571 research outputs found
New meanings from old buildings
The three modest house projects described here are by three fellow travellers — the two authors and David Lea — interested in the Organic side of Modernism. Conversational partners who have worked together in various capacities over many years, they share a common conviction about ‘working with the given
Exploring the Nature of Weak Chandra Sources near the Galactic Centre
We present results from the first near-IR imaging of the weak X-ray sources
discovered in the Chandra/ACIS-I survey (Wang et al. 2002) towards the Galactic
Centre (GC). These ~800 discrete sources, which contribute significantly to the
GC X-ray emission, represent an important and previously unknown population
within the Galaxy. From our VLT observations we will identify likely IR
counterparts to a sample of the hardest sources, which are most likely X-ray
binaries. With these data we can place constraints on the nature of the
discrete weak X-ray source population of the GC.Comment: In Proc. of ``Interacting Binaries: Accretion, Evolution, and
Outcomes'', eds. L. A. Antonelli et al., AIP, Cefalu, Sicily, 200
Spectral ageing analysis of the double-double radio galaxy J1453+3308
We present new radio observations at frequencies ranging from 240 to 4860 MHz
of the well-known, double-double radio galaxy (DDRG), J1453+3308, using both
the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA).
These observations enable us to determine the spectra of the inner and outer
lobes over a large frequency range and demonstrate that while the spectrum of
the outer lobes exhibits significant curvature, that of the inner lobes appears
practically straight. The break frequency, and hence the inferred synchrotron
age of the outer structure, determined from 16arcsec strips transverse to
the source axis, increases with distance from the heads of the lobes. The
maximum spectral ages for the northern and southern lobes are 47 and 58
Myr respectively. Because of the difference in the lengths of the lobes these
ages imply a mean separation velocity of the heads of the lobes from the
emitting plasma of 0.036c for both the northern and southern lobes. The
synchrotron age of the inner double is about 2 Myr which implies an advance
velocity of 0.1c, but these values have large uncertainties because the
spectrum is practically straight.Comment: 10 pages, 10 figures, 5 tables, accepted for publication in MNRA
Time-sequenced Multi-Radio-Frequency Observations of Cygnus X-3 in Flare
Multifrequency observations from the VLA, VLBA and OVRO Millimeter Array of a
major radio outburst of Cygnus X-3 in 2001 September are presented, measuring
the evolution of the spectrum of the source over three decades in frequency,
over a period of six days. Following the peak of the flare, as the intensity
declines the high-frequency spectrum at frequency nu steepens from nu^{-0.4} to
nu^{-0.6}, after which the spectral index remains at this latter terminal
value; a trend previously observed but hitherto not satisfactorily explained.
VLBA observations, for the first time, track over several days the expansion of
a sequence of knots whose initial diameters are approximately 8
milliarcseconds. The light-crossing time within these plasmons is of the same
order as the time-scale over which the spectrum is observed to evolve. We
contend that properly accounting for light-travel time effects in and between
plasmons which are initially optically thick, but which after expansion become
optically thin, explains the key features of the spectral evolution, for
example the observed timescale. Using the VLBA images, we have directly
measured for the first time the proper motions of individual knots, analysis of
which shows a two-sided jet whose axis is precessing. The best-fit jet speed is
roughly beta = 0.63 and the precession period is about 5 days, significantly
lower than fitted for a previous flare. Extrapolation of the positions of the
knots measured by the VLBA back to zero-separation shows this to occur
approximately 2.5 days after the detection of the rise in flux density of
Cygnus X-3.Comment: 23 pages, 10 figures, accepted by Ap
A multifrequency study of giant radio sources III. Dynamical age vs. spectral age of the lobes of selected sources
The dynamical ages of the opposite lobes of selected giant radio sources are
estimated using the DYNAGE algorithm of Machalski et al., and compared with
their spectral ages estimated and studied by Jamrozy et al. in Paper II. As
expected, the DYNAGE fits give slightly different dynamical ages and other
model's parameters for the opposite lobes modelled independently each other,
e.g. the age ratios are found between ~1.1 to ~1.4. Demanding similar values of
the jet power and the radio core density for the same source, we look for a
self-consistent solution for the opposite lobes, which results in different
density profiles along them found by the fit. We also show that a departure
from the equipartition conditions assumed in the model, justified by X-ray
observations of the lobes of some nearby radio galaxies, and a relevant
variation of the magnetic-field strengths may provide an equalisation of the
lobes' ages. A comparison of the dynamical and spectral ages shows that a ratio
of the dynamical age to the spectral age of the lobes of investigated giant
radio galaxies is between ~1 and ~5, i.e. is similar to that found for smaller
radio galaxies (e.g. Parma et al. 1999). Supplementing possible causes for this
effect already discussed in the literature, like uncertainty of assumed
parameters of the model, an influence of a possible departure from the energy
equipartition assumption, etc. Arguments are given to suggest that DYNAGE can
better take account of radiative effects at lower frequencies than the
spectral-ageing analysis.The DYNAGE algorithm is especially effective for
sources at high redshifts, for which an intrinsic spectral curvature is shifted
to low frequencies.Comment: Accepted to MNRAS, 12 pages, 6 figures, 4 table
Opacity effects and shock-in-jet modelling of low-level activity in Cygnus X-3
We present simultaneous dual-frequency radio observations of Cygnus X-3 during a phase of low-level activity. We constrain the minimum variability time-scale to be 20 min at 43 GHz and 30 min at 15 GHz, implying source sizes of 2-4 au. We detect polarized emission at a level of a few per cent at 43 GHz which varies with the total intensity. The delay of ∼10 min between the peaks of the flares at the two frequencies is seen to decrease with time, and we find that synchrotron self-absorption and free-free absorption by entrained thermal material play a larger role in determining the opacity than absorption in the stellar wind of the companion. A shock-in-jet model gives a good fit to the light curves at all frequencies, demonstrating that this mechanism, which has previously been used to explain the brighter, longer lived giant outbursts in this source, is also applicable to these low-level flaring events. Assembling the data from outbursts spanning over two orders of magnitude in flux density shows evidence for a strong correlation between the peak brightness of an event, and the time-scale and frequency at which this is attained. Brighter flares evolve on longer time-scales and peak at lower frequencies. Analysis of the fitted model parameters suggests that brighter outbursts are due to shocks forming further downstream in the jet, with an increased electron normalization and magnetic field strength both playing a role in setting the strength of the outburs
A method of estimation of the dynamical age of FR II-type radio sources from multifrequency data
We propose a new approach in determining ages of FR II type radio sources. We
apply the assumed dynamical model of Kaiser et al. (1997) to a number of FR II
type radio galaxies observed at different radio frequencies, and fit - for each
frequency separately - the model free parameters to the observed sources'
quantities. Such a procedure, using enlarged in fact a number of observables,
enables us to determine relatively precise ages and other crucial
characteristics of the analyzed sources. The resulting age estimates agree very
well with those obtained by means of `classical' spectral ageing method for
objects not older than 10 Myr, for which good-quality spectral data are
available. The presented method is however also applicable in the case of the
sources older than this, and/or the ones for which the only available
low-resolution radio data do not allow for detailed spectral ageing studies.
Our analysis indicates that the main factor precluding precise age
determination for FR II type radio galaxies regards the poorly known shape of
the initial electron energy distribution injected by the jet terminal shocks to
the expanding lobes/cocoons. We briefly consider this issue, and conclude that
the broad-band single power-law form assumed here may be accurate enough for
the presented estimates, although most likely it does not strictly correspond
to some well-defined realistic particle acceleration process. Instead, it
should be considered as a simplest model approximation of the initial electron
continuum, averaged over a very broad energy range and over the age of the
source, with the effective spectral index which may be different for different
sources.Comment: 15 pages, 9 figures included. Accepted for publication in A&
A multifrequency study of the large radio galaxies 3C46 and 3C452
We present low-frequency observations starting from ~150 MHz with the Giant
Metrewave Radio Telescope (GMRT), and high-frequency observations with the Very
Large Array (VLA) of two large radio galaxies 3C46 and 3C452. These
observations were made with the objectives of estimating their spectral ages
and examining any evidence of diffuse extended emission at low radio
frequencies due to an earlier cycle of activity. While no evidence of extended
emission due to an earlier cycle of activity has been found, the spectral ages
have been estimated to be ~15 and 27 Myr for the oldest relativistic plasma
seen in the regions close to the cores for 3C46 and 3C452 respectively. The
spectra in the vicinity of the hotspots are consistent with a straight spectrum
with injection spectral indices of ~1.0 and 0.78 respectively, somewhat steeper
than theoretical expectations.Comment: Accepted for publication in MNRAS, 10 pages, 8 figures and 4 table
Determining the nature of the faint X-ray source population near the Galactic Centre
We present results of a multi-wavelength program to study the faint discrete
X-ray source population discovered by Chandra in the Galactic Centre (GC). From
IR imaging obtained with the VLT we identify candidate K-band counterparts to
75% of the X-ray sources in our sample. By combining follow-up VLT K-band
spectroscopy of a subset of these candidate counterparts with the magnitude
limits of our photometric survey, we suggest that only a small percentage of
the sources are HMXBs, while the majority are likely to be canonical LMXBs and
CVs at the distance of the GC. In addition, we present our discovery of highly
structured small-scale (5-15") extinction towards the Galactic Centre. This is
the finest-scale extinction study of the Galactic Centre to date. Finally, from
these VLT observations we are able to place constraints on the stellar
counterpart to the ``bursting pulsar'' GRO J1744-28.Comment: 9 pages, in Proceedings of "VI Microquasar Workshop: Microquasars and
Beyond, 18-22 September 2006, Como, Italy; paper in PDF format with
full-resolution figures available at
http://www.astro.ufl.edu/~reba/mqw_rmb.pd
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