Study of the Local Interstellar Medium using Pulsar Scintillation

We present here the results from an extensive scintillation study of twenty pulsars in the dispersion measure (DM) range 3 - 35 pc cm^-3 carried out using the Ooty Radio Telescope, to investigate the distribution of ionized material in the local interstellar medium (LISM). Our analysis reveals several anomalies in the scattering strength, which suggest that the distribution of scattering material in the Solar neighborhood is not uniform. Our model suggests the presence of a low density bubble surrounded by a shell of much higher density fluctuations. We are able to put some 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: 5 pages, 3 figure

Plasma Turbulence in the Local Bubble

Turbulence in the Local Bubble could play an important role in the thermodynamics of the gas that is there. The best astronomical technique for measuring turbulence in astrophysical plasmas is radio scintillation. Measurements of the level of scattering to the nearby pulsar B0950+08 by Philips and Clegg in 1992 showed a markedly lower value for the line-of-sight averaged turbulent intensity parameter $$than is observed for other pulsars, consistent with radio wave propagation through a highly rarefied plasma. In this paper, we discuss the observational progress that has been made since that time. At present, there are four pulsars (B0950+08, B1133+16, J0437-4715, and B0809+74) whose lines of sight seem to lie mainly within the local bubble. The mean densities and line of sight components of the interstellar magnetic field along these lines of sight are smaller than nominal values for pulsars, but not by as much expected. Three of the four pulsars also have measurements of interstellar scintillation. The value of the parameter$$ is smaller than normal for two of them, but is completely nominal for the third. This inconclusive status of affairs could be improved by measurements and analysis of ``arcs'' in ``secondary spectra'' of pulsars.Comment: Submitted to Space Science Reviews as contribution to Proceedings of ISSI (International Space Science Institute) workshop "From the Heliosphere to the Local Bubble". Refereed version accepted for publicatio

Interstellar scintillation, AGN physics and the SKA

A large fraction of compact, extragalactic radio sources exhibit rapid variability at centimetre wavelengths as their radio emission is scattered by electron density fluctuations in the interstellar medium of the Galaxy. Next-generation wide-field radio telescopes will have to account for this in forming deep images of the radio sky. Interstellar scintillation offers a unique probe of very small-scale structure in both the ionized interstellar medium and the compact jets of the radio sources themselves. The effective resolution is two orders of magnitude higher than achievable with very long baseline interferometry. The recent Micro-Arcsecond Scintillation-Induced Variability Survey revealed a reduction in ISS at 4.9 GHz with increasing source redshift, implying either an increase in the apparent angular size of high-redshift radio cores beyond that expected due to a cosmological decrease in brightness, or a decrease in the microarcsecond-scale core dominance towards high redshift. The result could be due either to source-intrinsic evolution in the selected sample, or to scatter-broadening in the intergalactic medium

Exotic clouds in the local interstellar medium

The neutral interstellar medium (ISM) inside the Local Bubble (LB) has been known to have properties typical of the warm neutral medium (WNM). However, several recent neutral hydrogen (HI) absorption experiments show evidence for the existence of at least several cold diffuse clouds inside or at the boundary of the LB, with properties highly unusual relative to the traditional cold neutral medium. These cold clouds have a low HI column density, and AU-scale sizes. As the kinematics of cold and warm gas inside the LB are similar, this suggests a possibility of all these different flavors of the local ISM belonging to the same interstellar flow. The co-existence of warm and cold phases inside the LB is exciting as it can be used to probe the thermal pressure inside the LB. In addition to cold clouds, several discrete screens of ionized scattering material are clearly located inside the LB. The cold exotic clouds inside the LB are most likely long-lived, and we expect many more clouds with similar properties to be discovered in the future with more sensitive radio observations. While physical mechanisms responsible for the production of such clouds are still poorly understood, dynamical triggering of phase conversion and/or interstellar turbulence are likely to play an important role.Comment: 10 pages, refereed, accepted for publication in the proceedings of the "From the Outer Heliosphere to the Local Bubble: Comparisons of New Observations with Theory" conference, Space Science Review

Active Galactic Nuclei at the Crossroads of Astrophysics

Over the last five decades, AGN studies have produced a number of spectacular examples of synergies and multifaceted approaches in astrophysics. The field of AGN research now spans the entire spectral range and covers more than twelve orders of magnitude in the spatial and temporal domains. The next generation of astrophysical facilities will open up new possibilities for AGN studies, especially in the areas of high-resolution and high-fidelity imaging and spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These studies will address in detail a number of critical issues in AGN research such as processes in the immediate vicinity of supermassive black holes, physical conditions of broad-line and narrow-line regions, formation and evolution of accretion disks and relativistic outflows, and the connection between nuclear activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical Symposia Serie

Interstellar modulation of the flux density and arrival time of pulses from pulsar B 1937+214

International audienceObservations of the millisecond pulsar B1937+214 made at Nan\c cay over 6 years show 30% rms flux variations over 13 +/- 4 days due to Refractive Interstellar Scintillations. The arrival times (TOA) also show variations over a similar time scale 16 +/- 10 days with an rms amplitude of about 0.3mu secs. These ``rapid'' TOA variations are anti-correlated ( ~ -40%) with the flux and so are also caused by propagation through the ionized interstellar medium. The correlation is such that weak pulses tend to arrive late. While TOA modulations due to changing geometric delay should be positively correlated with flux, those due to small scale variations in the dispersive delay should be negatively correlated with the flux and so are presumed to be responsible in our observations. The level and time scales are shown to be consistent with expectations based on the Kolmogorov model of the interstellar density spectrum. However, in the data there is a sequence of about 5 discrete events, in which the flux remains low over 10-30 days and the TOA is on average late but also shows rapid variations. Assuming that these are indeed discrete events, we interpret them as due to isolated regions of enhanced plasma density crossing the line of sight. Such ``Extreme Scattering Events'' make a major contribution to the TOA variations and their anti-correlations with the observed flux. They are seen against a background of the normal refractive scintillation. A model is proposed in which discrete sheets of plasma cross the line of sight and cause a ``de-focussing'' event when aligned parallel to the line of sight. The statistics of the events imply a surprisingly large space density of the sheets; an alternative is that by chance we view PSR B1937+214 tangentially through a supernova shell which is fragmented and so causes multiple events

Plasma Diagnostics of the Interstellar Medium with Radio Astronomy

Contains fulltext : 119335.pdf (preprint version ) (Open Access