160 research outputs found
Tracing the merger-driven evolution of active galaxies using the CJF sample
In the context of the evolution of large structures in the Universe, it is
unclear whether active galaxies are a phase which each galaxy undergoes, and
what is the importance of the evolution of black holes in their centers. Binary
black hole (BBH) systems could play a key role in our understanding of the
above question.
We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample for
evidence in favor of the merger-driven evolution scheme of active galaxies and
search tracer-systems of AGN evolution and possible indications of BBH
candidates. We discuss the validity and ambiguity of such indications and
formulate a set of selection criteria for the detection of such systems. We
conduct an extensive literature search for all available multi-wavelength
information, concentrating on the optical and infrared regime, in addition to
morphological information of the CJF sources. We analyze the statistics of this
sample, in terms of these properties.
We find 1 ULIRG (Mrk 231) included in the CJF, prototype of a transitory
system. In total 28.6% of the CJF sources with z<0.4 are distorted or have a
companion. Given the unbiased sample used here, this provides strong evidence
for the ubiquity of the merger phenomenon in the context of active galaxies. We
find a correlation between the radio and the near-infrared luminosity for the
high-luminosity sources, interpreted in the context of the interplay between a
star-formation and AGN component. We find a connection between variability and
evolutionary transitory systems, as selected through their near-infrared
colors. We select 28 sources that trace the different evolution phases of an
AGN, as well as a number of the most promising BBH candidates. We find 4
sources with almost periodical variability in the optical and radio on similar
timescales.Comment: 18 pages, 6 figures, accepted for publication in A&A (updated to
match proofs
Physical Conditions and Variability Processes in AGN Jets through Multi-Frequency Linear and Circular Radio Polarization Monitoring
Radio polarimetry is an invaluable tool to investigate the physical
conditions and variability processes in active galactic nuclei (AGN) jets.
However, detecting their linear and circular polarization properties is a
challenging endeavor due to their low levels and possible depolarization
effects. We have developed an end-to-end data analysis methodology to recover
the polarization properties of unresolved sources with high accuracy. It has
been applied to recover the linear and circular polarization of 87 AGNs
measured by the F-GAMMA program from July 2010 to January 2015 with a mean
cadence of 1.3 months. Their linear polarization was recovered at four
frequencies between 2.64 and 10.45 GHz and the circular polarization at 4.85
and 8.35 GHz. The physical conditions required to reproduce the observed
polarization properties and the processes which induce their variability were
investigated with a full-Stokes radiative transfer code which emulates the
synchrotron emission of modeled jets. The model was used to investigate the
conditions needed to reproduce the observed polarization behavior for the
blazar 3C 454.3, assuming that the observed variability is attributed to
evolving internal shocks propagating downstream.Comment: 6 pages, 2 figure
Imaging an Event Horizon: Mitigation of Source Variability of Sagittarius A*
The black hole in the center of the Galaxy, associated with the compact
source Sagittarius A* (Sgr A*), is predicted to cast a shadow upon the emission
of the surrounding plasma flow, which encodes the influence of general
relativity in the strong-field regime. The Event Horizon Telescope (EHT) is a
Very Long Baseline Interferometry (VLBI) network with a goal of imaging nearby
supermassive black holes (in particular Sgr A* and M87) with angular resolution
sufficient to observe strong gravity effects near the event horizon. General
relativistic magnetohydrodynamic (GRMHD) simulations show that radio emission
from Sgr A* exhibits vari- ability on timescales of minutes, much shorter than
the duration of a typical VLBI imaging experiment, which usually takes several
hours. A changing source structure during the observations, however, violates
one of the basic assumptions needed for aperture synthesis in radio
interferometry imaging to work. By simulating realistic EHT observations of a
model movie of Sgr A*, we demonstrate that an image of the average quiescent
emission, featuring the characteristic black hole shadow and photon ring
predicted by general relativity, can nonetheless be obtained by observing over
multiple days and subsequent processing of the visibilities (scaling,
averaging, and smoothing) before imaging. Moreover, it is shown that this
procedure can be combined with an existing method to mitigate the effects of
interstellar scattering. Taken together, these techniques allow the black hole
shadow in the Galactic center to be recovered on the reconstructed image.Comment: 10 pages, 12figures, accepted for publication in Ap
3 mm GMVA Observations of Total and Polarized Emission from Blazar and Radio Galaxy Core Regions
We present total and linearly polarized 3 mm Global mm-VLBI Array images of a
sample of blazars and radio galaxies from the VLBA-BU-BLAZAR 7 mm monitoring
program designed to probe the innermost regions of active galactic nuclei (AGN)
jets and locate the sites of gamma-ray emission observed by the Fermi-LAT. The
lower opacity at 3 mm and improved angular resolution, on the order of 50
microarcseconds, allow us to distinguish features in the jet not visible in the
7 mm VLBA data. We also compare two different methods used for the calibration
of instrumental polarisation and we analyze the resulting images for some of
the sources in the sample.Comment: Polarised Emission from Astrophysical Jets, June 12-16, 2017,
Ierapetra, Greec
The dependence of optical polarisation of blazars on the synchrotron peak frequency
The RoboPol instrument and the relevant program was developed in order to conduct a systematic study of the optical polarisation variability of blazars. Driven by the discovery that long smooth rotations of the optical polarisation plane can be associated with the activity in other bands and especially in gamma rays, the program was meant to investigate the physical mechanisms causing them and quantify the optical polarisation behaviour in blazars. Over the first three nominal observing seasons (2013, 2014 and 2015) RoboPol detected 40 rotations in 24 blazars by observing a gamma–ray-loud and gamma–ray-quite unbiassed sample of blazars, providing a reliable set of events for exploring the phenomenon. The obtain datasets provided the ground for a systematic quantification of the variability of the optical polarisation in such systems. In the following after a brief review of the discoveries that relate to the gamma-ray loudness of the sources we move on to discuss a simple jet model that explains the observed dichotomy in terms of polarisation between gamma–ray-loud and quite sources and the dependence of polarisation and the stability of the polarisation angle on the synchrotron peak frequency
- …