arXiv subjects

Dipanjan Mitra

Publications and source records attributed to Dipanjan Mitra.

78 records · Page 5Linked to original sources

arXiv2003

The effect of HII regions on rotation measure of pulsars

We have obtained new rotation measure for 11 pulsars observed with the Effelsberg 100-m radio telescope, in the direction of the Perseus arm. Using a combination of 34 published and the 11 newly measured pulsar rotation measures we study the magnetic field structure towards the Perseus arm. We find that two pulsars towards l$\sim$ 149$^{\circ}$ (Region 1) and four pulsars towards l$\sim113^{\circ}$ (Region 2) lie behind HII regions which seriously affects the pulsar rotation measures. The rotation measure of PSR J2337+6151 seem to be affected by its passage through the supernova remnant G114.3+0.3. For Region 1, we are able to constrain the random component of the magnetic field to $5.7μ$G. For the large-scale component of the Galactic magnetic field we determine a field strength of $1.7\pm1.0μ$G. This average field is constant on Galactic scales lying within the Galactic longitude range of $85^{\circ} <$ l $ < 240^{\circ}$ and we find no evidence for large scale field reversal upto 5-6 kpc. We highlight the great importance to include the effects of foreground emission in any systematic study.

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arXiv2002

Toward An Empirical Theory of Pulsar Emission. VII. On the Spectral Behavior of Conal Beam Radii and Emission Heights

In this paper we return to the old problem of conal component-pair widths and profile dimensions. Observationally, we consider a set of 10 pulsars with prominent conal component pairs, for which well measured profiles exist over the largest frequency range now possible. Apart from some tendency to narrow at high frequency, the conal components exhibit almost constant widths. We use all three profile measures, the component separation as well as the outside half-power and 10% widths, to determine conal beam radii, which are the focus of our subsequent analysis. These radii at different frequencies are well fitted by a relationship introduced by Thorsett (1991), but the resulting parameters are highly correlated. Three different types of behavior are found: one group of stars exhibits a continuous variation of beam radius which can be extrapolated down to the stellar surface along the ``last open field lines''; a second group exhibits beam radii which asymptotically approach a minimum high frequency value that is 3--5 times larger; and a third set shows almost no spectral change in beam radius at all. The first two behaviors are associated with outer-cone component pairs; whereas the constant separation appears to reflect inner-cone emission.

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arXiv2002

Vacuum Gap Model for PSR B0943+10

PSR B0943+10 is known to show remarkably stable drifting subpulses, which can be interpreted in terms of a circumferential motion of 20 sparks, each completing one circulation around the periphery of the polar cap in 37 pulsar periods. We use this observational constraint and argue that the vacuum gap model can adequately describe the observed drift patterns. Further we demonstrate that {\em only} the presence of strong non-dipolar surface magnetic field can favor such vacuum gap formation. Subsequently, for the first time we are able to constrain the parameters of the surface magnetic field, and model the expected magnetic structure on the polar cap of PSR B0943+10 considering the inverse Compton scattering photon dominated vacuum gap.

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arXiv2001

Modelling of surface magnetic field in neutron stars: application to radio pulsars

We propose a vacuum gap (VG) model which can be applied uniformly for normal and high magnetic field pulsars. The model requires strong and non-dipolar surface magnetic field near the pulsar polar cap. We assume that the actual surface magnetic field in pulsars results from a superposition of global dipole field and crust-anchored small scale magnetic anomaly. We provide a numerical formalism for modelling such structures of surface magnetic field and explore it within the framework of VG model, which requires strong surface fields more than 10^{13} G.

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arXiv2000

Vacuum gaps in pulsars and PSR J2144-3933

In this paper we revisit the radio pulsar death line problem within the framework of curvature radiation and/or inverse compton scattering induced vacuum gap model above neutron star polar caps. Our special interest is in the recently detected pulsar PSR J2144-3933 with extremal period 8.5 seconds, which lies far beyond conventional death lines. We argue, that formation of vacuum gaps requires a complicated multipolar surface magnetic field, with a strenght $B_s$ much higher than the surface dipolar component $B_d$, and radii of curvature ${\cal R}$ much smaller than the neutron star radius $R=10^6$ cm. Such a multipolar surface field is also consistent with death lines including the extremal pulsar PSR J2144-3933. Since vacuum gap models produce sparks, our paper naturally supports the spark related models of subpulse drift phenomenon as well as to the spark associated models of coherent pulsar radio emission.

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arXiv1999

Evolution of Multipolar Magnetic Field in Isolated Neutron Stars

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. We find that except for multipoles of very high order ($l\gsim 25$) the evolution is similar to that of a dipole. Therefore no significant evolution is expected in pulse shape of isolated radio pulsars due to the evolution of the multipole structure of the magnetic field.

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