20 research outputs found
Emergence and disappearance of micro-arcsecond structure in the scintillating quasar J1819+3845
The 4.8 GHz lightcurves of the scintillating intra-day variable quasar
J1819+3845 during 2004-5 exhibit sharp structure, down to a time scale of 15
minutes, that was absent from lightcurves taken prior to this period and from
the 2006 lightcurves. Analysis of the lightcurve power spectra show that the
variations must be due to the emergence of new structure in the source. The
power spectra yield a scattering screen distance of 3.8 +/- 0.3 pc for a
best-fit v_ISS=59 +/- 0.5 km/s or 2.0 +/- 0.3 pc for the scintillation velocity
reported by Dennett-Thorpe & de Bruyn (2003). The turbulence is required to be
exceptionally turbulent, with C_N^2 > 0.7 Delta L_pc^{-1} m^{-20/3} for
scattering material of thickness Delta L_{pc} pc along the ray path. The 2004
power spectrum can be explained in terms of a double source with a component
separation 240 +/- 15 microas in 2004.Comment: MNRAS Lett (accepted), version with high-resolution figures at
http://www.astro.caltech.edu/~jpm/MdB1819.pd
On the evolution of young radio-loud AGN
This paper describes an investigation of the early evolution of extragalactic
radio sources using samples of faint and bright Gigahertz Peaked Spectrum (GPS)
and Compact Steep Spectrum (CSS) radio galaxies. Correlations found between
their peak frequency, peak flux density and angular size provide strong
evidence that synchrotron self absorption is the cause of the spectral
turnovers, and indicate that young radio sources evolve in a self-similar way.
In addition, the data seem to suggest that the sources are in equipartition
while they evolve. If GPS sources evolve to large size radio sources, their
redshift dependent birth-functions should be the same. Therefore, since the
lifetimes of radio sources are thought to be short compared to the Hubble time,
the observed difference in redshift distribution between GPS and large size
sources must be due to a difference in slope of their luminosity functions. We
argue that this slope is strongly affected by the luminosity evolution of the
individual sources. A scenario for the luminosity evolution is proposed in
which GPS sources increase in luminosity and large scale radio sources decrease
in luminosity with time. This evolution scenario is expected for a ram-pressure
confined radio source in a surrounding medium with a King profile density. In
the inner parts of the King profile, the density of the medium is constant and
the radio source builds up its luminosity, but after it grows large enough the
density of th e surrounding medium declines and the luminosity of the radio
source decreases. A comparison of the local luminosity function (LLF) of GPS
galaxies with that of extended sources is a good test for this evolution
scenario [abridged].Comment: LaTeX, 11 pages, 8 figures; Accepted by MNRAS. Related papers may be
found at http://www.ast.cam.ac.uk/~snellen . Valuable comments of referee
incorporated. More discussion on simulation
Radio Interferometric Calibration Using The SAGE Algorithm
Radio Interferometry is an essential method for astronomical observations.
Self-calibration techniques have increased the quality of the radio
astronomical observations (and hence the science) by orders of magnitude.
Recently, there is a drive towards sensor arrays built using inexpensive
hardware and distributed over a wide area acting as radio interferometers.
Calibration of such arrays poses new problems in terms of computational cost as
well as in performance of existing calibration algorithms. We consider the
application of the Space Alternating Generalized Expectation Maximization
(SAGE) \cite{Fess94} algorithm for calibration of radio interferometric arrays.
Application to real data shows that this is an improvement over existing
calibration algorithms that are based on direct, deterministic non linear
optimization. As presented in this paper, we can improve the computational cost
as well as the quality of the calibration using this algorithm.Comment: 6 pages, 3 figures, Appearing in 13th IEEE DSP workshop (IEEE Signal
Processing Society
The LOFAR Magnetism Key Science Project
Measuring radio waves at low frequencies offers a new window to study cosmic
magnetism, and LOFAR is the ideal radio telescope to open this window widely.
The LOFAR Magnetism Key Science Project (MKSP) draws together expertise from
multiple fields of magnetism science and intends to use LOFAR to tackle
fundamental questions on cosmic magnetism by exploiting a variety of
observational techniques. Surveys will provide diffuse emission from the Milky
Way and from nearby galaxies, tracking the propagation of long-lived cosmic-ray
electrons through magnetic field structures, to search for radio halos around
spiral and dwarf galaxies and for magnetic fields in intergalactic space.
Targeted deep-field observations of selected nearby galaxies and suspected
intergalactic filaments allow sensitive mapping of weak magnetic fields through
Rotation Measure (RM) grids. High-resolution observations of protostellar jets
and giant radio galaxies reveal structures on small physical scales and at high
redshifts, whilst pulsar RMs map large-scale magnetic structures of the
Galactic disk and halo in revolutionary detail. The MKSP is responsible for the
development of polarization calibration and processing, thus widening the
scientific power of LOFAR.Comment: Proceedings of "Magnetic Fields in the Universe: From Laboratory and
Stars to Primordial Structures", 2011 Aug. 21-27 in Zakopane/Poland, eds. M.
Soida et a
Hemispheric Imaging of Galactic Neutral Hydrogen with a Phased Array Antenna System
Abstract The thousand element array (THEA) system is a phased array system consisting of 1 m 2 tiles having 64 Vivaldi elements each arranged on a regular 8-by-8 grid, which has been developed as a demonstrator of technology and applicability for SKA. In this paper we present imaging results of galactic neutral hydrogen with THEA. Measurements have been taken using a dense 2-by-2 array of four tiles as a four tile adder. The results are compared with results from the Leiden-Dwingeloo Survey, showing qualitative agreement, but also indicating that further studies are needed on the instrumental characteristics
Episodic Star Formation Coupled to Reignition of Radio Activity in 3C 236
We present Hubble Space Telescope UV and optical imaging of the radio galaxy
3C 236, whose relic 4 Mpc radio jet lobes and inner 2 kpc CSS radio source are
evidence of multiple epochs of AGN activity. Our data confirm the presence of
four bright knots of FUV emission in an arc along the edge of the inner
circumnuclear dust disk in the galaxy's nucleus, as well as FUV emission
cospatial with the nucleus itself. We interpret these to be sites of recent or
ongoing star formation. We present photometry of these knots, as well as an
estimate for the internal extinction in the source using the Balmer decrement
from SDSS spectroscopy. We estimate the ages of the knots by comparing our
extinction-corrected photometry with stellar population synthesis models. We
find the four knots cospatial with the dusty disk to be young, of order 10^7 yr
old. The FUV emission in the nucleus is likely due to an episode of star
formation triggered ~10^9 yr ago. We argue that the young 10^7 yr old knots
stem from an episode of star formation that was roughly coeval with the event
resulting in reignition of radio activity, creating the CSS source. The 10^9 yr
old stars in the nucleus may be associated with the previous epoch of activity
that generated the 4 Mpc relic source, before it was cut off by exhaustion or
interruption. The ages of the knots, considered in context with the disturbed
morphology of the nuclear dust and the double-double morphology of the "old"
and "young" radio sources, present evidence for an episodic AGN/starburst
connection. We suggest that the AGN fuel supply was interrupted for ~10^7 yr
due to a minor merger event and has now been restored, and the resultant
non-steady flow of gas toward the nucleus is likely responsible for both the
new episode of infall-induced star formation and also the multiple epochs of
radio activity.Comment: 15 pages, 7 figures. Accepted for publication in ApJ
A very brief description of LOFAR - the Low Frequency Array
LOFAR (Low Frequency Array) is an innovative radio telescope optimized for
the frequency range 30-240 MHz. The telescope is realized as a phased aperture
array without any moving parts. Digital beam forming allows the telescope to
point to any part of the sky within a second. Transient buffering makes
retrospective imaging of explosive short-term events possible. The scientific
focus of LOFAR will initially be on four key science projects (KSPs): 1)
detection of the formation of the very first stars and galaxies in the universe
during the so-called epoch of reionization by measuring the power spectrum of
the neutral hydrogen 21-cm line (Shaver et al. 1999) on the ~5' scale; 2)
low-frequency surveys of the sky with of order expected new sources; 3)
all-sky monitoring and detection of transient radio sources such as gamma-ray
bursts, x-ray binaries, and exo-planets (Farrell et al. 2004); and 4) radio
detection of ultra-high energy cosmic rays and neutrinos (Falcke & Gorham 2003)
allowing for the first time access to particles beyond 10^21 eV (Scholten et
al. 2006). Apart from the KSPs open access for smaller projects is also
planned. Here we give a brief description of the telescope.Comment: 2 pages, IAU GA 2006, Highlights of Astronomy, Volume 14, K.A. van
der Hucht, e