Resolving the Structure at the Heart of BAL Quasars Through Microlensing Induced Polarisation Variability

While amongst the most luminous objects in the universe, many details regarding the inner structure of quasars remain unknown. One such area is the mechanism promoting increased polarisation in the broad absorption line troughs of certain quasars. This study shows how microlensing can be used to differentiate between two popular models that explain such polarisation through a realistic computational analysis. By examining a statistical ensemble of correlation data between two observables (namely image brightness and polarisation of the flux coming from the quasar), it was found that through spectropolarimetric monitoring it would be possible to discern between a model with an external scattering region and a model without one.Comment: 13 pages - accepted for publication in PASA. Quality of figures reduce

Boson Stars as Gravitational Lenses

We discuss boson stars as possible gravitational lenses and study the lensing effect by these objects made of scalar particles. The mass and the size of a boson star may vary from an individual Newtonian object similar to the Sun to the general relativistic size and mass of a galaxy close to its Schwarzschild radius. We assume boson stars to be transparent which allows the light to pass through them though the light is gravitationally deflected. We assume boson stars of the mass $M = 10^{10}M_\odot$ to be on non-cosmological distance from the observer. We discuss the lens equation for these stars as well as the details of magnification. We find that there are typically three images of a star but the deflection angles may vary from arcseconds to even degrees. There is one tangential critical curve (Einstein ring) and one radial critical curve for tangential and radial magnification, respectively. Moreover, the deflection angles for the light passing in the gravitational field of boson stars can be very large (even of the order of degrees) which reflects the fact they are very strong relativistic objects. We also propose a suitable formula for the lens equation for such large deflection angles, and with the reservation that large deflection angle images are highly demagnified but in the area of the tangential critical curve, their existence may help in observational detection of suitable lenses possessing characteristic features of boson stars which could also serve as a direct evidence for scalar fields in the universe.Comment: accepted by Astrophys. J., 31 pages, AASTeX, 6 figure

Nova Geminorum 1912 and the Origin of the Idea of Gravitational Lensing

Einstein's early calculations of gravitational lensing, contained in a scratch notebook and dated to the spring of 1912, are reexamined. A hitherto unknown letter by Einstein suggests that he entertained the idea of explaining the phenomenon of new stars by gravitational lensing in the fall of 1915 much more seriously than was previously assumed. A reexamination of the relevant calculations by Einstein shows that, indeed, at least some of them most likely date from early October 1915. But in support of earlier historical interpretation of Einstein's notes, it is argued that the appearance of Nova Geminorum 1912 (DN Gem) in March 1912 may, in fact, provide a relevant context and motivation for Einstein's lensing calculations on the occasion of his first meeting with Erwin Freundlich during a visit in Berlin in April 1912. We also comment on the significance of Einstein's consideration of gravitational lensing in the fall of 1915 for the reconstruction of Einstein's final steps in his path towards general relativity.Comment: 31 p

Microlensing

Microlensing observations have now become a useful tool in searching for non--luminous astrophysical compact objects (brown dwarfs, faint stars, neutron stars, black holes and even planets). Originally conceived for establishing whether the halo of the Galaxy is composed of this type of objects, the ongoing searches are actually also sensitive to the dark constituents of other Galactic components (thin and thick disks, outer spheroid, bulge). We discuss here the present searches for microlensing of stars in the Magellanic Clouds and in the Galactic bulge (EROS, MACHO, OGLE and DUO collaborations). We analyse the information which can be obtained regarding the spatial distribution and motion of the lensing objects as well as about their mass function and their overall contribution to the mass of the Galaxy. We also discuss the additional signals, such as the parallax due to the motion of the Earth, the effects due to the finite source size and the lensing events involving binary objects, which can further constrain the lens properties. We describe the future prospects for these searches and the further proposed observations which could help to elucidate these issues, such as microlensing of stars in the Andromeda galaxy, satellite parallax measurements and infrared observations.Comment: Latex file, 59 p., 13 ps figures, to appear in Physics Reports (postcript file replaced by latex file

Notes on Hidden Mirror World

A few remarks on Dark Matter (DM) models are presented. An example is Mirror Matter which is the oldest but still viable DM candidate, perhaps not in the purest form. It can serve as a test-bench for other analogous DM models, since the properties of macroscopic objects are quite firmly fixed for Mirror Matter. A pedagogical derivation of virial theorem is given and it is pointed out that concepts of virial velocity or virial temperature are misleading for some cases. It is shown that the limits on self-interaction cross-sections derived from observations of colliding clusters of galaxies are not real limits for individual particles if they form macroscopic bodies. The effect of the heating of interstellar medium by Mirror Matter compact stars is very weak but may be observable. The effect of neutron star heating by accretion of M-baryons may be negligible. Problems of MACHOs as Mirror Matter stars are touched upon.Comment: Latex, revtex, 24 pages, 1 figure, references updated and adde

Gravitational Lensing in Astronomy

Deflection of light by gravity was predicted by General Relativity and observationaly confirmed in 1919. In the following decades various aspects of the gravitational lens effect were explored theoretically, among them the possibility of multiple or ring-like images of background sources, the use of lensing as a gravitational telescope on very faint and distant objects, and the possibility to determine Hubble's constant with lensing. Only relatively recently gravitational lensing became an observational science after the discovery of the first doubly imaged quasar in 1979. Today lensing is a booming part of astrophysics. In addition to multiply-imaged quasars, a number of other aspects of lensing have been discovered since, e.g. giant luminous arcs, quasar microlensing, Einstein rings, galactic microlensing events, arclets, or weak gravitational lensing. By now literally hundreds of individual gravitational lens phenomena are known. Although still in its childhood, lensing has established itself as a very useful astrophysical tool with some remarkable successes. It has contributed significant new results in areas as different as the cosmological distance scale, the large scale matter distribution in the universe, mass and mass distribution of galaxy clusters, physics of quasars, dark matter in galaxy halos, or galaxy structure.Comment: Review article for "Living Reviews in Relativity", see http://www.livingreviews.org . 41 pages, latex, 22 figures (partly in GIF format due to size constraints). High quality postscript files can be obtained electronically at http://www.aip.de:8080/~jkw/review_figures.htm