• Volume 16, Issue 3-4

      December 1995,   pages  326-424

    • Preface

      G. Srinivasan

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    • Polarization-mode separation and the emission geometry of pulsar 0823 + 26: A new pattern of pulsar emission?

      Joanna M. Rankin N. Rathnasree

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      We explore the detailed polarization behaviour of pulsar 0823 + 26 using the technique of constructing partial ‘mode-separated’ profiles corresponding to the primary and secondary polarization modes. The characteristics of the two polarization modes in this pulsar are particularly interesting, both because they are anything but orthogonal and because the secondary mode exhibits a structure seen neither in the primary mode nor in the total profile. The new leading and trailing features in the secondary mode, which appear to represent a conal component pair, are interpreted geometrically on the basis of their width and the associated polarization-angle traverse as an outer cone.

      If the secondary-mode features are, indeed, an outer cone, then questions about the significance of the pulsar’s postcursor component become more pressing. It seems that 0823 + 26 has a very nearly equatorial geometry, in that both magnetic poles and the sightline all fall close to the rotational equator of the star. We thus associate the postcursor component with emission along those bundles of field lines which are also equatorial and which continue to have a tangent in the direction of our sight line for a significant portion of the star’s rotation cycle. It seems that in all pulsars with postcursor components, this emission follows the core component, and all may thus have equatorial emission geometries. No pulsars with precursors in this sense — including the Crab pulsar — are known.

      The distribution of power between the primary and secondary modes is very similar at both 430 and 1400 MHz. Our analysis shows that in this pulsar considerable depolarization must be occurring on time scales that are short compared to the time resolution of our observations, which is here some 0.5–1.0 milliseconds. One of the most interesting features of the modeseparated partial profiles is a phase offset between the primary and secondary modes. The secondary-mode ‘main pulse’ arrives some 1.5 ± 0.1‡ before the primary-mode one at 430 MHz and some 1.3 +0.1 ‡ at 21 cm. Given that the polar cap has an angular diameter of 3.36‡, we consider whether this is a geometric effect or an effect of differential propagation of the two modes in the inner magnetosphere of the pulsar.

    • Disk luminosity and angular momentum for accreting, weak field neutron stars in the ‘Slow’ rotation approximation

      Bhaskar Datta Arun V. Thampan Paul J. Wiita

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      For accretion on to neutron stars possessing weak surface magnetic fields and substantial rotation rates (corresponding to the secular instability limit), we calculate the disk and surface layer luminosities general relativistically using the Hartle & Thorne formalism, and illustrate these quantities for a set of representative neutron star equations of state. We also discuss the related problem of the angular momentum evolution of such neutron stars and give a quantitative estimate for this accretion driven change in angular momentum. Rotation always increases the disk luminosity and reduces the rate of angular momentum evolution. These effects have relevance for observations of low-mass X-ray binaries.

    • A numerical survey of neutron star crustal density profiles

      B. Datta A. V. Thampan D. Bhattacharya

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      An accurate numerical survey of the density profiles corresponding to the crusts of neutron stars for representative equation of state models is presented. This will find application in calculations of thermal and magnetic evolution of neutron stars.

    • Flux monitoring at 327 MHz during SL9-Jupiter collision

      Rakesh K. Malik Pradeep Gothoskar P. K. Manohran G. Swarup K. Subramanian V. Balasubramanian

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      Jupiter flux at 327 MHz was monitored using the Ooty radio telescope from July 12th to July 29th during the collision of comet Shoemaker-Levi 9 with Jupiter. Flux was found to increase steadily from July 17th to July 26th by ∼ 2–5 Jy, after which it declined to its pre-event value. The comparison of 327 MHz observations with those at 840 MHz and 2240 MHz indicates that the enhancement was mainly due to the increased synchrotron emission and the contribution of thermal emission was very small at metric-decimetric frequencies. The enhancement in radio emission was found to be more at 840 MHz than at 327 or 2240 MHz. The steepening of the spectrum between 327 and 840 MHz as well as between 2240 and 840 MHz was also noted.

    • Effect of strong magnetic field on cosmic quark-hadron phase transition and baryon inhomogeneity

      Somenath Chakrabarty

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      The effect of intense magnetic field on the cosmic quark-hadron phase transition and also on the baryon number inhomogeneity has been investigated using phenomenological MIT bag model for the quark sector. For the sake of simplicity an ideal gas equation of state has been considered for the hadronic phase.

    • VV Orionis – improved elements

      M. B. K. Sarma P. Vivekananda Rao

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      TheUBV light curves obtained by Duerbeck (1975) andHa (wide) and Ha (narrow) light curves obtained by Chambliss & Davan (1987) of the detached eclipsing binary VV Orionis (VV Ori) were analysed using the Wilson-Devinney method fixing the two parametersTh (25,000 K) and q(0.4172), resulting in the following absolute elements:A = 13.605 ± 0.03 LR,Rh = 5.03 ±0.03R, Rc = 2.43 ±0.02R,Mbol,h = -5.18 ± 0.11,Mbol,c = -1.54 ± 0.06,mh =10.81 + 0.42m andmc = 4.51 ± 0.41m. The de-reddened colours obtained from applying the reddening corrections ofE(B-V) = 0m.05 andE(U-B) = Om.04, and the derived temperatures of the components, gave spectral types ofB 1.5V for the primary and 54-5V with anUV excess of 0m·3 for the secondary component. A comparison of the logL and logTe of the components with the observed ZAMS shows the primary component to be a little above and the secondary component to be a little below/or on the ZAMS. A comparison of the properties of the components of VV Ori and a few other detached systems with the normal stars in the logL, logR and logTe versus logm planes, indicated a need for either a readjustment of the scales of the above parameters or modifications in the theoretical models. From the position of the components on the evolutionary tracks of Pop I composition computed by Schaller et al. (1992) it is noticed that while the primary component of W Ori had slightly evolved along the main-sequence, its secondary is still unevolved. The age of VV Ori is found to be 10 ± 1 million years and it is at a distance of 368 ± 10 pc.

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