arXiv subjects
Publications and source records attributed to D. Mitra.
The linear polarization position angle (PPA) of pulsar emission shows a wide variety of structures, which deviates from the classical rotating vector model. In this study we investigate the effect on the PPA due to emission arising from a range of heights. We demonstrate that by appropriately choosing the emission height we can reconstruct the PPA sweep across the integrated pulse profiles.
We present a study of single pulses from nine bright northern pulsars to investigate the behaviour of circular polarisation, V. The observations were conducted with the Effelsberg 100-m radio telescope at 1.41 GHz and 4.85 GHz and the Westerbork radio telescope at 352 MHz. For the first time, we present the average profile of the absolute circular polarisation |V| in the single pulses. We demonstrate that the average profile of |V| is the distinguishing feature between pulse components that exhibit low V in the single pulses and components that exhibit high V of either handedness, despite both cases resulting in a low mean. We also show that the |V| average profile remains virtually constant with frequency, which is not generally the case for V, leading us to the conclusion that |V| is a key quantity in the pulsar emission problem.
We report multifrequency measurements of scatter broadening times for nine highly dispersed pulsars over a wide frequency range (0.6 -- 4.9 GHz). We find the scatter broadening times to be larger than expected and to scale with frequency with an average power-law index of $3.44\pm 0.13$, i.e. significantly less than that expected from standard theories. Such possible discrepancies have been predicted very recently by Cordes & Lazio.
We have measured the scatter broadening of pulsars in the direction of the Gum nebula. For the first time, our observations show clear variations of scattering properties across the Gum nebula. The IRAS-Vela shell is shown to be a high scattering region. Our revised estimations of distances to these pulsars are consistently less by a factor of 2--3, which has very important consequences for the deduced values of radio luminosity and transverse velocity of pulsars.
The evolution of the multipolar structure of the magnetic field of isolated neutron stars is studied assuming the currents to be confined to the crust. Lower orders ($\le 25$) of multipole are seen to evolve in a manner similar to the dipole suggesting little or no evolution of the expected pulse shape. We also study the multifrequency polarization position angle traverse of PSR B0329+54 and find a significant frequency dependence above 2.7 GHz. We interpret this as an evidence of strong multipolar magnetic field present in the radio emission region.
Characterizing the shape and evolution of pulsar radio emission beams is important for understanding the observed emission. The various attempts by earlier workers investigating beam shapes have resulted in widely different conclusions. Using a carefully selected subset of the recently published multifrequency polarimetry observations of 300 radio pulsars (Gould & Lyne, 1998), we attempt to model the shape of pulsar beams. Assuming that the beam shape is elliptical, in general, and that it may depend on the angle between the rotation and the magnetic axes, we seek a consistent model where we also solve for the dependence of the beam size on frequency. From the six-frequency data on conal triple and multiple component profiles, we show that a) the pulsar emission beams follow a nested cone structure with at least three distinct cones, although only one or more of the cones may be active in a given pulsar; b) each emission cone is illuminated in the form of an annular ring of width typically about 20% of the cone radius. Although some slight preference is evident for a model where the beam is circular for an aligned rotator & latitudinally compressed for an orthogonal rotator, the possibility that the beam shape is circular at all inclinations is found to be equally consistent with the data. While the overall size scales as $P^{-0.5}$ (where $P$ is the pulsar period) as expected from the notion of dipolar open field lines, we see no evidence in support of the beam shape evolution with pulsar period.