21 research outputs found
The magnetic field and geometry of the oblique shock in the jet of 3C 346
We investigate the brightest regions of the kpc-scale jet in the powerful
radio galaxy 3C 346, using new optical HST ACS/F606W polarimetry together with
Chandra X-ray data and 14.9 GHz and 22.5 GHz VLA radio polarimetry. The jet
shows a close correspondence in optical and radio morphology, while the X-ray
emission shows an 0.80 +/- 0.17 kpc offset from the optical and radio peak
positions. Optical and radio polarimetry show the same apparent magnetic field
position angle and fractional polarization at the brightest knot, where the jet
undergoes a large kink of almost 70 degrees in the optical and radio images.
The apparent field direction here is well-aligned with the new jet direction,
as predicted by earlier work that suggested the kink was the result of an
oblique shock. We have explored models of the polarization from oblique shocks
to understand the geometry of the 3C 346 jet, and find that the upstream flow
is likely to be highly relativistic (0.91 +0.05 / -0.07 c), where the plane of
the shock front is inclined at an angle of 51 (+/- 11) degrees to the upstream
flow which is at an angle 14 (+8 / -7) degrees to our line of sight. The actual
deflection angle of the jet in this case is only 22 degrees.Comment: 11 pages, 5 figures. Accepted by MNRA
The structure of the jet in 3C 15 from multi-band polarimetry
We investigate the structure of the kpc-scale jet in the nearby (z = 0.073)
radio galaxy 3C 15, using new optical Hubble Space Telescope (HST) ACS/F606W
polarimetry together with archival multi-band HST imaging, Chandra X-ray data
and 8.4 GHz VLA radio polarimetry. The new data confirm that synchrotron
radiation dominates in the optical. With matched beams, the jet is generally
narrower in the optical than in the radio, suggesting a stratified flow. We
examine a simple two-component model comprising a highly relativistic spine and
lower-velocity sheath. This configuration is broadly consistent with
polarization angle differences seen in the optical and radio data. The base of
the jet is relatively brighter in the ultraviolet and X-ray than at lower
energies, and the radio and optical polarization angles vary significantly as
the jet brightens downstream. Further out, the X-ray intensity rises again and
the apparent magnetic field becomes simpler, indicating a strong shock.
Modelling the synchrotron spectrum of this brightest X-ray knot provides an
estimate of its minimum internal pressure, and a comparison with the thermal
pressure from X-ray emitting gas shows that the knot is overpressured and
likely to be a temporary, expanding feature.Comment: 12 pages, 7 figures, accepted by MNRA
A GPU-based survey for millisecond radio transients using ARTEMIS
Astrophysical radio transients are excellent probes of extreme physical
processes originating from compact sources within our Galaxy and beyond. Radio
frequency signals emitted from these objects provide a means to study the
intervening medium through which they travel. Next generation radio telescopes
are designed to explore the vast unexplored parameter space of high time
resolution astronomy, but require High Performance Computing (HPC) solutions to
process the enormous volumes of data that are produced by these telescopes. We
have developed a combined software /hardware solution (code named ARTEMIS) for
real-time searches for millisecond radio transients, which uses GPU technology
to remove interstellar dispersion and detect millisecond radio bursts from
astronomical sources in real-time. Here we present an introduction to ARTEMIS.
We give a brief overview of the software pipeline, then focus specifically on
the intricacies of performing incoherent de-dispersion. We present results from
two brute-force algorithms. The first is a GPU based algorithm, designed to
exploit the L1 cache of the NVIDIA Fermi GPU. Our second algorithm is CPU based
and exploits the new AVX units in Intel Sandy Bridge CPUs.Comment: 4 pages, 7 figures. To appear in the proceedings of ADASS XXI, ed.
P.Ballester and D.Egret, ASP Conf. Se
Analysing the impact of far-out sidelobes on the imaging performance of the SKA-LOW telescope
The Square Kilometre Array’s Low Frequency instrument (SKA-LOW) will operate in the undersampled regime for most of the frequency band where grating lobes pose particular challenges. To achieve the expected level of sensitivity for SKA-LOW, it is particularly important to understand how interfering sources in both near and far side-lobes of the station beam affect the imaging performance. In this study, we discuss options for station designs, and adopting a random element layout, we assess its effectiveness by investigating how sources far from the main lobe of the station beam degrade images of the target field. These sources have the effect of introducing a noise-like corruption to images, which is called the far sidelobe confusion noise (FSCN). Using , a software simulator accelerated using graphics processing units, we carried out end-to-end simulations using an all-sky model and telescope configuration representative of the SKA-LOW instrument. The FSCN is a function of both the station beam and the interferometric point spread function, and decreases with increasing observation time until the coverage of the aperture plane no longer improves. Using apodization to reduce the level of near-in sidelobes of the station beam had a notable improvement on the level of the FSCN at low frequencies. Our results indicate that the effects of picking up sources in the sidelobes are worse at low frequencies, where the array is less sparse.This work used the Wilkes GPU cluster at the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc., NVIDIA and Mellanox, and part funded by STFC with industrial sponsorship from Rolls Royce and Mitsubishi Heavy Industries
Extragalactic jets with helical magnetic fields: relativistic MHD simulations
Extragalactic jets are inferred to harbor dynamically important, organized
magnetic fields which presumably aid in the collimation of the relativistic jet
flows. We here explore by means of grid-adaptive, high resolution numerical
simulations the morphology of AGN jets pervaded by helical field and flow
topologies. We concentrate on morphological features of the bow shock and the
jet beam behind the Mach disk, for various jet Lorentz factors and magnetic
field helicities. We investigate the influence of helical magnetic fields on
jet beam propagation in overdense external medium. We use the AMRVAC code,
employing a novel hybrid block-based AMR strategy, to compute ideal plasma
dynamics in special relativity. The helicity of the beam magnetic field is
effectively transported down the beam, with compression zones in between
diagonal internal cross-shocks showing stronger toroidal field regions. In
comparison with equivalent low-relativistic jets which get surrounded by
cocoons with vortical backflows filled by mainly toroidal field, the high speed
jets demonstrate only localized, strong toroidal field zones within the
backflow vortical structures. We find evidence for a more poloidal, straight
field layer, compressed between jet beam and backflows. This layer decreases
the destabilizing influence of the backflow on the jet beam. In all cases, the
jet beam contains rich cross-shock patterns, across which part of the kinetic
energy gets transferred. For the high speed reference jet considered here,
significant jet deceleration only occurs beyond distances exceeding , as the axial flow can reaccelerate downstream to the internal
cross-shocks. This reacceleration is magnetically aided, due to field
compression across the internal shocks which pinch the flow.Comment: 16 pages, Astronomy and Astrophysics accepted for publicatio
Extended soft X-ray emission in 3CR radio galaxies at z < 0.3: High Excitation and Broad Line Galaxies
We analyze Chandra observations of diffuse soft X-ray emission associated
with a complete sample of 3CR radio galaxies at z < 0.3. In this paper we focus
on the properties of the spectroscopic sub-classes of high excitation galaxies
(HEGs) and broad line objects (BLOs). Among the 33 HEGs we detect extended (or
possibly extended) emission in about 40% of the sources; the fraction is even
higher (8/10) restricting the analysis to the objects with exposure times
larger than 10 ks. In the 18 BLOs, extended emission is seen only in 2 objects;
this lower detection rate can be ascribed to the presence of their bright X-ray
nuclei that easily outshine any genuine diffuse emission.
A very close correspondence between the soft X-ray and optical line
morphology emerges. We also find that the ratio between [O III] and extended
soft X-ray luminosity is confined within a factor of 2 around a median value of
5. Both results are similar to what is seen in Seyfert galaxies.
We discuss different processes that could explain the soft X-ray emission and
conclude that the photoionization of extended gas, coincident with the narrow
line region, is the favored mechanism.Comment: 15 pages, 11 figures. Accepted for publication on A&
OSKAR: Simulating digital beamforming for the SKA aperture array
Digital beamforming for the aperture array components of the Square Kilometre Array (SKA) poses considerable computational challenges. We propose a hierarchical scheme aimed at tackling them and introduce OSKAR, a beamforming simulator which implements these ideas and algorithms. The simulator continues to be developed to investigate possible designs for the custom devices envisaged for phase 1 of the SKA construction. © 2010 IEEE
Science pipelines for the Square Kilometre Array
The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and the largest Big Data project in the known Universe. The first phase of the project will generate on the order of five zettabytes of data per year. A critical task for the SKA will be its ability to process data for science, which will need to be conducted by science pipelines. Together with polarization data from the LOFAR Multifrequency Snapshot Sky Survey (MSSS), we have been developing a realistic SKA-like science pipeline that can handle the large data volumes generated by LOFAR at 150 MHz. The pipeline uses task-based parallelism to image, detect sources and perform Faraday tomography across the entire LOFAR sky. The project thereby provides a unique opportunity to contribute to the technological development of the SKA telescope, while simultaneously enabling cutting-edge scientific results. In this paper, we provide an update on current efforts to develop a science pipeline that can enable tight constraints on the magnetised large-scale structure of the Universe