arXiv subjects

R. T. Gangadhara

Publications and source records attributed to R. T. Gangadhara.

25 records · Page 2Linked to original sources

arXiv2001

Influence of Rotation on Pulsar Radiation Characteristics

We present a relativistic model for pulsar radio emission by including the effect of rotation on coherent curvature radiation by bunches. We find that rotation broadens the width of leading component compared to the width of trailing component. We estimate the component widths in the average pulse profiles of about 24 pulsars, and find that 19 of them have a broader leading component. We explain this difference in the component widths by using the nested cone emission geometry. We estimate the effect of pulsar spin on the Stokes parameters, and find that the inclination between the rotation and magnetic axes can introduce an asymmetry in the circular polarization of the conal components. We analyze the single pulse polarization data of PSR B0329+54 at 606 MHz, and find that in its conal components, one sense of circular polarization dominates in the leading component while the other sense dominates in the trailing component. Our simulation shows that changing the sign of the impact parameter changes the sense of circular polarization as well as the swing of polarization angle.

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arXiv2001

Understanding the radio emission geometry of PSR B0329+54

We have analyzed high-quality single pulse data of PSR B0329+54 at 325 MHz and 606 MHz to study the structure of the emission beam. Using the window-threshold technique, which is suitable for detecting weak emission components, we have detected 4 additional emission components in the pulse window. Three of these are new components and the fourth is a confirmation of a recently proposed component. Hence PSR B0329+54 is now known to have 9 emission components - the highest among all known pulsars. The distribution of the pulse components around the central core component indicates that the emission beam consists of four nested cones. The asymmetry in the location of the conal components in the leading versus trailing parts of the profile is interpreted as being due to aberration and retardation in the pulsar magnetosphere. These measurements allow us to determine the precise location of the 4 conal rings of emission. We find that the successive outer cones are emitted at higher altitudes in the magnetosphere. Further, for any given cone, the emission height at the lower frequency is found to be more than that at the higher frequency. The inferred heights range from ~160 km to ~1150 km. The set of ``active'' field lines, from which most of the conal radiation appears to originate, are found to be confined to a region located within ~0.5 to ~0.6 of the polar cap radius. We discuss the implications of our new findings on our understanding of the pulsar emission geometry and its impact on the emission mechanisms.

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arXiv1999

Detection of new emission components in PSR B0329+-54

To study the structure of emission beam, we have analysed the single pulse data of PSR B0329+-54 at 325 and 606 MHz. In order to unambiguously detect the weak emission components, we have developed a new data analysis technique, which we term ``window-thresholding''. By applying this technique to the data, we have detected three new emission components, and also confirmed the presence of a component which was proposed earlier. Hence our analysis indicates that PSR B0329+-54 has nine components, which is among the highest of all the known pulsars. The symmetric distribution of pulse components about the pulse centre, indicates that the emission beam is conal.

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arXiv1999

Stimulated Raman scattering of water maser lines in astrophysical plasmas

Radiative transfer equations are derived and solved for the stimulated Raman scattering of water maser lines in the astrophysical plasmas with electron density of about 10^6 - 10^7 cm-3. In stimulated Raman scattering, the energy of water maser line is transferred to the side band modes: Stokes mode and anti-Stokes mode. The Stokes mode is easily produced by backward Raman scattering while the anti-Stokes mode is created by the interacting intersecting masers in the plasma. The intensity of the Stokes mode is higher than that of the anti-Stokes mode. These side band modes are proposed as explanation for the extreme velocity features observed in the galaxy NGC 4258. The threshold value of the brightness temperature for the Raman scattering is about 10^16 - 10^19 K, and it is satisfied in the case of NGC 4258.

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arXiv1999

Propagation effect on polarization of pulsar radio emission

We consider the role of a propagation effect such as the stimulated Raman scattering on the polarization of radio pulses. When an intense electromagnetic wave with frequency close to the plasma frequency interacts with the plasma in the pulsar magnetosphere, the incident wave undergoes stimulated Raman scattering. Using typical plasma and magnetic field parameters, we compute the growth rate and estimate the polarization properties of the scattered mode. At some conditions, we find that the polarization properties of the scattered mode can become significantly different from those of the incident wave. The frequencies, at which strong Raman scattering occurs in the outer parts of magnetosphere, fall into the observed radio band. In some pulsars, for example, PSR B0628-28 and 1914+13, a large and symmetric type of circular polarization has been observed. We propose that such an unusual circular polarization is produced by the propagation effects.

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arXiv1997

On the nonthermal emission in active galactic nuclei

We consider the role of centrifugal force on the energetics of electrons moving along the magnetic field lines of spinning active galactic nuclei. We find the energy gained by charged particles against inverse Compton scattering and/ synchrotron radiation losses, become quite significant in the region close to the light cylinder. The particles accelerated by the centrifugal force become a part of the jet material. The scattering of UV-photons against the energetic electrons lead to the generation of X-ray and gamma-ray photons within the light cylinder, and outside, they radiate the non-thermal optical radiation via the synchrotron emission.

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arXiv1997

Orthogonal polarization mode phenomenon in pulsars

We consider the polarization properties of radiation emitted by relativistic charged particles while moving along the curved magnetic field lines in a pulsar manetosphere. We propose that the radiation emitted by positrons and electrons while moving along the curved magnetic field lines is orthogonally polarized. The polarization angle of each orthogonal polarization mode is well described by the rotating vector model. However, the polarization angle swings observed in micropulses and subpulses are often found to be contrary to the rotating vector model, and such swings are expected to arise due to the coherent superposition of orthogonal polarization modes. As an application of our model, we discuss the polarization of pulsar PSR B0950+08 at the frequency 1.71 GHz. Data on individual pulses, obtained using the 100-m Effelsberg radiotelescope, was statistically analysed. We find that pulsars emit radiation mainly in the form of two independent orthogonal modes. It seems, they exist at all pulse longitudes but at some longitudes one mode dominates over the other. The polarization angle gray-plot indicates two most favoured angles separated by approximately 90 degree at each pulse longitude. The depolarization is mainly caused by the incoherent superposition of orthogonal modes. We infer from this study that pulsar radiation consists of two major elliptically polarized electromagnetic waves with orthogonal polarization angles. Our model predicts that such waves could be radiated by the positrons and the electrons accelerated along the curved magnetic field lines. The sense of circular polarization of these modes depend upon from which side of particle trajectory the radiation is received.

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