128 research outputs found
Shocks, Outflows and Bubbles: New Views on Pulsars and their Winds
A typical young pulsar slows down at an imperceptible rate, its spin period
increasing by less than 10 microseconds over the course of a year. However, the
inertia of a pulsar is so extreme that to effect this tiny change in rotation
rate, the star must dissipate about 10^46 ergs of kinetic energy. Observations
of pulsars and their surroundings demonstrate that this ``spin-down energy'' is
expelled into the pulsar's surroundings in spectacular fashion, in the form of
a relativistic wind of charged particles and magnetic fields. In this review I
highlight some recent observational results on pulsar winds at radio, X-ray and
optical wavelengths, and explain what we can learn from these data about shock
structure, particle acceleration and the interstellar medium.Comment: 8 pages, 4 embedded EPS figures, uses ws-procs9x6.cls. To appear in
proceddings of "Texas in Tuscany" (XXI Symposium on Relativistic
Astrophysics
Cosmic Magnetism with the Square Kilometre Array and its Pathfinders
One of the five key science projects for the Square Kilometre Array (SKA) is
"The Origin and Evolution of Cosmic Magnetism", in which radio polarimetry will
be used to reveal what cosmic magnets look like and what role they have played
in the evolving Universe. Many of the SKA prototypes now being built are also
targeting magnetic fields and polarimetry as key science areas. Here I review
the prospects for innovative new polarimetry and Faraday rotation experiments
with forthcoming facilities such as ASKAP, LOFAR, the ATA, the EVLA, and
ultimately the SKA. Sensitive wide-field polarisation surveys with these
telescopes will provide a dramatic new view of magnetic fields in the Milky
Way, in nearby galaxies and clusters, and in the high-redshift Universe.Comment: 7 pages, including 2 colour figures. To appear in proceedings of IAU
Symposium 259: "Cosmic Magnetic Fields: From Planets, To Stars and Galaxies",
Tenerife, Nov 200
Barrels, jets and smoke-rings: Understanding the bizarre shapes of radio supernova remnants
This thesis considers the various morphologies of radio supernova remnants (SNRs), and attempts to determine whether their appearance results from the properties of the progenitor star and its supernova explosion, or from the structure of the interstellar medium (ISM) and ambient magnetic field into which a SNR consequently expands. High-resolution observations of Supernova 1987A show a young remnant whose appearance and evolution are completely dominated by the structure of its progenitor wind. A statistical study of the Galactic population of bilateral SNRs demonstrates that the symmetry axes of these remnants run parallel to the Galactic Plane. This result can be explained by the interaction of main sequence stellar wind-bubbles with the ambient magnetic field; expansion of SNRs into the resulting elongated cavities results in a bilateral appearance with the observed alignment. Radio observations of SNR G296.8-00.3 show a double-ringed morphology which is best explained by expansion either into an anisotropic main-sequence progenitor wind or into multiple cavities in the ISM. Data on SNRs G309.2-00.6 and G320.4-01.2 (MSH 15-52) make a strong case that the appearance of both remnants is significantly affected by collimated outflows from a central source; for G309.2-00.6 the source itself is not detected, but for G320.4-01.2 there is now compelling evidence that the remnant is associated with and is interacting with the young pulsar PSR B1509-58. I conclude that, while the youngest SNRs are shaped by their progenitor's circumstellar material, the appearance of most SNRs reflects the properties of the local ISM and magnetic field. Remnants which interact with an associated pulsar or binary system appear to be rare, and are easily distinguished by their unusual and distorted morphologies
"A Search For The Young and Energetic Pulsar in G328.4+0.2"
The pulsar-powered nebula G328.4+0.2 is one of the largest and most luminous such sources known. The nature of G328.4+0.2 has been a source of controversy - the object s flat radio spectral index has been used to argue that this object is a young pulsar wind nebula (PWN), while others have used radial protrusions in the magnetic field orientation along the source s outer edge to argue that it is an old supernova remnant (SNR). In the first interpretation, the X-ray nebula inside this radio source would be located inside the central "bar" detected in the radio. In the second interpretation, the expectation is that the X-ray PWN would be located at either end of this central "bar". The goals of our XMM observation were to try and detect the pulsar, and to use its location and other properties to distinguish between the above two possibilities
Bow shocks around pulsars and neutron stars
Pulsar wind nebulae are now well established as important probes both of
neutron stars' relativistic winds and of the surrounding interstellar medium.
Amongst this diverse group of objects, pulsar bow shocks have long been
regarded as an oddity, only seen around a handful of rapidly moving neutron
stars. However, recent efforts at optical, radio and X-ray wavelengths have
identified many new pulsar bow shocks, and these results have consequently
motivated renewed theoretical efforts to model these systems. Here I review the
new results and ideas which have emerged on these spectacular systems, and
explain how bow shocks and "Crab-like" nebulae now form a consistent picture
within our understanding of pulsar winds.Comment: 12 pages, 2 embedded EPS figures, 1 GIF figure. Advances in Space
Research, in pres
Low Frequency Radio Constraints on the Synchrotron Cosmic Web
We present a search for the synchrotron emission from the synchrotron cosmic
web by cross correlating 180MHz radio images from the Murchison Widefield Array
with tracers of large scale structure (LSS). We use two versions of the radio
image covering degrees with point sources brighter than
0.05 Jy subtracted, with and without filtering of Galactic emission. As tracers
of the LSS we use the Two-Micron-All-Sky-Survey (2MASS) and the Widefield
InfraRed Explorer (WISE) redshift catalogues to produce galaxy number density
maps. The cross correlation functions all show peak amplitudes at zero degrees,
decreasing with varying slopes towards zero correlation over a range of one
degree. The cross correlation signals include components from point source,
Galactic, and extragalactic diffuse emission. We use models of the diffuse
emission from smoothing the density maps with Gaussians of sizes 1-4 Mpc to
find limits on the cosmic web components. From these models we find surface
brightness 99.7 per cent upper limits in the range of 0.09-2.20 mJy beam
(average beam size of 2.6 arcmin), corresponding to 0.01-0.30 mJy
arcmin. Assuming equipartition between energy densities of cosmic rays
and the magnetic field, the flux density limits translate to magnetic field
strength limits of 0.03-1.98 G, depending heavily on the spectral index.
We conclude that for a 3 detection of 0.1 G magnetic field
strengths via cross correlations, image depths of sub-mJy to sub-Jy are
necessary. We include discussion on the treatment and effect of extragalactic
point sources and Galactic emission, and next steps for building on this work.Comment: 25 pages, 18 figures, 4 tables, accepted for publication in MNRA
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