The AGN Population in Radio and Gamma-rays: origins and present perspective

C.C. Cheung ( NRC/NRL )

Radio observations have long provided exquisite sensitivity and resolution over a broad wavelength range for studies of relativistic jets in AGN. Complementarily, gamma-ray observations can independently probe jet properties via measurements of inverse Compton emission, variability, and absorption effects. While the substantial overlap in the radio-loud and `gamma-ray loud' AGN population was already established in the 1990's by CGRO/EGRET, this radio-gamma connection can now be fully exploited with the advent of the Fermi-LAT and current ground-based Cherenkov TeV observatories. In the context of radio/gamma-ray observations, I provide an overview of the recent advances being made in our understanding of AGN jets and a personal perspective on key areas where substantial progress can be made over the coming years.

RadioAstron Space VLBI mission: early results

Nikolai Kardashev, Yuri Kovalev ( ASC Lebedev )

The 10-meter space radio telescope Spektr-R was successfully launched on July 18, 2011, and unfurled several days later. The space element of the ground-space VLB interferometer RadioAstron covers four frequency bands from 92 to 1.3 cm and provides baselines up to 300,000 km. This will allow to study space objects with a resolution as high as about 10 microarcsecond. Current status of the mission and early results of RadioAstron space VLBI observations of active galactic nuclei will be presented.

The Parsec-Scale Radio Jets of Gamma-Ray Blazars

Matthew Lister ( Purdue University )

The MOJAVE program is a long term NRAO Key Project that is investigating the structure and evolution of highly beamed radio jets on parsec scales at 15 GHz with the VLBA. By obtaining regular observations of over 300 jets, we are able to examine statistical trends among blazar sub-samples that are complete with respect to gamma-ray and radio emission. I report on our recent kinematic and structural analysis of gamma-ray blazars, which finds distinct differences in trends seen among BL Lac objects versus quasars. The evidence supports a scenario where the brightest BL Lac jets are less beamed and have slower jets than quasars, and have a high energy spectrum that is dominated by synchrotron self-Compton emission.

High precision position measurements of the cores in 3C66A and 3C66B

Zhiqiang Shen

High-resolution VLBI observations make it possible to determine the position of

compact VLBI components at an accuracy of milli-arcsecond or better. We will report

the frequency-dependent core position shift of 3C66A based on three frequencies

(8, 15 and, 22 GHz) VLBA images made at its flare state in 2006. We will also

present the results from four-epoch three-frequency (2, 8 and, 22GHz) VLBA phase

referencing observations of 3C66B in 2004-2005, from which a large difference

in the core position was found compared to the previous measurements in 2001-

2002. The possible nuclear opacity change and its application to the astrometric and

astrophysical work will be discussed.

The AGN Population in Radio and Gamma-rays: Theoretical Perspective

Lukasz Stawarz ( ISAS/JAXA )

In this talk I will present a brief summary of the theoretical models regarding the production of non-thermal emission in active galaxies, and the very basics of the AGN unification scheme. First I will discuss the early predictions regarding high-energy radiation of AGN, including the X-ray and gamma-ray domains, following the radio studies prior to the era of the Compton Gamma-Ray Observatory (CGRO). Next I will summarize the main highlights from the CGRO mission, which revolutionized our understanding of the hard X-ray/soft gamma-ray accretion-related emission of radio-quiet AGN, and the gamma-ray jet-related emission of radio-loud AGN. Then I will discuss the most recent highlights since the successful launch of the Fermi Gamma-ray Space Telescope and the operation of modern ground-based Cherenkov Telescopes (HESS, MAGIC, VERITAS), summarizing the current view on the production of gamma-ray emission in relativistic jets, and on the radio/high-energy connection in active galaxies. I will conclude by discussing the future perspectives, commenting on what sort of different AGN classes are expected to be detected by the upcoming high-energy missions, and which open questions regarding the physics of AGN and supermassive black holes in general can be answered in the future by means of joint radio and gamma-ray studies involving Fermi-LAT, Cherenkov Telescope Array (CTA), ALMA, LOFAR, etc.

Very high-energy gamma-ray radiogalaxies and blazars

Helene SOL (1), Catherine BOISSON (2), Matteo CERUTTI (2), Andreas ZECH (2) (1 CNRS, Paris Observatory, 2 Paris Observatory)

The emerging family of very high-energy (VHE) radiogalaxies recently discovered by ground-based gamma-ray telescopes (HESS, MAGIC, VERITAS) gathers a few sources that appear quite different from each other regarding their radio, optical and X-ray properties. These AGN provide an interesting example where the connection between radio and high- energy emission seems particularly odd, in contrast to genuine BL Lac and blazars where correlations between radio and gamma-rays are often found.

After a short review of the current observational results on VHE radiogalaxies, we will compare them with the case of some remarkable VHE blazars with respect to their spectral energy distribution, correlation between multi-wavelength light curves from radio to gamma- rays and VLBI evolution when available, and show how slightly modified hadronic and leptonic scenarios inspired by blazar modelling can be applied to describe low or high states of VHE radiogalaxies.

Indeed, all VHE radiogalaxies discovered up to now have shown some transient BL Lac phenomena reported in the literature. We will argue that they may be radio-loud AGN with intermediate viewing angles, members of the still elusive transition population between beamed BL Lacs and unbeamed FR I radiogalaxies.

Assessing AGN Variability and Cross-waveband Correlations in the Era of High-Quality Monitoring Data in Low and High Energies

Talvikki Hovatta (Caltech)

During the past few years there has been an explosion of large-sample, high-quality AGN monitoring data across the electromagnetic spectrum, prompted largely by the operation of Fermi, which provides continuous

all-sky monitoring at gamma-ray energies. These datasets present an unprecedented opportunity for studies of AGN variability and the relation of AGN activity between different wavebands. In response to this wealth of

data, new methods are being developed to address various systematic effects that arise during the statistical analysis of lightcurves resulting from monitoring programs. I will discuss such methods and the problems they have allowed us to address in both amplitude and time domain correlation studies.

Gamma-ray emission along the radio jet: studies with Planck, Metsähovi and Fermi data.

Esko Valtaoja (Tuorla Observatory, University of Turku, Finland)

We study the connections between millimeter and sub-millimeter radio emission and gamma-ray activity in AGN using data from the Metsähovi radio monitoring program, the Planck satellite, and the Fermi satellite, as well as additional ground-based and space observations obtained within our Planck LFI Consortium collaboration.

The most important conclusion is that strong gamma-ray flares in quasars are closely connected with growing shocks in the jet, placing the gamma-ray emission site at, or down-stream of, the radio core. However, other sites of gamma-ray emission may also exist. In BL Lacs a different gamma-ray emission site or mechanism may be dominant.

Counterparts to Fermi LAT sources from the ATPMN 5 and 8 GHz catalogue of southern radio sources

David McConnell(1), Tara Murphy(2), Elaine Sadler(2), Ron Ekers(1) (1 ATNF/CASS CSIRO, 2 University of Sydney)

We investigate the radio-gamma-ray connection in AGN using a new radio catalogue, the ATPMN, in conjunction with the Fermi LAT 1FGL and 2FGL catalogues.The ATPMN (McConnell et al. 2012) contains southern radio sources that has been produced from archival ATCA data taken at 5 and 8 GHz in 1992-1994 as a follow-up survey of PMN radio source positions. The new catalogue presents positions, angular sizes, flux-densities and polarization properties for around 9000 sources in the declination range -87.5 < delta < - 38.5 degrees; the typical resolution for all measurements is 1 arcsec. We identify a subset of sources that have extreme radio spectral indices or are highly variable;many of which have gamma-ray counterparts. The ATPMN has 127 sources that are likely counterparts to gamma-ray sources in the 2FGL and 80 counterparts in the 1FGL, but only 57 appear in both gamma-ray lists. We will describe the source population contained in ATPMN, and will present analyses of the radio – gamma-ray comparisons with both the 1FGL and 2FGL and provide some understanding of the sources with radio counterparts that appear in only one of the two Fermi catalogues.

The radio counterparts of the 2009 exceptional gamma-ray flares in 3C273.

Zulema Abraham, Pedro Paulo Beaklini ( IAG/University of Sao Paulo )

The correlation between gamma-ray flares and changes in the structure and intensity of the radio emission in AGNs is a well-known phenomenon, but because in many cases the delays between the occurrences of variability at the different frequencies is similar to the time interval between different flares, it is not always possible to determine their exact correspondence.

This situation was reverted for the exceptionally luminous gamma-ray flares observed in 3C273 by the Fermi Gamma Ray Telescope in September 2009. We present here the detection of the 43 GHz counterparts of these flares, delayed by 170 days, from observations performed with the Itapetinga radiotelescope, in Brazil. Although the radio intensity of these flares was not very different from that of flares occurring at other epochs, their temporal structure was very well correlated to that of the gamma-ray flares, supporting the assumption of a common origin.

We interpret the radio flares as synchrotron and the gamma rays as Synchrotron Self Compton emission from relativistic electrons accelerated in compact plasma components ejected by the core of the quasar or in shock waves propagating along an ultra-relativistic jet forming a small angle with the line of sight. We revisit the precessing model for the pc- scale jet of 3C273 (Abraham & Romero 1999, A&A 344, 61) and claim that at the epoch in which the flares were seen, the boosting of the gamma-ray emission attained its maximum value, because the angle of the jet with the line of sight reached its minimum value while the Doppler factor was maximum. According to the model, which was based on the differences in the superluminal velocities of the components ejected at different epochs, as well as in their position angles in the plane of the sky, the jet in 3C273 is precessing with a period of 16 years, an aperture angle of 5.4 degrees around an axis the forms and angle of 14 degrees with the line of sight. The negligible increase in the radio emission relative to other flares can be also explained by this model, because the boosting produced by the increase in the Doppler factor is compensated by the decrease due to the optically thick spectral index.

On the location of the gamma-ray emission in blazars

Juri Poutanen (University of Oulu, Finland), Boris Stern (Institute for Nuclear Research, Moscow, Russia)

Fermi Gamma-ray Space Telescope has detected hundreds of AGNs, most of them are blazars. The GeV spectra of the flat spectra radio quasars (FSRQ) cannot be described by a simple power law model or any smoothly curved models.

A much better description is obtained with a broken power law, with the break energies of a few GeV. The sharpness and the position of the breaks can be well reproduced by absorption of gamma-rays via photon-photon pair production on He II and H I Lyman recombination continuum (LyC) and lines. We study spectral variability during flares of the brightest blazar 3C 454.3 and discover anti-correlation between the column density of the He II LyC and the gamma-ray flux. This implies that the gamma-ray emission zone lies close to the boundary of the high- ionization part of the broad-line region and moves away from the black hole when the flux increases. Identification of the gamma-ray production with the relativistic jet, implies that the jet is already accelerated at sub-parsec distances from the central black hole, which favours the Blandford-Znajek process as the jet launching mechanism. We also produce a combined spectrum of weaker blazars by blue-shifting them first according to their redshifts and show that the break at 5 GeV is extremely significant. This proves that in the majority FSRQ have the breaks caused by He II LyC.

What Sets the Power of Jets from Accreting Black Holes?

Alexander Tchekhovskoy (1), Jonathan McKinney (2) (1 Princeton University, 2 Stanford University )

Recent advances in numerical modeling of black hole accretion systems provide us with unprecedented insights into the physics of accretion and jets in active galactic nuclei and other black hole accretion systems. I will review the current understanding of the connection between jets, accretion, and black hole spin. Only recently, numerical simulations allowed us to determine the maximum outflow efficiency with which radiatively inefficient accretion flows can produce energy in the form of relativistic jets and winds. I will present the dependence of this maximum efficiency on black hole spin and accretion disk thickness and discuss observational implications.

To be or not to be a blazar. The case of the gamma-ray Narrow-Line Seyfert 1 SBS 0846+513.

Filippo D'Ammando ( Dip. Fisica, Univ. Perugia )

The strong and variable radio emission, and the flat radio spectrum suggested the presence of a relativistic jet in some radio-loud Narrow-Line Seyfert 1s, now confirmed by the Fermi-LAT detection in gamma rays. In June 2011 a strong gamma-ray flare from SBS 0846+513 was observed by Fermi-LAT reaching a gamma-ray luminosity (0.1–300 GeV) of 10^48 erg s−1, comparable to that of the brightest flat spectrum radio quasars. Apparent superluminal velocity in the jet was inferred from 2011-2012 VLBA images, suggesting the presence of a highly relativistic jet. Both the power released by this object during the flaring activity and the apparent superluminal velocity are strong indicators of the presence of a relativistic jet as powerful as those in blazars. In addition variability and spectral properties in radio and gamma-ray bands indicate a blazar-like behaviour, suggesting that, except for some optical peculiar characteristics, SBS 0846+513 could be a young blazar at the low end of the blazar’s BH mass distribution.

In this talk I discuss the radio-to-gamma-rays properties of this new gamma-ray emitting Narrow-Line Seyfert 1 in the context of the blazar scenario and the other gamma-ray AGNs detected by Fermi-LAT.

On the connection between radio and gamma rays. The extraordinary case of the flaring blazar PKS 1510-089

Monica Orienti (1), Filippo D'Ammando (2), Marcello Giroletti (3), Gabriele Giovannini (3), Daniele Dallacasa (4), Tiziana Venturi (1), Shoko Koyama (5), Hiroshi Nagai (5), Motoki Kino (5) (1 INAF - IRA, 2 University of Perugia, 3 INAF -IRA, 4 University of Bologna, 5 NAOJ )

Powerful radio emission is not commonly found in active galactic nuclei (AGN) and is associated with the presence of outflows of relativistic plasma produced in the innermost region. When these jets are aligned close to the line of sight, they show typical characteristics like superluminal motion, strong variability across the whole electromagnetic spectrum, and high polarization percentage. An intriguing aspect still unclear is the connection between the regions responsible for the emission at the various wavelengths, with a particular interest between the two extremes of the spectrum: radio and gamma rays. The gamma-ray emission from AGN is detected in radio-loud objects suggesting a strong connection between radio and gamma-ray emission.

In this presentation I will discuss results on the flaring radio source PKS 1510-089. Since the advent of AGILE and Fermi many gamma-ray flaring episodes have been detected from PKS 1510-089. After July 2011 this object has been undergoing very high activity states in both radio and gamma rays, with an extraordinary flaring activity detected in October 2011. By means of very high resolution observations with VERA and VLBA, and monitoring campaign performed with the Medicina single-dish, I will investigate the radio variability and relate it to the gamma-ray activity.

Variability of blazars: probing emission regions and acceleration processes

Fabrizio Tavecchio ( INAF/OAB, Italy )

Among the several uncertainties afflicting our knowledge of blazars (and, more generally, relativistic jets), that related to the location of the emission region is particularly acute. Recent multifrequency observations challenged the traditional scheme and triggered renewed activity. I will review the current observational status and the possible interpretations. I will focus in particular on the evidence of very rapid gamma-ray variability of flat spectrum radio quasars, discussing the scenario based on strong re-collimation of the flow beyond the broad line region.

Fermi Gamma-ray Bubbles and the Connection between Radio, Microwave, and Gamma-ray

MENG Su ( Harvard University/MIT )

Data from Fermi Gamma-ray Space Telescope reveal two large gamma-ray bubbles, extending 50 degrees above and below the Galactic center, with a width of about 40 degrees in longitude. The gamma-ray emission associated with these bubbles has a significantly harder spectrum (dN/dE ~ E^{−2}) up to ~100 GeV than the IC emission from electrons in the Galactic disk, or the gamma-rays produced by decay of pions from proton-ISM collisions. The bubbles are spatially correlated with the hard-spectrum microwave excess known as the WMAP haze; the edges of the bubbles also line up with features in the ROSAT X-ray maps at 1.5 − 2 keV. I will summarize observational evidence of the Fermi bubbles. These Galactic gamma-ray bubbles are probably ongoing shocks and were most likely created by some large episode of energy injection in the Galactic center, such as past accretion events onto the central massive black hole, or a nuclear starburst in the last 10 Myr. We recently found large scale collimated jet structure penetrating through the bubbles from the Galactic center, which provides further evidence of this picture. We will combine radio and microwave observations with gamma-ray data to constrain the nature of Fermi bubbles.

γ-rays in flat-spectrum AGN: revisiting the fast jet hypothesis

Marios Karouzos (1), Silke Britzen (2), Anton Zensus (2), Andreas Eckart (3) (1 CEOU-Seoul National University, 2 Max Planck Institute for Radio Astronomy, 3 University of Cologne)

We use the first Fermi-LAT AGN catalog to identify gamma-detected sources in the Caltech-Jodrell Bank flat-spectrum (CJF) sample of radio-loud flat-spectrum AGN. The rich VLBI dataset available for the CJF is used to study possible correlations between the gamma- ray and radio properties of these sources. We find that there is a substantial population of flat-spectrum gamma-detected AGN that show subluminal or mildly superluminal pc-scale jet speeds, contrary to previous studies. In addition we find a tentative correlation between gamma-ray and radio luminosities. We argue that a two-zone, spine-sheath, configuration can explain our results.

VLBI core flux density and position angle analysis of the MOJAVE AGN

Xiang Liu , Ligong Mi, Baorong Liu, Quanwei Li ( Xinjiang Astronomical Observatory, CAS )

In order to investigate the VLBI core properties we model-fitted 15GHz VLBA cores for about 60 core-dominated AGN from the MOJAVE sample, we will show the long term variability of flux density and position angle of core, and result of statistical analysis.

A deep view on the mm/radio properties of AGN

Sascha Trippe ( Seoul National University )

I report the results of our IRAM/PdBI monitoring program of radio-bright AGN at mm/radio frequencies. From 14-year long flux monitoring we find variability on timescales of years. A statistical analysis unveils the occurrence of distinct "quiescent" and "flare" emission activity phases. We find our sources to be substantially polarized with median degrees of linear polarization of approx. 4%. For a few selected targets, we observe strong polarization variability; in one case, we find the highest rotation measure ever reported for an extragalactic radio source.