The core shift effect in the blazar 3C 454.3 Kutkin, A. M; Sokolovsky, K. V; Lisakov, M. M ...
Monthly notices of the Royal Astronomical Society,
02/2014, Volume:
437, Issue:
4
Journal Article
Peer reviewed
Open access
Opacity-driven shifts of the apparent very long baseline interferometry (VLBI) core position with frequency (the 'core shift' effect) probe physical conditions in the innermost parts of jets in ...active galactic nuclei. We present the first detailed investigation of this effect in the brightest γ-ray blazar 3C 454.3 using direct measurements from simultaneous 4.6-43 GHz very long baseline array observations, and a time lag analysis of 4.8-37 GHz light curves from the University of Michigan Radio Observatory, Crimean Astrophysical Observatory and Metsähovi observations in 2007-2009. The results support the standard Königl model of jet physics in the VLBI core region. The distance of the core from the jet origin r
c(ν), the core size W(ν) and the light curve time lag ΔT(ν) all depend on the observing frequency ν as r
c(ν) ∝ W(ν) ∝ ΔT(ν) ∝ ν−1/k
. The obtained range of k = 0.6-0.8 is consistent with the synchrotron self-absorption being the dominating opacity mechanism in the jet. The similar frequency dependence of r
c(ν) and W(ν) suggests that the external pressure gradient does not dictate the jet geometry in the cm-band core region. Assuming equipartition, the magnetic field strength scales with distance r as B = 0.4(r/1 pc)−0.8 G. The total kinetic power of electron/positron jet is about 1044 ergs s−1.
ABSTRACT
Geometry of relativistic jets in active galaxies provides important information about mechanisms of launching, collimation, and acceleration of plasma flow. We propose a new method to probe ...a boundary shape of a jet on parsec scales – in the vicinity of its radio core. Apparent speed of an outflow is derived from variability time delays and core shifts measured at the same jet region, providing a self-consistent estimate of the Lorentz factor Γ. We link together the distance along the jet z with its transverse size assuming a constant flow acceleration. Our results indicate that jets have parabolic shape and sustain an effective acceleration in the core region, consistent with the Lorentz factor dependency Γ ∝ z0.5. The proposed method can be applied to the sources observed at small viewing angles as well as to the distant sources when direct measurements are impossible due to a limited angular resolution.
Context.
Detection of the electromagnetic emission from coalescing binary neutron stars (BNS) is important for understanding the merger and afterglow.
Aims.
We present a search for a radio ...counterpart to the gravitational-wave (GW) source GW190425, a BNS merger, using Apertif on the Westerbork Synthesis Radio Telescope (WSRT).
Methods.
We observed a field of high probability in the associated localisation region for three epochs at Δ
T
= 68, 90, 109 d post merger. We identified all sources that exhibit flux variations consistent with the expected afterglow emission of GW190425. We also looked for possible transients. These are sources that are only present in one epoch. In addition, we quantified our ability to search for radio afterglows in the fourth and future observing runs of the GW detector network using Monte Carlo simulations.
Results.
We found 25 afterglow candidates based on their variability. None of these could be associated with a possible host galaxy at the luminosity distance of GW190425. We also found 55 transient afterglow candidates that were only detected in one epoch. All of these candidates turned out to be image artefacts. In the fourth observing run, we predict that up to three afterglows will be detectable by Apertif.
Conclusions.
While we did not find a source related to the afterglow emission of GW190425, the search validates our methods for future searches of radio afterglows.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
ABSTRACT
The Blandford and Königl model of active galactic nuclei (AGN) jets predicts that the position of the apparent opaque jet base – the core – changes with frequency. This effect is observed ...with radio interferometry and is widely used to infer parameters and structure of the innermost jet regions. The position of the radio core is typically estimated by fitting a Gaussian template to the interferometric visibilities. This results in a model approximation error, i.e. a bias that can be detected and evaluated through simulations of observations with a realistic jet model. To assess the bias, we construct an artificial sample of sources based on the AGN jet model evaluated on a grid of the parameters derived from a real VLBI flux-density-limited sample and create simulated VLBI data sets at 2.3, 8.1, and 15.4 GHz. We found that the core position shifts from the true jet apex are generally overestimated. The bias is typically comparable to the core shift random error and can reach a factor of 2 for jets with large apparent opening angles. This observational bias depends mostly on the ratio between the true core shift and the image resolution. This implies that the magnetic field, the core radial distance, and the jet speed inferred from the core shift measurements are overestimated. We present a method to account for the bias.
Super massive black holes at the centres of galaxies can cycle through periods of activity and quiescence. Characterising the duty cycle of active galactic nuclei (AGN) is crucial for understanding ...the impact of the energy they release on the host galaxy. For radio AGN, this can be done by identifying dying (remnant) and restarted radio galaxies from their radio spectral properties. Using the combination of the images at 1400 MHz produced by Apertif, the new phased-array feed receiver installed on the Westerbork Synthesis Radio Telescope, and images at 150 MHz provided by LOFAR, we have derived resolved spectral index images (at a resolution of ~15 arcsec) for all the sources within an approximately 6 deg
2
area of the Lockman Hole region. In this way, we were able to select 15 extended radio sources with emission (partly or entirely) characterised by extremely steep spectral indices (steeper than 1.2). These objects represent cases of radio sources in the remnant or the restarted phases of their life cycle. Our findings confirm that these objects are not as rare as previously thought, suggesting a relatively fast cycle. They also show a variety of properties that can be relevant for modelling the evolution of radio galaxies. For example, the restarted activity can occur while the remnant structure from a previous phase of activity is still visible. This provides constraints on the duration of the “off” (dying) phase. In extended remnants with ultra-steep spectra at low frequencies, the activity likely stopped a few hundred megayears ago, and they correspond to the older tail of the age distribution of radio galaxies, in agreement with the results of simulations of radio source evolution. We find remnant radio sources with a variety of structures (from double-lobed to amorphous), possibly suggesting different types of progenitors. The present work sets the stage for exploiting the powerful tool of low-frequency spectral index studies of extended sources by taking advantage of the large areas common to the LOFAR and the Apertif surveys.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Abstract
The high brightness temperatures, Tb ≳ 1013 K, detected in several active galactic nuclei by RadioAstron space VLBI observations challenge theoretical limits. Refractive scattering by the ...interstellar medium may affect such measurements. We quantify the scattering properties and the sub-mas scale source parameters for the quasar B0529+483. Using RadioAstron correlated flux density measurements at 1.7, 4.8, and 22 GHz on projected baselines up to 240 000 km we find two characteristic angular scales in the quasar core, about 100 and 10 μas. Some indications of scattering substructure are found. Very high brightness temperatures, Tb ≥ 1013 K, are estimated at 4.8 and 22 GHz even taking into account the refractive scattering. Our findings suggest a clear dominance of the particle energy density over the magnetic field energy density in the core of this quasar.
We present the serendipitous detection of the two main OH maser lines at 1667 and 1665 MHz associated with IRAS 10597+5926 at
z
⊙
= 0.19612 in the untargeted Apertif Wide-area Extragalactic imaging ...Survey (AWES), and the subsequent measurement of the OH 1612 MHz satellite line in the same source. With a total OH luminosity of log(
L
/
L
⊙
) = 3.90 ± 0.03, IRAS 10597+5926 is the fourth brightest OH megamaser (OHM) known. We measure a lower limit for the 1667/1612 ratio of
R
1612
> 45.9, which is the highest limiting ratio measured for the 1612 MHz OH satellite line to date. OH satellite line measurements provide a potentially valuable constraint by which to compare detailed models of OH maser pumping mechanisms. Optical imaging shows that the galaxy is likely a late-stage merger. Based on published infrared and far ultraviolet fluxes, we find that the galaxy is an ultra-luminous infrared galaxy (ULIRG) with log(
L
TIR
/
L
⊙
) = 12.24 that is undergoing a starburst with an estimated star formation rate of 179 ± 40
M
⊙
yr
−1
. These host galaxy properties are consistent with the physical conditions responsible for very bright OHM emission. Finally, we provide an update on the predicted number of OH masers that may be found in AWES and estimate the total number of OH masers that will be detected in each of the individual main and satellite OH 18 cm lines.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Abstract
Clues to the physical conditions in radio cores of blazars come from measurements of brightness temperatures as well as effects produced by intrinsic opacity. We study the properties of the ...ultra-compact blazar AO 0235+164 with RadioAstron ground–space radio interferometer, multifrequency VLBA, EVN, and single-dish radio observations. We employ visibility modelling and image stacking for deriving structure and kinematics of the source, and use Gaussian process regression to find the relative multiband time delays of the flares. The multifrequency core size and time lags support prevailing synchrotron self-absorption. The intrinsic brightness temperature of the core derived from ground-based very long baseline interferometry (VLBI) is close to the equipartition regime value. In the same time, there is evidence for ultra-compact features of the size of less than 10 μas in the source, which might be responsible for the extreme apparent brightness temperatures of up to 1014 K as measured by RadioAstron. In 2007–2016 the VLBI components in the source at 43 GHz are found predominantly in two directions, suggesting a bend of the outflow from southern to northern direction. The apparent opening angle of the jet seen in the stacked image at 43 GHz is two times wider than that at 15 GHz, indicating a collimation of the flow within the central 1.5 mas. We estimate the Lorentz factor Γ = 14, the Doppler factor δ = 21, and the viewing angle θ = 1.7° of the apparent jet base, derive the gradients of magnetic field strength and electron density in the outflow, and the distance between jet apex and the core at each frequency.
Opacity, variability, and kinematics of AGN jets Kutkin, A M; Pashchenko, I N; Sokolovsky, K V ...
Monthly notices of the Royal Astronomical Society,
06/2019, Volume:
486, Issue:
1
Journal Article
Peer reviewed
Open access
ABSTRACT
Synchrotron self-absorption in active galactic nuclei (AGN) jets manifests itself as a time delay between flares observed at high and low radio frequencies. It is also responsible for the ...observing frequency-dependent change in size and position of the apparent base of the jet, aka the core shift effect, detected with very long baseline interferometry (VLBI). We measure the time delays and the core shifts in 11 radio-loud AGN to estimate the speed of their jets without relying on multi-epoch VLBI kinematics analysis. The 15–8 GHz total flux density time lags are obtained using Gaussian process regression, the core shift values are measured using VLBI observations and adopted from the literature. A strong correlation is found between the apparent core shift and the observed time delay. Our estimate of the jet speed is higher than the apparent speed of the fastest VLBI components by the median coefficient of 1.4. The coefficient ranges for individual sources from 0.5 to 20. We derive Doppler factors, Lorentz factors, and viewing angles of the jets, as well as the corresponding de-projected distance from the jet base to the core. The results support evidence for acceleration of the jets with bulk motion Lorentz factor Γ ∝ R0.52±0.03 on de-projected scales R of 0.5–500 parsecs.
Galaxy mergers have been observed to trigger nuclear activity by feeding gas to the central supermassive black hole. One such class of objects are Ultra Luminous InfraRed Galaxies (ULIRGs), which are ...mostly late stage major mergers of gas-rich galaxies. Recently, large-scale (∼100 kpc) radio continuum emission has been detected in a select number of ULIRGs, all of which also harbour powerful Active Galactic Nuclei (AGN). This hints at the presence of large-scale radio emission being evidence for nuclear activity. Exploring the origin of this radio emission and its link to nuclear activity requires high sensitivity multi-frequency data. We present such an analysis of the ULIRG Mrk 273. Using the International LOFAR telescope (ILT), we detected spectacular large-scale arcs in this system. This detection includes, for the first time, a giant ∼190 kpc arc in the north. We propose these arcs are fuelled by a low power radio AGN triggered by the merger. We also identified a bright ∼45 kpc radio ridge, which is likely related to the ionised gas nebula in that region. We combined this with high sensitivity data from APERture Tile In Focus (Apertif) and archival data from the Very Large Array (VLA) to explore the spectral properties. The ILT simultaneously allowed us to probe the nucleus at a resolution of ∼0.3″, where we detected three components, and, for the first time, diffuse emission around these components. Combining this with archival high frequency VLA images of the nucleus allowed us to detect absorption in one component, and a steep spectrum radio AGN in another. We then extrapolate from this case study to the importance of investigating the presence of radio emission in more ULIRGs and what it can tell us about the link between mergers and the presence of radio activity.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
PDF
