature at ∼0.1 mas. The interaction between the superluminal component and a recollimation shock at ∼0.1 mas could have triggered the multi-wavelength flares. The variability Doppler factor associated with such an interaction is large enough to explain the high-energy emission and the remarkable optical flare occurred very close in time.Accepted manuscrip
Monitoring and Forecasting the Ionosphere Over Europe: The DIAS Project
Knowledge of the state of the upper atmosphere, and in particular its ionospheric part, is very important in several
applications affected by space weather, especially the communications and navigation systems that rely on radio
transmission. To better classify the ionosphere and forecast its disturbances over Europe, a data collection endeavour
called the European Digital Upper Atmosphere Server (DIAS) was initiated in 2004 by a consortium formed around
several European ionospheric stations that transmit in real-time ionospheric parameters automatically scaled. The
DIAS project is a collaborative venture of eight institutions funded by the European Commission eContent
Programme. The project seeks to improve access to digital information collected by public European institutes and to
expand its use.
The main objective of the DIAS project is to develop a pan-European digital data collection describing the state of
the upper atmosphere, based on real-time information and historical data collections provided by most of the
operating ionospheric stations in Europe. Various groups of users require data specifying upper atmospheric
conditions over Europe for nowcasting and forecasting purposes. The DIAS system is designed to distribute such
information.
The successful operation of DIAS is based on the effective use of observational data in operational applications
through the development of new added-value ionospheric products and services that best fit the needs of the market.
DIAS is a unique European system, and its continuous operation will efficiently support radio propagation services
with the most reliable information. DIAS began providing services to users in August 2006
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