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

Spontaneous Time Asymmetry due to Horizon

We show that quantized matter fields in the presence of background metrics with Horizon exhibit spontaneous time asymmetry. All quantized matter fields have to vanish at the horizon. Some phenemenological applications of this in the context of black holes and early universe are considered.Comment: 4 pages, Revte