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3C 84 (NGC 1275) is the radio source at the centre of the Perseus cluster and exhibits a bright radio jet. We observed the source with the Global Millimeter VLBI Array (GMVA) between 2008 and 2015, with a typical angular resolution of ∼50 μas. The observations revealed a consistent double nuclear structure separated by ∼770 gravitational radii assuming a black hole mass of 3.2 × 108 M⊙. The region is likely too broad and bright to be the true jet base anchored in the accretion disc or black hole ergosphere. A cone and parabola were fit to the stacked (time averaged) image of the nuclear region. The data did not strongly prefer either fit, but combined with a jet/counter-jet ratio analysis, an upper limit on the viewing angle to the inner jet region of ≤35° was found. This provides evidence for a variation of the viewing angle along the jet (and therefore a bent jet) within ∼0.5 pc of the jet launching region. In the case of a conical jet, the apex is located ∼2400 gravitational radii upstream of the bright nuclear region and up to ∼600 gravitational radii upstream in the parabolic case. We found a possible correlation between the brightness temperature and relative position angle of the double nuclear components, which may indicate rotation within the jet. © 2021 The Author(s)., This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2021R1A6A3A01086420). JAH was supported by Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT, South Korea (2018H1D3A1A02032824) and the research grant (2021R1C1C1009973). S-SL was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (2020R1A2C2009003). J-YK is supported for this research by the International Max-Planck Research School (IMPRS) for Astronomy and Astrophysics at the University of Bonn and Cologne. This research was supported by an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Universities Space Research Association through a contract with NASA. RL acknowledges the support of the Spanish Ministerio de Economía y Competitividad (grant PID2019-108995GB-C21), the Consejería de Economía, Conocimiento, Empresas y Universidad, Junta de Andalucía (grant P18-FR-1769), and the Consejo Superior de Investigaciones Científicas (grant 2019AEP112). GFP is supported for this research by the International Max-Planck Research School (IMPRS) for Astronomy and Astrophysics at the University of Bonn and Cologne. ST and MK acknowledge support via NRF grant 2019R1F1A1059721. This research has made use of data obtained with the Global Millimeter VLBI Array (GMVA), which consists of telescopes operated by the MPIfR, IRAM, Onsala, Metsahovi, Yebes, the Korean VLBI Network, the Green Bank Observatory, and the Very Long Baseline Array (VLBA). The VLBA is an instrument of the NRAO, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The data were correlated at the correlator of the Max Planck Institute for Radioastronomy (MPIfR) in Bonn, Germany. This work made use of the Swinburne University of Technology software correlator, developed as part of the Australian Major National Research Facilities Programme and operated under licence., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

CTA 102 is a blazar implying that its relativistic jet points towards Earth and emits synchrotron radiation produced by energetic particles gyrating in the magnetic field. This study aims to figure out the physical origins of radio flares in the jet, including the connection between the magnetic field and the radio flares. The data set in the range of 2.6–343.5 GHz was collected over a period of ∼5.5 yr (2012 November 20–2018 September 23). During the data collection period, seven flares at 15 GHz with a range of the variability time-scale of roughly 76–227 d were detected. The quasi-simultaneous radio data were used to investigate the synchrotron spectrum of the source. We found that the synchrotron radiation is self-absorbed. The turnover frequency and the peak flux density of the synchrotron self-absorption (SSA) spectra are in the ranges of ∼42–172 GHz and ∼0.9–10.2 Jy, respectively. From the SSA spectra, we derived the SSA magnetic field strengths to be ∼9.20, ∼12.28, and ∼50.97 mG on 2013 December 24, 2014 February 28, and 2018 January 13, respectively. We also derived the equipartition magnetic field strengths to be in the range of ∼24–109 mG. The equipartition magnetic field strengths are larger than the SSA magnetic field strengths in most cases, which indicates that particle energy mainly dominates in the jet. Our results suggest that the flares in the jet of CTA 102 originated due to particle acceleration. We propose the possible mechanisms of particle acceleration. © 2021 The Author(s)., We are grateful to the staff of the KVN who helped to operate the array and to correlate the data. The KVN is a facility operated by the KASI (Korea Astronomy and Space Science Institute). The KVN observations and correlations are supported through the high-speed network connections among the KVN sites provided by the KREONET (Korea Research Environment Open NETwork), which is managed and operated by the KISTI (Korea Institute of Science and Technology Information). This research has made use of data from the OVRO 40-m monitoring programme that was supported in part by NASA grants NNX08AW31G, NNX11A043G, and NNX14AQ89G, and NSF grants AST-0808050 and AST-1109911, and private funding from Caltech and the MPIfR. This publication makes use of data obtained at the Metsähovi Radio Observatory, operated by Aalto University in Finland. IA acknowledges financial support from the Spanish ‘Ministerio de Ciencia e Innovación’ (MCINN) through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía-CSIC (SEV-2017-0709). Acquisition and reduction of the MAPCAT data was supported in part by MICINN through grants AYA2016-80889-P and PID2019-107847RB-C44. The POLAMI observations were carried out at the IRAM 30m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). The Submillimetre Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00001.CAL. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI(Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. This research has made use of data from the MOJAVE data base that is maintained by the MOJAVE team (Lister et al. 2018). This study makes use of VLBA data from the VLBA-BU Blazar Monitoring Program (BEAM-ME and VLBA-BU-BLAZAR; http://www.bu.edu/blazars/BEAM-ME.html), funded by NASA through the Fermi Guest Investigator Program. The VLBA is an instrument of the National Radio Astronomy Observatory. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated by Associated Universities, Inc. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (2020R1A2C2009003).

3C 84 (NGC 1275) is the bright radio core of the Perseus Cluster. Even in the absence of strong relativistic effects, the source has been detected at Gamma-rays up to TeV energies. Despite its intensive study, the physical processes responsible for the high-energy emission in the source remain unanswered. We present a detailed kinematics study of the source and its connection to Gamma-ray emission. The sub-parsec scale radio structure is dominated by slow-moving features in both the eastern and western lanes of the jet. The jet appears to have accelerated to its maximum speed within less than 125 000 gravitational radii. The fastest reliably detected speed in the jet was ~0.9 c. This leads to a minimum Lorentz factor of ~1.35. Our analysis suggests the presence of multiple high-energy sites in the source. If Gamma-rays are associated with kinematic changes in the jet, they are being produced in both eastern and western lanes in the jet. Three Gamma-ray flares are contemporaneous with epochs where the slowly moving emission region splits into two sub-regions. We estimate the significance of these events being associated as ~2-3 sigma. We tested our results against theoretical predictions for magnetic reconnection-induced mini-jets and turbulence and find them compatible., 15 pages, 21 figures, 13 tables, Accepted to ApJ

Controversial studies on the jet collimation profile of BL Lacertae (BL Lac), the eponymous blazar of BL Lac objects class, complicate the scenario in this already puzzling class of objects. Understanding the jet geometry, in connection with the jet kinematics and the physical conditions in the surrounding medium, is fundamental to better constrain the formation, acceleration and collimation mechanisms in extragalactic jets. With the aim of investigating the jet geometry in the innermost regions of the jet of BL Lac, and solving the controversy, we explore the radio jet in this source, using high resolution millimeter-wave VLBI data. We collect 86GHz GMVA and 43GHz VLBA data to obtain stacked images that we use to infer the jet collimation profile by means of two comparable methods. We analyze the kinematics at 86GHz, and we discuss it in the context of the jet expansion. Finally we consider a possible implication of the Bondi sphere in shaping the different expanding region observed along the jet. We found that the jet in BL Lac expands with an overall conical geometry. A higher expanding rate region is observed between ~5 and 10 pc (de-projected) from the black hole. Such a region is associated with the decrease in brightness usually observed in high-frequency VLBI images of BL Lac. The jet retrieves the original jet expansion around 17 pc, where the presence of a recollimation shock is supported by both the jet profile and the 15GHz kinematics (MOJAVE survey). The change in the jet expansion profile occurring at ~5 pc could be associated with a change in the external pressure profile in correspondence of the Bondi radius (~3.3X10$^5$$R_s$)., Comment: 10 pages, 12 figures. Accepted for publication in Astronomy & Astrophysics

The flat spectrum radio quasar (FSRQ) PKS 1510−089 is known for its complex multiwavelength behaviour and it is one of only a few FSRQs detected in very-high-energy (VHE, E > 100 GeV) γ rays. The VHE γ-ray observations with H.E.S.S. and MAGIC in late May and early June 2016 resulted in the detection of an unprecedented flare, which revealed, for the first time, VHE γ-ray intranight variability for this source. While a common variability timescale of 1.5 h has been found, there is a significant deviation near the end of the flare, with a timescale of ∼20 min marking the cessation of the event. The peak flux is nearly two orders of magnitude above the low-level emission. For the first time, a curvature was detected in the VHE γ-ray spectrum of PKS 1510–089, which can be fully explained by the absorption on the part of the extragalactic background light. Optical R-band observations with ATOM revealed a counterpart of the γ-ray flare, even though the detailed flux evolution differs from the VHE γ-ray light curve. Interestingly, a steep flux decrease was observed at the same time as the cessation of the VHE γ-ray flare. In the high-energy (HE, E > 100 MeV) γ-ray band, only a moderate flux increase was observed with Fermi-LAT, while the HE γ-ray spectrum significantly hardens up to a photon index of 1.6. A search for broad-line region (BLR) absorption features in the γ-ray spectrum indicates that the emission region is located outside of the BLR. Radio very-long-baseline interferometry observations reveal a fast-moving knot interacting with a standing jet feature around the time of the flare. As the standing feature is located ∼50 pc from the black hole, the emission region of the flare may have been located at a significant distance from the black hole. If this is indeed a true correlation, the VHE γ rays must have been produced far down in the jet, where turbulent plasma crosses a standing shock., Astronomy & Astrophysics, 648, ISSN:0004-6361, ISSN:1432-0746

Recent Fermi-Large Area Telescope (LAT) light curves indicate an active $\gamma$-ray state spanning about five months from 2016 June to 2016 October in the BL Lac object 1749+096 (OT 081). During this period, we find two notable $\gamma$-ray events: an exceptionally strong outburst followed by a significant enhancement (local peak). In this study, we analyze multi-waveband light curves (radio, optical, X-ray, and $\gamma$-ray) plus very-long baseline interferometry (VLBI) data to investigate the nature of the $\gamma$-ray events. The $\gamma$-ray outburst coincides with flux maxima at longer wavelengths. We find a spectral hardening of the $\gamma$-ray photon index during the $\gamma$-ray outburst. The photon index shows a transition from a softer-when-brighter to a harder-when-brighter trend at around 1.8 $\times$ $10^{-7}$ ph cm$^{-2}$ s$^{-1}$. We see indication that both the $\gamma$-ray outburst and the subsequent enhancement precede the propagation of a polarized knot in a region near the VLBI core. The highest polarized intensity, 230\,mJy, and an electric vector position angle rotation, by $\sim$32$^{\circ}$, are detected about 12 days after the $\gamma$-ray outburst. We conclude that both $\gamma$-ray events are caused by the propagation of a disturbance in the mm-wave core., Comment: 11 pages, 7 figures, 2 tables. To appear in MNRAS (submitted: 2018 May 9; accepted: 2018 July 15)

In this paper, we propose a new approach to determining cosmological distances to active galactic nuclei (AGN) via light travel-time arguments, which can be extended from nearby sources to very high redshift sources. The key assumption is that the variability seen in AGN is constrained by the speed of light and therefore provides an estimate of the linear size of an emitting region. This can then be compared with the angular size measured with very long baseline interferometry (VLBI) in order to derive a distance. We demonstrate this approach on a specific well studied low redshift (z = 0.0178) source 3C84 (NGC 1275), which is the bright radio core of the Perseus Cluster. We derive an angular diameter distance including statistical errors of $D_{A} = 72^{+5}_{-6}$ Mpc for this source, which is consistent with other distance measurements at this redshift. Possible sources of systematic errors and ways to correct for them are discussed., Comment: Accepted for publication in MNRAS Letters, 5 pages, 2 figures, 1 table

The flat spectrum radio quasar 4C 38.41 showed a significant increase of its radio flux density during the period 2012 March - 2015 August which correlates with gamma-ray flaring activity. Multi-frequency simultaneous VLBI observations were conducted as part of the interferometric monitoring of gamma-ray bright active galactic nuclei (iMOGABA) program and supplemented with additional monitoring observations at various bands across the electromagnetic spectrum. The epochs of the maxima for the two largest gamma-ray flares coincide with the ejection of two respective new VLBI components and the evolution of the physical properties seem to be in agreement with the shock-in-jet model. Derived synchrotron self absorption magnetic fields, of the order of 0.1 mG, do not seem to dramatically change during the flares, and are much smaller, by a factor 10,000, than the estimated equipartition magnetic fields, indicating that the source of the flare may be associated with a particle dominated emitting region.

We present the jet kinematics of the flat spectrum radio quasar (FSRQ) 4C +21.35 using time-resolved KaVA very long baseline interferometry array radio maps obtained from September 2014 to July 2016. During two out of three observing campaigns, observations were performed bi-weekly at 22 and 43 GHz quasi-simultaneously. At 22 GHz, we identified three jet components near the core with apparent speeds up to (14.4+/-2.1)c. The timing of the ejection of a new component detected in 2016 is consistent with a gamma-ray flare in November 2014. At 43 GHz, we found four inner jet (, 11 pages, 9 figures, 3 tables. To appear in MNRAS (submitted 2018 June 7; accepted 2019 March 28)

Context. Investigating the magnetic field structure in the innermost regions of relativistic jets is fundamental to understanding the crucial physical processes giving rise to jet formation, as well as to their extraordinary radiation output up to γ-ray energies. Aims. We study the magnetic field structure of the quasar CTA 102 with 3 and 7 mm VLBI polarimetric observations, reaching an unprecedented resolution (∼50 μas). We also investigate the variability and physical processes occurring in the source during the observing period, which coincides with a very active state of the source over the entire electromagnetic spectrum. Methods. We perform the Faraday rotation analysis using 3 and 7 mm data and we compare the obtained rotation measure (RM) map with the polarization evolution in 7 mm VLBA images. We study the kinematics and variability at 7 mm and infer the physical parameters associated with variability. From the analysis of γ-ray and X-ray data, we compute a minimum Doppler factor value required to explain the observed high-energy emission. Results. Faraday rotation analysis shows a gradient in RM with a maximum value of ∼6 × 10 4 rad m -2 and intrinsic electric vector position angles (EVPAs) oriented around the centroid of the core, suggesting the presence of large-scale helical magnetic fields. Such a magnetic field structure is also visible in 7 mm images when a new superluminal component is crossing the core region. The 7 mm EVPA orientation is different when the component is exiting the core or crossing a stationary feature at ∼0.1 mas. The interaction between the superluminal component and a recollimation shock at ∼0.1 mas could have triggered the multi-wavelength flares. The variability Doppler factor associated with such an interaction is large enough to explain the high-energy emission and the remarkable optical flare occurred very close in time. © ESO 2019., DB acknowledges support from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme under grant agreement No 771282. IA acknowledges support by a Ramon y Cajal grant of the Ministerio de Ciencia, Innovacion y Universidades (MICINU) of Spain. The research at the IAA-CSIC was supported in part by the MICINU through grant AYA2016-80889-P.

Located at the center of the Perseus cluster, 3C 84 is an extremely bright and nearby radio galaxy. Because of the strong diffuse thermal emission from the cluster in X-rays, the detailed properties and the origin of a power-law component from the central active galactic nucleus (AGN) remains unclear in the source. We report here the first NuSTAR observations of 3C 84. The source was observed for 24.2 and 32 ks on February 01 and 04, 2018, respectively. NuSTAR observations spectrally decompose the power-law AGN component above 10 keV. The power-law component dominates the spectrum above 20 keV with a photon index $\sim$1.9 and an energy flux F$_{20-30 keV}$ = 1.0 $\times$10$^{-11}$ erg cm$^{-2}$ s$^{-1}$, corresponding to an isotropic luminosity, $L_{20-30 keV}$ = 7.4$\times$10$^{42}$ erg s$^{-1}$. We discuss possible emitting sites for the power-law component. The expected thermal emission from the accretion disk is not hot enough to account for the hard X-rays detected from the source. Similar X-ray and $\gamma$-ray photon indices and long-term flux variations, the absence of cutoff energy in the hard X-ray spectrum of the source, correlated hard X-ray flux and hardness ratio variations, and the similarity of optical-X-ray slope to blazar rather than Seyfert galaxies supports the hard X-ray power-law component originating from the jet., Comment: accepted for publication in ApJ Letters

The flat spectrum radio quasar 1633+382 (4C~38.41) showed a significant increase of its radio flux density during the period 2012 March - 2015 August which correlates with gamma-ray flaring activity. Multi-frequency simultaneous VLBI observations were conducted as part of the interferometric monitoring of gamma-ray bright active galactic nuclei (iMOGABA) program and supplemented with additional radio monitoring observations with the OVRO 40m telescope, the Boston University VLBI program, and the Submillimeter Array. The epochs of the maxima for the two largest gamma-ray flares coincide with the ejection of two respective new VLBI components. Analysis of the spectral energy distribution indicates a higher turnover frequency after the flaring events. The evolution of the flare in the turnover frequency - turnover flux density plane probes the adiabatic losses in agreement with the shock-in-jet model. Derived synchrotron self absorption magnetic fields, of the order of 0.1mG, do not seem to dramatically change during the flares, and are much smaller, by a factor $10^4$, than the estimated equipartition magnetic fields, indicating that the source of the flare may be associated with a particle dominated emitting region., 12 pages, 7 figures, 4 tables. Accepted for publication in ApJ

Understanding the launching, acceleration, and collimation of jets powered by active galactic nuclei remains an outstanding problem in relativistic astrophysics. This is partly because observational tests of jet formation models suffer from the limited angular resolution of ground-based very long baseline interferometry that has thus far been able to probe the transverse jet structure in the acceleration and collimation zone of only two sources. Here we report radio interferometric observations of 3C 84 (NGC 1275), the central galaxy of the Perseus cluster, made with an array including the orbiting radio telescope of the RadioAstron mission. The obtained image transversely resolves the edge-brightened jet in 3C 84 only 30 microarcseconds from the core, which is ten times closer to the central engine than what has been possible in previous ground-based observations, and it allows us to measure the jet collimation profile from ~ 100 to ~10000 gravitational radii from the black hole. The previously found, almost cylindrical jet profile on scales larger than a few thousand r_g is now seen to continue at least down to a few hundred r_g from the black hole and we find a broad jet with a transverse radius larger than about 250 r_g at only 350 r_g from the core. If the bright outer jet layer is launched by the black hole ergosphere, it has to rapidly expand laterally on scales smaller than 100 r_g. If this is not the case, then this jet sheath is likely launched from the accretion disk., Unedited, revised manuscript of the Letter published as Advance Online Publication (AOP) on the Nature Astronomy website on 02 April 2018. 15 pages, 3 figures

We study the centimeter- to millimeter-wavelength synchrotron spectrum of the core of the radio galaxy M87 at $\lesssim0.8\,{\rm mas}~\sim110R_{s}$ spatial scales using four years of fully simultaneous, multi-frequency VLBI data obtained by the Korean VLBI Network (KVN). We find a core spectral index $\alpha$ of $\gtrsim-0.37$ ($S\propto \nu^{+\alpha}$) between 22GHz and 129GHz. By combining resolution-matched flux measurements from the Very Long Baseline Array (VLBA) at 15GHz and taking the Event Horizon Telescope (EHT) 230GHz core flux measurements in epochs 2009 and 2012 as lower limits, we find evidence of a nearly flat core spectrum across 15GHz and 129GHz, which could naturally connect the 230GHz VLBI core flux. The extremely flat spectrum is a strong indication that the jet base does not consist of a simple homogeneous plasma, but of inhomogeneous multi-energy components, with at least one component with the turn-over frequency $\gtrsim100$GHz. The spectral shape can be qualitatively explained if both the strongly (compact, optically thick at $>$100GHz) and the relatively weakly magnetized (more extended, optically thin at $, Comment: 5 pages, 5 figures, accepted for A&A letters

We present multi-frequency simultaneous VLBI radio observations of the flat spectrum radio quasar 1633+382 (4C~38.41) as part of the interferometric monitoring of gamma-ray bright active galactic nuclei (iMOGABA) program combined with additional observations in radio, optical, X-rays and $\gamma-$rays carried out between the period 2012 March - 2015 August. The monitoring of this source reveals a significant long-lived increase in its activity since approximately two years in the radio bands, which correlates with a similar increase on all other bands from sub-millimeter to $\gamma-$rays. A significant correlation is also found between radio fluxes and simultaneous spectral indices during this period. The study of the discrete correlation function (DCF) indicates time lags smaller than the $\sim40$ days uncertainties among both radio bands and also high-energy bands, and a time lag of $\sim$70 days, with $\gamma-$rays leading radio. We interpret that the high-energy and radio fluxes are arising from different emitting regions, located at $1\pm12$ and $40\pm13$ pc from the central engine respectively., Comment: 16 pages, 15 figures, and 5 tables. Accepted for publication in ApJ. Small typo in the abstract corrected

3C 84 is a nearby Active Galactic Nucleus (AGN) that is unique in that is believed that we are observing near the true jet launching region - unlike blazars. The source is active in Gamma rays and has been detected with Fermi since its launch in 2008, including being detected at TeV energies with other instruments. Due to the relative proximity of the source (z=0.018), it provides a unique opportunity to pinpoint the location of the $\gamma$-ray emission by combining the Gamma ray data with very long baseline inteferometry (VLBI) data. A study using the Korean VLBI network (KVN) showed that the Gamma rays occur in both downstream jet emission and the region near where the jet is launched. Further analysis of the kinematics using Wavelet Image Segmentation and Evaluation (WISE) algorithm, which uses 2-dimensional cross-correlations to statistically derive the kinematics of high-resolution 7 mm VLBA data show that the Gamma ray emission is caused by a fast-travelling shock catching a slower moving shock and then interacting with the external medium, in behaviour reminiscent of a long duration gamma-ray burst (GRB). This could explain why such high energy flaring is seen in such low Doppler boosted sources. Finally, we show some early results from a study of the jet launching region using the Global mm-VLBI Array (GMVA). The nucleus appears to have a consistent double nuclear structure that is likely too broad to be the true jet base., Comment: 6 pages, 3 figures, Fermi symposium proceedings, 2017

We present total and linearly polarized 3 mm Global mm-VLBI Array images of a sample of blazars and radio galaxies from the VLBA-BU-BLAZAR 7 mm monitoring program designed to probe the innermost regions of active galactic nuclei (AGN) jets and locate the sites of gamma-ray emission observed by the Fermi-LAT. The lower opacity at 3 mm and improved angular resolution, on the order of 50 microarcseconds, allow us to distinguish features in the jet not visible in the 7 mm VLBA data. We also compare two different methods used for the calibration of instrumental polarisation and we analyze the resulting images for some of the sources in the sample., Polarised Emission from Astrophysical Jets, June 12-16, 2017, Ierapetra, Greece

We study the kinematics of the M87 jet using the first year data of the KVN and VERA Array (KaVA) large program, which has densely monitored the jet at 22 and 43 GHz since 2016. We find that the apparent jet speeds generally increase from $\approx0.3c$ at $\approx0.5$ mas from the jet base to $\approx2.7c$ at $\approx20$ mas, indicating that the jet is accelerated from subluminal to superluminal speeds on these scales. We perform a complementary jet kinematic analysis by using archival Very Long Baseline Array monitoring data observed in $2005-2009$ at 1.7 GHz and find that the jet is moving at relativistic speeds up to $\approx5.8c$ at distances of $200-410$ mas. We combine the two kinematic results and find that the jet is gradually accelerated over a broad distance range that coincides with the jet collimation zone, implying that conversion of Poynting flux to kinetic energy flux takes place. If the jet emission consists of a single streamline, the observed trend of jet acceleration ($\Gamma\propto z^{0.16\pm0.01}$) is relatively slow compared to models of a highly magnetized jet. This indicates that Poynting flux conversion through the differential collimation of poloidal magnetic fields may be less efficient than expected. However, we find a non-negligible dispersion in the observed speeds for a given jet distance, making it difficult to describe the jet velocity field with a single power-law acceleration function. We discuss the possibility that the jet emission consists of multiple streamlines following different acceleration profiles, resulting in jet velocity stratification., Comment: Accepted for publication in ApJ; This is a new version after updating Figure 11 in the manuscript

PKS 1510--089 is a bright and active $\gamma$-ray source that showed strong and complex $\gamma$-ray flares in mid-2015 during which the Major Atmospheric Gamma Imaging Cherenkov telescopes detected variable very high energy (VHE; photon energies $>$100 GeV) emission. We present long-term multi-frequency radio, optical, and $\gamma$-ray light curves of PKS 1510--089 from 2013 to 2018, and results of an analysis of the jet kinematics and linear polarization using 43 GHz Very Long Baseline Array data observed between late 2015 and mid-2017. We find that a strong radio flare trails the $\gamma$-ray flares in 2015, showing an optically thick spectrum at the beginning and becoming optically thin over time. Two laterally separated knots of emission are observed to emerge from the radio core nearly simultaneously during the $\gamma$-ray flares. We detect an edge-brightened linear polarization near the core in the active jet state in 2016, similar to the quiescent jet state in 2008--2013. These observations indicate that the $\gamma$-ray flares may originate from compression of the knots by a standing shock in the core and the jet might consist of multiple complex layers showing time-dependent behavior, rather than of a simple structure of a fast jet spine and a slow jet sheath., Comment: 11 pages, 7 figures, To appear in ApJ

The Gamma-ray BL Lac object OJ 287 is known to exhibit inner-parsec "jet-wobbling", high degrees of variability at all wavelengths and quasi-stationary features including an apparent (~100 deg) position angle change in projection on the sky plane. Sub-50 micro-arcsecond resolution 86 GHz observations with the global mm-VLBI array (GMVA) supplement ongoing multi-frequency VLBI blazar monitoring at lower frequencies. Using these maps together with cm/mm total intensity and Gamma-ray observations from Fermi/LAT from 2008-2014, we aimed to determine the location of Gamma-ray emission and to explain the inner-mas structural changes. Observations with the GMVA offer approximately double the angular resolution compared with 43 GHz VLBA observations and allow us to observe above the synchrotron self-absorption peak frequency. The jet was spectrally decomposed at multiple locations along the jet. From this we derived estimates of the magnetic field. How the field decreases down the jet allowed an estimate of the distance to the jet apex and an estimate of the magnetic field strength at the jet apex and in the broad line region. Combined with accurate kinematics we attempt to locate the site of Gamma-ray activity, radio flares and spectral changes. Strong Gamma-ray flares appeared to originate from either the "core" region, a downstream stationary feature, or both, with Gamma-ray activity significantly correlated with radio flaring in the downstream quasi-stationary feature. Magnetic field estimates were determined at multiple locations along the jet, with the magnetic field found to be >1.6 G in the "core" and >0.4 G in the downstream quasi-stationary feature. We therefore found upper limits on the location of the "core" as >6.0 pc from the jet apex and determined an upper limit on the magnetic field near the jet base of the order of thousands of Gauss., 30 pages, 20 figures

We study the linear polarization of the radio cores of eight blazars simultaneously at 22, 43, and 86 GHz with observations obtained by the Korean VLBI Network (KVN) in three epochs between late 2016 and early 2017 in the frame of the Plasma-physics of Active Galactic Nuclei (PAGaN) project. We investigate the Faraday rotation measure (RM) of the cores; the RM is expected to increase with observing frequency if core positions depend on frequency due to synchrotron self-absorption. We find a systematic increase of RMs at higher observing frequencies in our targets. The RM--$\nu$ relations follow power-laws with indices distributed around 2, indicating conically expanding outflows serving as Faraday rotating media. Comparing our KVN data with contemporaneous optical polarization data from the Steward Observatory for a few sources, we find indication that the increase of RM with frequency saturates at frequencies of a few hundreds GHz. This suggests that blazar cores are physical structures rather than simple $\tau=1$ surfaces. A single region, e.g. a recollimation shock, might dominate the jet emission downstream of the jet launching region. We detect a sign change in the observed RMs of CTA 102 on a time scale of $\approx$1 month, which might be related to new superluminal components emerging from its core undergoing acceleration/deceleration and/or bending. We see indication for quasars having higher core RMs than BL Lac objects, which could be due to denser inflows/outflows in quasars., Comment: 22 pages, 10 figures, 3 tables; to appear in ApJ (submitted 2018 March 7; accepted 2018 May 11)

One of the most intriguing and challenging quests of current astrophysics is to understand the physical conditions and processes responsible for production of high-energy particles, and emission of \gamma-rays. A combination of high-resolution Very Long Baseline Interferometry (VLBI) images with broadband flux variability measurements is a unique way to probe the emission mechanisms at the bases of jets. Our analysis of \gamma-ray flux variability observed by the Fermi-LAT (Large Area Telescope) along with the parsec-scale jet kinematics suggests that the $\gamma$-ray emission in blazar S5 0716+714 has a significant correlation with the mm-VLBI core flux and the orientation of jet outflow on parsec scales. These results indicate that the inner jet morphology has a tight connection with the observed $\gamma$-ray flares. An overview of our current understanding on high-energy radiation processes, their origin, and location is presented here., Comment: 5 pages, 2 figures, to be published in the Journal of Physics: Conference Series

Active Galactic Nuclei are the luminous centres of active galaxies that produce powerful relativistic jets from central super massive black holes (SMBH). When these jets are oriented towards the observer’s line-of-sight, they become very bright, very variable and very energetic. These sources are known as blazars and Very Long Baseline Interferometry (VLBI) provides a direct means of observing into the heart of these objects. VLBI performed at 3 mm with the Global mm-VLBI Array (GMVA) and 7 mm VLBI performed with the Very Long Baseline Array (VLBA), allows some of the highest angular resolution images of blazars to be produced. In this thesis, we present the first results of an ongoing monitoring program of blazars known to emit at γ-ray energies. The physical processes that produce these jets and the γ-ray emission are still not well known. The jets are thought to be produced by converting gravitational energy around the black hole into relativistic particles that are accelerated away at near the speed of light. However, the exact mechanisms for this and the role that magnetic fields play is not fully clear. Similarly, γ-rays have been long known to have been emitted from blazars and that their production is often related to the up-scattering of synchrotron radiation from the jet. However, the origin of seed photons for the up-scattering (either from within the jet itself or from an external photon field) and the location of the γ-ray emission regions has remained inconclusive. In this thesis, we aim to describe the likely location of γ-ray emission in jets, the physical structure of blazar jets, the location of the VLBI features relative to the origin of the jet and the nature of the magnetic field, both of the VLBI scale jet and in the region where the jet is produced. We present five sources that have been monitored at 3 mm using the GMVA from 2008 until 2012. These sources have been analysed with near-in-time 7 mm maps from the Very Long Baseline Array (VLBA), γ-ray light curves from the Fermi/LAT space telescope and cm to mm-wave total-intensity light curves. In one source, OJ 287, the source has additionally been analysed with monthly imaging at 7 mm with the VLBA and near-in-time 2 cm VLBI maps. We use these resources to analyse high angular resolution structural and spectral changes and see if they correlate with flaring (both radio and γ-ray) activity and with VLBI component ejections. By spectrally decomposing sources, we can determine the spatially resolved magnetic field structure in the jets at the highest yet performed resolutions and at frequencies that are near or above the turnover frequency for synchrotron self-absorption (SSA). We compute the magnetic field estimates from SSA theory and by assuming equipartition between magnetic fields and relativistic particle energies. v All sources analysed exhibit downstream quasi-stationary features which sometimes exhibit higher brightness temperatures and flux density variability than the VLBI “core”, which we interpret as being recollimation or oblique shocks. We find that γ-ray flaring, mm-wave radio flaring and changes in opacity from optically thick to optically thin, is in many cases consistent with component ejections past both the VLBI “core” and these quasi-stationary downstream features. We find decreasing apparent brightness temperatures and Doppler factors as a function of increased “core” separation, which is interpreted as consistent with a slowly accelerating jet over the de-projected inner ∼10-20 pc. Assuming equipartition between magnetic energy and relativistic particle energy, the magnetic field strengths within the jets at these scales are, on average, between B ∼ 0.3 − 0.9 G, with the highest strengths found within the VLBI “core”. From the observed gradient in magnetic field strengths, we can place the mmwave “core” ∼1-3 pc downstream of the base of the jet. Additionally, we estimate the the magnetic field is Bapex ∼ 3000 − 18000 G at the base of the jet. We computed theoretical estimates based on jet production under magnetically arrested disks (MAD) and find our estimates to be consistent. In the BL Lac source OJ 287, we included monthly 7 mm and near-in-time 2 cm VLBA maps to provide full kinematics and increased spectral coverage. Following a previously reported radical change in inner-jet PA of ∼100◦we find unusually discrepant PAs compared with the previous jet direction, that follow very different trajectories. The source exhibits a downstream quasi-stationary feature that at times has higher brightness temperatures than the “core”. The source also exhibited a large change in apparent component speeds as compared with previous epochs, which we propose could be due to changes in jet pressure causing changes in the location of downstream recollimation or oblique shocks and hence their line-of-sight viewing angle. The addition of 2 cm VLBA data allows for a comparison of magnetic fields derived from SSA and equipartition. The magnetic field estimates are consistent within 20%, with BSSA ≥ 1.6 G and Bequi ≥ 1.2 G in the “core” and BSSA ≤ 0.4 G and Bequi ≤ 0.3 G in the stationary feature. Gamma-ray emission appears to originate in the “core” and the stationary feature. The decrease in magnetic field strengths places the mmwave “core” downstream of the jet base by ≤6 pc and likely outside of the broad line region (BLR). This, combined with the results in other sources are consistent with γ-rays being produced in the vicinity of the VLBI “core” of in further downstream stationary features, which are likely over a parsec downstream of the central black hole, favouring the scenario of photons being up-scattered within the relativistic jet.

Context. Blazars are among the most energetic objects in the Universe. In 2008 August, Fermi/LAT detected the blazar PKS 1502+106 showing a rapid and strong gamma-ray outburst followed by high and variable flux over the next months. This activity at high energies triggered an intensive multi-wavelength campaign covering also the radio, optical, UV, and X-ray bands indicating that the flare was accompanied by a simultaneous outburst at optical/UV/X-rays and a delayed outburst at radio bands. Aims: In the current work we explore the phenomenology and physical conditions within the ultra-relativistic jet of the gamma-ray blazar PKS 1502+106. Additionally, we address the question of the spatial localization of the MeV/GeV-emitting region of the source. Methods: We utilize ultra-high angular resolution mm-VLBI observations at 43 and 86 GHz complemented by VLBI observations at 15 GHz. We also employ single-dish radio data from the F-GAMMA program at frequencies matching the VLBI monitoring. Results: PKS 1502+106 shows a compact core-jet morphology and fast superluminal motion with apparent speeds in the range 5--22 c. Estimation of Doppler factors along the jet yield values between ~7 up to ~50. This Doppler factor gradient implies an accelerating jet. The viewing angle towards the source differs between the inner and outer jet, with the former at ~3 degrees and the latter at ~1 degree, after the jet bends towards the observer beyond 1 mas. The de-projected opening angle of the ultra-fast, magnetically-dominated jet is found to be (3.8 +/- 0.5) degrees. A single jet component can be associated with the pronounced flare both at high-energies and in radio bands. Finally, the gamma-ray emission region is localized at less than 5.9 pc away from the jet base., Comment: 22 pages, 12 figures. Accepted for publication in A&A

We present a high-frequency very long baseline interferometry (VLBI) kinematical study of the BL Lac object S5 0716+714 over the time period of September 2008 to October 2010. The aim of the study is to investigate the relation of the jet kinematics to the observed broadband flux variability. We find significant non-radial motions in the jet outflow of the source. In the radial direction, the highest measured apparent speed is \sim37 c, which is exceptionally high, especially for a BL Lac object. Patterns in the jet flow reveal a roughly stationary feature \sim0.15 mas downstream of the core. The long-term fits to the component trajectories reveal acceleration in the sub-mas region of the jet. The measured brightness temperature, T_{B}, follows a continuous trend of decline with distance, T_B \propto r_{jet}^{-(2.36\pm0.41)}, which suggests a gradient in Doppler factor along the jet axis. Our analysis suggest that a moving disturbance (or a shock wave) from the base of the jet produces the high-energy (optical to \gamma-ray) variations upstream of the 7 mm core, and then later causes an outburst in the core. Repetitive optical/\gamma-ray flares and the curved trajectories of the associated components suggest that the shock front propagates along a bent trajectory or helical path. Sharper \gamma-ray flares could be related to the passage of moving disturbances through the stationary feature. Our analysis suggests that the \gamma-ray and radio emission regions have different Doppler factors., Comment: 12 pages, 8 figures, accepted for publication in A&A

Using millimeter-very long baseline interferometry (VLBI) observations of the BL Lac object S5 0716+714 from August 2008 to September 2013, we investigate variations in the core flux density and orientation of the sub-parsec scale jet i.e. position angle. The gamma-ray data obtained by the Fermi-LAT (Large Area Telescope) are used to investigate the high-energy flux variations over the same time period. For the first time in any blazar, we report a significant correlation between the gamma-ray flux variations and the position angle (PA) variations in the VLBI jet. The cross-correlation analysis also indicates a positive correlation such that the mm-VLBI core flux density variations are delayed with respect to the gamma-ray flux by 82$\pm$32 days. This suggests that the high-energy emission is coming from a region located $\geq$(3.8$\pm$1.9) parsecs upstream of the mm-VLBI core (closer to the central black hole). These results imply that the observed inner jet morphology has a strong connection with the observed gamma-ray flares., Comment: 5 pages, 6 figures, accepted for publication in A&A Letters

We describe a new tool for studying the structure and physical characteristics of ultracompact active galactic nucleus (AGN) jets and their surroundings with ?as precision. This tool is based on the frequency dependence of the light curves observed for intra-day variable radio sources, where the variability is caused by interstellar scintillation. We apply this method to PKS 1257-326 to resolve the core-shift as a function of frequency on scales well below ~12 ?as. We find that the frequency dependence of the position of the scintillating component is r??0.1 ? 0.24 (99% confidence interval) and the frequency dependence of the size of the scintillating component is d??0.64 ? 0.006. Together, these results imply that the jet opening angle increases with distance along the jet: with nd > 1.8. We show that the flaring of the jet, and flat frequency dependence of the core position is broadly consistent with a model in which the jet is hydrostatically confined and traversing a steep pressure gradient in the confining medium with and np 7. Such steep pressure gradients have previously been suggested based on very long baseline interferometry studies of the frequency dependent core shifts in AGNs.

Blazars are among the most variable objects in the universe. They feature energetic jets of plasma that launch from the cores of these active galactic nuclei (AGN), triggering activity from radio up to gamma-ray energies. Spatial localization of the region of their MeV/GeV emission is a key question in understanding the blazar phenomenon. The flat spectrum radio quasar (FSRQ) PKS 1502+106 has exhibited extreme and correlated, radio and high-energy activity that triggered intense monitoring by the Fermi-GST AGN Multi-frequency Monitoring Alliance (F-GAMMA) program and the Global Millimeter VLBI Array (GMVA) down to $\lambda$3 mm (or 86 GHz), enabling the sharpest view to date towards this extreme object. Here, we report on preliminary results of our study of the gamma-ray loud blazar PKS 1502+106, combining VLBI and single dish data. We deduce the critical aspect angle towards the source to be $\theta_{\rm c} = 2.6^{\circ}$, calculate the apparent and intrinsic opening angles and constrain the distance of the 86 GHz core from the base of the conical jet, directly from mm-VLBI but also through a single dish relative timing analysis. Finally, we conclude that gamma rays from PKS 1502+106 originate from a region between ~1-16 pc away from the base of the hypothesized conical jet, well beyond the bulk of broad-line region (BLR) material of the source., Comment: 4 pages, 1 figure, proceedings of the 12th European VLBI Network Symposium and Users Meeting - EVN 2014, 7-10 October 2014, Cagliari, Italy. Published online in PoS, PoS(EVN 2014)087

Global VLBI imaging at millimeter and sub-millimeter wavelength overcomes the opacity barrier of synchrotron self-absorption in AGN and opens the direct view into sub-pc scale regions not accessible before. Since AGN variability is more pronounced at short millimeter wavelength, mm-VLBI can reveal structural changes in very early stages after outbursts. When combined with observations at longer wavelength, global 3mm and 1mm VLBI adds very detailed information. This helps to determine fundamental physical properties at the jet base, and in the vicinity of super-massive black holes at the center of AGN. Here we present new results from multi-frequency mm-VLBI imaging of OJ287 during a major outburst. We also report on a successful 1.3mm VLBI experiment with the APEX telescope in Chile. This observation sets a new record in angular resolution. It also opens the path towards future mm-VLBI with ALMA, which aims at the mapping of the black hole event horizon in nearby galaxies, and the study of the roots of jets in AGN., Comment: 6 pages, to appear in 11th European VLBI Network Symposium, ed. P. Charlot et al., Bordeaux (France), October 9-12, 2012