61 research outputs found
VLBI and total flux density investigations of the structure of active galactic nuclei
This doctoral thesis studies the structure of active galactic nuclei (AGN) using very long baseline interferometry (VLBI) at high radio frequencies (22 - 86 GHz). The VLBI observations are complemented using information from total flux density (TFD) observations at the corresponding frequencies and the TFD variations are compared to the structure changes seen in the VLBI images.
The basis of this work is the data from the three epoch VLBI monitoring project, which produced 45 images and parameters of 15 extragalactic sources at 22 GHz.
The maximum intrinsic brightness temperature for synchrotron sources is assumed to be limited by the inverse Compton catastrophe to ≅ 1012 K. We have presented two new methods to estimate this limit using total flux density observations, synchrotron-Self-Compton X-ray fluxes and shock parameters from VLBI observations. Using several data sets, we find that both methods yield a value of ≤ 1011 K, which is consistent with the equipartition limit of 1010 - 1011 K.
We have developed a new method for estimating the geometry of the Universe using the linear sizes of shocks in AGN jets. The angular sizes of the shocks from VLBI observations are normalized using intrinsic diameter estimates from TFD observations. Using a very small set of test data, we verify that the accuracy of the derived values compare favourably with traditional methods using much larger samples.
A clear connection between radio and gamma-ray flares and VLBI component ejections was found. Furthermore, it was shown that the origin of the gamma-ray flares is in the shocks and that they can not be produced close to the core of the AGN.
A helical structure was found in the jet of CTA 102, a High Polarised Quasar (HPQ). The apparent proper motions in the jet varied from 4.2 to 13.8 times the speed of the light. This work combined the work of several years of observation, covering wavelengths from 1.3 to 12 mm and angular resolutions from 50 to 500 microarcseconds.
The first 2mm VLBI experiment between Pico Veleta and Metsähovi was performed and correlation fringes were detected on a baseline of 1.55 billion wavelengths. The success of this experiment shows that VLBI observations are possible at very high frequencies leading to very high resolution and enabling to produce images of the fine structure of the cores of the AGN.reviewe
Time variability of the core-shift effect in the blazar 3C 454.3
Using VLBI to measure a so-called core shift effect is a common way of
obtaining estimates of the jet magnetic field strength. The VLBI core is
typically identified as the bright feature at the jet's base, and the position
of the core changes with the observed frequency, . In this work, we investigated the time variability of the
core-shift effect in the blazar 3C 454.3. We employed self-referencing analysis
of multi-frequency (5, 8, 15, 22-24, and 43 GHz) VLBA data covering 19 epochs
from 2005 until 2010. We found significant core shift variability ranging from
0.27 to 0.86 mas between 5 and 43 GHz, confirming the core-shift variability
phenomenon observed before. Time variability of the core-shift index ()
was found typically below one, with an average value of and a
standard deviation of . values were found during flaring and
quiescent states and our results indicate that commonly assumed conical jet
shape and equipartition conditions do not always hold simultaneously. Still,
these conditions are often assumed when deriving magnetic field strengths from
core shift measurements, leading to unreliable results if significantly
deviates from unity. Therefore, it is important to verify that holds
before using core shift values and the equipartition assumption to derive
physical parameters in the jets. When epochs are selected in the case
of 3C 454.3, the magnetic field estimates are indeed quite consistent, even
though the core shift varies with time. Additionally, our estimations of the
jet's magnetic flux in 3C 454.3 show that the source is indeed in the
magnetically arrested disk state. Finally, we found a good correlation of the
core position with the core flux density, , which is consistent with increased particle density
during the flares.Comment: 53 pages, 6 tables, 57 figures. Article submitted to Astronomy and
Astrophysic
Significant Limb-Brightening in the Inner Parsec of Markarian 501
We present three 43 GHz images and a single 86 GHz image of Markarian 501
from VLBA observations in 2005. The 86 GHz image shows a partially resolved
core with a flux density of about 200 mJy and a size of about 300 Schwarzschild
radii, similar to recent results by Giroletti et al. Extreme limb-brightening
is found in the inner parsec of the jet in the 43 GHz images, providing strong
observational support for a `spine-layer' structure at this distance from the
core. The jet is well resolved transverse to its axis, allowing Gaussian model
components to be fit to each limb of the jet. The spine-layer brightness ratio
and relative sizes, the jet opening angle, and a tentative detection of
superluminal motion in the layer are all discussed.Comment: Astrophysical Journal Letters, accepted for January 1, 2009 issu
Intrinsic brightness temperatures of blazar jets at 15 GHz
We have developed a new Bayesian Markov Chain Monte Carlo method to deconvolve light curves of blazars into individual flares, including proper estimation of the fit errors. We use the method to fit 15GHz light curves obtained within the OVRO 40-m blazar monitoring program where a large number of AGN have been monitored since 2008 in support of the Fermi Gamma-Ray Space Telescope mission. The time scales obtained from the fitted models are used to calculate the variability brightness temperature of the sources. Additionally, we have calculated brightness temperatures of a sample of these objects using Very Long Baseline Array data from the MOJAVE survey. Combining these two data sets enables us to study the intrinsic brightness temperature distribution in these blazars at 15 GHz. Our preliminary results indicate that the mean intrinsic brightness temperature in a sample of 14 sources is near the equipartition brightness temperature of ~ 10^(11)K
Optical polarization angle and VLBI jet direction in the binary black hole model of OJ287
We study the variation of the optical polarization angle in the blazar OJ287
and compare it with the precessing binary black hole model with a 'live'
accretion disk. First, a model of the variation of the jet direction is
calculated, and the main parameters of the model are fixed by the long term
optical brightness evolution. Then this model is compared with the variation of
the parsec scale radio jet position angle in the sky. Finally, the variation of
the polarization angle is calculated using the same model, but using a magnetic
field configuration which is at a constant angle relative to the optical jet.
It is found that the model fits the data reasonably well if the field is almost
parallel to the jet axis. This may imply a steady magnetic field geometry, such
as a large-scale helical field.Comment: to appear in Monthly Notices of Royal Astronomical Societ
Hunting for dark halo substructure using submilliarcsecond-scale observations of macrolensed radio jets
Dark halo substructure may reveal itself through secondary, small-scale
gravitational lensing effects on light sources that are macrolensed by a
foreground galaxy. Here, we explore the prospects of using Very Long Baseline
Interferometry (VLBI) observations of multiply-imaged quasar jets to search for
submilliarcsecond-scale image distortions produced by various forms of dark
substructures in the 1e3-1e8 Msolar mass range. We present lensing simulations
relevant for the angular resolutions attainable with the existing European VLBI
Network (EVN), the global VLBI array, and an upcoming observing mode in which
the Atacama Large Millimeter Array (ALMA) is connected to the global VLBI
array. While observations of this type would not be sensitive to standard cold
dark matter subhalos, they can be used to detect more compact forms of halo
substructure predicted in alternative structure formation scenarios. By mapping
~5 strongly lensed systems, it should be possible to detect or robustly rule
out primordial black holes in the 1e3-1e6 Msolar mass range if they constitute
>1% percent of the dark matter in these lenses. Ultracompact minihalos are
harder to detect using this technique, but 1e6-1e8 Msolar ultracompact
minihalos could in principle be detected if they constitute >10% of the dark
matter.Comment: 13 pages, 8 figures; v.2 accepted for publication in MNRA
A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger
Tidal disruption events (TDEs) are transient flares produced when a star is ripped apart by the gravitational field of a supermassive black hole (SMBH). We have observed a transient source in the western nucleus of the merging galaxy pair Arp 299 that radiated >1.5 × 10 erg at infrared and radio wavelengths but was not luminous at optical or x-ray wavelengths. We interpret this as a TDE with much of its emission reradiated at infrared wavelengths by dust. Efficient reprocessing by dense gas and dust may explain the difference between theoretical predictions and observed luminosities of TDEs. The radio observations resolve an expanding and decelerating jet, probing the jet formation and evolution around a SMBH. Copyright © 2018, American Association for the Advancement of ScienceS.M. acknowledges financial support from the Academy of Finland (pmject 8120503). The research leading to these mats has received funding from the European.. Commission Seventh Framework Programme (FP/2007-2013) under grant agreement number & 227290, 283393 (RadioNc-t) and 60725 (HELP). AA., M.P.-T., N.R.-O. and R.H.T. acknowledge support from the Spanish MINECO through grants AYA2012-38491-002-02 and AYA2015 63939 C2 1 P. P.G.J. acknowledges support from European Research Council Consolidator Grant 647208. C.R.-C. acknowledges support by the Ministry of Economy, Development and Tourism's Millennium Science Initiative through grant 10120009, awarded to The Millennium. Institute of Astrophysics, MAS Chile, and from CONICYT through FONDECYT grant 3150238 and China-CON1CYT fund CAS160313. P.M. and M.A.A. acknowledge support from the ERC research grant CAMAP-250276, and partial support from the Spanish MINECO grant AYA2015-66889C2-1P Lard the local Valencia government ghat PROMETE0-11-2014069. FIE. acknowledges support from a Science Foundation Ireland-Royal Society University Research Fellowship. D.L.C. acknowledges support from grants ST/0001901/4 ST/J001368/1, ST/ K001051/1, and st/N0001138/1. P.V. acknowledges support from the National Research Foundation of South Africa.. J.H. acknowledges financial support tom the Finis h ChAth ral Fouridation and the Virile), YIP and Kahle Vais8I8 Foundation. J.K. acknowledges financial support from the Academy of Finland (grant 311138)
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