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

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

MUSE observations of a changing-look AGN I: The re-appearance of the broad emission lines

Optical changing-look Active Galactic Nuclei (AGN) are a class of sources that change type within a short timescale of years or decades. This change is characterised by the appearance or disappearance of broad emission lines, often associated with dramatic AGN continuum flux changes that are orders of magnitude larger than those expected from typical AGN variability. In this work we study for the first time the host galaxy of a changing-look AGN, Mrk 590, using high spatial resolution optical and near-infrared observations. We discover that after ~ 10 yr absence, the optical broad emission lines of Mrk 590 have reappeared. The AGN optical continuum flux however, is still ~ 10 times lower than that observed during the most luminous state in the 1990s. The host galaxy shows a 4.5 kpc radius star-forming ring with knots of ionised and cold molecular gas emission. Extended ionised and warm molecular gas emission are detected in the nucleus, indicating that there is a reservoir of gas as close as 60 pc from the black hole. We observe a nuclear gas spiral between radii r ~ 0.5 - 2 kpc, which has been suggested as a dynamical mechanism able to drive the necessary gas to fuel AGN. We also discover blue-shifted and high velocity dispersion [O III] emission out to a radius of 1 kpc, tracing a nuclear gas outflow. The gas dynamics in Mrk 590 suggest a complex balance between gas inflow and outflow in the nucleus of the galaxy.Comment: Accepted for publication in MNRA

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

Automatic, fast and robust characterization of noise distributions for diffusion MRI

Knowledge of the noise distribution in magnitude diffusion MRI images is the centerpiece to quantify uncertainties arising from the acquisition process. The use of parallel imaging methods, the number of receiver coils and imaging filters applied by the scanner, amongst other factors, dictate the resulting signal distribution. Accurate estimation beyond textbook Rician or noncentral chi distributions often requires information about the acquisition process (e.g. coils sensitivity maps or reconstruction coefficients), which is not usually available. We introduce a new method where a change of variable naturally gives rise to a particular form of the gamma distribution for background signals. The first moments and maximum likelihood estimators of this gamma distribution explicitly depend on the number of coils, making it possible to estimate all unknown parameters using only the magnitude data. A rejection step is used to make the method automatic and robust to artifacts. Experiments on synthetic datasets show that the proposed method can reliably estimate both the degrees of freedom and the standard deviation. The worst case errors range from below 2% (spatially uniform noise) to approximately 10% (spatially variable noise). Repeated acquisitions of in vivo datasets show that the estimated parameters are stable and have lower variances than compared methods.Comment: v2: added publisher DOI statement, fixed text typo in appendix A

Identifying Biomarkers to Select Patients with Borderline Resectable and Locally Advanced Pancreatic Ductal Adenocarcinoma (BRPC, LAPC) for Radiotherapy (RT)

https://openworks.mdanderson.org/sumexp22/1036/thumbnail.jp

Time domain studies of Active Galactic Nuclei with the Square Kilometre Array

Variability of radio-emitting active galactic nuclei can be used to probe both intrinsic variations arising from shocks, flares, and other changes in emission from regions surrounding the central supermassive black hole, as well as extrinsic variations due to scattering by structures in our own Galaxy. Such interstellar scattering also probes the structure of the emitting regions, with microarcsecond resolution. Current studies have necessarily been limited to either small numbers of objects monitored over long periods of time, or large numbers of objects but with poor time sampling. The dramatic increase in survey speed engendered by the Square Kilometre Array will enable precision synoptic monitoring studies of hundreds of thousands of sources with a cadence of days or less. Statistics of variability, in particular concurrent observations at multiple radio frequencies and in other bands of the electromagnetic spectrum, will probe accretion physics over a wide range of AGN classes, luminosities, and orientations, as well as enabling a detailed understanding of the structures responsible for radio wave scattering in the Galactic interstellar medium