Alignment of Quasar Polarizations on Large Scales Explained by Warped Cosmic Strings

The recently discovered alignment of quasar polarizations on very large scales could possibly be explained by considering cosmic strings on a warped five dimensional spacetime. Compact objects, such as cosmic strings, could have tremendous mass in the bulk, while their warped manifestations in the brane can be consistent with general relativity in 4D. The self-gravitating cosmic string induces gravitational wavelike disturbances which could have effects felt on the brane, i.e., the massive effective 4D modes (Kaluza-Klein modes) of the perturbative 5D graviton. This effect is amplified by the time dependent part of the warp factor. Due to this warp factor, disturbances don’t fade away during the expansion of the universe. From a nonlinear perturbation analysis it is found that the effective Einstein 4D equations on an axially symmetric spacetime, contain a “back-reaction” term on the righthand side caused by the projected 5D Weyl tensor and can act as a dark energy term. The propagation equations to first order for the metric components and scalar-gauge fields contain -dependent terms, so the approximate wave solutions are no longer axially symmetric. The disturbances, amplified by the warp factor, can possess extremal values for fixed polar angles. This could explain the two preferred polarization vectors mod .

Multifrequency Polarization Variations in the Quasar 0917+624

We present a detailed analysis of multi-frequency observations of linear polarization in the intraday variable quasar 0917+624 (z = 1.44). The observations were made in May 1989 at five frequencies (1.4, 2.7, 5.0, 8.3 and 15GHz) with the VLA and the Effelsberg 100 m-telescope and in December 1988 at two frequencies (2.7 and 5.0 GHz) with the latter. It is shown that the relationship between the variations of the polarized and total flux density is highly wavelength dependent, and the multi-frequency polarization behavior may be essential for investigating the mechanisms causing these variations. It is shown that the variations observed at 20 cm can be interpreted in terms of refractive interstellar scintillation. However, after subtracting the variation due to scintillation, three 'features' emerged in the light-curve of the polarized flux density, indicating an additional variable component. Interestingly, these features are shown to be correlated with the variations at 2-6 cm, thus indicating that these features and the associated variations are due to some intrinsic causes. Moreover, a very rapid polarization angle swing of - 180° observed in December 1988 which cannot be explained by refractive interstellar scintillation, may also be produced by an intrinsic mechanism. Accordingly, we use a shock model to explain the polarization variations observed at the higher frequencies, although scintillation could also exist. The shock model can explain not only the variation of intensity, but also the time variation of its degree and angle of polarization, including the rapid swing of the polarization angle. It is shown that the degree and angle of polarization of the shock need only vary slightly in order to account for the observed complicated behaviour of polarization.

Refractive Focusing by Interstellar Clouds and the Rapid Polarization Angle Swing in QSO 1150+812

A very rapid polarization position angle swing of -180° （with a time scale of ,-6 hours） observed at 2 cm in QSO 1150＋812 （z = 1.25） was reported by Kochenov ＆ Gabuzda. This very rare event is difficult to explain. We found a possible interpretation in the framework of a source model consisting of three polarized components, in which two compact polarized components are nearly simultaneously occulted by an interstellar cloud, with consequent focusing-defocusing effects. A specific plasma-lens model is proposed which can reasonably fit the polarized flux density curve with results derived for the two lensed components. Some physical parameters of the plasma-lens and the source components are estimated. The two compact polarized components are estimated to have brightness temperatures of -6 ×10^12 K. Thus a bulk relativistic motion with a Lorentz factor less than 10 is required to meet the inverse-Compton limit.

Refractive Focusing of Interstellar Clouds and Intraday Polarization Angle Swings

Intraday variations of compact extragalactic radio sources in flux density and polarization are generally interpreted in terms of refractive scintillation from the continuous interstellar medium of our Galaxy. However, continuous polarization angle swings of - 180° （for example, the one observed in the QSO 0917＋624） could not be interpreted in this way. Qian et al. have shown that the polarization angle swing observed in the QSO 1150＋812 can be explained in terms of focusing-defocusing effect by an interstellar cloud, which occults two closely-placed polarized components. Here we further show that the polarization angle swing event observed in the QSO 0917＋624 can also be explained in this way. We also found evidence for the cloud eclipsing a non-polarized （core） component during a short period out- side the swing. A particular （and specific） plasma-lens model is proposed to model-fit the polarization swing event of 0917＋624. Some physical parameters related to the plasma-lens and the source components are estimated. The brightness temperatures of the two lensed components are estimated to be -1.6× 10^13 K. Thus bulk relativistic motion with a Lorentz factor less than -20 may be sufficient to avoid the inverse - Compton catastrophe.