The microarcsecond structure of an active galactic nucleus jet via interstellar scintillation

We describe a new tool for studying the structure and physical characteristics of ultracompact AGN jets and their surroundings with microarcsecond precision. This tool is based on the frequency dependence of the light curves observed for intra-day variable radio sources, where the variability is caused by interstellar scintillation. We apply this method to PKS1257-326 to resolve the core-shift as a function of frequency on scales well below ~12 microarcseconds. We find that the frequency dependence of the position of the scintillating component is r \propto \nu^{-0.1 \pm 0.24} (99% confidence interval) and the frequency dependence of the size of the scintillating component is d \propto \nu^{-0.64 \pm 0.006}. Together, these results imply that the jet opening angle increases with distance along the jet: d \propto r^{n_d}$ with n_d > 1.8. We show that the flaring of the jet, and flat frequency dependence of the core position is broadly consistent with a model in which the jet is hydrostatically confined and traversing a steep pressure gradient in the confining medium with p \propto r^{-n_p} and n_p > 7. Such steep pressure gradients have previously been suggested based on VLBI studies of the frequency dependent core shifts in AGN.Comment: accepted for publication in Ap

The Deep X-ray Radio Blazar Survey (DXRBS). II. New Identifications

We have searched the archived, pointed ROSAT Position Sensitive Proportional Counter data for blazars by correlating the WGACAT X-ray database with several publicly available radio catalogs, restricting our candidate list to serendipitous X-ray sources with a flat radio spectrum (alpha_r <= 0.70). This makes up the Deep X-ray Radio Blazar Survey (DXRBS). Here we present new identifications and spectra for 106 sources, including 86 radio-loud quasars, 11 BL Lacertae objects, and 9 narrow-line radio galaxies. Together with our previously published objects and already known sources, our sample now contains 298 identified objects: 234 radio-loud quasars (181 flat-spectrum quasars: FSRQ [alpha_r <= 0.50] and 53 steep-spectrum quasars: SSRQ), 36 BL Lacs, and 28 narrow-line radio galaxies. Redshift information is available for 96% of these. Thus our selection technique is ~ 90% efficient at finding radio-loud quasars and BL Lacs. Reaching 5 GHz radio fluxes ~ 50 mJy and 0.1-2.0 keV X-ray fluxes a few x 10^-14 erg/cm^2/s, DXRBS is the faintest and largest flat-spectrum radio sample with nearly complete (~ 85%) identification. We review the properties of the DXRBS blazar sample, including redshift distribution and coverage of the X-ray-radio power plane for quasars and BL Lacs. Additionally, we touch upon the expanded multiwavelength view of blazars provided by DXRBS. By sampling for the first time the faint end of the radio and X-ray luminosity functions, this sample will allow us to investigate the blazar phenomenon and the validity of unified schemes down to relatively low powers.Comment: 33 pages, 5 figures. Accepted for publication in MNRAS. Postscript file also available at http://www.stsci.edu/~padovani/survey.htm

Extremely Anisotropic Scintillations

A small number of quasars exhibit interstellar scintillation on time-scales less than an hour; their scintillation patterns are all known to be anisotropic. Here we consider a totally anisotropic model in which the scintillation pattern is effectively one-dimensional. For the persistent rapid scintillators J1819+3845 and PKS1257-326 we show that this model offers a good description of the two-station time-delay measurements and the annual cycle in the scintillation time-scale. Generalising the model to finite anisotropy yields a better match to the data but the improvement is not significant and the two additional parameters which are required to describe this model are not justified by the existing data. The extreme anisotropy we infer for the scintillation patterns must be attributed to the scattering medium rather than a highly elongated source. For J1819+3845 the totally anisotropic model predicts that the particular radio flux variations seen between mid July and late August should repeat between late August and mid November, and then again between mid November and late December as the Earth twice changes its direction of motion across the scintillation pattern. If this effect can be observed then the minor-axis velocity component of the screen and the orientation of that axis can both be precisely determined. In reality the axis ratio is finite, albeit large, and spatial decorrelation of the flux pattern along the major axis may be observable via differences in the pairwise fluxes within this overlap region; in this case we can also constrain both the major-axis velocity component of the screen and the magnitude of the anisotropy.Comment: 5 pages, 4 figures, MNRAS submitte

Parsec-scale radio morphology and variability of a changing-look AGN: the case of Mrk 590

We investigate the origin of the parsec-scale radio emission from the changing-look active galactic nucleus (AGN) of Mrk 590, and examine whether the radio power has faded concurrently with the dramatic decrease in accretion rates observed between the 1990s and the present. We detect a compact core at 1.6 GHz and 8.4 GHz using new Very Long Baseline Array observations, finding no significant extended, jet-like features down to $\sim$1 pc scales. The flat spectral index ($\alpha_{1.6}^{8.4} = 0.03$) and high brightness temperature ($T_{\rm b} \sim 10^{8}\,\rm K$) indicate self-absorbed synchrotron emission from the AGN. The radio to X-ray luminosity ratio of ${\rm log}(L_{\rm R}/L_{\rm X}) \sim -5$, similar to that in coronally active stars, suggests emission from magnetized coronal winds, although unresolved radio jets are also consistent with the data. Comparing new Karl G. Jansky Very Large Array measurements with archival and published radio flux densities, we find $46\%$, $34\%$, and (insignificantly) $13\%$ flux density decreases between the 1990s and the year 2015 at 1.4 GHz, 5 GHz and 8.4 GHz respectively. This trend, possibly due to the expansion and fading of internal shocks within the radio-emitting outflow after a recent outburst, is consistent with the decline of the optical-UV and X-ray luminosities over the same period. Such correlated variability demonstrates the AGN accretion-outflow connection, confirming that the changing-look behaviour in Mrk 590 originates from variable accretion rates rather than dust obscuration. The present radio and X-ray luminosity correlation, consistent with low/hard state accretion, suggests that the black hole may now be accreting in a radiatively inefficient mode.Comment: 14 pages, 5 tables, 5 figures, accepted for publication in MNRA

Dual-Frequency Observations of 140 Compact, Flat-Spectrum Active Galactic Nuclei for Scintillation-Induced Variability

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.Comment: 30 pages, 14 figures, accepted for publication in the Astronomical Journa

EVN & MERLIN studies of a new sample of BL Lac objects

The recent Deep X-ray Radio Blazar Survey (DXRBS) has identified a sample of BL Lac objects spanning the intermediate range of spectral energy distributions between “classical” X-ray selected and radio-selected samples of BL Lacs. Detailed studies of such samples are needed to answer some of the currently open questions regarding the nature of BL Lacs and their place in a unified model of AGN. High-resolution radio imaging provides direct information on jet evolution and beaming parameters. We present some preliminary results from EVN & MERLIN observations of sources in the DXRBS BL Lac sample for which little or no high-resolution radio data were previously available

Observations of Intrahour Variable Quasars: Scattering in our Galactic Neighbourhood

Interstellar scintillation (ISS) has been established as the cause of the random variations seen at centimetre wavelengths in many compact radio sources on timescales of a day or less. Observations of ISS can be used to probe structure both in the ionized insterstellar medium of the Galaxy, and in the extragalactic sources themselves, down to microarcsecond scales. A few quasars have been found to show large amplitude scintillations on unusually rapid, intrahour timescales. This has been shown to be due to weak scattering in very local Galactic ``screens'', within a few tens of parsec of the Sun. The short variability timescales allow detailed study of the scintillation properties in relatively short observing periods with compact interferometric arrays. The three best-studied ``intrahour variable'' quasars, PKS 0405-385, J1819+3845 and PKS 1257-326, have been instrumental in establishing ISS as the principal cause of intraday variability at centimetre wavelengths. Here we review the relevant results from observations of these three sources.Comment: 10 pages, 4 figures, to appear in Astronomical and Astrophysical Transaction

HI emission and absorption in nearby, gas-rich galaxies II. -- sample completion and detection of intervening absorption in NGC 5156

We present the results of a survey for intervening 21cm HI absorption in a sample of 10 nearby, gas-rich galaxies selected from the HI Parkes All-Sky Survey (HIPASS). This follows the six HIPASS galaxies searched in previous work and completes our full sample. In this paper we searched for absorption along 17 sightlines with impact parameters between 6 and 46 kpc, making one new detection. We also obtained simultaneous HI emission-line data, allowing us to directly relate the absorption-line detection rate to the HI distribution. From this we find the majority of the non-detections in the current sample are because sightline does not intersect the HI disc of the galaxy at sufficiently high column density, but that source structure is also an important factor. The detected absorption-line arises in the galaxy NGC 5156 ($z = 0.01$) at an impact parameter of 19 kpc. The line is deep and narrow with an integrated optical depth of 0.82 km s$^{-1}$. High resolution Australia Telescope Compact Array (ATCA) images at 5 and 8 GHz reveal that the background source is resolved into two components with a separation of 2.6 arcsec (500 pc at the redshift of the galaxy), with the absorption likely occurring against a single component. We estimate that the ratio of the spin temperature and covering factor, $T_{\mathrm{S}}/f$, is approximately 950 K in the outer disc of NGC 5156, but further observations using VLBI would allow us to accurately measure the covering factor and spin temperature of the gas.Comment: 31 pages, 11 figure

Why Do Compact Active Galactic Nuclei at High Redshift Scintillate Less?

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 < 110\muas at 4.9 GHz with 99% confidence for all lines of sight, and as low as < 8\muas for sight-lines to the most compact, \sim 10\muas sources.Comment: 38 pages, 13 figures, accepted for publication in The Astrophysical Journa