On 27 June 2012, an eruptive solar prominence was observed in the extreme ultraviolet (EUV) and radio wavebands. At the Aalto University Metsähovi Radio Observatory (MRO) it was observed at 37 GHz. ...It was the first time that the MRO followed a radio prominence with dense sampling in the millimetre wavelengths. This prompted us to study the connection of the 37 GHz event with other wavelength domains. At 37 GHz, the prominence was tracked to a height of around
1.6
R
⊙
, at which the loop structure collapsed. The average velocity of the radio prominence was
55
±
6
km
s
−
1
. The brightness temperature of the prominence varied between
800
±
100
K and
3200
±
100
K. We compared our data with the
Solar Dynamic Observatory
(SDO)/
Atmospheric Imaging Assembly
(AIA) instrument’s 304 Å EUV data, and found that the prominence behaves very similarly in both wavelengths. The EUV data also reveal flaring activity nearby the prominence. We present a scenario in which this flare works as a trigger that causes the prominence to move from a stable stage to an acceleration stage.
The connection between solar radio and soft X-ray emission has earlier been studied at various radio frequencies. For instance, the intensity peak times during solar flares have been compared between ...these two wavelength regimes. It has been reported that solar radio emission peaks before soft X-ray emission during a flare. However, opposite results have also been presented. In this study, we compare millimetre (8 mm) solar and soft X-ray emissions (0.5–4 Å and 1–8 Å). The radio observations were made at Metsähovi Radio Observatory of Aalto University in Finland between 2015 and 2019. The soft X-ray data were observed with GOES-15 (Geostationary Operational Environmental Satellite). The data show that the solar millimetre emission can peak either before or after soft X-ray peak emission. In this study, we present two different scenarios, which could explain the peaking time differences and behaviour. The first scenario proposes a tight connection between the millimetre (8 mm) and soft X-ray emissions, the second one is for cases where the emission mechanisms are more separate.
We compare the γ-ray photon flux variability of northern blazars in the Fermi/LAT First Source Catalog with 37 GHz radio flux density curves from the Metsähovi quasar monitoring program. We find that ...the relationship between simultaneous millimeter (mm) flux density and γ-ray photon flux is different for different types of blazars. The flux relation between the two bands is positively correlated for quasars and does no exist for BLLacs. Furthermore, we find that the levels of γ-ray emission in high states depend on the phase of the high frequency radio flare, with the brightest γ-ray events coinciding with the initial stages of a mm flare. The mean observed delay from the beginning of a mm flare to the peak of the γ-ray emission is about 70 days, which places the average location of the γ-ray production at or downstream of the radio core. We discuss alternative scenarios for the production of γ-rays at distances of parsecs along the length of the jet.
We present RadioAstron Space VLBI imaging observations of the BL Lac object S5 0716+71 made on 2015 January 3-4 at a frequency of 22 GHz (wavelength λ = 1.3 cm). The observations were made in the ...framework of the AGN Polarization Key Science Program. The source was detected on projected space-ground baselines up to 70,833 km (5.6 Earth diameters) for both parallel-hand and cross-hand interferometric visibilities. We have used these detections to obtain a full-polarimetric image of the blazar at an unprecedented angular resolution of 24 as, the highest for this source to date. This enabled us to estimate the size of the radio core to be <12 × 5 as and to reveal a complex structure and a significant curvature of the blazar jet in the inner 100 as, which is an indication that the jet viewing angle lies inside the opening angle of the jet conical outflow. Fairly highly (15%) linearly polarized emission is detected in a jet region 19 as in size, located 58 as downstream from the core. The highest brightness temperature in the source frame is estimated to be >2.2 × 1013 K for the blazar core. This implies that the inverse-Compton limit must be violated in the rest frame of the source, even for the largest Doppler factor δ ∼ 25 reported for 0716+714.
Context.
Supermassive black holes can launch highly relativistic jets with velocities reaching Lorentz factors of as high as Γ > 50. How the jets accelerate to such high velocities and where along ...the jet they reach terminal velocity are open questions that are tightly linked to their structure as well as their launching and dissipation mechanisms.
Aims.
Changes in the beaming factor along the jets could potentially reveal jet acceleration, deceleration, or bending. We aim to (1) quantify the relativistic effects in multiple radio frequencies and (2) study possible jet velocity–viewing angle variations at parsec scales.
Methods.
We used the state-of-the-art code
Magnetron
to model light curves from the University of Michigan Radio Observatory and the Metsähovi Radio Observatory’s monitoring programs in five frequencies covering about 25 years of observations in the 4.8 to 37 GHz range for 61 sources. We supplement our data set with high-frequency radio observations in the 100–340 GHz range from ALMA, CARMA, and SMA. For each frequency we estimate the Doppler factor which we use to quantify possible changes in the relativistic effects along the jets.
Results.
The majority of our sources do not show any statistically significant difference in their Doppler factor across frequencies. This is consistent with constant velocity in a conical jet structure, as expected at parsec scales. However, our analysis reveals 17 sources where relativistic beaming changes as a function of frequency. In the majority of cases, the Doppler factor increases towards lower frequencies. Only 1253–053 shows the opposite behavior. By exploring their jet properties we find that the jet of 0420–014 is likely bent across the 4.8–340 GHz range. For 0212+735, the jet is likely parabolic, and still accelerating in the 4.8–37 GHz range. We discuss possible interpretations for the trends found in the remaining sources.
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, Letnik:
437, Številka:
4
Journal Article
Recenzirano
Odprti dostop
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
Among active galactic nuclei, blazars show extreme variability properties. We here investigate the case of the BL Lac object S4 0954+65 with data acquired in 2019–2020 by the Transiting ...Exoplanet Survey Satellite (TESS) and by the Whole Earth Blazar Telescope (WEBT) Collaboration. The 2-min cadence optical light curves provided by TESS during three observing sectors of nearly 1 month each allow us to study the fast variability in great detail. We identify several characteristic short-term time-scales, ranging from a few hours to a few days. However, these are not persistent, as they differ in the various TESS sectors. The long-term photometric and polarimetric optical and radio monitoring undertaken by the WEBT brings significant additional information, revealing that (i) in the optical, long-term flux changes are almost achromatic, while the short-term ones are strongly chromatic; (ii) the radio flux variations at 37 GHz follow those in the optical with a delay of about 3 weeks; (iii) the range of variation of the polarization degree and angle is much larger in the optical than in the radio band, but the mean polarization angles are similar; (iv) the optical long-term variability is characterized by a quasi-periodicity of about 1 month. We explain the source behaviour in terms of a rotating inhomogeneous helical jet, whose pitch angle can change in time.
We performed a long-term optical (B, V, R bands), infrared (J and K bands), and radio band (15, 22, 37 GHz band) study of the flat spectrum radio quasar, 3C 454.3, using data collected over a period ...of more than 8 yr (MJD 54500-57500). The temporal variability, spectral properties, and interwaveband correlations were studied by dividing the available data into smaller segments with more regular sampling. This helped us constrain the size and the relative locations of the emission regions for different wavebands. Spectral analysis of the source revealed the interplay between the accretion disk and jet emission. The source predominantly showed a redder-when-brighter trend, though we observed a bluer-when-brighter trend at high flux levels, which could be a signature of particle acceleration and radiative cooling. Significant correlations with near-zero lag were seen between various optical and infrared bands, indicating that these emission regions are cospatial. Correlations with a time lag of about 10-100 days are seen between the optical/infrared and radio bands indicating these emissions arise from different regions. We also observe the DCF peak lag change from year to year. We try to explain these differences using a curved jet model where the different emission regions have different viewing angles resulting in a frequency-dependent Doppler factor. This variable Doppler factor model explains the variability timescales and the variation in DCF peak lag between the radio and optical emissions in different segments. Lags of 6-180 days are seen between emissions in various radio bands, indicating a core-shift effect.
Abstract
The blazar 3C 454.3 exhibited a strong flare seen in γ-rays, X-rays and optical/near-infrared bands during 2009 December 3–12. Emission in the V and J bands rose more gradually than did the ...γ-rays and soft X-rays, though all peaked at nearly the same time. Optical polarization measurements showed dramatic changes during the flare, with a strong anticorrelation between optical flux and degree of polarization (which rose from ∼3 to ∼20 per cent) during the declining phase of the flare. The flare was accompanied by large rapid swings in polarization angle of ∼170°. This combination of behaviours appears to be unique. We have cm-band radio data during the same period but they show no correlation with variations at higher frequencies. Such peculiar behaviour may be explained using jet models incorporating fully relativistic effects with a dominant source region moving along a helical path or by a shock-in-jet model incorporating three-dimensional radiation transfer if there is a dominant helical magnetic field. We find that spectral energy distributions at different times during the flare can be fit using modified one-zone models where only the magnetic field strength and particle break frequencies and normalizations need change. An optical spectrum taken at nearly the same time provides an estimate for the central black hole mass of ∼2.3 × 109 M⊙. We also consider two weaker flares seen during the ∼200 d span over which multiband data are available. In one of them, the V and J bands appear to lead the γ-ray and X-ray bands by a few days; in the other, all variations are simultaneous.
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.
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