Impulsive radio bursts that are detectable across cosmological distances constitute extremely powerful probes of the ionized Inter-Galactic Medium (IGM), intergalactic magnetic fields, and the properties of space-time itself. Their dispersion measure
s (DMs) will enable us to detect the missing baryons in the low-redshift Universe and make the first measurements of the mean galaxy halo profile, a key parameter in models of galaxy formation and feedback. Impulsive bursts can be used as cosmic rulers at redshifts exceeding 2, and constrain the dark energy equation-of-state parameter, $w(z)$ at redshifts beyond those readily accessible by Type Ia SNe. Both of these goals are realisable with a sample of $sim 10^4$ fast radio bursts (FRBs) whose positions are localized to within one arcsecond, sufficient to obtain host galaxy redshifts via optical follow-up. It is also hypothesised that gravitational wave events may emit coherent emission at frequencies probed by SKA1-LOW, and the localization of such events at cosmological distances would enable their use as cosmological standard sirens. To perform this science, such bursts must be localized to their specific host galaxies so that their redshifts may be obtained and compared against their dispersion measures, rotation measures, and scattering properties. The SKA can achieve this with a design that has a wide field-of-view, a substantial fraction of its collecting area in a compact configuration (80% within a 3,km radius), and a capacity to attach high-time-resolution instrumentation to its signal path.
Using the new wideband capabilities of the Australia Telescope Compact Array (ATCA), we obtain spectra for PKS 1718-649, a well-known gigahertz-peaked spectrum radio source. The observations, between approximately 1 and 10 GHz over three epochs spann
ing approximately 21 months, reveal variability both above the spectral peak at ~3 GHz and below the peak. The combination of the low and high frequency variability cannot be easily explained using a single absorption mechanism, such as free-free absorption or synchrotron self-absorption. We find that the PKS 1718-649 spectrum and its variability are best explained by variations in the free-free optical depth on our line-of-sight to the radio source at low frequencies (below the spectral peak) and the adiabatic expansion of the radio source itself at high frequencies (above the spectral peak). The optical depth variations are found to be plausible when X-ray continuum absorption variability seen in samples of Active Galactic Nuclei is considered. We find that the cause of the peaked spectrum in PKS 1718-649 is most likely due to free-free absorption. In agreement with previous studies, we find that the spectrum at each epoch of observation is best fit by a free-free absorption model characterised by a power-law distribution of free-free absorbing clouds. This agreement is extended to frequencies below the 1 GHz lower limit of the ATCA by considering new observations with Parkes at 725 MHz and 199 MHz observations with the newly operational Murchison Widefield Array. These lower frequency observations argue against families of absorption models (both free-free and synchrotron self-absorption) that are based on simple homogenous structures.
We investigate the feasibility of detecting and probing various components of the ionized intergalactic medium (IGM) and their turbulent properties at radio frequencies through observations of scatter broadening of compact sources. There is a strong
case for conducting targeted observations to resolve scatter broadening (where the angular size scales as $sim u^{-2}$) of compact background sources intersected by foreground galaxy haloes and rich clusters of galaxies to probe the turbulence of the ionized gas in these objects, particularly using Space VLBI with baselines of 350,000 km at frequencies below 800 MHz. The sensitivity of the Square Kilometre Array (SKA) allows multifrequency surveys of interstellar scintillation (ISS) of $sim 100 ,mu$Jy sources to detect or place very strong constraints on IGM scatter broadening down to $sim 1, mu$as scales at 5 GHz. Scatter broadening in the warm-hot component of the IGM with typical overdensities of $sim 30$ cannot be detected, even with Space VLBI or ISS, and even if the outer scales of turbulence have an unlikely low value of $sim 1$ kpc. Nonetheless, intergalatic scatter broadening can be of order $sim 100, mu$as at 1 GHz and $sim 3, mu$as at 5 GHz for outer scales $sim 1$ kpc, assuming a sufficiently high source redshift that most sight-lines intersect within a virial radius of at least one galaxy halo ($z gtrsim 0.5$ and $z gtrsim 1.4$ for $10^{10} {rm M}_odot$ and $10^{11} {rm M}_odot$ systems, following McQuinn (2014)). Both Space VLBI and multiwavelength ISS observations with the SKA can easily test such a scenario, or place strong constraints on the outer scale of the turbulence in such regions.
We examine the long-term evolution of the intra-hour variable quasar, J1819+3845, whose variations have been attributed to interstellar scintillation by extremely local turbulent plasma, located only 1-3pc from Earth. The variations in this source ce
ased some time between June 2006 and February 2007. The evolution of the source spectrum and the long-term lightcurve, and the persistent compactness of the source VLBI structure indicates that the cessation of rapid variability was associated with the passage of the scattering material out of the line of sight to the quasar. We present an analysis of the linear polarization variations and their relation to total intensity variations. The proper motion of polarized features in the quasar jet is found to be subluminal. Systematic time delays between Stokes I, Q and U, in combination with the structure of the source obtained from 8.4GHz VLBI data, confirm the estimate of the screen distance: 1-2pc, making the screen one of the nearest objects to the Solar System. We determine the physical properties of this scattering material. The electron density in the scattering region is extremely high with respect to the warm ionized ISM, with an estimated density of $n_e sim 97 , l_0^{1/3} {Delta L}_{100}^{-1/2}$cm$^{-3}$, where $l_0$ is the outer scale of the turbulence in AU and $Delta L = 100 Delta L_{100}$ AU is the depth of the scattering region. If this plasma is in pressure balance with the local magnetic field, one expects a ~2 rad/m^2 rotation measure change associated with the passage of this material past the quasar. We examine the rotation measures of sources and the diffuse polarized emission in the surrounding region. We place a limit of 10 rad/m^2 on the RM change. The variability of sources near J1819+3845 is used to deduce that the screen must therefore be either very small (~100 AU) or patchy.
The fraction of compact active galactic nuclei (AGNs) that exhibit interstellar scintillation (ISS) at radio wavelengths, as well as their scintillation amplitudes, have been found to decrease significantly for sources at redshifts z > 2. This can be
attributed to an increase in the angular sizes of the muas-scale cores or a decrease in the flux densities of the compact muas cores relative to that of the mas-scale components with increasing redshift, possibly arising from (1) the space-time curvature of an expanding Universe, (2) AGN evolution, (3) source selection biases, (4) scatter broadening in the ionized intergalactic medium (IGM) and intervening galaxies, or (5) gravitational lensing. We examine the frequency scaling of this redshift dependence of ISS to determine its origin, using data from a dual-frequency survey of ISS of 128 sources at 0 < z < 4. We present a novel method of analysis which accounts for selection effects in the source sample. We determine that the redshift dependence of ISS is partially linked to the steepening of source spectral indices ({alpha}^8.4_4.9) with redshift, caused either by selection biases or AGN evolution, coupled with weaker ISS in the {alpha}^8.4_4.9 < -0.4 sources. Selecting only the -0.4 < {alpha}^8.4_4.9 < 0.4 sources, we find that the redshift dependence of ISS is still significant, but is not significantly steeper than the expected (1+z)^0.5 scaling of source angular sizes due to cosmological expansion for a brightness temperature and flux-limited sample of sources. We find no significant evidence for scatter broadening in the IGM, ruling it out as the main cause of the redshift dependence of ISS. We obtain an upper limit to IGM scatter broadening of < 110muas at 4.9 GHz with 99% confidence for all lines of sight, and as low as < 8muas for sight-lines to the most compact, sim 10muas sources.
The extreme, intra-hour and > 10% rms flux density scintillation observed in AGNs such as PKS 0405-385, J1819+3845 and PKS 1257-326 at cm wavelengths has been attributed to scattering in highly turbulent, nearby regions in the interstellar medium. Su
ch behavior has been found to be rare. We searched for rapid scintillators among 128 flat spectrum AGNs and analyzed their properties to determine the origin of such rapid and large amplitude radio scintillation. The sources were observed at the VLA at 4.9 and 8.4 GHz simultaneously at two hour intervals over 11 days. We detected six rapid scintillators with characteristic time-scales of < 2 hours, none of which have rms variations > 10%. We found strong lines of evidence linking rapid scintillation to the presence of nearby scattering regions, estimated to be < 12 pc away for ~ 200 muas sources and < 250 pc away for ~ 10 muas sources. We attribute the scarcity of rapid and large amplitude scintillators to the requirement of additional constraints, including large source compact fractions. J1819+3845 was found to display ~ 2% rms variations at ~ 6 hour time-scales superposed on longer > 11 day variations, suggesting that the highly turbulent cloud responsible for its extreme scintillation has moved away, with its scintillation now caused by a more distant screen ~ 50 to 150 pc away.
The 4.9 GHz Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey detected a drop in Interstellar Scintillation (ISS) for sources at redshifts z > 2, indicating an apparent increase in angular diameter or a decrease in flux density of the
most compact components of these sources, relative to their extended emission. This can result from intrinsic source size effects or scatter broadening in the Intergalactic Medium (IGM), in excess of the expected (1+z)^0.5 angular diameter scaling of brightness temperature limited sources due to cosmological expansion. We report here 4.9 GHz and 8.4 GHz observations and data analysis for a sample of 140 compact, flat-spectrum sources which may allow us to determine the origin of this angular diameter-redshift relation by exploiting their different wavelength dependences. In addition to using ISS as a cosmological probe, the observations provide additional insight into source morphologies and the characteristics of ISS. As in the MASIV Survey, the variability of the sources is found to be significantly correlated with line-of-sight H-alpha intensities, confirming its link with ISS. For 25 sources, time delays of about 0.15 to 3 days are observed between the scintillation patterns at both frequencies, interpreted as being caused by a shift in core positions when probed at different optical depths. Significant correlation is found between ISS amplitudes and source spectral index; in particular, a large drop in ISS amplitudes is observed at spectral indices of < -0.4 confirming that steep spectrum sources scintillate less. We detect a weakened redshift dependence of ISS at 8.4 GHz over that at 4.9 GHz, with the mean variance at 4-day timescales reduced by a factor of 1.8 in the z > 2 sources relative to the z < 2 sources, as opposed to the factor of 3 decrease observed at 4.9 GHz. This suggests scatter broadening in the IGM.
We report results from a deep high-frequency search for pulsars within the central parsec of Sgr A* using the Green Bank Telescope. The observing frequency of 15 GHz was chosen to maximize the likelihood of detecting normal pulsars (i.e. with periods
of $sim 500$,ms and spectral indices of $sim -1.7$) close to Sgr A*, that might be used as probes of gravity in the strong-field regime; this is the highest frequency used for such pulsar searches of the Galactic Center to date. No convincing candidate was detected in the survey, with a $10sigma$ detection threshold of $sim 10 mu$Jy achieved in two separate observing sessions. This survey represents a significant improvement over previous searches for pulsars at the Galactic Center and would have detected a significant fraction ($gtrsim 5%) of the pulsars around Sgr A*, if they had properties similar to those of the known population. Using our best current knowledge of the properties of the Galactic pulsar population and the scattering material toward Sgr A*, we estimate an upper limit of 90 normal pulsars in orbit within the central parsec of Sgr A*.
