F. D’Ammando

Very Long Baseline polarimetry and the γ-ray connection in Markarian 421 during the broadband campaign in 2011

Context. This is the third paper in a series devoted to the analysis of the multiwavelength data from a campaign on the nearby (z = 0.03) TeV blazar Mrk 421 during 2011. Aims. We investigate the structure of the high angular resolution polarization, the magnetic topology, the total intensity light curve, the γ-ray flux, and the photon index. We describe how they evolve and how they are connected. Methods. We analyzed data in polarized intensity obtained with the Very Long Baseline Array (VLBA) at twelve epochs (one observation per month from January to December 2011) at 15, 24, and 43 GHz. For the absolute orientation of the electric vector position angles (EVPA) we used the D-terms method; we also confirm its accuracy. We also used γ-ray data from the Fermi Large Area Telescope on weekly time bins throughout 2011. Results. The source shows polarized emission, and its properties vary with time, frequency, and location along the jet. The core mean polarization fraction is generally between 1% and 2%, with a 4% peak at 43 GHz in March; the polarization angle is variable, mainly at 15 GHz, where it changes frequently, and less so at 43 GHz, where it oscillates in the range 114 ◦ −173 ◦ . The jet polarization properties are more stable, with a fractional polarization of around 16% and a polarization angle nearly perpendicular to the jet axis. The average flux and photon index at γ-ray energies are (17.4 ± 0.5) ×10 −8 ph cm −2 s −1 and Γ= 1.77 ± 0.02. The γ-ray light curve shows variability, with a main peak of (38 ± 11) × 10 −8 ph cm −2 s −1 at the beginning of March and two later peaks centered on September 8 and November 13. The first γ-ray peak appears to be associated with the peak in the core polarized emission at 43 GHz, as well as with the total intensity light curve. A discrete correlation function analysis yields a correlation coefficient of 0.54 at zero delay, with a significance level >99.7%. Conclusions. With this multifrequency study, we accurately determine the polarization properties of Mrk 421, both in the core and in the jet region. The radio and γ-ray light curves are correlated. The observed EVPA variability at 15 GHz is partly due to opacity and partly to a variable Faraday rotation effect. To explain the residual variability of the intrinsic polarization angle and the low degree of polarization in the core region, we invoke a blend of variable cross-polarized subcomponents with different polarization properties within the beam.

VLBA observations of radio faint Fermi-LAT sources above 10 GeV

The first Fermi-LAT High-energy source catalog (1FHL), containing gamma-ray sources detected above 10 GeV, is an ideal sample to characterize the physical properties of the most extreme gamma-ray sources. We investigate the pc scale properties of a sub-sample of radio faint 1FHL sources with the aim to confirm the proposed blazar associations, by revealing a compact high brightness temperature radio core, and we propose new low-frequency counterparts for the unassociated gamma-ray sources (UGS). Moreover, we increase the number of 1FHL sources with high resolution observations to explore the possible connection between radio and gamma rays at E >10 GeV. We observed 84 1FHL sources, mostly blazars of High Synchrotron Peaked (HSP) type, in the northern sky with the Very Long Baseline Array (VLBA) at 5 GHz. These sources lack high resolution radio observations and have at least one NVSS counterpart within the 95% confidence radius. For those sources without a well identified radio counterpart we exploit the VLBA multiple phase-center correlation capability to discern among the possible low-frequency candidates. For about 93% of the sources of our sample we reveal a compact high brightness temperature radio core, supporting their proposed blazar association. The vast majority of the detected sources are radio weak, with a median VLBI flux density value of 16.3 mJy. For the detected sources we obtain an average brightness temperature of the order of

Exploring the connection between radio and GeV-TeV γ-ray emission in the 1FHL and 2FHL AGN samples

2times10^{10} , rm{K}

VERA monitoring of the radio jet 3C 84 in the period of 2007–2013: Detection of non-linear motion

. We find a compact component for 16 UGS, for which we propose a new low-frequency association. We find brightness temperature values which do not require high Doppler factors, and are in agreement with the expected values for the equipartition of energy between particles and magnetic field. We find strong indications about the blazar nature of all of the detected UGS, for which we propose new low-frequency associations.

Multiband Observations of the Quasar PKS 2326–502 during Active and Quiescent Gamma-Ray States in 2010–2012

Context. With the advent of the Fermi Large Area Telescope (LAT) it was revealed that blazars, representing the most extreme radio-loud active galactic nuclei (AGN) population, dominate the census of the γ -ray sky, and a significant correlation was found between radio and γ -ray emission in the 0.1−100u2009GeV energy range. However, the possible connection between radio and very high energy (VHE, E > 0.1u2009TeV) emission still remains elusive, owing to the lack of a homogeneous coverage of the VHE sky. Aims. The main goal of this work is to quantify and assess the significance of a possible connection between the radio emission on parsec scale measured by the very long baseline interferometry (VLBI) and GeV-TeV γ -ray emission in blazars, which is a central issue for understanding blazar physics and the emission processes in these objects. Methods. We investigate the radio VLBI and high energy γ -ray emission by using two large and unbiased AGN samples extracted from the first and second Fermi -LAT catalogs of hard γ -ray sources detected above 10u2009GeV (1FHL) and 50u2009GeV (2FHL). For comparison, we perform the same correlation analysis by using the 0.1−300u2009GeV γ -ray energy flux provided by the third Fermi -LAT source catalog (3FGL). We assess the correlation’s statistical significance by means of a method based on permutations of the luminosities, by taking into account the various observational biases, which may apparently enhance or spoil any intrinsic correlation. Results. We find that the correlation strength and significance depend on the γ -ray energy range, with a different behavior among the blazar sub-classes. Overall, the radio and γ -ray emission above 10u2009GeV turns out to be uncorrelated for the full samples and for all of the blazar sub-classes with the exception of high synchrotron peaked (HSP) objects, which show a strong and significant correlation. On the contrary, when 0.1−300u2009GeV γ -ray energies are considered, a strong and significant correlation is found for the full blazar sample as well as for all of the blazar sub-classes. Conclusions. We interpret and explain this correlation behavior within the framework of the blazar spectral energy distribution properties. In the most powerful blazars, which are in general of low synchrotron peaked type, the high energy emission component peaks at energies lower than those sampled by the LAT. On the contrary, in HSP blazars the part of the high energy spectrum affected by cooling effects is well beyond the energy range sampled by the LAT, showing a rising spectrum both in the 3FGL and 1FHL/2FHL energy ranges.

X-Ray and GeV Gamma-Ray Variability of the Radio Galaxy NGC 1275

We present a kinematic study of the subparsec-scale radio jet of the radio galaxy 3C 84/NGC 1275 with the VLBI Exploration of Radio Astrometry (VERA) array at 22 GHz for 80 epochs from 2007 October to 2013 December. The averaged radial velocity of the bright component C3 with reference to the radio core is found to be

The Radio/Gamma-Ray Connection from 120 MHz to 230 GHz

0.27 pm 0.02c

Investigating the Nature of Late-time High-energy GRB Emission through Joint Fermi/Swift Observations

between 2007 October and 2013 December. This constant velocity of C3 is naturally explained by the advancing motion of the head of the mini-radio lobe. We also find a non-linear component in the motion of C3 with respect to the radio core. We briefly discuss possible origins of this non-linear motion.