José L. Gómez

Polarimetric Observations of 15 Active Galactic Nuclei at High Frequencies: Jet Kinematics from Bimonthly Monitoring with the Very Long Baseline Array

We present total and polarized intensity images of 15 active galactic nuclei obtained with the Very Long Baseline Array at 7 mm wavelength at 17 epochs from 1998 March to 2001 April. At some epochs the images are accompanied by nearly simultaneous polarization measurements at 3 mm, 1.35/0.85 mm, and optical wavelengths. Here we analyze the 7 mm images to define the properties of the jets of two radio galaxies, five BL Lac objects, and eight quasars on angular scales 0.1 mas. We determine the apparent velocities of 106 features in the jets. For many of the features we derive Doppler factors using a new method based on a comparison of the timescale of decline in flux density with the light-travel time across the emitting region. This allows us to estimate the Lorentz factors (Γ), intrinsic brightness temperatures, and viewing angles of 73 superluminal knots, as well as the opening angle of the jet for each source. The Lorentz factors of the jet flows in the different blazars range from Γ ~ 5 to 40 with the majority of the quasar components having Γ ~ 16–18, while the values in the BL Lac objects are more uniformly distributed. The brightest knots in the quasars have the highest apparent speeds, while the more slowly moving components are pronounced in the BL Lac objects. The quasars in our sample have similar opening angles and marginally smaller viewing angles than the BL Lacs. The two radio galaxies have lower Lorentz factors and wider viewing angles than the blazars. Opening angle and Lorentz factor are inversely proportional, as predicted by gasdynamical models. The brightness temperature drops more abruptly with distance from the core in the BL Lac objects than in the quasars and radio galaxies, perhaps owing to stronger magnetic fields in the former resulting in more severe synchrotron losses of the highest energy electrons. In nine sources we detect statistically meaningful deviations from ballistic motion, with the majority of components accelerating with distance from the core. In six sources we identify jet features with characteristics of trailing shocks that form behind the primary strong perturbations in jet simulations. The apparent speeds of these components increase with distance from the core, suggestive of acceleration of the underlying jet.

Flaring Behavior of the Quasar 3C 454.3 Across the Electromagnetic Spectrum

We analyze the behavior of the parsec-scale jet of the quasar 3C 454.3 during pronounced flaring in 2005-2008. Three major disturbances propagated down the jet along different trajectories with Lorentz factors Γ > 10. The disturbances show a clear connection with millimeter-wave outbursts, in 2005 May/June, 2007 July, and 2007 December. High-amplitude optical events in the R-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and γ-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the millimeter-wave core lies at the end of the jets acceleration and collimation zone. We infer that the X-ray emission is produced via inverse Compton scattering by relativistic electrons of photons both from within the jet (synchrotron self-Compton) and external to the jet (external Compton, or EC); which one dominates depends on the physical parameters of the jet. A broken power-law model of the γ-ray spectrum reflects a steepening of the synchrotron emission spectrum from near-IR to soft UV wavelengths. We propose that the γ-ray emission is dominated by the EC mechanism, with the sheath of the jet supplying seed photons for γ-ray events that occur near the millimeter-wave core.

LOCATION OF γ-RAY FLARE EMISSION IN THE JET OF THE BL LACERTAE OBJECT OJ287 MORE THAN 14 pc FROM THE CENTRAL ENGINE

We combine time-dependent multi-waveband flux and linear polarization observations with submilliarcsecond-scale polarimetric images at ? = 7?mm of the BL Lacertae type blazar OJ287 to locate the ?-ray emission in prominent flares in the jet of the source >14?pc from the central engine. We demonstrate a highly significant correlation between the strongest ?-ray and millimeter-wave flares through Monte Carlo simulations. The two reported ?-ray peaks occurred near the beginning of two major millimeter-wave outbursts, each of which is associated with a linear polarization maximum at millimeter wavelengths. Our very long baseline array observations indicate that the two millimeter-wave flares originated in the second of two features in the jet that are separated by >14?pc. The simultaneity of the peak of the higher-amplitude ?-ray flare and the maximum in polarization of the second jet feature implies that the ?-ray and millimeter-wave flares are cospatial and occur >14?pc from the central engine. We also associate two optical flares, accompanied by sharp polarization peaks, with the two ?-ray events. The multi-waveband behavior is most easily explained if the ?-rays arise from synchrotron self-Compton scattering of optical photons from the flares. We propose that flares are triggered by interaction of moving plasma blobs with a standing shock. The ?-ray and optical emission is quenched by inverse Compton losses as synchrotron photons from the newly shocked plasma cross the emission region. The millimeter-wave polarization is high at the onset of a flare, but decreases as the electrons emitting at these wavelengths penetrate less polarized regions.

Multiwaveband polarimetric observations of 15 active galactic nuclei at high frequencies: correlated polarization behavior

We report on multifrequency linear polarization monitoring of 15 active galactic nuclei containing highly relativistic jets with apparent speeds from ~4c to >40c. The measurements were obtained at optical, 1 mm, and 3 mm wavelengths, and at 7 mm with the Very Long Baseline Array. The data show a wide range in degree of linear polarization among the sources, from 30%, and interday polarization variability in individual sources. The polarization properties suggest separation of the sample into three groups with low, intermediate, and high variability of polarization in the core at 7 mm (LVP, IVP, and HVP, respectively). The groups are partially associated with the common classification of active galactic nuclei as radio galaxies and quasars with low optical polarization (LVP), BL Lacertae objects (IVP), and highly optically polarized quasars (HVP). Our study investigates correlations between total flux, fractional polarization, and polarization position angle at the different wavelengths. We interpret the polarization properties of the sources in the sample through models in which weak shocks compress turbulent plasma in the jet. The differences in the orientation of sources with respect to the observer, jet kinematics, and abundance of thermal matter external to the jet near the core can account for the diversity in the polarization properties. The results provide strong evidence that the optical polarized emission originates in shocks, most likely situated between the 3 and 7 mm VLBI cores. They also support the idea that the 1 mm core lies at the edge of the transition zone between electromagnetically dominated and turbulent hydrodynamic sections of the jet.

Faraday Rotation and Polarization Gradients in the Jet of 3C?120: Interaction with the External Medium and a Helical Magnetic Field?

We present a sequence of 12 monthly polarimetric 15, 22, and 43 GHz VLBA observations of the radio galaxy 3C 120 revealing a systematic presence of gradients in Faraday rotation and degree of polarization across and along the jet. The degree of polarization increases with distance from the core and toward the jet edges and has an asymmetric profile in which the northern side of the jet is more highly polarized. The Faraday rotation measure is also stratified across the jet width, with larger values for the southern side. We find a localized region of high Faraday rotation measure superposed on this structure between approximately 3 and 4 mas from the core, with a peak of ~6000 rad m−2. Interaction of the jet with the external medium or a cloud would explain the confined region of enhanced Faraday rotation, as well as the stratification in degree of polarization and the flaring of superluminal knots when crossing this region. The data are also consistent with a helical field in a two-fluid jet model, consisting of an inner, emitting jet and a sheath containing nonrelativistic electrons. However, this helical magnetic field model cannot by itself explain the localized region of enhanced Faraday rotation. The polarization electric vectors, predominantly perpendicular to the jet axis once corrected for Faraday rotation, require a dominant component parallel to the jet axis (in the frame of the emitting plasma) for the magnetic field in the emitting region.

Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron. I. Imaging BL Lacertae at 21 μas resolution

This research has been supported by the Spanish Ministry of Economy and Competitiveness grant AYA2013-40825-P, by the Russian Foundation for Basic Research (projects 13-02-12103, 14-02-31789, and 15-02-00949), and St. Petersburg University research grant 6.38.335.2015. The research at Boston University (BU) was funded in part by NASA Fermi Guest Investigator grant NNX14AQ58G. Y.M. acknowledges support from the ERC Synergy Grant >BlackHoleCam-Imaging the Event Horizon of Black Holes> (Grant 610058). Part of this work was supported by the COST Action MP1104 >Polarization as a tool to study the Solar System and beyond.> The RadioAstron project is led by the Astro Space Center of the Lebedev Physical Institute of the Russian Academy of Sciences and the Lavochkin Scientific and Production Association under a contract with the Russian Federal Space Agency, in collaboration with partner organizations in Russia and other countries.

Three-dimensional Simulations of Relativistic Precessing Jets Probing the Structure of Superluminal Sources

We present the results of a three-dimensional, relativistic, hydrodynamic simulation of a precessing jet into which a compact blob of matter is injected. A comparison of synthetic radio maps computed from the hydrodynamic model, taking into account the appropriate light-travel time delays, with those obtained from observations of actual superluminal sources shows that the variability of the jet emission is the result of a complex combination of phase motions, viewing angle selection effects, and nonlinear interactions between perturbations and the underlying jet and/or the external medium. These results question the hydrodynamic properties inferred from observed apparent motions and radio structures and reveal that shock-in-jet models may be overly simplistic.

ON THE SOURCE OF FARADAY ROTATION IN THE JET OF THE RADIO GALAXY 3C 120

The source of Faraday rotation in the jet of the radio galaxy 3C 120 is analyzed through Very Long Baseline Array observations carried out between 1999 and 2007 at 86, 43, 22, 15, 12, 8, 5, 2, and 1.7 GHz. Comparison of observations from 1999 to 2001 reveals uncorrelated changes in the linear polarization of the underlying jet emission and the Faraday rotation screen: while the rotation measure (RM) remains constant between approximately 2 and 5 mas from the core, the RM-corrected electric vector position angles (EVPAs) of two superluminal components are rotated by almost 90° when compared to other components moving through similar jet locations. On the other hand, the innermost 2 mas experiences a significant change in RM—including a sign reversal—but without variations in the RM-corrected EVPAs. Similarly, observations in 2007 reveal a double sign reversal in RM along the jet, while the RM-corrected EVPAs remain perpendicular to the jet axis. Although the observed coherent structure and gradient of the RM along the jet support the idea that the Faraday rotation is produced by a sheath of thermal electrons that surrounds the emitting jet, the uncorrelated changes in the RM and RM-corrected EVPAs indicate that the emitting jet and the source of Faraday rotation are not closely connected physically and have different configurations for the magnetic field and/or kinematical properties. Furthermore, the existence of a region of enhanced RM whose properties remain constant over three years requires a localized source of Faraday rotation, favoring a model in which a significant fraction of the RM originates in foreground clouds.

RECOLLIMATION SHOCKS IN MAGNETIZED RELATIVISTIC JETS

We have performed two-dimensional special-relativistic magnetohydrodynamic simulations of non-equilibrium over-pressured relativistic jets in cylindrical geometry. Multiple stationary recollimation shock and rarefaction structures are produced along the jet by the nonlinear interaction of shocks and rarefaction waves excited at the interface between the jet and the surrounding ambient medium. Although initially the jet is kinematically dominated, we have considered axial, toroidal and helical magnetic fields to investigate the effects of different magnetic-field topologies and strengths on the recollimation structures. We find that an axial field introduces a larger effective gas-pressure and leads to stronger recollimation shocks and rarefactions, resulting in larger flow variations. The jet boost grows quadratically with the initial magnetic field. On the other hand, a toroidal field leads to weaker recollimation shocks and rarefactions, modifying significantly the jet structure after the first recollimation rarefaction and shock. The jet boost decreases systematically. For a helical field, instead, the behaviour depends on the magnetic pitch, with a phenomenology that ranges between the one seen for axial and toroidal magnetic fields, respectively. In general, however, a helical magnetic field yields a more complex shock and rarefaction substructure close to the inlet that significantly modifies the jet structure. The differences in shock structure resulting from different field configurations and strengths may have observable consequences for disturbances propagating through a stationary recollimation shock.

THE CONNECTION BETWEEN THE RADIO JET AND THE GAMMA-RAY EMISSION IN THE RADIO GALAXY 3C 120

We present the analysis of the radio jet evolution of the radio galaxy 3C 120 during a period of prolonged -ray activity detected by the Fermi satellite between December 2012 and October 2014. We nd a clear connection between the -ray and radio emission, such that every period of -ray activity is accompanied by the aring of the mm-VLBI core and subsequent ejection of a new superluminal component. However, not all ejections of components are associated with -ray events detectable by Fermi. Clear ray detections are obtained only when components are moving in a direction closer to our line of sight. This suggests that the observed -ray emission depends not only on the interaction of moving components with the mm-VLBI core, but also on their orientation with respect to the observer. Timing of the -ray detections and ejection of superluminal components locate the -ray production to within 0.13 pc from the mm-VLBI core, which was previously estimated to lie about 0.24 pc from the central black hole. This corresponds to about twice the estimated extension of the broad line region, limiting the external photon eld and therefore suggesting synchrotron self Compton as the most probable mechanism for the production of the -ray emission. Alternatively, the interaction of components with the jet sheath can provide the necessary photon eld to produced the observed -rays by Compton scattering. Subject headings: galaxies: active | galaxies: jet | galaxies: individual (3C120) | radio continuum: galaxies