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Register a new userTwo families of elliptical plasma lenses
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Plasma lensing is the refraction of low-frequency electromagnetic rays due to free electrons in the interstellar medium. Although the phenomenon has a distinct similarity to gravitational lensing, particularly in its mathematical description, plasma lensing introduces other additional features, such as wavelength dependence, radial rather than tangential image distortions, and strong demagnification of background sources. Axisymmetrical models of plasma lenses have been well-studied in the literature, but density distributions with more complicated shapes can provide new and exotic image configurations and increase the richness of the magnification properties. As a first step towards non-axisymmetrical distributions, we study two families of elliptical plasma lens, softened power-law and exponential plasma distributions. We perform numerical studies on each lens model, and present them over a parameter space. In addition to deriving elliptical plasma lens formulae, we also investigate the number of critical curves that the lens can produce by studying the lens parameter space, in particular the dependence on the lensing ellipticity. We find that the introduction of ellipticity into the plasma distribution can enhance the lensing effects as well as the complexity of the magnification map.
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Spherical plasma lens models are known to suffer from a severe over-pressure problem, with some observations requiring lenses with central pressures up to millions of times in excess of the ambient ISM. There are two ways that lens models can solve the over-pressure problem: a confinement mechanism exists to counter the internal pressure of the lens, or the lens has a unique geometry, such that the projected column-density appears large to an observer. This occurs with highly asymmetric models, such as edge-on sheets or filaments, with potentially low volume-density. In the first part of this work we investigate the ability of non-magnetized plasma filaments to mimic the magnification of sources seen behind spherical lenses and we extend a theorem from gravitational lens studies regarding this model degeneracy. We find that for plasma lenses, the theorem produces unphysical charge density distributions. In the second part of the work, we consider the plasma lens over-pressure problem. Using magnetohydrodynamics, we develop a non self-gravitating model filament confined by a helical magnetic field. We use toy models in the force-free limit to illustrate novel lensing properties. Generally, magnetized filaments may act as lenses in any orientation with respect to the observer, with the most high density events produced from filaments with axes near the line of sight. We focus on filaments that are perpendicular to the line of sight that show the toroidal magnetic field component may be observed via the lens rotation measure.
Compact radio sources sometimes exhibit intervals of large, rapid changes in their flux-density, due to lensing by interstellar plasma crossing the line-of-sight. A novel survey program has made it possible to discover these Extreme Scattering Events (ESEs) in real time, resulting in a high-quality dynamic spectrum of an ESE observed in PKS 1939-315. Here we present a method for determining the column-density profile of a plasma lens, given only the dynamic radio spectrum of the lensed source, under the assumption that the lens is either axisymmetric or totally anisotropic. Our technique relies on the known, strong frequency dependence of the plasma refractive index in order to determine how points in the dynamic spectrum map to positions on the lens. We apply our method to high-frequency (4.2-10.8 GHz) data from the Australia Telescope Compact Array of the PKS 1939-315 ESE. The derived electron column-density profiles are very similar for the two geometries we consider, and both yield a good visual match to the data. However, the fit residuals are substantially above the noise level, and deficiencies are evident when we compare the predictions of our model to lower-frequency (1.6-3.1 GHz) data on the same ESE, thus motivating future development of more sophisticated inversion techniques.
Extreme scattering events (ESEs) are distinctive fluctuations in the brightness of astronomical radio sources caused by occulting plasma lenses in the interstellar medium. The inferred plasma pressures of the lenses are $sim 10^3$ times the ambient pressure, challenging our understanding of gas conditions in the Milky Way. Using a new survey technique, we have discovered an ESE while it was in progress. We report radio and optical follow-up observations. Modelling of the radio data demonstrates that the lensing structure is a density enhancement and that the lens is diverging, ruling out one of two competing physical models. Our technique will uncover many more ESEs, addressing a long-standing mystery of the small-scale gas structure of the Galaxy.
Recent progress is summarized on the determination of the density distributions of stars and dark matter, stellar kinematics, and stellar population properties, in the extended, low surface brightness halo regions of elliptical galaxies. With integral field absorption spectroscopy and with planetary nebulae as tracers, velocity dispersion and rotation profiles have been followed to ~4 and ~5-8 effective radii, respectively, and in M87 to the outer edge at ~150 kpc. The results are generally consistent with the known dichotomy of elliptical galaxy types, but some galaxies show more complex rotation profiles in their halos and there is a higher incidence of misalignments, indicating triaxiality. Dynamical models have shown a range of slopes for the total mass profiles, and that the inner dark matter densities in ellipticals are higher than in spiral galaxies, indicating earlier assembly redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals and comparison with dynamical mass determinations indicates that non-thermal components to the pressure may be important in the inner ~10 kpc, and that the properties of these systems are closely related to their group environments. First results on the outer halo stellar population properties do not yet give a clear picture. In the halo of one bright galaxy, lower [alpha/Fe] abundances indicate longer star formation histories pointing towards late accretion of the halo. This is consistent with independent evidence for on-going accretion, and suggests a connection to the observed size evolution of elliptical galaxies with redshift.
In the standard gravitational lensing scenario, rays from a background source are bent in the direction of a foreground lensing mass distribution. Diverging lens behaviour produces deflections in the opposite sense to gravitational lensing, and is also of astrophysical interest. In fact, diverging lensing due to compact distributions of plasma has been proposed as an explanation for the extreme scattering events (ESEs) that produce frequency-dependent dimming of extra-galactic radio sources, and may also be related to the refractive radio-wave phenomena observed to affect the flux density of pulsars. In this work we study the behaviour of two families of astrophysical diverging lenses in the geometric optics limit, the power-law and the exponential plasma lenses. Generally, the members of these model families show distinct behaviour in terms of image formation and magnification, however the inclusion of a finite core for certain power-law lenses can produce a caustic and critical curve morphology that is similar to the well-studied Gaussian plasma lens. Both model families can produce dual radial critical curves, a novel distinction from the tangential distortion usually produced by gravitational (converging) lenses. The deflection angle and magnification of a plasma lens varies with the observational frequency, producing wavelength-dependent magnifications that alter the amplitudes and the shape of the light curves. Thus, multi-wavelength observations can be used to physically constrain the distribution of the electron density in such lenses.
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