Marios Karouzos | Seoul National University (original) (raw)

Papers by Marios Karouzos

Astrophysical Journal

Activity at the centers of galaxies, during which the central supermassive black hole is accretin... more Activity at the centers of galaxies, during which the central supermassive black hole is accreting material, is nowadays accepted to be rather ubiquitous and most probably a phase of every galaxy's evolution. It has been suggested that galactic mergers and interactions may be the culprits behind the triggering of nuclear activity. We use near-infrared data from the new Infrared Medium-Deep Survey (IMS) and the Deep eXtragalactic Survey (DXS) of the VIMOS-SA22 field and radio data at 1.4 GHz from the FIRST survey and a deep VLA survey to study the environments of radio-AGN over an area of ~25 sq. degrees and down to a radio flux limit of 0.1 mJy and a J-band magnitude of 23 mag AB. Radio-AGN are predominantly found in environments similar to those of control galaxies at similar redshift, J-band magnitude, and U-R rest-frame absolute color. However, a sub-population of radio-AGN is found in environments up to 100 times denser than their control sources. We thus preclude merging as the dominant triggering mechanism of radio-AGN. Through the fitting of the broadband spectral energy distribution of radio-AGN in the least and most dense environments, we find that those in the least dense environments show higher radio-loudness, higher star formation efficiencies, and higher accretion rates, typical of the so-called high-excitation radio-AGN. These differences tend to disappear at z>1. We interpret our results in terms of a different triggering mechanism for these sources that is driven by mass-loss through winds of young stars created during the observed ongoing star formation.

Very long baseline interferometry (VLBI) allows for high-resolution and high-sensitivity observat... more Very long baseline interferometry (VLBI) allows for high-resolution and high-sensitivity observations of relativistic jets, that can reveal periodicities of several years in their structure. We perform an analysis of long-term VLBI data of the quasar S5 1928+738 in terms of a geometric model of a helical structure projected onto the plane of the sky. We monitor the direction of the jet axis through its inclination and position angles. We decompose the variation of the inclination of the inner 2 mas of the jet of S5 1928+738 into a periodic term with amplitude of ˜0.89° and a linear decreasing trend with rate of ˜0.05 yr-1. We also decompose the variation of the position angle into a periodic term with amplitude of ˜3.39° and a linear increasing trend with rate of ˜0.24 yr-1. We interpret the periodic components as arising from the orbital motion of a binary black hole inspiralling at the jet base and derive corrected values of the mass ratio and separation from the accumulated 18 yr of VLBI data. Then, we identify the linear trends in the variations as due to the slow reorientation of the spin of the jet emitter black hole induced by the spin-orbit precession and we determine the precession period TSO = 4852 ± 646 yr of the more massive black hole, acting as the jet emitter. Our study provides indications, for the first time from VLBI jet kinematics, for the spinning nature of the jet-emitting black hole.

Several observational studies and numerical simulations suggest that mergers must contribute to t... more Several observational studies and numerical simulations suggest that mergers must contribute to the evolution of galaxies; however, the role that they play is not yet fully understood. In this paper we study a sample of 52 double nucleus disk galaxies that are considered as candidates for a minor merger event. The luminosity of each of the nuclei and their relative separation are derived from a multi-component photometric fit of the galaxies in the Sloan Digital Sky Survey optical images. We find that the nuclei in most of the sources have projected separations <=4 kpc. The ratio of nuclear luminosities indicates that most of the systems are likely in the coalescence stage of a major merger. This is supported by the existence of a single galaxy disk in 65% of the systems studied and the finding of a correlation between nuclear luminosity and host luminosity for the single-disk systems: those sources fitted with as single disk are in a more evolved stage of the merger and present an enhancement of the nuclear luminosity compared to the double-disk systems, as expected from simulations of galaxy mergers. Finally, we identify a sample of 19 double nucleus disk galaxies in which the two nuclei are physically separated by <=1 kpc and constitute thus a sample of sub-kpc binary active galactic nucleus candidates.

Monthly Notices of the Royal Astronomical Society, Mar 2014

There is an ongoing debate concerning the driver of nuclear activity in galaxies, with active gal... more There is an ongoing debate concerning the driver of nuclear activity in galaxies, with active galactic nuclei (AGN) either being triggered by major or minor galactic mergers or, alternatively, through secular processes like cold gas accretion and/or formation of bars. We investigate the close environment of active galaxies selected in the X-ray, the radio and the mid-IR. We utilize the first data release of the new near-IR VISTA Deep Extragalactic Observations (VIDEO) survey of the XMM-Large Scale Structure field. We use two measures of environment density, namely counts within a given aperture and a finite redshift slice (pseudo-3D density) and closest neighbour density measures Σ2 and Σ5. We select both AGN and control samples, matching them in redshift and apparent Ks-band magnitude. We find that AGN are found in a range of environments, with a subset of the AGN samples residing in overdense environments. Seyfert-like X-ray AGN and flat-spectrum radio-AGN are found to inhabit significantly overdense environments compared to their control sample. The relation between overdensities and AGN luminosity does not however reveal any positive correlation. Given the absence of an environment density-AGN luminosity relation, we find no support for a scheme where high-luminosity AGN are preferentially triggered by mergers. On the contrary, we find that AGN likely trace over dense environments at high redshift due to the fact that they inhabit the most massive galaxies, rather than being an AGN.

Astrophysical Journal, Apr 2014

Several lines of argument support the existence of a link between activity at the nuclei of galax... more Several lines of argument support the existence of a link between activity at the nuclei of galaxies, in the form of an accreting supermassive black hole, and star formation activity in these galaxies. Radio jets have long been argued to be an ideal mechanism that allows active galactic nuclei (AGNs) to interact with their host galaxies and affect star formation. We use a sample of radio sources in the North Ecliptic Pole (NEP) field to study the nature of this putative link, by means of spectral energy distribution (SED) fitting. We employ the excellent spectral coverage of the AKARI infrared space telescope and the rich ancillary data available in the NEP to build SEDs extending from UV to far-IR wavelengths. We find a significant AGN component in our sample of relatively faint radio sources (<mJy). A positive correlation is found between the luminosity of the AGN component and that of star formation in the host galaxy, independent of the radio luminosity. In contrast, for narrow redshift and AGN luminosity ranges, we find that increasing radio luminosity leads to a decrease in the specific star formation rate. The most radio-loud AGNs are found to lie on the main sequence of star formation for their respective redshifts. For the first time, we potentially see such a two-sided feedback process in the same sample. We discuss the possible suppression of star formation, but not total quenching, in systems with strong radio jets, that supports the maintenance nature of feedback from radio AGN jets.

In this paper, we describe the first data release of the Visible and Infrared Survey Telescope fo... more In this paper, we describe the first data release of the Visible and Infrared Survey Telescope for Astronomy (VISTA) Deep Extragalactic Observations (VIDEO) survey. VIDEO is a ˜12 deg2 survey in the near-infrared Z, Y, J, H and Ks bands, specifically designed to enable the evolution of galaxies and large structures to be traced as a function of both epoch and environment from the present day out to z = 4, and active galactic nuclei (AGNs) and the most massive galaxies up to and into the epoch of reionization. With its depth and area, VIDEO will be able to fully explore the period in the Universe where AGN and starburst activity were at their peak and the first galaxy clusters were beginning to virialize. VIDEO therefore offers a unique data set with which to investigate the interplay between AGN, starbursts and environment, and the role of feedback at a time when it was potentially most crucial.
We provide data over the VIDEO-XMM3 tile, which also covers the Canada-France-Hawaii Telescope Legacy Survey Deep-1 field (CFHTLS-D1). The released VIDEO data reach a 5σ AB-magnitude depth of Z = 25.7, Y = 24.5, J = 24.4, H = 24.1 and Ks = 23.8 in 2 arcsec diameter apertures (the full depth of Y = 24.6 will be reached within the full integration time in future releases). The data are compared to previous surveys over this field and we find good astrometric agreement with the Two Micron All Sky Survey, and source counts in agreement with the recently released UltraVISTA survey data. The addition of the VIDEO data to the CFHTLS-D1 optical data increases the accuracy of photometric redshifts and significantly reduces the fraction of catastrophic outliers over the redshift range 0 < z < 1 from 5.8 to 3.1 per cent in the absence of an i-band luminosity prior. However, we expect that the main improvement in photometric redshifts will come in the redshift range 1 < z < 4 due to the sensitivity to the Balmer and 4000 Å breaks provided by the near-infrared VISTA filters. All images and catalogues presented in this paper are publicly available through ESO's phase 3 archive and the VISTA Science Archive.

A new paradigm for active galactic jet kinematics has emerged through detailed investigations of ... more A new paradigm for active galactic jet kinematics has emerged through detailed investigations of BL Lac objects using very long baseline radio interferometry. In this new scheme, most, if not all, jet components appear to remain stationary with respect to the core but show significant non-radial motions. This paper presents results from our kinematic investigation of the jets of a statistically complete sample of radio-loud flat-spectrum active galaxies, focusing on the comparison between the jet kinematic properties of BL Lacs and flat-spectrum radio-quasars. It is shown that there is a statistically significant difference between the kinematics of the two AGN classes, with BL Lacs showing more bent jets, that are wider and show slower movement along the jet axis, compared to flat-spectrum radio-quasars. This is interpreted as evidence for helically structured jets.

Context. Recent VLBI studies of the morphology and kinematics of individual BL Lac objects (S5 18... more Context. Recent VLBI studies of the morphology and kinematics of individual BL Lac objects (S5 1803+784, PKS 0735+178, etc.) have revealed a new paradigm for the pc-scale jet kinematics of these sources. Unlike the apparent superluminal outward motions usually observed in blazars, most, if not all, jet components in these sources appear to be stationary with respect to the core, while exhibiting strong changes in their position angles. As a result, the jet ridge lines of these sources evolve substantially, at times forming a wide channel-flow.
Aims: We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample of radio-loud active galaxies to study this new kinematic scenario for flat-spectrum AGN. Comparing BL Lac objects and quasars in the CJF, we look for differences in the kinematics and morphologies of their jet ridge lines. The large number of sources in the CJF sample, together with the excellent kinematical data available, allow us to perform a robust statistical analysis in this context.
Methods: We develop a number of tools to extract information about the apparent linear and angular evolution of the CJF jet ridge lines, as well as their morphology. We study both the radial and non-radial apparent motions in the CJF jets. A statistical analysis of the extracted information allows us to test this new kinematic scenario and assess the importances of the radial and non-radial motions in flat-spectrum AGN jets in general and especially in BL Lacs compared to quasars. We also use these tools to check the kinematics for (multi-wavelength) variable AGN.
Results: We find that approximately half of the sample display appreciable apparent jet widths (>10 degrees), with BL Lac jet ridge lines showing significantly larger apparent widths than those of both quasars and radio galaxies. In addition, BL Lac jet ridge lines are found to change their apparent width more strongly. Finally, BL Lac jet ridge lines show the least apparent linear evolution, which translates into the smallest apparent expansion speeds for their components. We find compelling evidence supporting a substantially different kinematic scenario for flat-spectrum radio-AGN jets and in particular for BL Lac objects. In addition, we find that variability is closely related to the properties of a source's jet ridge line. Variable quasars are found to have "BL Lac like" behavior, relative to their non-variable counterparts.

Astronomy & Astrophysics, May 2011

Context. The recent release of the First Fermi-LAT Source Catalog solidified the predominant asso... more Context. The recent release of the First Fermi-LAT Source Catalog solidified the predominant association of extragalactic γ-ray emitters to active galaxies, in particular blazars. A tight connection between AGN jet kinematics and γ-ray properties has been argued for, attributing the energetic emission from active galaxies to their highly relativistic outflows.
Aims: We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample to study the connection between AGN jet kinematics and their γ-ray properties. The high number of sources included in the sample, in addition to the excellent kinematic data available, allows us to investigate the origin of γ-ray emission in AGN.
Methods: We identify the CJF sources detected in γ-rays (by Fermi-LAT and EGRET). We use γ-ray luminosities and the available VLBI kinematic data to look for correlations between γ-ray and kinematic properties, as well as for differences between AGN classes (quasars, BL Lacs, radio galaxies). We also check the kinematics of the TeV sources in the CJF.
Results: 21.8% of the CJF has been detected in the γ-ray regime. We find the detectability of BL Lacs significantly higher compared to quasars. Sources detected in the γ-rays show a wider apparent jet velocity distribution compared to the non-detected ones, but the maxima of both distributions are at similar values. No strong link between γ-ray detectability and fast apparent jet speeds is found. A tentative correlation is found between γ-ray luminosity and maximum apparent jet speeds, stronger for BL Lac and γ-variable sources. We find non-radial jet motions to be important to γ-ray emission. We suggest two-zone, spine-sheath models as a possible explanation for our results. Two out of four CJF TeV sources show superluminal jet speeds, in contrast to previous studies.

Astronomy & Astrophysics, Sep 2010

Context. For the evolution of large-scale structures in the Universe, it is unclear whether activ... more Context. For the evolution of large-scale structures in the Universe, it is unclear whether active galaxies represent a phase that each galaxy undergoes, and whether and to which extent the evolution of black holes at their centers is important. Binary black hole (BBH) systems may play a key role in our understanding of the above questions.
Aims: We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample to identify evidence that supports the merger-driven evolution scheme of active galaxies, and search for tracer-systems of active galactic nucleus (AGN) evolution and possible BBH candidates. We discuss the strength of and uncertainty in the evidence and formulate a set of selection criteria to detect such tracer-systems.
Methods: We conduct an extensive literature search for all available multiwavelength data, particularly in the optical and infrared regime, and morphological information about the CJF sources. We perform a statistical analysis of the properties of this sample.
Results: 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 of the ubiquity of the merger phenomenon in the context of active galaxies. We find a correlation between the radio and the near-infrared luminosity of 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 on the basis of 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.

Context. The recent release of the First Fermi-LAT Source Catalog solidified the predominant asso... more Context. The recent release of the First Fermi-LAT Source Catalog solidified the predominant association of extragalactic γ-ray emitters to active galaxies, in particular blazars. A tight connection between AGN jet kinematics and γ-ray properties has been argued for, attributing the energetic emission from active galaxies to their highly relativistic outflows. Aims. We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample to study the connection between AGN jet kinematics and their γ-ray properties. The high number of sources included in the sample, in addition to the excellent kinematic data available, allows us to investigate the origin of γ-ray emission in AGN. Methods. We identify the CJF sources detected in γ-rays (by Fermi-LAT and EGRET). We use γ-ray luminosities and the available VLBI kinematic data to look for correlations between γ-ray and kinematic properties, as well as for differences between AGN classes (quasars, BL Lacs, radio galaxies). We also check the kinematics of the TeV sources in the CJF. Results. 21.8% of the CJF has been detected in the γ-ray regime. We find the detectability of BL Lacs significantly higher compared to quasars. γ-detected sources show a wider apparent jet velocity distribution compared to the non-detected ones, but the maxima of both distributions are at similar values. No strong link between γ-ray detectability and fast apparent jet speeds is found. A tentative correlation is found between γ-ray luminosity and maximum apparent jet speeds, stronger for BL Lac and γ-variable sources. We find non-radial jet motions to be important to γ-ray emission. We suggest two-zone, spine-sheath, models as a possible explanation to our results. We find 2 out of 4 CJF TeV sources show superluminal jet speeds, in contrast to previous studies. M. Karouzos et al.: γ-rays in flat-spectrum AGN: Revisiting the fast jet hypothesis with the CJF sample Lister et al. (2009b) study the kinematic properties of the 3-month Fermi-LAT detected sources of the MOJAVE sample.

We investigate the jet morphology and kinematics of a statistically complete radio-loud AGN sampl... more We investigate the jet morphology and kinematics of a statistically complete radio-loud AGN sample in terms of the gamma-ray properties of the sources. Gamma-ray detected AGN dominate the high end of the jet apparent speed distribution of the total sample. Gamma-variable sources show stronger evolution in their jet morphology. A 5.1% of the sources show large (> 15 degrees) swings in their jet ejection angle.

Astronomy & Astrophysics, 2010

Context. Flat-spectrum radio sources often show a core-jet structure on pc-scales. Individual jet... more Context. Flat-spectrum radio sources often show a core-jet structure on pc-scales. Individual jet components reveal predominantly outward directed motion. For the BL Lac object S5 0716+714 conflicting apparent velocities have been reported in the literature. This object is an intra-day variable source and suited to investigate a possible correlation between kinematic properties and flux-density variability on different timescales. Aims. We study the kinematics in the pc-scale jet of S5 0716+714 to determine the apparent speeds of the jet components based on a much improved data set. In addition, we search for correlations between the radio flux-density light curves and the morphological changes detected along the VLBI jet. Methods. We (re-)analyze 50 VLBI observations obtained with the VLBA at 5 different frequencies (5 -43 GHz) between 1992.73 and 2006.32. The data have been parameterized using circular Gaussian components. We analyze the jet component motion in detail taking care not only to account for motion in the radial but also in the orthogonal direction. We study the evolution of the jet ridge line and investigate the spectral properties of the individual components. We search for correlations between radio band light curves and the kinematic properties of the jet components. Results. We present an alternative kinematic scenario for jet component motion in S5 0716+714. We present evidence for the apparent stationarity of jet components (with regard to their core separation) with time. Jet components, however, do seem to move significantly non-radially with regard to their position angle and in a direction perpendicular to the major axis of the jet. We discuss a possible correlation between the long-term radio flux-density variability and apparent jet component motions. Conclusions. In S5 0716+714 an alternative motion scenario is proposed. With regard to the core separation, rather stationary components can fit the VLBI observations well. A new model to explain the observed motion with regard to the position angle is required. Based on the correlation between the longterm radio flux-density variability and the position angle evolution of a jet component, we conclude that a geometric contribution to the origin of the long-term variability might not be negligible. Subluminal motion has been reported for most of the TeV blazars. Our analysis also confirms this finding for the case of S5 0716+714. This result increases the number of TeV blazars showing apparent subluminal motion to 7.

Astronomy & Astrophysics, 2011

Context. The quasar B0605−085 (OH 010) shows a hint for probable periodical variability in the ra... more Context. The quasar B0605−085 (OH 010) shows a hint for probable periodical variability in the radio total flux-density light curves. Aims. We study the possible periodicity of B0605−085 in the total flux-density, spectra and opacity changes in order to compare it with jet kinematics on parsec scales. Methods. We have analyzed archival total flux-density variability at ten frequencies (and 230 GHz) together with the archival high-resolution very long baseline interferometry data at 15 GHz from the MOJAVE monitoring campaign. Using the Fourier transform and discrete autocorrelation methods we have searched for periods in the total flux-density light curves. In addition, spectral evolution and changes of the opacity have been analyzed. Results. We found a period in multi-frequency total flux-density light curves of 7.9 ± 0.5 yrs. Moreover, a quasi-stationary jet component C1 follows a prominent helical path on a similar time scale of 8 years. We have also found that the average instantaneous speeds of the jet components show a clear helical pattern along the jet with a characteristic scale of 3 mas. Taking into account average speeds of jet components, this scale corresponds to a time scale of about 7.7 years. Jet precession can explain the helical path of the quasi-stationary jet component C1 and the periodical modulation of the total flux-density light curves. We have fitted a precession model to the trajectory of the jet component C1, with a viewing angle φ 0 = 2.6 • ± 2.2 • , aperture angle of the precession cone Ω = 23.9 • ± 1.9 • and fixed precession period (in the observers frame) P = 7.9 yrs.

Astronomy & Astrophysics, 2010

Context. BL Lac objects show core-jet structures with features moving outwards along the jet. We ... more Context. BL Lac objects show core-jet structures with features moving outwards along the jet. We present a kinematic analysis of jet component motion in the pc-scale jet of the BL Lac object S5 1803+784, which does not reveal long-term outward motion for most of the components. Aims. S5 1803+784 shows complex kinematic phenomena; understanding these provides new insights into the emission processes in BL Lac objects and possibly into the differences between quasars and BL Lac objects. Methods. The blazar S5 1803+784 has been studied with VLBI at ν=1. 6, 2.3, 5, 8.4, and 15 GHz between 1993.88 and 2005.68 in 26 observing runs. We (re)analyzed the data and present Gaussian model-fits. We collected the already published kinematic information for this source from the literature and re-identified the components according to the new scenario presented in this paper. Altogether, 94 epochs of observations have been investigated. Results. A careful study of the long-term kinematics reveals a new picture for jet component motion in S5 1803+784. In contrast to previously discussed motion scenarios, we find that the jet structure within 12 mas of the core can most easily be described by the coexistence of several bright jet features that remain on the long-term at roughly constant core separations (in addition to the already known "stationary" jet component ∼ 1.4 mas) and one faint component moving with an apparent superluminal speed (∼19c, based on 3 epochs). While most of the components maintain long-term roughly constant distances from the core, we observe significant, smooth changes in their position angles. We report on an evolution of the whole jet ridge line with time over the almost 12 years of observations. The width of the jet changes periodically with a period of ∼8-9 years. We find a correlation between changes in the position angle and maxima in the total flux-density light-curves. We present evidence for a geometric origin of the observed phenomena and discuss possible models. Conclusions. We find evidence for a significantly different scenario of jet component motion in S5 1803+784 compared to the generally accepted one of outwardly moving jet components, and conclude that the observed phenomena (evolution of the jet ridge line, roughly constant component core separations but with significant position angle changes) can most easily be explained within a geometric model.

Monthly Notices of The Royal Astronomical Society, 2009

We present the result of our extensive intranight optical monitoring of the well known low-energy... more We present the result of our extensive intranight optical monitoring of the well known low-energy peaked BL Lac (LBL) object PKS 0735+178. This long-term follow-up consists of R-band monitoring for a minimum duration of ∼ 4 hours, on 17 nights spanning 11 years (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008). Using the CCD as an N-star photometer, a detection limit of around 1% was attained for the intranight optical variability (INOV). Remarkably, an INOV amplitude of 3% on hour-like time scale was not observed on any of the 17 nights, even though the likelihood of a typical LBL showing such INOV levels in a single session of 4 hours duration is known to be high (∼ 50%). Our observations have thus established a peculiar long-term INOV quiescence of this radio-selected BL Lac object. Moreover, the access to unpublished optical monitoring data of similarly high sensitivity, acquired in another programme, has allowed us to confirm the same anomalous INOV quiescence of this LBL all the way back to 1989, the epoch of its historically largest radio outburst. Here, we present observational evidence revealing the very unusual INOV behaviour of this classical BL Lac object and discuss this briefly in the context of its other known exceptional properties.

Context. The quasar B0605−085 (OH 010) shows a hint for probable periodical variability in the ra... more Context. The quasar B0605−085 (OH 010) shows a hint for probable periodical variability in the radio total flux-density light curves. Aims. We study the possible periodicity of B0605−085 in the total flux-density, spectra and opacity changes in order to compare it with jet kinematics on parsec scales. Methods. We have analyzed archival total flux-density variability at ten frequencies (and 230 GHz) together with the archival high-resolution very long baseline interferometry data at 15 GHz from the MOJAVE monitoring campaign. Using the Fourier transform and discrete autocorrelation methods we have searched for periods in the total flux-density light curves. In addition, spectral evolution and changes of the opacity have been analyzed. Results. We found a period in multi-frequency total flux-density light curves of 7.9 ± 0.5 yrs. Moreover, a quasi-stationary jet component C1 follows a prominent helical path on a similar time scale of 8 years. We have also found that the average instantaneous speeds of the jet components show a clear helical pattern along the jet with a characteristic scale of 3 mas. Taking into account average speeds of jet components, this scale corresponds to a time scale of about 7.7 years. Jet precession can explain the helical path of the quasi-stationary jet component C1 and the periodical modulation of the total flux-density light curves. We have fitted a precession model to the trajectory of the jet component C1, with a viewing angle φ 0 = 2.6 • ± 2.2 • , aperture angle of the precession cone Ω = 23.9 • ± 1.9 • and fixed precession period (in the observers frame) P = 7.9 yrs.

Astronomy & Astrophysics, 2008

We present a new method to fit the variations of both coordinates of a VLBI component as a functi... more We present a new method to fit the variations of both coordinates of a VLBI component as a function of time, assuming that the nucleus of the radio source contains a binary black hole system (BBH system). The presence of a BBH system produces 2 perturbations of the trajectory of the ejected VLBI components. By using only the VLBI coordinates, the problem we have to solve reduces to an astrometric problem. Knowledge of the variations of the VLBI coordinates as a function of time contains the kinematical information, thus we are able to deduce the inclination angle of the source and the bulk Lorentz factor of the ejected component. Generally, there is a family of the BBH system producing the same fit to our data. To illustrate this method, we apply it to the source 1807+784. We find that the inclination of the source is io = 5.8 o +1.7 −1.8 and the VLBI component is ejected with a bulk Lorentz factor of γ = 3.7 +0.3 −0.2 . We determine the family of the BBH system which provides the best fit, assuming at first that the masses of the 2 black holes are equal and then that the masses are different. Each family of BBH systems is characterized by Tp/T b ≈ 1.967, where Tp and T b are the precession period of the accretion disk of the black hole ejecting the VLBI component and the orbiting period of the BBH system.

Astronomy & Astrophysics, 2009

Context. Flat-spectrum radio sources often show a core-jet structure on pc-scales. Individual jet... more Context. Flat-spectrum radio sources often show a core-jet structure on pc-scales. Individual jet components reveal predominantly outward directed motion. For the BL Lac object S5 0716+714 conflicting apparent velocities have been reported in the literature. This object is an intra-day variable source and suited to investigate a possible correlation between kinematic properties and flux-density variability on different timescales. Aims. We study the kinematics in the pc-scale jet of S5 0716+714 to determine the apparent speeds of the jet components based on a much improved data set. In addition, we search for correlations between the radio flux-density light curves and the morphological changes detected along the VLBI jet. Methods. We (re-)analyze 50 VLBI observations obtained with the VLBA at 5 different frequencies (5 -43 GHz) between 1992.73 and 2006.32. The data have been parameterized using circular Gaussian components. We analyze the jet component motion in detail taking care not only to account for motion in the radial but also in the orthogonal direction. We study the evolution of the jet ridge line and investigate the spectral properties of the individual components. We search for correlations between radio band light curves and the kinematic properties of the jet components. Results. We present an alternative kinematic scenario for jet component motion in S5 0716+714. We present evidence for the apparent stationarity of jet components (with regard to their core separation) with time. Jet components, however, do seem to move significantly non-radially with regard to their position angle and in a direction perpendicular to the major axis of the jet. We discuss a possible correlation between the long-term radio flux-density variability and apparent jet component motions. Conclusions. In S5 0716+714 an alternative motion scenario is proposed. With regard to the core separation, rather stationary components can fit the VLBI observations well. A new model to explain the observed motion with regard to the position angle is required. Based on the correlation between the longterm radio flux-density variability and the position angle evolution of a jet component, we conclude that a geometric contribution to the origin of the long-term variability might not be negligible. Subluminal motion has been reported for most of the TeV blazars. Our analysis also confirms this finding for the case of S5 0716+714. This result increases the number of TeV blazars showing apparent subluminal motion to 7.

Astrophysical Journal

Activity at the centers of galaxies, during which the central supermassive black hole is accretin... more Activity at the centers of galaxies, during which the central supermassive black hole is accreting material, is nowadays accepted to be rather ubiquitous and most probably a phase of every galaxy's evolution. It has been suggested that galactic mergers and interactions may be the culprits behind the triggering of nuclear activity. We use near-infrared data from the new Infrared Medium-Deep Survey (IMS) and the Deep eXtragalactic Survey (DXS) of the VIMOS-SA22 field and radio data at 1.4 GHz from the FIRST survey and a deep VLA survey to study the environments of radio-AGN over an area of ~25 sq. degrees and down to a radio flux limit of 0.1 mJy and a J-band magnitude of 23 mag AB. Radio-AGN are predominantly found in environments similar to those of control galaxies at similar redshift, J-band magnitude, and U-R rest-frame absolute color. However, a sub-population of radio-AGN is found in environments up to 100 times denser than their control sources. We thus preclude merging as the dominant triggering mechanism of radio-AGN. Through the fitting of the broadband spectral energy distribution of radio-AGN in the least and most dense environments, we find that those in the least dense environments show higher radio-loudness, higher star formation efficiencies, and higher accretion rates, typical of the so-called high-excitation radio-AGN. These differences tend to disappear at z>1. We interpret our results in terms of a different triggering mechanism for these sources that is driven by mass-loss through winds of young stars created during the observed ongoing star formation.

Very long baseline interferometry (VLBI) allows for high-resolution and high-sensitivity observat... more Very long baseline interferometry (VLBI) allows for high-resolution and high-sensitivity observations of relativistic jets, that can reveal periodicities of several years in their structure. We perform an analysis of long-term VLBI data of the quasar S5 1928+738 in terms of a geometric model of a helical structure projected onto the plane of the sky. We monitor the direction of the jet axis through its inclination and position angles. We decompose the variation of the inclination of the inner 2 mas of the jet of S5 1928+738 into a periodic term with amplitude of ˜0.89° and a linear decreasing trend with rate of ˜0.05 yr-1. We also decompose the variation of the position angle into a periodic term with amplitude of ˜3.39° and a linear increasing trend with rate of ˜0.24 yr-1. We interpret the periodic components as arising from the orbital motion of a binary black hole inspiralling at the jet base and derive corrected values of the mass ratio and separation from the accumulated 18 yr of VLBI data. Then, we identify the linear trends in the variations as due to the slow reorientation of the spin of the jet emitter black hole induced by the spin-orbit precession and we determine the precession period TSO = 4852 ± 646 yr of the more massive black hole, acting as the jet emitter. Our study provides indications, for the first time from VLBI jet kinematics, for the spinning nature of the jet-emitting black hole.

Several observational studies and numerical simulations suggest that mergers must contribute to t... more Several observational studies and numerical simulations suggest that mergers must contribute to the evolution of galaxies; however, the role that they play is not yet fully understood. In this paper we study a sample of 52 double nucleus disk galaxies that are considered as candidates for a minor merger event. The luminosity of each of the nuclei and their relative separation are derived from a multi-component photometric fit of the galaxies in the Sloan Digital Sky Survey optical images. We find that the nuclei in most of the sources have projected separations <=4 kpc. The ratio of nuclear luminosities indicates that most of the systems are likely in the coalescence stage of a major merger. This is supported by the existence of a single galaxy disk in 65% of the systems studied and the finding of a correlation between nuclear luminosity and host luminosity for the single-disk systems: those sources fitted with as single disk are in a more evolved stage of the merger and present an enhancement of the nuclear luminosity compared to the double-disk systems, as expected from simulations of galaxy mergers. Finally, we identify a sample of 19 double nucleus disk galaxies in which the two nuclei are physically separated by <=1 kpc and constitute thus a sample of sub-kpc binary active galactic nucleus candidates.

Monthly Notices of the Royal Astronomical Society, Mar 2014

There is an ongoing debate concerning the driver of nuclear activity in galaxies, with active gal... more There is an ongoing debate concerning the driver of nuclear activity in galaxies, with active galactic nuclei (AGN) either being triggered by major or minor galactic mergers or, alternatively, through secular processes like cold gas accretion and/or formation of bars. We investigate the close environment of active galaxies selected in the X-ray, the radio and the mid-IR. We utilize the first data release of the new near-IR VISTA Deep Extragalactic Observations (VIDEO) survey of the XMM-Large Scale Structure field. We use two measures of environment density, namely counts within a given aperture and a finite redshift slice (pseudo-3D density) and closest neighbour density measures Σ2 and Σ5. We select both AGN and control samples, matching them in redshift and apparent Ks-band magnitude. We find that AGN are found in a range of environments, with a subset of the AGN samples residing in overdense environments. Seyfert-like X-ray AGN and flat-spectrum radio-AGN are found to inhabit significantly overdense environments compared to their control sample. The relation between overdensities and AGN luminosity does not however reveal any positive correlation. Given the absence of an environment density-AGN luminosity relation, we find no support for a scheme where high-luminosity AGN are preferentially triggered by mergers. On the contrary, we find that AGN likely trace over dense environments at high redshift due to the fact that they inhabit the most massive galaxies, rather than being an AGN.

Astrophysical Journal, Apr 2014

Several lines of argument support the existence of a link between activity at the nuclei of galax... more Several lines of argument support the existence of a link between activity at the nuclei of galaxies, in the form of an accreting supermassive black hole, and star formation activity in these galaxies. Radio jets have long been argued to be an ideal mechanism that allows active galactic nuclei (AGNs) to interact with their host galaxies and affect star formation. We use a sample of radio sources in the North Ecliptic Pole (NEP) field to study the nature of this putative link, by means of spectral energy distribution (SED) fitting. We employ the excellent spectral coverage of the AKARI infrared space telescope and the rich ancillary data available in the NEP to build SEDs extending from UV to far-IR wavelengths. We find a significant AGN component in our sample of relatively faint radio sources (<mJy). A positive correlation is found between the luminosity of the AGN component and that of star formation in the host galaxy, independent of the radio luminosity. In contrast, for narrow redshift and AGN luminosity ranges, we find that increasing radio luminosity leads to a decrease in the specific star formation rate. The most radio-loud AGNs are found to lie on the main sequence of star formation for their respective redshifts. For the first time, we potentially see such a two-sided feedback process in the same sample. We discuss the possible suppression of star formation, but not total quenching, in systems with strong radio jets, that supports the maintenance nature of feedback from radio AGN jets.

In this paper, we describe the first data release of the Visible and Infrared Survey Telescope fo... more In this paper, we describe the first data release of the Visible and Infrared Survey Telescope for Astronomy (VISTA) Deep Extragalactic Observations (VIDEO) survey. VIDEO is a ˜12 deg2 survey in the near-infrared Z, Y, J, H and Ks bands, specifically designed to enable the evolution of galaxies and large structures to be traced as a function of both epoch and environment from the present day out to z = 4, and active galactic nuclei (AGNs) and the most massive galaxies up to and into the epoch of reionization. With its depth and area, VIDEO will be able to fully explore the period in the Universe where AGN and starburst activity were at their peak and the first galaxy clusters were beginning to virialize. VIDEO therefore offers a unique data set with which to investigate the interplay between AGN, starbursts and environment, and the role of feedback at a time when it was potentially most crucial.
We provide data over the VIDEO-XMM3 tile, which also covers the Canada-France-Hawaii Telescope Legacy Survey Deep-1 field (CFHTLS-D1). The released VIDEO data reach a 5σ AB-magnitude depth of Z = 25.7, Y = 24.5, J = 24.4, H = 24.1 and Ks = 23.8 in 2 arcsec diameter apertures (the full depth of Y = 24.6 will be reached within the full integration time in future releases). The data are compared to previous surveys over this field and we find good astrometric agreement with the Two Micron All Sky Survey, and source counts in agreement with the recently released UltraVISTA survey data. The addition of the VIDEO data to the CFHTLS-D1 optical data increases the accuracy of photometric redshifts and significantly reduces the fraction of catastrophic outliers over the redshift range 0 < z < 1 from 5.8 to 3.1 per cent in the absence of an i-band luminosity prior. However, we expect that the main improvement in photometric redshifts will come in the redshift range 1 < z < 4 due to the sensitivity to the Balmer and 4000 Å breaks provided by the near-infrared VISTA filters. All images and catalogues presented in this paper are publicly available through ESO's phase 3 archive and the VISTA Science Archive.

A new paradigm for active galactic jet kinematics has emerged through detailed investigations of ... more A new paradigm for active galactic jet kinematics has emerged through detailed investigations of BL Lac objects using very long baseline radio interferometry. In this new scheme, most, if not all, jet components appear to remain stationary with respect to the core but show significant non-radial motions. This paper presents results from our kinematic investigation of the jets of a statistically complete sample of radio-loud flat-spectrum active galaxies, focusing on the comparison between the jet kinematic properties of BL Lacs and flat-spectrum radio-quasars. It is shown that there is a statistically significant difference between the kinematics of the two AGN classes, with BL Lacs showing more bent jets, that are wider and show slower movement along the jet axis, compared to flat-spectrum radio-quasars. This is interpreted as evidence for helically structured jets.

Context. Recent VLBI studies of the morphology and kinematics of individual BL Lac objects (S5 18... more Context. Recent VLBI studies of the morphology and kinematics of individual BL Lac objects (S5 1803+784, PKS 0735+178, etc.) have revealed a new paradigm for the pc-scale jet kinematics of these sources. Unlike the apparent superluminal outward motions usually observed in blazars, most, if not all, jet components in these sources appear to be stationary with respect to the core, while exhibiting strong changes in their position angles. As a result, the jet ridge lines of these sources evolve substantially, at times forming a wide channel-flow.
Aims: We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample of radio-loud active galaxies to study this new kinematic scenario for flat-spectrum AGN. Comparing BL Lac objects and quasars in the CJF, we look for differences in the kinematics and morphologies of their jet ridge lines. The large number of sources in the CJF sample, together with the excellent kinematical data available, allow us to perform a robust statistical analysis in this context.
Methods: We develop a number of tools to extract information about the apparent linear and angular evolution of the CJF jet ridge lines, as well as their morphology. We study both the radial and non-radial apparent motions in the CJF jets. A statistical analysis of the extracted information allows us to test this new kinematic scenario and assess the importances of the radial and non-radial motions in flat-spectrum AGN jets in general and especially in BL Lacs compared to quasars. We also use these tools to check the kinematics for (multi-wavelength) variable AGN.
Results: We find that approximately half of the sample display appreciable apparent jet widths (>10 degrees), with BL Lac jet ridge lines showing significantly larger apparent widths than those of both quasars and radio galaxies. In addition, BL Lac jet ridge lines are found to change their apparent width more strongly. Finally, BL Lac jet ridge lines show the least apparent linear evolution, which translates into the smallest apparent expansion speeds for their components. We find compelling evidence supporting a substantially different kinematic scenario for flat-spectrum radio-AGN jets and in particular for BL Lac objects. In addition, we find that variability is closely related to the properties of a source's jet ridge line. Variable quasars are found to have "BL Lac like" behavior, relative to their non-variable counterparts.

Astronomy & Astrophysics, May 2011

Context. The recent release of the First Fermi-LAT Source Catalog solidified the predominant asso... more Context. The recent release of the First Fermi-LAT Source Catalog solidified the predominant association of extragalactic γ-ray emitters to active galaxies, in particular blazars. A tight connection between AGN jet kinematics and γ-ray properties has been argued for, attributing the energetic emission from active galaxies to their highly relativistic outflows.
Aims: We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample to study the connection between AGN jet kinematics and their γ-ray properties. The high number of sources included in the sample, in addition to the excellent kinematic data available, allows us to investigate the origin of γ-ray emission in AGN.
Methods: We identify the CJF sources detected in γ-rays (by Fermi-LAT and EGRET). We use γ-ray luminosities and the available VLBI kinematic data to look for correlations between γ-ray and kinematic properties, as well as for differences between AGN classes (quasars, BL Lacs, radio galaxies). We also check the kinematics of the TeV sources in the CJF.
Results: 21.8% of the CJF has been detected in the γ-ray regime. We find the detectability of BL Lacs significantly higher compared to quasars. Sources detected in the γ-rays show a wider apparent jet velocity distribution compared to the non-detected ones, but the maxima of both distributions are at similar values. No strong link between γ-ray detectability and fast apparent jet speeds is found. A tentative correlation is found between γ-ray luminosity and maximum apparent jet speeds, stronger for BL Lac and γ-variable sources. We find non-radial jet motions to be important to γ-ray emission. We suggest two-zone, spine-sheath models as a possible explanation for our results. Two out of four CJF TeV sources show superluminal jet speeds, in contrast to previous studies.

Astronomy & Astrophysics, Sep 2010

Context. For the evolution of large-scale structures in the Universe, it is unclear whether activ... more Context. For the evolution of large-scale structures in the Universe, it is unclear whether active galaxies represent a phase that each galaxy undergoes, and whether and to which extent the evolution of black holes at their centers is important. Binary black hole (BBH) systems may play a key role in our understanding of the above questions.
Aims: We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample to identify evidence that supports the merger-driven evolution scheme of active galaxies, and search for tracer-systems of active galactic nucleus (AGN) evolution and possible BBH candidates. We discuss the strength of and uncertainty in the evidence and formulate a set of selection criteria to detect such tracer-systems.
Methods: We conduct an extensive literature search for all available multiwavelength data, particularly in the optical and infrared regime, and morphological information about the CJF sources. We perform a statistical analysis of the properties of this sample.
Results: 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 of the ubiquity of the merger phenomenon in the context of active galaxies. We find a correlation between the radio and the near-infrared luminosity of 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 on the basis of 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.

Context. The recent release of the First Fermi-LAT Source Catalog solidified the predominant asso... more Context. The recent release of the First Fermi-LAT Source Catalog solidified the predominant association of extragalactic γ-ray emitters to active galaxies, in particular blazars. A tight connection between AGN jet kinematics and γ-ray properties has been argued for, attributing the energetic emission from active galaxies to their highly relativistic outflows. Aims. We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample to study the connection between AGN jet kinematics and their γ-ray properties. The high number of sources included in the sample, in addition to the excellent kinematic data available, allows us to investigate the origin of γ-ray emission in AGN. Methods. We identify the CJF sources detected in γ-rays (by Fermi-LAT and EGRET). We use γ-ray luminosities and the available VLBI kinematic data to look for correlations between γ-ray and kinematic properties, as well as for differences between AGN classes (quasars, BL Lacs, radio galaxies). We also check the kinematics of the TeV sources in the CJF. Results. 21.8% of the CJF has been detected in the γ-ray regime. We find the detectability of BL Lacs significantly higher compared to quasars. γ-detected sources show a wider apparent jet velocity distribution compared to the non-detected ones, but the maxima of both distributions are at similar values. No strong link between γ-ray detectability and fast apparent jet speeds is found. A tentative correlation is found between γ-ray luminosity and maximum apparent jet speeds, stronger for BL Lac and γ-variable sources. We find non-radial jet motions to be important to γ-ray emission. We suggest two-zone, spine-sheath, models as a possible explanation to our results. We find 2 out of 4 CJF TeV sources show superluminal jet speeds, in contrast to previous studies. M. Karouzos et al.: γ-rays in flat-spectrum AGN: Revisiting the fast jet hypothesis with the CJF sample Lister et al. (2009b) study the kinematic properties of the 3-month Fermi-LAT detected sources of the MOJAVE sample.

We investigate the jet morphology and kinematics of a statistically complete radio-loud AGN sampl... more We investigate the jet morphology and kinematics of a statistically complete radio-loud AGN sample in terms of the gamma-ray properties of the sources. Gamma-ray detected AGN dominate the high end of the jet apparent speed distribution of the total sample. Gamma-variable sources show stronger evolution in their jet morphology. A 5.1% of the sources show large (> 15 degrees) swings in their jet ejection angle.

Astronomy & Astrophysics, 2010

Context. Flat-spectrum radio sources often show a core-jet structure on pc-scales. Individual jet... more Context. Flat-spectrum radio sources often show a core-jet structure on pc-scales. Individual jet components reveal predominantly outward directed motion. For the BL Lac object S5 0716+714 conflicting apparent velocities have been reported in the literature. This object is an intra-day variable source and suited to investigate a possible correlation between kinematic properties and flux-density variability on different timescales. Aims. We study the kinematics in the pc-scale jet of S5 0716+714 to determine the apparent speeds of the jet components based on a much improved data set. In addition, we search for correlations between the radio flux-density light curves and the morphological changes detected along the VLBI jet. Methods. We (re-)analyze 50 VLBI observations obtained with the VLBA at 5 different frequencies (5 -43 GHz) between 1992.73 and 2006.32. The data have been parameterized using circular Gaussian components. We analyze the jet component motion in detail taking care not only to account for motion in the radial but also in the orthogonal direction. We study the evolution of the jet ridge line and investigate the spectral properties of the individual components. We search for correlations between radio band light curves and the kinematic properties of the jet components. Results. We present an alternative kinematic scenario for jet component motion in S5 0716+714. We present evidence for the apparent stationarity of jet components (with regard to their core separation) with time. Jet components, however, do seem to move significantly non-radially with regard to their position angle and in a direction perpendicular to the major axis of the jet. We discuss a possible correlation between the long-term radio flux-density variability and apparent jet component motions. Conclusions. In S5 0716+714 an alternative motion scenario is proposed. With regard to the core separation, rather stationary components can fit the VLBI observations well. A new model to explain the observed motion with regard to the position angle is required. Based on the correlation between the longterm radio flux-density variability and the position angle evolution of a jet component, we conclude that a geometric contribution to the origin of the long-term variability might not be negligible. Subluminal motion has been reported for most of the TeV blazars. Our analysis also confirms this finding for the case of S5 0716+714. This result increases the number of TeV blazars showing apparent subluminal motion to 7.

Astronomy & Astrophysics, 2011

Context. The quasar B0605−085 (OH 010) shows a hint for probable periodical variability in the ra... more Context. The quasar B0605−085 (OH 010) shows a hint for probable periodical variability in the radio total flux-density light curves. Aims. We study the possible periodicity of B0605−085 in the total flux-density, spectra and opacity changes in order to compare it with jet kinematics on parsec scales. Methods. We have analyzed archival total flux-density variability at ten frequencies (and 230 GHz) together with the archival high-resolution very long baseline interferometry data at 15 GHz from the MOJAVE monitoring campaign. Using the Fourier transform and discrete autocorrelation methods we have searched for periods in the total flux-density light curves. In addition, spectral evolution and changes of the opacity have been analyzed. Results. We found a period in multi-frequency total flux-density light curves of 7.9 ± 0.5 yrs. Moreover, a quasi-stationary jet component C1 follows a prominent helical path on a similar time scale of 8 years. We have also found that the average instantaneous speeds of the jet components show a clear helical pattern along the jet with a characteristic scale of 3 mas. Taking into account average speeds of jet components, this scale corresponds to a time scale of about 7.7 years. Jet precession can explain the helical path of the quasi-stationary jet component C1 and the periodical modulation of the total flux-density light curves. We have fitted a precession model to the trajectory of the jet component C1, with a viewing angle φ 0 = 2.6 • ± 2.2 • , aperture angle of the precession cone Ω = 23.9 • ± 1.9 • and fixed precession period (in the observers frame) P = 7.9 yrs.

Astronomy & Astrophysics, 2010

Context. BL Lac objects show core-jet structures with features moving outwards along the jet. We ... more Context. BL Lac objects show core-jet structures with features moving outwards along the jet. We present a kinematic analysis of jet component motion in the pc-scale jet of the BL Lac object S5 1803+784, which does not reveal long-term outward motion for most of the components. Aims. S5 1803+784 shows complex kinematic phenomena; understanding these provides new insights into the emission processes in BL Lac objects and possibly into the differences between quasars and BL Lac objects. Methods. The blazar S5 1803+784 has been studied with VLBI at ν=1. 6, 2.3, 5, 8.4, and 15 GHz between 1993.88 and 2005.68 in 26 observing runs. We (re)analyzed the data and present Gaussian model-fits. We collected the already published kinematic information for this source from the literature and re-identified the components according to the new scenario presented in this paper. Altogether, 94 epochs of observations have been investigated. Results. A careful study of the long-term kinematics reveals a new picture for jet component motion in S5 1803+784. In contrast to previously discussed motion scenarios, we find that the jet structure within 12 mas of the core can most easily be described by the coexistence of several bright jet features that remain on the long-term at roughly constant core separations (in addition to the already known "stationary" jet component ∼ 1.4 mas) and one faint component moving with an apparent superluminal speed (∼19c, based on 3 epochs). While most of the components maintain long-term roughly constant distances from the core, we observe significant, smooth changes in their position angles. We report on an evolution of the whole jet ridge line with time over the almost 12 years of observations. The width of the jet changes periodically with a period of ∼8-9 years. We find a correlation between changes in the position angle and maxima in the total flux-density light-curves. We present evidence for a geometric origin of the observed phenomena and discuss possible models. Conclusions. We find evidence for a significantly different scenario of jet component motion in S5 1803+784 compared to the generally accepted one of outwardly moving jet components, and conclude that the observed phenomena (evolution of the jet ridge line, roughly constant component core separations but with significant position angle changes) can most easily be explained within a geometric model.

Monthly Notices of The Royal Astronomical Society, 2009

We present the result of our extensive intranight optical monitoring of the well known low-energy... more We present the result of our extensive intranight optical monitoring of the well known low-energy peaked BL Lac (LBL) object PKS 0735+178. This long-term follow-up consists of R-band monitoring for a minimum duration of ∼ 4 hours, on 17 nights spanning 11 years (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008). Using the CCD as an N-star photometer, a detection limit of around 1% was attained for the intranight optical variability (INOV). Remarkably, an INOV amplitude of 3% on hour-like time scale was not observed on any of the 17 nights, even though the likelihood of a typical LBL showing such INOV levels in a single session of 4 hours duration is known to be high (∼ 50%). Our observations have thus established a peculiar long-term INOV quiescence of this radio-selected BL Lac object. Moreover, the access to unpublished optical monitoring data of similarly high sensitivity, acquired in another programme, has allowed us to confirm the same anomalous INOV quiescence of this LBL all the way back to 1989, the epoch of its historically largest radio outburst. Here, we present observational evidence revealing the very unusual INOV behaviour of this classical BL Lac object and discuss this briefly in the context of its other known exceptional properties.

Context. The quasar B0605−085 (OH 010) shows a hint for probable periodical variability in the ra... more Context. The quasar B0605−085 (OH 010) shows a hint for probable periodical variability in the radio total flux-density light curves. Aims. We study the possible periodicity of B0605−085 in the total flux-density, spectra and opacity changes in order to compare it with jet kinematics on parsec scales. Methods. We have analyzed archival total flux-density variability at ten frequencies (and 230 GHz) together with the archival high-resolution very long baseline interferometry data at 15 GHz from the MOJAVE monitoring campaign. Using the Fourier transform and discrete autocorrelation methods we have searched for periods in the total flux-density light curves. In addition, spectral evolution and changes of the opacity have been analyzed. Results. We found a period in multi-frequency total flux-density light curves of 7.9 ± 0.5 yrs. Moreover, a quasi-stationary jet component C1 follows a prominent helical path on a similar time scale of 8 years. We have also found that the average instantaneous speeds of the jet components show a clear helical pattern along the jet with a characteristic scale of 3 mas. Taking into account average speeds of jet components, this scale corresponds to a time scale of about 7.7 years. Jet precession can explain the helical path of the quasi-stationary jet component C1 and the periodical modulation of the total flux-density light curves. We have fitted a precession model to the trajectory of the jet component C1, with a viewing angle φ 0 = 2.6 • ± 2.2 • , aperture angle of the precession cone Ω = 23.9 • ± 1.9 • and fixed precession period (in the observers frame) P = 7.9 yrs.

Astronomy & Astrophysics, 2008

We present a new method to fit the variations of both coordinates of a VLBI component as a functi... more We present a new method to fit the variations of both coordinates of a VLBI component as a function of time, assuming that the nucleus of the radio source contains a binary black hole system (BBH system). The presence of a BBH system produces 2 perturbations of the trajectory of the ejected VLBI components. By using only the VLBI coordinates, the problem we have to solve reduces to an astrometric problem. Knowledge of the variations of the VLBI coordinates as a function of time contains the kinematical information, thus we are able to deduce the inclination angle of the source and the bulk Lorentz factor of the ejected component. Generally, there is a family of the BBH system producing the same fit to our data. To illustrate this method, we apply it to the source 1807+784. We find that the inclination of the source is io = 5.8 o +1.7 −1.8 and the VLBI component is ejected with a bulk Lorentz factor of γ = 3.7 +0.3 −0.2 . We determine the family of the BBH system which provides the best fit, assuming at first that the masses of the 2 black holes are equal and then that the masses are different. Each family of BBH systems is characterized by Tp/T b ≈ 1.967, where Tp and T b are the precession period of the accretion disk of the black hole ejecting the VLBI component and the orbiting period of the BBH system.

Astronomy & Astrophysics, 2009

Context. Flat-spectrum radio sources often show a core-jet structure on pc-scales. Individual jet... more Context. Flat-spectrum radio sources often show a core-jet structure on pc-scales. Individual jet components reveal predominantly outward directed motion. For the BL Lac object S5 0716+714 conflicting apparent velocities have been reported in the literature. This object is an intra-day variable source and suited to investigate a possible correlation between kinematic properties and flux-density variability on different timescales. Aims. We study the kinematics in the pc-scale jet of S5 0716+714 to determine the apparent speeds of the jet components based on a much improved data set. In addition, we search for correlations between the radio flux-density light curves and the morphological changes detected along the VLBI jet. Methods. We (re-)analyze 50 VLBI observations obtained with the VLBA at 5 different frequencies (5 -43 GHz) between 1992.73 and 2006.32. The data have been parameterized using circular Gaussian components. We analyze the jet component motion in detail taking care not only to account for motion in the radial but also in the orthogonal direction. We study the evolution of the jet ridge line and investigate the spectral properties of the individual components. We search for correlations between radio band light curves and the kinematic properties of the jet components. Results. We present an alternative kinematic scenario for jet component motion in S5 0716+714. We present evidence for the apparent stationarity of jet components (with regard to their core separation) with time. Jet components, however, do seem to move significantly non-radially with regard to their position angle and in a direction perpendicular to the major axis of the jet. We discuss a possible correlation between the long-term radio flux-density variability and apparent jet component motions. Conclusions. In S5 0716+714 an alternative motion scenario is proposed. With regard to the core separation, rather stationary components can fit the VLBI observations well. A new model to explain the observed motion with regard to the position angle is required. Based on the correlation between the longterm radio flux-density variability and the position angle evolution of a jet component, we conclude that a geometric contribution to the origin of the long-term variability might not be negligible. Subluminal motion has been reported for most of the TeV blazars. Our analysis also confirms this finding for the case of S5 0716+714. This result increases the number of TeV blazars showing apparent subluminal motion to 7.