Aris Karastergiou

Abstract:

We present an empirical model for single pulses of radio emission from pulsars based on gaussian probability distributions for relevant variables. The radiation at a specific pulse phase is represented as the superposition of radiation in two (approximately) orthogonally polarized modes (OPMs) from one or more subsources in the emission region of the pulsar. For each subsource, the polarization states are drawn randomly from statistical distributions, with the mean and the variance on the Poincar\'e sphere as free parameters. The intensity of one OPM is chosen from a log-normal distribution, and the intensity of the other OPM is assumed to be partially correlated, with the degree of correlation also chosen from a gaussian distribution. The model is used to construct simulated data described in the same format as real data: distributions of the polarization of pulses on the Poincar\'e sphere and histograms of the intensity and other parameters. We concentrate on the interpretation of data for specific phases of PSR B0329+54 for which the OPMs are not orthogonal, with one well defined and the other spread out around an annulus on the Poincar\'e sphere at some phases. The results support the assumption that the radiation emerges in two OPMs with closely correlated intensities, and that in a statistical fraction of pulses one OPM is invisible.

Abstract:

We present here a direct comparison of the polarisation position angle (PA) profiles of 17 pulsars, observed at 1.4 and 3.1 GHz. Absolute PAs are obtained at each frequency, permitting a measurement of the difference in the profiles. By doing this, we obtain more precise rotation measure (RM) values for some of the pulsars in the current catalogue. We find that, apart from RM corrections, there are small, pulse longitude dependent differences in PA with frequency. Such differences go beyond the interpretation of a geometrical origin. We describe in detail the PA evolution between the two frequencies and discuss possible causes, such as orthogonal and non-orthogonal polarisation modes of emission. We also use the PA and total power profiles to estimate the difference in emission height at which the two frequencies originate. In our data sample, there are changes in the relative strengths of different pulse components, especially overlapping linearly polarised components, which coincide with intrinsic changes of the PA profile, resulting in interesting PA differences between the two frequencies.

Abstract:

We present polarization profiles for 48 southern pulsars observed with the new 10-cm receiver at the Parkes telescope. We have exploited the low system temperature and high bandwidth of the receiver to obtain profiles which have good signal-to-noise for most of our sample at this relatively high frequency. Although, as expected, a number of profiles are less linearly polarized at 3.1 GHz than at lower frequencies, we identify some pulsars and particular components of profiles in other pulsars which have increased linear polarization at this frequency. We discuss the dependence of linear polarization with frequency in the context of a model in which emission consists of the superposition of two, orthogonally polarized modes. We show that a simple model, in which the orthogonal modes have different spectral indices, can explain many of the observed properties of the frequency evolution of both the linear polarization and the total power, such as the high degree of linear polarization seen at all frequencies in some high spin-down, young pulsars. Nearly all the position angle profiles show deviations from the rotating vector model; this appears to be a general feature of high-frequency polarization observations. © 2005 RAS.

Abstract:

We present polarisation profiles for 48 southern pulsars observed with the new 10-cm receiver at the Parkes telescope. We have exploited the low system temperature and high bandwidth of the receiver to obtain profiles which have good signal to noise for most of our sample at this relatively high frequency. Although, as expected, a number of profiles are less linearly polarised at 3.1 GHz than at lower frequencies, we identify some pulsars and particular components of profiles in other pulsars which have increased linear polarisation at this frequency. We discuss the dependence of linear polarisation with frequency in the context of a model in which emission consists of the superposition of two, orthogonally polarised modes. We show that a simple model, in which the orthogonal modes have different spectral indices, can explain many of the observed properties of the frequency evolution of both the linear polarisation and the total power, such as the high degree of linear polarisation seen at all frequencies in some high spin-down, young pulsars. Nearly all the position angle profiles show deviations from the rotating vector model; this appears to be a general feature of high-frequency polarisation observations.

Abstract:

In most pulsars, the circularly polarized component, Stokes V, is weak in the average pulse profiles. By forming the average profile of |V| from single pulses we can distinguish between pulsars where V is weak in the individual pulses and those where large V of variable handedness is observed from one pulse to the other. We show how |V| profiles depend on the signal-to-noise ratio of V in the single pulses and demonstrate that it is possible to simulate the observed, broad distributions of V by assuming a model where |V| is distributed around a mean value and the handedness of V is permitted to change randomly. The |V| enhanced profiles of 1.3 pulsars are shown, five observed at 1.41 GHz and eight observed at 4.85 GHz, to complement the set in Karastergiou et al. (2003b). It is argued that the degree of circular polarization in the single pulses is related to the orthogonal polarization mode phenomenon and not to the classification of the pulse components as cone or core.