Pulsar Scintillation and the Local Bubble

We present here the results from an extensive scintillation study of twenty pulsars in the dispersion measure (DM) range 3 - 35 pc cm^-3 caried out using the Ooty Radio Telescope (ORT) at 327 MHz, to investigate the distribution of ionized material in the local interstellar medium. Observations were made during the period January 1993 to August 1995, in which the dynamic scintillation spectra of these pulsars were regularly monitored over 10 - 90 epochs spanning 100 days. Reliable and accurate estimates of strengths of scattering have been deduced from the scintillation parameters averaged out for their long-term fluctuations arising from refractive scintillation (RISS) effects. Our analysis reveals several anomalies in the scattering strength, which suggest tht the distribution of scattering material in the Solar neighborhood is not uniform. We have modelled these anomalous scattering effects in terms of inhomogeneities in the distribution of electron dnsity fluctuations in the local interstellar medium (LISM). Our model suggests the presence of a low density bubble surrounded by a shell of much higher density fluctuations. We are able to put constraints on geometrical and scattering properties of such a structure, and find it to be morphologically similar to the Local Bubble known from other studies.Comment: 35 pages, 12 figure

Long-Term Scintillation Studies of Pulsars: III. Testing Theoretical Models of Refractive Scintillation

Refractive interstellar scintillation (RISS) is thought to be the cause behind a variety of phenomena seen at radio wavelengths in pulsars and compact radio sources. Though there is substantial observational data to support several consequences of it, the quantitative predictions from theories have not been thoroughly tested. In this paper, data from our long-term scintillation study of 18 pulsars are used to test the predictions. The fluctuations of decorrelation bandwidth ($\nu_d$), scintillation time scale ($\tau_d$) and flux density (F) are examined for their cross-correlations and compared with the predictions. The theory predicts a strong correlation between $\nu_d$ and $\tau_d$, and strong anti-correlations between $\nu_d$ and F, and $\tau_d$ and F. For 5 pulsars, we see a reasonable agreement. There is considerable difficulty in reconciling the results for the rest of the pulsars. Our analysis shows the underlying noise sources can sometimes reduce the correlation, but cannot cause an absence of correlation. It is also unlikely that the poor flux correlations arise from a hitherto unrecognized intrinsic flux variations. For PSR B0834+06, which shows anomalous behaviour of persistent drift slopes, positive correlation is found between $\tau_d$ and the drift-corrected $\nu_d$. Many pulsars show an anti-correlation between $\nu_d$ and the drift slope, and this is in accordance with the simple models of RISS. The detections of correlated variations of observables and a reasonable agreement between the predicted and measured correlations for some pulsars confirm RISS as the primary cause of the observed fluctuations. However, the complexity seen with the detailed results suggests the necessity of more comprehensive theoretical treatments for describing refractive fluctuations and their correlations.Comment: 27 pages, 6 Figures, 6 Tables. Accepted for publication in The Astrophysical Journa

Women Reproductive Rights in India: Prospective Future.

Reproductive rights were established as a subset of the human rights. Parents have a basic human right to determine freely and responsibly the number and the spacing of their children. Issues regarding the reproductive rights are vigorously contested, regardless of the population’s socioeconomic level, religion or culture. Following review article discusses reproductive rights with respect to Indian context focusing on socio economic and cultural aspects. Also discusses sensitization of government and judicial agencies in protecting the reproductive rights with special focus on the protecting the reproductive rights of people with disability (mental illness and mental retardation)

Charge order suppression and antiferromagnetic to ferromagnetic switch over in Pr_0.5Ca_0.5MnO_3 nanowires

We have prepared crystalline nanowires (diameter ~ 50 nm, length ~ a few microns) of the charge ordering manganite Pr_0.5Ca_0.5Mn_O3 using a low reaction temperature hydrothermal method and characterized them using X-ray diffraction, transmission electron microscopy, SQUID magnetometry and electron magnetic resonance measurements. While the bulk sample shows a charge ordering transition at 245 K and an antiferromagnetic transition at 175 K, SQUID magnetometry and electron magnetic resonance experiments reveal that in the nanowires phase, a ferromagnetic transition occurs at ~ 105 K. Further, the antiferromagnetic transition disappears and the charge ordering transition is suppressed. This result is particularly significant since the charge order in Pr_0.5Ca_0.5MnO_3 is known to be very robust, magnetic fields as high as 27 T being needed to melt it.Comment: 12 pages including 4 figures. submitted to Applied Physics Letter

Long-term scintillation studies of pulsars. II. Refractive effects and the spectrum of plasma density fluctuations

Refractive scintillation effects in pulsars are powerful techniques for discriminating between different models proposed for the electron density fluctuation spectrum in the interstellar medium. Data from our long-term scintillation study of 18 pulsars in the dispersion measure range 3-35 pc cm-3 (Paper I) are used to investigate two important observable effects of refractive scintillation, viz., (1) modulations of diffractive scintillation observables and flux density, and (2) drifting bands in dynamic scintillation spectra. Our data provide simultaneous measurements of decorrelation bandwidth, scintillation timescale, flux density, and drift rate of patterns. The observed modulations of the first three are compared with the available theoretical predictions, and constraints are placed on the power spectrum of plasma density fluctuations. The measured modulation indices are found to be larger than predicted by a Kolmogorov form of density spectrum. The properties of the drift rate of patterns along with the diffractive scintillation parameters have been used to estimate independently the slope of the density power spectrum, which is found to be consistent with a Kolmogorov form for several pulsars. The contradictory results from these two independent methods of constraining the electron density spectrum are not reconcilable with the simple theoretical models based on power-law forms of density spectrum. Our observations show anomalous scintillation behavior such as persistent drifting bands for some pulsars. This can be interpreted as an excess power in the low wavenumber range (~10-12 to 10-13 m-1) compared to the Kolmogorov expectations, or the existence of localized density structures. The results from our observations are discussed in combination with those from earlier studies in an attempt to understand the overall nature of the density spectrum. The emerging picture is a Kolmogorov-like spectrum (α≈11/3) in the wavenumber range ~10-6 m-1 to ~10-11 m-1, which either steepens or has a bump near ~10-12 to 10-13 m-1. The accumulated data also suggest the existence of discrete density structures along some lines of sight. We also discuss the possible implications of our results for the theoretical models

Multiple imaging of PSR B1133+16 by the interstellar medium

Refraction of pulsar radiation by electron density irregularities in the interstellar medium sometimes produces multiple imaging of pulsars, which can lead to periodic oscillations of intensity in pulsar dynamic spectra records. Such events can be used as tools to resolve the emission regions in pulsar magnetospheres. Here we describe results from the recent observation of a double imaging event for PSR B1133+16, which place fairly tight constraints on the location of the emission regions. Our analysis constrains the location of the scattering object to the shell of the Local Bubble. The phase of the oscillations shows significant variations across the pulse. The minimum value for the transverse separation of the emitting regions at the two edges of the pulse is inferred to be 3 × 105 m. This translates to a minimum emission altitude of 2.6 × 106 m. The nonmonotonic variations of the fringe phase with pulse longitude are interpreted as variations of the altitude of the emission regions for the orthogonal polarization modes of this pulsar. This is in agreement with theories where propagation effects, such as refraction, are responsible for the orthogonal modes