Dipanjan Mitra - National Center for Radio Astrophysics, TIFR, Pune, India

Dipanjan Mitra
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Dipanjan Mitra
National Center for Radio Astrophysics, TIFR, Pune, India

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High Energy Astrophysical Phenomena (13)
Astrophysics (12)
Astrophysics of Galaxies (12)
Solar and Stellar Astrophysics (2)

Publications Authored By Dipanjan Mitra

Radio pulsar PSR B1946+35 is a classical example of a core/cone triple pulsar where the observer's line-of-sight cuts the emission beam centrally. In this paper we perform a detailed single-pulse polarimetric analysis of B1946+35 using sensitive Arecibo archival and new observations at 1.4 and 4. Read More

We constrain the X-ray properties of the nearby $(360\,{\rm pc})$, old ($5\,{\rm Myr}$) pulsar B1133+16 with $\sim 100\,{\rm ks}$ effective exposure time by {\it XMM-Newton}. The observed pulsar flux in the 0.2-3 keV energy range is $\sim 10^{-14} \, {\rm erg \, cm}^{-2} \, {\rm s}^{-1}$, which results in the recording of $\sim 600$ source counts with the EPIC pn and MOS detectors. Read More

We have conducted the Meterwavelength Single-pulse Polarimetric Emission Survey to study the radio emission properties of normal pulsars. A total of 123 pulsars with periods between 0.1 seconds and 8. Read More

A large sample of pulsars was observed as part of the Meterwavelength Single-pulse Polarimetric Emission Survey. We carried out a detailed fluctuation spectral analysis which revealed periodic features in 46% pulsars including 22 pulsars where drifting characteristics were reported for the first time. The pulsar population can be categorized into three distinct groups, pulsars which show systematic drift motion within the pulse window, the pulsars showing no systematic drift but periodic amplitude fluctuation and pulsars with no periodic variations. Read More

Radio pulsar B1933+16 is brightest core-radiation dominated pulsar in the Arecibo sky, and here we carry out a comprehensive high resolution polarimetric study of its radiation at both 1.5 and 4.6 GHz. Read More

We review a physical model where the high brightness temperature of 10$^{25}-10^{30}$ K observed in pulsar radio emission is explained by coherent curvature radiation excited in the relativistic electron-positron plasma in the pulsar magnetosphere. Read More

Bright single pulses of many radio pulsars show rapid intensity fluctuations (called microstructure) when observed with time resolutions of tens of microseconds. Here, we report an analysis of Arecibo 59.5 $\mu$sec-resolution polarimetric observations of 11 P-band and 32 L-band pulsars with periods ranging from 150 msec to 3. Read More

We present the measurements of scatter broadening time-scales ($\tau_{sc}$) for 124 pulsars at 327 MHz, using the upgraded Ooty Radio Telescope (ORT). These pulsars lie in the dispersion measure range of 37 $-$ 503 pc cm$^{-3}$ and declination ($\delta$) range of $-$57$^{\circ} < \delta< 60^{\circ}$. New $\tau_{sc}$ estimates for 58 pulsars are presented, increasing the sample of all such measurements by about 40% at 327 MHz. Read More

Precursors and postcursors (PPCs) are rare emission components detected in a handful of pulsars that appear beyond the main pulse emission, in some cases far away from it. In this paper we attempt to characterize the PPC emission in relation to the pulsar main pulse geometry. In our analysis we find that PPC components have properties very different from that of outer conal emission. Read More

The pulsar radio emission is generated in the near magnetosphere of the neutron star and it has to propagate through the rest of it to emerge into the interstellar medium. An important issue is whether this propagation affects the planes of polarization of the generated radiation. Observationally, there is a sufficient evidence that the emerging radiation is polarized parallel or perpendicular to the magnetic field line planes that should be associated with the ordinary O and extraordinary X plasma modes respectively, excited by some radiative process. Read More

We have examined the cyclotron resonance instability developing in the relativistic out-flowing plasma in the pulsar magnetosphere. The instability condition leads to radio emission in the sub-GHz frequency regime which is likely to be seen as off-pulse emission. Recent studies have shown the presence of off-pulse emission in long period pulsars, and we demonstrate this plasma process to be an energetically viable mechanism. Read More

Searching for the physical mechanism that can excite the coherent radio emission in pulsars is still an enigmatic problem. A wealth of high quality observations exist, which over the years have been instrumental in putting stringent constraints to pulsar emission models. In this article we will discuss the observational results that strongly suggests that pulsar radio emission is excited by coherent curvature radiation. Read More

The research presented here examines an 8-hour observation of pulsar B1822-09,taken by the Giant Metrewave Radio Telescope. B1822-09 has been known to exhibit two stable emission modes, the B-mode, where the precursor (PC) `turns-on', and the Q-mode, which is defined by interpulse (IP) emission. The results of our analysis, of this extremely long observation, have shown that B1822-09 exhibits at least three other emission behaviors that have not been seen before in other similar pulsars or in other observations of B1822-09. Read More

In Basu et al. 2011 we reported the detection of Off-pulse emission from two long period pulsars B0525+21 and B2045-16. The pulsars were observed at a single epoch using the 325 MHz frequency band of the Giant Meterwave Radio Telescope (GMRT). Read More

We study the radio--FIR correlation between the nonthermal (synchrotron) radio continuum emission at \lambda 90 cm (333 MHz) and the far infrared emission due to cool (~20 K) dust at \lambda 70\mu m in spatially resolved normal galaxies at scales of ~1 kpc. The slope of the radio--FIR correlation significantly differs between the arm and interarm regions. However, this change is not evident at a lower wavelength of \lambda 20 cm (1. Read More

We report the detection of off-pulse emission from two long period pulsars PSR B0525+21 and PSR B2045-16 using the Giant Metrewave Radio Telescope (GMRT). We recorded high time resolution interferometric data at 325 MHz and gated the data offline to separate the on- and off-pulse sections of the pulsar. On imaging the two temporal sections separately, we detected a point source in the off-pulse images of both the pulsars coincident with the pulsar locations in the on-pulse images. Read More

We report analysis of an 8 hr observation of PSR B0943+10 at 325 MHz performed at the Giant Metrewave Radio Telescope (GMRT) in India. B0943+10 is well known for displaying regular sub-pulse drifting and two emission modes. We investigate the modal behavior of B0943+10. Read More

We report Giant Meterwave Radio Telescope (GMRT) continuum observations of six nearby normal galaxies at 333 MHz. The galaxies are observed with angular resolutions better than ~20" (corresponding to a linear scale of about 0.4 - 1 kpc). Read More

Lyne & Manchester (1988) identified a group of some 50 pulsars they called "partial cones" which they found difficult to classify and interpret. They were notable for their asymmetric average profiles and asymmetric polarization position-angle (PPA) traverses, wherein the steepest gradient (SG) point fell toward one edge of the total intensity profile. Over the last two decades, this population of pulsars has raised cautions regarding the core/cone model of the radio pulsar-emission beam which implies a high degree of order, symmetry and geometric regularity. Read More

This paper reports new observations of pulsars B0943+10 and B1822--09 carried out with the Arecibo Observatory (AO) and the Giant Metrewave Radio Telescope (GMRT), respectively. Both stars exhibit two stable emission modes. We report the discovery in B0943+10 of a highly linearly polarized precursor component that occurs primarily in only one mode. Read More

Forty years have passed since the discovery of pulsars, yet the physical mechanism of their coherent radio emission is a mystery. Recent observational and theoretical studies strongly suggest that the radiation outcoming from the pulsar magnetosphere consists mainly of extraordinary waves polarized perpendicular to the planes of pulsar dipolar magnetic field. However, the fundamental question whether these waves are excited by maser or coherent curvature radiation, remains open. Read More

We have observed a total of 67 pulsars at five frequencies ranging from 243 to 3100 MHz. Observations at the lower frequencies were made at the Giant Metre Wave Telescope in India and those at higher frequencies at the Parkes Telescope in Australia. We present profiles from 34 of the sample with the best signal to noise ratio and the least scattering. Read More

New GMRT observations of the five-component pulsar B1857--26 provide detailed insight into its pulse-sequence modulation phenomena for the first time. The outer conal components exhibit a 7.4-rotation-period, longitude-stationary modulation. Read More

Affiliations: 1Inter-University Centre for Astronomy and Astrophysics, Pune, India, 2National Center for Radio Astrophysics, TIFR, Pune, India, 3National Center for Radio Astrophysics, TIFR, Pune, India
Category: Astrophysics

In an earlier paper (Ahuja, et al, 2005), based on simultaneous multi-frequency observations with the Giant Metrewave Radio Telescope (GMRT), we reported the variation of pulsar dispersion measures (DMs) with frequency. A few different explanations are possible for such frequency dependence, and a possible candidate is the effect of pulse shape evolution on the DM estimation technique. In this paper we describe extensive simulations we have done to investigate the effect of pulse profile evolution on pulsar DM estimates. Read More

In this paper we report multifrequency single pulse polarization observations of the PSR B0329+54 normal mode using the Giant Meterwave Radio Telescope at 325 and 610 MHz and the Effelsberg Observatory at 2695 MHz. Our observations show that towards the central part of the polarization position-angle traverse there is an unusual ``arc''-like structure, which is comprised of a broadband ``kink'' and a frequency-dependent ``spot.'' The features are intimately connected with the intensity dependence of the core component: the stronger emission arrives earlier and its linear polarization is displaced farther along the ``kink''. Read More

We use a set of carefully selected published average multifrequency polarimetric observations for six bright cone dominated pulsars and devise a method to combine the multifrequency polarization position angle (PPA) sweep traverses. We demonstrate that the PPA traverse is in excellent agreement with the rotating vector model over this broad frequency range confirming that radio emission emanates from perfectly dipolar field lines. For pulsars with central core emission in our sample, we find the peak of central core component to lag the steepest gradient of the PPA traverse at several frequencies. Read More

Affiliations: 1MPI fuer Radioastronomie, Bonn, Germany, 2MPI fuer Radioastronomie, Bonn, Germany, 3MPI fuer Radioastronomie, Bonn, Germany
Category: Astrophysics

Combining dispersion measures, distances and emission measures for 157 pulsars lying above \mid b \mid > 5 degree and between 60 degree < l < 360 degree we find the mean volume filling factor (\bar{f_v}) of the diffused ionized gas in the Milky Way. This filling factor is inversely related to the mean electron density (\bar{n_c}) in the clouds, \bar{f_v} = (0.0184 +/- 0. Read More

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. Read More

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. Read More

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. Read More

Affiliations: 1Institute of Astronomy, University of Zielona Gora, Poland, 2Institute of Astronomy, University of Zielona Gora, Poland, 3Max-Planck Institute for Radioastronomy, Bonn, Germany
Category: Astrophysics

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. Read More

Affiliations: 1J. Kepler Astronomical Centre, 2Raman Research Institut
Category: Astrophysics

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. Read More

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. Read More