Emergence and disappearance of micro-arcsecond structure in the scintillating quasar J1819+3845

The 4.8 GHz lightcurves of the scintillating intra-day variable quasar J1819+3845 during 2004-5 exhibit sharp structure, down to a time scale of 15 minutes, that was absent from lightcurves taken prior to this period and from the 2006 lightcurves. Analysis of the lightcurve power spectra show that the variations must be due to the emergence of new structure in the source. The power spectra yield a scattering screen distance of 3.8 +/- 0.3 pc for a best-fit v_ISS=59 +/- 0.5 km/s or 2.0 +/- 0.3 pc for the scintillation velocity reported by Dennett-Thorpe & de Bruyn (2003). The turbulence is required to be exceptionally turbulent, with C_N^2 > 0.7 Delta L_pc^{-1} m^{-20/3} for scattering material of thickness Delta L_{pc} pc along the ray path. The 2004 power spectrum can be explained in terms of a double source with a component separation 240 +/- 15 microas in 2004.Comment: MNRAS Lett (accepted), version with high-resolution figures at http://www.astro.caltech.edu/~jpm/MdB1819.pd

Charting the transient universe using radio continuum surveys

The field of radio transients is exploding with the discovery of diverse new phenomena fuelled by recent advances in telescope and computational capabilities. The desire to maximise time on sky to detect ever more and rarer events drives us to share telescope time with large continuum surveys. We discuss here the advantages of a symbiotic relationship between transients and continuum surveys, and show how an understanding of the time domain can constitute an important facet of continuum survey data with regard to quality control, the interpretation of flux density and spectral information, and the origin of the radio emission. One example at centimetre wavelengths is the presence of intra-day variability, which sifts for the presence of 1-100µas structure and potentially serves as a discriminant of AGN and starburst-related radio emission. We identify and discuss four main issues for the successful integration of transients and continuum surveys

Optimization of survey strategies for detecting slow radio transients

We investigate the optimal tradeoff between sensitivity and field of view in surveys for slow radio transients using the event detection rate as the survey metric. This tradeoff bears implications for the design of surveys conducted with upcoming widefield radio interferometers, such as the ASKAP VAST survey and the MeerKAT TRAPUM survey. We investigate (i) a survey in which the events are distributed homogeneously throughout a volume centred on the Earth, (ii) a survey in which the events are homogeneously distributed, but are only detectable beyond a certain minimum distance, and (iii) a survey in which all the events occur at an identical distance, as is appropriate for a targetted survey of a particular field which subtends Npoint telescope pointings. For a survey of fixed duration, Tobs, we determine the optimal tradeoff between number of telescope pointings, N, and integration time per field. We consider a population in which the event luminosity distribution follows a power law with index − α, and tslew is the slewing time between fields or, for a drift scan, the time taken for the telescope drift by one beamwidth. Several orders of magnitude improvement in detection rate is possible by optimization of the survey parameters. The optimal value of N for case (i) is Nmax ~ Tobs/4tslew, while for case (iii) we find Nmax = (Lmax/L0)2[(3 − α)/2]2/(α − 1), where Lmax is the maximum luminosity of a transient event and L0 is the minimum luminosity event detectable in an integration of duration Tobs. (The instance Nmax > Npoint in (iii) implies re-observation of fields over the survey area, except when the duration of transient events exceeds that between re-observations of the same field, where Nmax = Npoint applies instead.) We consider the balance in survey optimization between telescope field of view, Ω, and sensitivity, characterised by the minimum detectable flux density, S0. For homogeneously distributed events (i), the detection rate scales as NΩS−3/20, while for targetted events (iii) it scales as NΩS1 − α0. However, if the targetted survey is optimised for N the event detection rate scales instead as ΩS−20. This analysis bears implications for the assessment of telescope designs: the quantity ΩS−20 is often used as the metric of telescope performance in the SKA transients literature, but only under special circumstances is it the metric that optimises the event detection rate

Scatter broadening of compact radio sources by the ionized intergalacticmedium: prospects for detection with Space VLBI and the SquareKilometre Array

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 ~ν−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 very long baseline interferometry (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 ~100 μJy sources to detect or place very strong constraints on IGM scatter broadening down to ~1 μas scales at 5 GHz. Scatter broadening in the warm–hot component of the IGM with typical overdensities of ~30 cannot be detected, even with Space VLBI or ISS, and even if the outer scales of turbulence have an unlikely low value of ~1 kpc. None the less, intergalactic scatter broadening can be of the order of ~100 μas at 1 GHz and ~3 μas at 5 GHz for outer scales ~1 kpc, assuming a sufficiently high-source redshift that most sight-lines intersect within a virial radius of at least one galaxy halo (z >~ 0.5 and 1.4 for 10 10 and 10 11 M⊙ 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

Fast radio burst event rate counts - I. Interpreting the observations

The fluence distribution of the fast radio burst (FRB) population (the 'source count' distribution, N ( > F) αF α ), is a crucial diagnostic of its distance distribution, and hence the progenitor evolutionary history. We critically reanalyse current estimates of the FRB source count distribution. We demonstrate that the Lorimer burst (FRB 010724) is subject to discovery bias, and should be excluded from all statistical studies of the population. We re-examine the evidence for flat, α > -1, source count estimates based on the ratio of single-beam to multiple-beam detections with the Parkes multibeam receiver, and show that current data imply only a very weak constraint of α ≲ -1.3. A maximum-likelihood analysis applied to the portion of the Parkes FRB population detected above the observational completeness fluence of 2 Jy ms yields α = -2.6 -1.3 +0.7 . Uncertainties in the location of each FRB within the Parkes beam render estimates of the Parkes event rate uncertain in both normalizing survey area and the estimated post-beam-corrected completeness fluence; this uncertainty needs to be accounted for when comparing the event rate against event rates measured at other telescopes

FRB event rate counts - II. Fluence, redshift, and dispersion measure distributions

We examine how the various observable statistical properties of the fast radio burst (FRB) population relate back to their fundamental physical properties in amodel-independent manner. We analyse the flux density and fluence distributions of FRBs as a tool to investigate their luminosity distance distribution and the evolution of their prevalence throughout cosmic history. We examine in detail particular scenarios in which the burst population follows some power of the cosmic star formation rate. FRBs present an important additional measurable over source counts of existing cosmological populations, namely the dispersion measure. Based on the known redshift of FRB121102 (the repeater) we expect at least 50 per cent of the dispersion measure to be attributable to the intergalactic medium and hence it can be used as a proxy for distance. We develop the framework to interpret the dispersion measure distribution, and investigate how the effect of Helium reionization in the intergalactic medium is evident in this distribution. Examination of existing data suggests that the FRB luminosity function is flatter than a critical slope, making FRBs easily detectable to large distances; in this regime the reduction in flux density with distance is outweighed by the increase in the number of bright bursts within the search volume

Interstellar scattering - New diagnostics of pulsars and the ISM

Extreme Scattering Events and pulsar secondary spectra have highlighted fundamental problems in our understanding of the dynamics of interstellar turbulence. We describe some of these problems in detail and present the theory behind the technique of speckle imaging, which offers a prospect of revealing fundamental properties of the turbulence. It also offers the prospect of resolving pulsar magnetospheres on ~ 10 nas scales

Absorption variability as a probe of the multiphase interstellar media surrounding active galaxies

© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. We examine a model for the variable free-free and neutral hydrogen absorption inferred towards the cores of some compact radio galaxies in which a spatially fluctuating medium drifts in front of the source. We relate the absorption-induced intensity fluctuations to the statistics of the underlying opacity fluctuations. We investigate models in which the absorbing medium consists of either discrete clouds or a power-law spectrum of opacity fluctuations. We examine the variability characteristics of a medium comprised of Gaussian-shaped clouds in which the neutral and ionized matter are co-located, and in which the clouds comprise spherical constant-density neutral cores enveloped by ionized sheaths. The cross-power spectrum indicates the spatial relationship between neutral and ionized matter, and distinguishes the two models, with power in the Gaussian model declining as a featureless power-law, but that in the ionized sheath model oscillating between positive and negative values. We show how comparison of the H I and free-free power spectra reveals information on the ionization and neutral fractions of the medium. The background source acts as a low-pass filter of the underlying opacity power spectrum, which limits temporal fluctuations to frequencies ? ? ???/?src, where ??? is the angular drift speed of the matter in front of the source, and it quenches the observability of opacity structures on scales smaller than the source size ?src. For drift speeds of ~103 km s-1 and source brightness temperatures ~1012 K, this limitation confines temporal opacity fluctuations to time-scales of order several months to decades

The Rotation Measure and 3.5mm Polarization of Sgr A*

We report the detection of variable linear polarization from Sgr A* at a wavelength of 3.5mm, the longest wavelength yet at which a detection has been made. The mean polarization is 2.1 +/- 0.1% at a position angle of 16 +/- 2 deg with rms scatters of 0.4% and 9 deg over the five epochs. We also detect polarization variability on a timescale of days. Combined with previous detections over the range 150-400GHz (750-2000 microns), the average polarization position angles are all found to be consistent with a rotation measure of -4.4 +/- 0.3 x 10^5 rad/m^2. This implies that the Faraday rotation occurs external to the polarized source at all wavelengths. This implies an accretion rate ~0.2 - 4 x 10^-8 Msun/yr for the accretion density profiles expected of ADAF, jet and CDAF models and assuming that the region at which electrons in the accretion flow become relativistic is within 10 R_S. The inferred accretion rate is inconsistent with ADAF/Bondi accretion. The stability of the mean polarization position angle between disparate polarization observations over the frequency range limits fluctuations in the accretion rate to less than 5%. The flat frequency dependence of the inter-day polarization position angle variations also makes them difficult to attribute to rotation measure fluctuations, and suggests that both the magnitude and position angle variations are intrinsic to the emission.Comment: Ap.J.Lett. accepte