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Bryan M. Gaensler

Publications and source records attributed to Bryan M. Gaensler.

At least 55 records · Page 3Linked to original sources
arXiv2011

Low-Mach-number turbulence in interstellar gas revealed by radio polarization gradients

The interstellar medium of the Milky Way is multi-phase, magnetized and turbulent. Turbulence in the interstellar medium produces a global cascade of random gas motions, spanning scales ranging from 100 parsecs to 1000 kilometres. Fundamental parameters of interstellar turbulence such as the sonic Mach number (the speed of sound) have been difficult to determine because observations have lacked the sensitivity and resolution to directly image the small-scale structure associated with turbulent motion. Observations of linear polarization and Faraday rotation in radio emission from the Milky Way have identified unusual polarized structures that often have no counterparts in the total radiation intensity or at other wavelengths, and whose physical significance has been unclear. Here we report that the gradient of the Stokes vector (Q,U), where Q and U are parameters describing the polarization state of radiation, provides an image of magnetized turbulence in diffuse ionized gas, manifested as a complex filamentary web of discontinuities in gas density and magnetic field. Through comparison with simulations, we demonstrate that turbulence in the warm ionized medium has a relatively low sonic Mach number, M_s <~ 2. The development of statistical tools for the analysis of polarization gradients will allow accurate determinations of the Mach number, Reynolds number and magnetic field strength in interstellar turbulence over a wide range of conditions.

astro-ph.GA↗
arXiv2011

Magnetic Fields in Large Diameter HII Regions Revealed by the Faraday Rotation of Compact Extragalactic Radio Sources

We present a study of the line-of-sight magnetic fields in five large-diameter Galactic HII regions. Using the Faraday rotation of background polarized radio sources, as well as dust-corrected H-alpha surface brightness as a probe of electron density, we estimated the strength and orientation of the magnetic field along 93 individual sight-lines through the HII regions. Each of the HII regions displayed a coherent magnetic field. The magnetic field strength (line-of-sight component) in the regions ranges from 2 to 6 microgauss, which is similar to the typical magnetic field strength in the diffuse interstellar medium. We investigated the relationship between magnetic field strength and electron density in the 5 HII regions. The slope of magnetic field vs. density in the low-density regime (0.8 < n_e < 30 per cubic cm) is very slightly above zero. We also calculated the ratio of thermal to magnetic pressure, beta_th, for each data point, which fell in the range 1.01 < beta_th < 25. Finally, we studied the orientation of the magnetic field in the solar neighborhood (d < 1.1 kpc) using our data from 5 HII regions along with existing measurements of the line-of-sight magnetic field strength from polarized pulsars whose distances have been determined from their annual parallax. We identify a net direction for the magnetic field in the solar neighborhood, but find no evidence for a preferred vertical direction of the magnetic field above or below the Galactic plane.

astro-ph.GA↗
arXiv2011

Visibility stacking in the quest for SNIa radio emission

We describe the process of stacking radio interferometry visibilities to form a deep composite image and its application to the observation of transient phenomena. We apply "visibility stacking" to 46 archival Very Large Array observations of nearby type Ia supernovae (SNeIa). This new approach provides an upper limit on the SNIa ensemble peak radio luminosity of 1.2x10^{25}erg/s/Hz at 5GHz, which is 5-10 times lower than previously measured. This luminosity implies an upper limit on the average companion stellar wind mass loss rate of 1.3x10^{-7}M_o/yr. This mass loss rate is consistent with the double degenerate scenario for SNeIa and rules out intermediate and high mass companions in the single degenerate scenario. In the era of time domain astronomy, techniques such as visibility stacking will be important in extracting the maximum amount of information from observations of populations of short lived events.

astro-ph.SR↗
arXiv2010

A 22-year Southern Sky Survey for Transient and Variable Radio Sources using the Molonglo Observatory Synthesis Telescope

We describe a 22-year survey for variable and transient radio sources, performed with archival images taken with the Molonglo Observatory Synthesis Telescope (MOST). This survey covers $2775 \unit{deg^2}$ of the sky south of $δ< -30\degree$ at an observing frequency of 843 MHz, an angular resolution of $45 \times 45 \csc | δ| \unit{arcsec^2}$ and a sensitivity of $5 σ\geq 14 \unit{mJy beam^{-1}}$. We describe a technique to compensate for image gain error, along with statistical techniques to check and classify variability in a population of light curves, with applicability to any image-based radio variability survey. Among radio light curves for almost 30000 sources, we present 53 highly variable sources and 15 transient sources. Only 3 of the transient sources, and none of the variable sources have been previously identified as transient or variable. Many of our variable sources are suspected scintillating Active Galactic Nuclei. We have identified three variable sources and one transient source that are likely to be associated with star forming galaxies at $z \simeq 0.05$, but whose implied luminosity is higher than the most luminous known radio supernova (SN1979C) by an order of magnitude. We also find a class of variable and transient source with no optical counterparts.

astro-ph.CO↗
arXiv2010

Spatial, Temporal and Spectral Properties of X-ray Emission from the Magnetar SGR~0501+4516

SGR~0501+4516 was discovered with the Swift satellite on 2008 August 22, after it emitted a series of very energetic bursts. Since then, the source was extensively monitored with Swift, the Rossi X-ray Timing Explorer (RXTE) and observed with Chandra and XMM-Newton, providing a wealth of information about its outburst behavior and burst induced changes of its persistent X-ray emission. Here we report the most accurate location of SGR~0501+4516 (with an accuracy of 0.11'') derived with Chandra. Using the combined RXTE, Swift/X-ray Telescope, Chandra and XMM-Newton observations we construct a phase connected timing solution with the longest time baseline (~240 days) to date for the source. We find that the pulse profile of the source is energy dependent and exhibits remarkable variations associated with the SGR~0501+4516 bursting activity. We also find significant spectral evolution (hardening) of the source persistent emission associated with bursts. Finally, we discuss the consequences of the SGR~0501+4516 proximity to the supernova remnant, SNR G160.9+2.6 (HB9).

astro-ph.HE↗
arXiv2009

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.

astro-ph.IM↗
arXiv2007

Revealing Cosmic Magnetism with Radio Polarimetry

While gravitation sustains the on-going evolution of the cosmos, it is magnetism that breaks gravity's symmetry and that provides the pathway to the non-thermal Universe. By enabling processes such as anisotropic pressure support, particle acceleration, and jet collimation, magnetism has for billions of years regulated the feedback vital for returning matter to the interstellar and intergalactic medium. After reviewing recent results that demonstrate the unique view of magnetic fields provided by radio astronomy, I explain how the Square Kilometre Array will provide data that will reveal what cosmic magnets look like, how they formed, and what role they have played in the evolving Universe.

astro-ph↗
arXiv2006

The Square Kilometre Array: A new probe of cosmic magnetism

Magnetic fields are a fundamental part of many astrophysical phenomena, but the evolution, structure and origin of magnetic fields are still unresolved problems in physics and astrophysics. When and how were the first fields generated? Are present-day magnetic fields the result of standard dynamo action, or do they represent rapid or recent field amplification through other processes? What role do magnetic fields play in turbulence, cosmic ray acceleration and structure formation? I explain how the Square Kilometre Array (SKA), a next-generation radio telescope, can deliver stunning new data-sets that will address these currently unanswered issues. The foundation for these experiments will be an all-sky survey of rotation measures, in which Faraday rotation toward >10^7 background sources will provide a dense grid for probing magnetism in the Milky Way, nearby galaxies, and in distant galaxies, clusters and protogalaxies. Using these data, we can map out the evolution of magnetized structures from redshifts z > 3 to the present, can distinguish between different origins for seed magnetic fields in galaxies, and can develop a detailed model of the magnetic field geometry of the intergalactic medium and of the overall Universe. In addition, the SKA will certainly discover new magnetic phenomena beyond what we can currently predict or imagine.

astro-ph↗
arXiv2006

The Evolution and Structure of Pulsar Wind Nebulae

Pulsars steadily dissipate their rotational energy via relativistic winds. Confinement of these outflows generates luminous pulsar wind nebulae, seen across the electromagnetic spectrum in synchrotron and inverse Compton emission, and in optical emission lines when they shock the surrounding medium. These sources act as important probes of relativistic shocks, particle acceleration and of interstellar gas. We review the many recent advances in the study of pulsar wind nebulae, with particular focus on the evolutionary stages through which these objects progress as they expand into their surroundings, and on morphological structures within these nebulae which directly trace the physical processes of particle acceleration and outflow. We conclude by considering some exciting new probes of pulsar wind nebulae, including the study of TeV gamma-ray emission from these sources, and observations of pulsar winds in close binary systems.

astro-ph↗
arXiv2005

Magnetic fields in the Southern Galactic Plane Survey

The Southern Galactic Plane Survey (SGPS) is a 1.4 GHz radio polarization and HI survey in a large part of the inner Galactic plane at a resolution of about an arcmin. Depolarization and Faraday rotation of polarized radiation from diffuse Galactic synchrotron emission, pulsars, and extragalactic sources can be used to infer information about the strength and structure of the Galactic magnetic field. Here, we discuss science results of the polarization data from the SGPS. We show from statistical analysis of rotation measures of polarized extragalactic sources that fluctuations in the magneto-ionized medium of the spiral arms are probably mainly caused by HII regions, while the rotation measure fluctuations in the interarm regions may be connected to the interstellar turbulent cascade. Furthermore, the variations of rotation measure with Galactic longitude enable modeling of the large-scale component of the Galactic magnetic field, including determination of the number and location of magnetic field reversals. Finally, the SGPS is an excellent way to study subparsec-scale structure in the ionized ISM by way of depolarization studies in HII regions.

astro-ph↗
arXiv2005

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.

astro-ph↗
arXiv2004

The Radio Variability of the Gravitational Lens PMN J1838-3427

We present the results of a radio variability study of the gravitational lens PMN J1838-3427. Our motivation was to determine the Hubble constant by measuring the time delay between variations of the two quasar images. We monitored the system for 4 months (approximately 5 times longer than the expected delay) using the Australia Telescope Compact Array at 9 GHz. Although both images were variable on a time scale of a few days, no correlated intrinsic variability could be identified, and therefore no time delay could be measured. Notably, the fractional variation of the fainter image (8%) was greater than that of the brighter image (4%), whereas lensed images of a point source would have the same fractional variation. This effect can be explained, at least in part, as the refractive scintillation of both images due to the turbulent interstellar medium of the Galaxy.

astro-ph↗
arXiv2004

Pulsar Wind Nebulae in EGRET Error Boxes

A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET gamma-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least 6 new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be associated with steady EGRET sources with hard spectra, typical for gamma-ray pulsars. Their toroidal morphologies can help determine the geometry of the pulsar which is useful for constraining models of pulsed gamma-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM.

astro-ph↗
arXiv2004

Ain't No Crab, PWN Got A Brand New Bag: Correlated radio and X-ray Structures in Pulsar Wind Nebulae

The traditional view of radio pulsar wind nebulae (PWN), encouraged by the Crab nebula's X-ray and radio morphologies, is that they are a result of the integrated history of their pulsars' wind. The radio emission should therefore be largely unaffected by recent pulsar activity, and hence minimally correlated with structures in the X-ray nebulae. Observations of several PWN, both stationary (sPWN) and rapidly moving (rPWN), now show clear morphological relationships between structures in the radio and X-ray with radio intensity variations on the order of unity. We present high-resolution X-ray and radio images of several PWN of both types and discuss the morphological relationships between the two wavebands.

astro-ph↗
arXiv2004

Shocks and Wind Bubbles Around Energetic Pulsars

The Crab Nebula demonstrates that neutron stars can interact with their environments in spectacular fashion, their relativistic winds generating nebulae observable across the electromagnetic spectrum. At many previous conferences, astronomers have discussed, debated and puzzled over the complicated structures seen in the Crab, but have been limited to treating most other pulsar wind nebulae (PWNe) as simple calorimeters for a pulsar's spin-down energy. However, with the wealth of high-quality data which have now become available, this situation has changed dramatically. I here review some of the main observational themes which have emerged from these new measurements. Highlights include the ubiquity of pulsar termination shocks, the unambiguous presence of relativistic jets in PWNe, complicated time variability seen in PWN structures, and the use of bow shocks to probe the interaction of pulsar winds with the ambient medium.

astro-ph↗
arXiv2004

X-ray Sources and their Optical Counterparts in the Globular Cluster M4

We report on the Chandra X-ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the core and 19 more within the cluster half-mass radius. The limiting luminosity of this observation is Lx~10e29 erg/sec for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify 6 X-ray sources with known objects and use ROSAT observations to show that the brightest X-ray source is variable. Archival data from the Hubble Space Telescope allow us to identify optical counterparts to 16 X-ray sources. Based on the X-ray and optical properties of the identifications and the information from the literature, we classify two (possibly three) sources as cataclysmic variables, one X-ray source as a millisecond pulsar and 12 sources as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass.

astro-ph↗
arXiv2004

Are Supershells Powered by Multiple Supernovae? Modeling the Radio Pulsar Population Produced by OB Associations

Traditional searches for radio pulsars have targeted individual small regions such as supernova remnants or globular clusters, or have covered large contiguous regions of the sky. None of these searches has been specifically directed towards giant supershells, some of which are likely to have been produced by multiple supernova (SN) explosions from an OB association. Here we perform a Montecarlo simulation of the pulsar population associated with supershells powered by multiple SNe. We predict that several tens of radio pulsars could be detected with current instruments associated with the largest Galactic supershells (with kinetic energies >~ 10^{53} ergs), and a few pulsars with the smaller ones. We test these predictions for some of the supershells which lie in regions covered by past pulsar surveys. For the smaller supershells, our results are consistent with the few detected pulsars per bubble. For the giant supershell GSH 242-03+37, we find the multiple SN hypothesis inconsistent with current data at the 95% level. We stress the importance of undertaking deep pulsar surveys in correlation with supershells. Failure to detect any pulsar enhancement in the largest of them would put serious constraints on the multiple SN origin for them. Conversely, the discovery of the pulsar population associated with a supershell would allow a different/independent approach to the study of pulsar properties.

astro-ph↗