The majority of Fermi-LAT detected (2FGL) sources are AGN, mostly blazars. However, the second largest category in the 2FGL are unassociated sources (~30% or 575 sources), whose multi-wavelength counterpart is either inconclusive or absent. Follow-up observations and archival data at X-ray, optical, and radio frequencies suggest that many unassociated 2FGL sources are strong candidates to be AGN. Typical observed characteristics of 2FGL detected AGN include variability at all frequencies and a spectral energy distribution (SED) with two “bumps”; a low-frequency synchrotron peak in the radio to optical/X-ray region and a high-frequency peak, possibly due to synchrotron self-Compton or Inverse Compton processes, that extends up to TeV energies. We present optical follow-up observations of a sample of Fermi unassociated sources with one or more potential X-ray counterparts detected within the LAT error circle.

We previously reported discovering that blazars have distinctive infrared colors as seen in the WISE mission. Of particular note are γ-ray blazars, the locus of whose colors roughly overlaps with QSOs contours but which is more tightly delimited and bluer than ULIRGs and Seyferts. Since a large fraction of γ-ray sources are unassociated with any point source, but are expected to be blazars, identifying a WISE-candidate blazar in the γ-ray source field enables efficient followup studies. In an effort to better understand why blazars have these colors, we reduced and analyzed Spitzer Infrared Spectrometer (IRS) measurements on 73 blazars of varying types, some at several epochs. These spectra can be used to study the continuum emission (non-thermal), and search for atomic or other spectral features.

We have been monitoring some well-known bright blazars with short LOFAR observations since February 2013 with fortnightly cadence using the full available bandwidth of the High Band Antennas (i.e., 48 MHz centred at 226 MHz). The sources were chosen to be bright at low frequencies and to exhibit strong GHz-frequency radio variability on timescales of weeks to years. None of the five objects selected have been monitored previously in the MHz band. Here we report some preliminary results on flux variability obtained so far with LOFAR. These observations are scientifically valuable in their own right and also act as a proof of principle for broader, higher-cadence monitoring of the extragalactic sky with LOFAR and possibly SKA.

Symmetric and triangle-shaped flux variability in X-ray and gamma-ray light curves is observed from many blazars. We derived the X-ray spectrum changing in time by using a kinetic equation of high energy electrons. Giving linearly changing the injection of low energy electrons into accelerating and emitting region, we obtained the preliminary results that represent the characteristic X-ray variability of the linear flux increase with hardening in the rise phase and the linear decrease with softening in the decay phase.

Feedback provided by relativistic jets may be effective in shaping the galaxy luminosity function. The quenching mode (quasar mode) at redshifts ~2-3 potentially disperses gas in star-forming galaxies. The maintenance mode (radio mode) heats the gas in galaxy clusters counteracting cooling flows. A number of authors have examined the effect of relativistic jets in dispersing clouds in the kpc-scale inhomogeneous interstellar medium of evolving galaxies. We have also investigated a particular case of maintenance-mode feedback in our simulation of the iconic radio galaxy / cooling flow cluster Hydra A. Modelling of the knots produced by the jets in the inner 10 kpc provides an estimate of 0.8 – 0.9 c for the velocities of the jets in agreement with other velocity estimates for FR1 jets. The addition of jet precession provides realistic simulations of the morphology of the Hydra A radio source and raises interesting questions as to the role of black hole and disk precession, in general, in galaxy formation.

It has long been known that kiloparsec-scale jets in radio galaxies can be divided into two flavours: strong (found in powerful sources, narrow and terminating in compact hot-spots) and weak (found in low-luminosity sources, flaring, unable to form hot-spots and terminating in diffuse lobes or tails). Both flavours are initially relativistic, but weak jets decelerate to sub-relativistic, transonic speeds by entraining external gas while strong jets remain relativistic and supersonic until they terminate. Much is now known about the kinematics of weak-flavour jets, which can be modelled as intrinsically symmetrical, decelerating relativistic flows, and we summarize the results of our work in this area. Strong-flavour jets are relatively faint and narrow, so it has hitherto proved difficult to obtain the necessary deep, transverse-resolved images in total intensity and linear polarization. The spectacular jets in the radio galaxy NGC 6251 appear to represent a transition case between weak and strong flavours: the jets show no clear evidence for deceleration, but are relatively wide. VLA observations hint at transverse velocity structure with a very fast (Lorentz factor >5) spine surrounded by a slower shear layer. New observations with the upgraded VLA should be able to test this picture.

We present a high resolution polarimetry and variability study of the M87 jet using VLA and HST data taken during 2002 to 2008. Both data-sets have an angular resolution as high as 0.06”, which is 2-3 times better than previous observations. New morphological details are revealed in both the optical and radio, which can help to reveal the energetic and magnetic field structure of the jet. By comparing the data with previously published HST and VLA observations, we show that the jet's morphology in total and polarized light is changing significantly on timescales of ~1 decade. We compare the evolution of the inner jet (particularly the nucleus and knot HST-1), when our observations overlap with the multi-wavelength monitoring campaigns conducted with HST and Chandra. We use these data to comment on particle acceleration and main emission processes.

A single measurement of linear polarization of a nonthermal source provides direct information about the mean direction and level of ordering of the magnetic field. Monitoring of the polarization in blazars, combined with millimeter-wave VLBI imaging in both total and polarized intensity, has the potential to determine the geometry of the magnetic field. This is a key probe of the physical processes in the relativistic jet, such as ordered field components, turbulence, magnetic reconnections, magnetic collimation and acceleration of the jet flow, particle acceleration, and radiative processes that produce extremely luminous, highly variable nonthermal emission. Well-sampled monitoring observations of multi-waveband flux and radio-optical polarization of blazars show a variety of behavior. In some cases, the observed polarization patterns appear systematic, while in others randomness dominates. Explanations involve helical magnetic fields, turbulence, and perhaps particle acceleration that depends on the angle between the magnetic field and shock fronts that might be present. Simulations from the author's TEMZ model, with turbulent plasma crossing a standing conical shock in the jet, show that a mixture of turbulent and toroidal magnetic field can produce the level of polarization variability that is observed, even when the two field components are roughly equal.

We present Faraday rotation measure (RM) properties of seven active galactic nuclei (AGN), observed with the NRAO VLA at three epochs in 2012-2014. Data was taken at 1.4, 2.2, 5.0, 8.2, 15.4, 22.4, 33.5 and 43.1 GHz quasi simultaneously in full polarization mode. For the first time RMs were calculated in a range of wavelengths covering more than one order of magnitude: from 21 cm up to 6 mm. We measured RM for each source and showed a tendency to increase its value toward high frequencies according to the law |RM|~νa with a = 1.6±0.1. For 0710+439, we observed an increase over the frequency range of 4 orders of magnitude and measured one of the highest RM ever, (−89±1)⋅103 rad/m2. Analysis of different epochs shows variations of the value and the sign of RM on short and long time-scales. This may be caused by changing physical conditions in the compact regions of the AGN jets, e.g., strength of magnetic field, particle density and so on.

In this talk we present a novel way to use optical polarimetric observations to provide independent constraints and guide to the modelling of the spectral energy distribution (SED) of blazars. The approach is particularly useful to judge when a two-zone model description of the SED of the source is required and is able to provide with extra information that helps breaking some of the degeneracies on the fit parameters of the two SSC model components. The method will be presented in some detail and will be subsequently applied to the 2008 multi-wavelength campaign of PKS 2155-304 as an illustration of its potential.

Several narrow-line Seyfert 1 galaxies (NLS1s) have now been detected in gamma rays, providing firm evidence that at least some of this class of active galactic nuclei (AGN) produce relativistic jets. The presence of jets in NLS1s is surprising, as these sources are typified by comparatively small black hole masses and near- or super-Eddington accretion rates. This challenges the current understanding of the conditions necessary for jet production. Comparing the properties of the jets in NLS1s with those in more familiar jetted systems is thus essential to improve jet production models. We present early results from our campaign to monitor the kinematics and polarization of the parsec-scale jets in a sample of 15 NLS1s through multifrequency observations with the Very Long Baseline Array. These observations are complemented by fast-cadence 15 GHz monitoring with the Owens Valley Radio Observatory 40 m telescope and optical spectroscopic monitoring with with the 2 m class telescope at the Guillermo Haro Astrophysics Observatory in Cananea, Mexico.

We have used the broadband backend available at the ATCA to study the fast interstellar scintillation (ISS) of quasar PKS 1257–326, resolving the core shift as a function of frequency on scales less than 10 microarcseconds. In this short paper we discuss the jet direction implied from the microarcsecond-scale core shift in PKS 1257–326.

The jet of BL Lac displays transverse patterns that propagate downstream superluminally. We suggest that they are transverse Alfvén waves propagating on the longitudinal component of a helical magnetic field. The speed of the wave adds relativistically to the speed of the beam, and the apparent speed of the pattern is greater than the beam speed. Models for the jet and the MHD waves give values for the Lorentz factor of the beam of 3–4.4 and pitch angle of the helical magnetic field of 43° - 65°. These are consistent with other estimates, if the beam and pattern speeds are allowed to differ.

The high-energy emission from blazars and other relativistic jet sources indicates that electrons are accelerated to ultra-relativistic (GeV - TeV) energies in these systems. This paper summarizes recent results from numerical studies of two fundamentally different particle acceleration mechanisms potentially at work in relativistic jets: Magnetic-field generation and relativistic particle acceleration in relativistic shear layers, which are likely to be present in relativistic jets, is studied via Particle-in-Cell (PIC) simulations. Diffusive shock acceleration at relativistic shocks is investigated using Monte-Carlo simulations. The resulting magnetic-field configurations and thermal + non-thermal particle distributions are then used to predict multi-wavelength radiative (synchrotron + Compton) signatures of both acceleration scenarios. In particular, we address how anisotropic shear-layer acceleration may be able to circumvent the well-known Lorentz-factor crisis, and how the self-consistent evaluation of thermal + non-thermal particle populations in diffusive shock acceleration simulations provides tests of the bulk Comptonization model for the Big Blue Bump observed in the SEDs of several blazars.

The emission of steady compact jets observed in the hard spectral state of X-ray binaries is likely to be powered by internal shocks caused by fluctuations of the outflow velocity. The dynamics of the internal shocks and the resulting spectral energy distribution (SED) of the jet is very sensitive to the shape of the Power Spectral Density (PSD) of the fluctuations of the jet Lorentz factor. It turns out that Lorentz factor fluctuations injected at the base of the jet with a flicker noise power spectrum (i.e. P(f) ∝1/f) naturally produce the canonical flat SED observed from radio to IR band in X-ray binary systems in the hard state. This model also predicts a strong, wavelength dependent, variability that resembles the observed one. In particular, strong sub-second variability is predicted in the infrared and optical bands. The assumed fluctuations of the jet Lorentz factor are likely to be triggered by the variability of the accretion flow which is best traced by the X-ray emission. In the case of GX339-4 for which high quality and simultaneous multi-wavelength data are available, we performed simulations assuming that the fluctuation of the jet Lorentz factor have the same PSD as the observed X-ray PSD. The synthetic SED calculated under this assumption provides a remarkable match to the observed radio to IR SED. In this case the model also produces strong mid-infrared spectral variability that is comparable to that reported in this source.

We investigate the systematics of the properties of highly relativistic jets at multiple frequencies, including Fermi, and MOJAVE data. We test the hypothesis that the blazar divide constitutes a dichotomy. We also explore possible measures of the Doppler factor for these highly Doppler-beamed active galactic nuclei.

Fanaroff-Riley I radiogalaxies have been observed in TeV gamma-rays during the last decades. The origin of the emission processes related with this energy band is still under debate. Here we consider the case of the two closest Fanaroff-Riley I objects: Centaurus A and M87. Their entire broadband spectral energy distributions and variability fluxes show evidences that leptonic models are not sufficient to explain their fluxes above 100 GeV. Indeed, both objects have been imaged by LAT instrument aboard of Fermi telescope with measured spectra well connected with one-zone leptonic models. However, to explain the TeV spectra obtained with campaigns by H.E.S.S., for Centaurus A, and by VERITAS, MAGIC and H.E.S.S. for M87, different emission processes must be introduced. In this work we introduce hadronic scenarios to describe the TeV gamma-ray fluxes observed and to obtain the expected neutrino counterparts for each considered TeV campaign. With the obtained neutrino spectra we calculate, through Monte Carlo simulations, the expected neutrino event rate in a hypothetical Km3 neutrino telescope and we compare the results with what has been observed by IceCube experiment up to now.

The nearby active galaxy NGC 1275, has been widely detected from radio to gamma rays. Its spectral energy distribution (SED) shows a double-peak feature, which is well explained by synchrotron self-Compton (SSC) model. However, recent TeV detections might suggest that very-high-energy γ-rays (E⩾100 GeV) may not have a leptonic origin. We test a lepto-hadronic model to describe the whole SED through SSC emission and neutral pion decay resulting from pγ interactions. Also, we estimate the neutrino events expected in a km3 Cherenkov telescope.

Markarian 421 (Mrk 421) is one of the brightest, fastest and closest BL Lac objects known. Its very high energy (VHE) spectrum has been successfully modeled with both leptonic and hadronic models and no conclusive results have been achieved yet about the origin of its VHE emission. Here we investigate the possibility that a fraction of the VHE flares of Mrk 421 are due to hadronic processes and calculate the expected associated neutrino flux. We introduce the obtained neutrino flux in a Monte Carlo simulation to see the expectation for a km3 Cherenkov neutrino telescope.

In this work, we study the acceleration of hot plasma to relativistic speed through the Compton rocket effect which is viable in the two-flow paradigm.