We present the results of a spectral analysis using simultaneous multifrequency (22, 43, 86, and 129 GHz) very long baseline interferometry (VLBI) observations of the Korean VLBI Network (KVN) on BL Lac object, Markarian 421 (Mrk 421). The data we used was obtained from January 2013 to June 2018. The light curves showed several flux enhancements with global decreases. To separate the variable and quiescent components in the multifrequency light curves for milliarcsecond-scale emission regions, we assumed that the quiescent radiation comes from the emission regions radiating constant optically-thin synchrotron emissions (i.e., a minimum flux density with an optically thin spectral index). The quiescent spectrum determined from the multifrequency light curves was subtracted from the total CLEAN flux density, yielding a variable component in the flux that produces the time-dependent spectrum. We found that the observed spectra were flat at 22-43 GHz, and relatively steep at 43-86 GHz, whereas the quiescent-corrected spectra are sometimes quite different from the observed spectra (e.g., sometimes inverted at 22-43 GHz ). The quiescent-corrected spectral indices were much more variable than the observed spectral indices. This spectral investigation implies that the quiescent-spectrum correction can significantly affect the multifrequency spectral index of variable compact radio sources such as blazars. Therefore, the synchrotron self-absorption B-field strength (B_SSA) can be significantly affected because B_SSA is proportional to the fifth power of turnover frequency.

In this work, we studied the Galactic Center supermassive black hole (SMBH), Sagittarius A* (Sgr A*), with the KVN and VERA Array (KaVA)/East Asian VLBI Network (EAVN) monitoring observations. Especially in 13 May 2019, Sgr A* experienced an unprecedented bright near infra-red (NIR) flare; so, we find a possible counterpart at 43 GHz (7 mm). As a result, a large temporal variation of the flux density at the level ∼15.4%, with the highest flux density of 2.04 Jy, is found on 11 May 2019. Interestingly, the intrinsic sizes are also variable, and the area and major-axis size show marginal correlation with flux density with ≳2 σ. Thus, we interpret that the emission region at 43 GHz follows the larger-when-brighter relation in 2019. The possible origins are discussed with an emergence of a weak jet/outflow component and the position angle change of the rotation axis of the accretion disk in time.

The nearby radio galaxy M87 offers a unique opportunity to explore the connections between the central supermassive black hole and relativistic jets. Previous studies of the inner region of M87 revealed a wide opening angle for the jet originating near the black hole(1-4). The Event Horizon Telescope resolved the central radio source and found an asymmetric ring structure consistent with expectations from general relativity(5). With a baseline of 17 years of observations, there was a shift in the jet's transverse position, possibly arising from an 8- to 10-year quasi-periodicity(3). However, the origin of this sideways shift remains unclear. Here we report an analysis of radio observations over 22 years that suggests a period of about 11 years for the variation in the position angle of the jet. We infer that we are seeing a spinning black hole that induces the Lense-Thirring precession of a misaligned accretion disk. Similar jet precession may commonly occur in other active galactic nuclei but has been challenging to detect owing to the small magnitude and long period of the variation.

We present the results of a radio multifrequency ($\rm 3{-}340~GHz$) study of the blazar 3C 454.3. After subtracting the quiescent spectrum corresponding to optically thin emission, we found two individual synchrotron self-absorption (SSA) features in the wide-band spectrum. The one SSA had a relatively low turnover frequency (νm) in the range of $\rm 3{-}37~GHz$ (lower νm SSA spectrum, LSS), and the other one had a relatively high νm of $\rm 55{-}124~GHz$ (higher νm SSA spectrum, HSS). Using the SSA parameters, we estimated B-field strengths at the surface where optical depth τ = 1. The estimated B-field strengths were $\rm \gt 7$ and $\rm \gt 0.2~mG$ for the LSS and HSS, respectively. The LSS-emitting region was magnetically dominated before the 2014 June γ-ray flare. The quasi-stationary component (C), ∼0.6 mas apart from the 43 -GHz radio core, became brighter than the core with decreasing observing frequency, and we found that component C was related to the LSS. A decrease in jet width was found near component C. As a moving component, K14 approached component C, and the flux density of the component was enhanced while the angular size decreased. The high intrinsic brightness temperature in the fluid frame was obtained as TB, int ≈ (7.0 ± 1.0) × 1011 K from the jet component after the 2015 August γ-ray flare, suggesting that component C is a high-energy emitting region. The observed local minimum of jet width and re-brightening behaviour suggest a possible recollimation shock in component C.

Because of its proximity and the large size of its black hole, M87 is one of the best targets for studying the launching mechanism of active galactic nucleus jets. Currently, magnetic fields are considered to be an essential factor in the launching and accelerating of the jet. However, current observational estimates of the magnetic field strength of the M87 jet are limited to the innermost part of the jet or to HST-1. No attempt has yet been made to measure the magnetic field strength in between. We aim to infer the magnetic field strength of the M87 jet out to a distance of several thousand r_s by tracking the distance-dependent changes in the synchrotron spectrum of the jet from high-resolution very long baseline interferometry observations. In order to obtain high-quality spectral index maps, quasi-simultaneous observations at 22 and 43 GHz were conducted using the KVN and VERA Array (KaVA) and the VLBA. We compared the spectral index distributions obtained from the observations with a model and placed limits on the magnetic field strengths as a function of distance. The overall spectral morphology is broadly consistent over the course of these observations. The observed synchrotron spectrum rapidly steepens from α_22-43 GHz -0.7 at 2 mas to α_22-43 GHz -2.5 at 6 mas. A spectral index model in which nonthermal electron injections inside the jet decrease with distance can adequately reproduce the observed trend. This suggests the magnetic field strength of the jet at a distance of 2 - 10 mas ( 900 r_s - 4500 r_s in the deprojected distance) has a range of B=(0.3 - 1.0 G)(z/2 mas)^-0.73. Extrapolating to the EHT scale yields consistent results, suggesting that the majority of the magnetic flux of the jet near the black hole is preserved out to 4500 r_s without significant dissipation.

Sagittarius A* (Sgr A*), the Galactic Center supermassive black hole (SMBH), is one of the best targets in which to resolve the innermost region of an SMBH with very long baseline interferometry (VLBI). In this study, we have carried out observations toward Sgr A* at 1.349 cm (22.223 GHz) and 6.950 mm (43.135 GHz) with the East Asian VLBI Network, as a part of the multiwavelength campaign of the Event Horizon Telescope (EHT) in 2017 April. To mitigate scattering effects, the physically motivated scattering kernel model from Psaltis et al. (2018) and the scattering parameters from Johnson et al. (2018) have been applied. As a result, a single, symmetric Gaussian model well describes the intrinsic structure of Sgr A* at both wavelengths. From closure amplitudes, the major-axis sizes are ∼704 ± 102 μas (axial ratio ∼ 1.19 − 0.19 + 0.24 ) and ∼300 ± 25 μas (axial ratio ∼1.28 ± 0.2) at 1.349 cm and 6.95 mm, respectively. Together with a quasi-simultaneous observation at 3.5 mm (86 GHz) by Issaoun et al. (2019), we show that the intrinsic size scales with observing wavelength as a power law, with an index ∼1.2 ± 0.2. Our results also provide estimates of the size and compact flux density at 1.3 mm, which can be incorporated into the analysis of the EHT observations. In terms of the origin of radio emission, we have compared the intrinsic structures with the accretion flow scenario, especially the radiatively inefficient accretion flow based on the Keplerian shell model. With this, we show that a nonthermal electron population is necessary to reproduce the source sizes.

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 dataset in the range 2.6-343.5 GHz was collected over a period of 5.5 years (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 26-171 days 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 42-167 GHz and 0.9-10.2 Jy, respectively. From the SSA spectra, we derived the SSA magnetic field strengths to be 9.20 mG, 12.28 mG, 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 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.

The East Asian VLBI Network (EAVN) is an international VLBI facility in East Asia and is operated under mutual collaboration between East Asian countries, as well as part of Southeast Asian and European countries. EAVN currently consists of 16 radio telescopes and three correlators located in China, Japan, and Korea, and is operated mainly at three frequency bands, 6.7, 22, and 43 GHz with the longest baseline length of 5078 km, resulting in the highest angular resolution of 0.28 milliarcseconds at 43 GHz. One of distinct capabilities of EAVN is multi-frequency simultaneous data reception at nine telescopes, which enable us to employ the frequency phase transfer technique to obtain better sensitivity at higher observing frequencies. EAVN started its open-use program in the second half of 2018, providing a total observing time of more than 1100 h in a year. EAVN fills geographical gap in global VLBI array, resulting in enabling us to conduct contiguous high-resolution VLBI observations. EAVN has produced various scientific accomplishments especially in observations toward active galactic nuclei, evolved stars, and star-forming regions. These activities motivate us to initiate launch of the 'Global VLBI Alliance' to provide an opportunity of VLBI observation with the longest baselines on the earth.

The East Asian Very Long Baseline Interferometry (VLBI) Network (EAVN) is a rapidly evolving international VLBI array that is currently promoted under joint efforts among China, Japan and Korea. EAVN aims at forming a joint VLBI Network by combining a large number of radio telescopes distributed over East Asian regions. After the combination of the Korean VLBI Network (KVN) and the VLBI Exploration of Radio Astrometry (VERA) into KaVA, further expansion with the joint array in East Asia is actively promoted. Here we report the first imaging results (at 22 and 43 GHz) of bright radio sources obtained with KaVA connected to Tianma 65-m and Nanshan 26-m Radio Telescopes in China. To test the EAVN imaging performance for different sources, we observed four active galactic nuclei (AGN) having different brightness and morphology. As a result, we confirmed that the Tianma 65-m Radio Telescope (TMRT) significantly enhances the overall array sensitivity, a factor of 4 improvement in baseline sensitivity and 2 in image dynamic range compared to the case of KaVA only. The addition of the Nanshan 26-m Radio Telescope (NSRT) further doubled the east-west angular resolution. With the resulting high-dynamic-range, high-resolution images with EAVN (KaVA+TMRT+NSRT), various fine-scale structures in our targets, such as the counter-jet in M87, a kink-like morphology of the 3C 273 jet and the weak emission in other sources are successfully detected. This demonstrates the powerful capability of EAVN to study AGN jets and to achieve other science goals in general. Ongoing expansion of EAVN will further enhance the angular resolution, detection sensitivity and frequency coverage of the network.

We present the results of simultaneous multifrequency imaging observations at 22, 43, 86, and 129 GHz of OJ 287. We used the Korean Very Long Baseline Interferometry Network as part of the Interferometric Monitoring of Gamma-ray Bright active galactic nuclei (iMOGABA). The iMOGABA observations were performed during 31 epochs from 2013 January 16 to 2016 December 28. We also used 15 GHz OVRO and 225 GHz SMA flux density data. We analyzed four flux enhancements in the light curves. The estimated timescales of three flux enhancements were similar with timescales of ∼50 days at two frequencies. A fourth flux enhancement had a variability timescale approximately twice as long. We found that 225 GHz enhancements led the 15 GHz enhancements by a range of 7–30 days in the time delay analysis. We found the fractional variability did not change with frequency between 43 and 86 GHz. We could reliably measure the turnover frequency, , of the core of the source in three epochs. This was measured to be in a range from 27 to 50 GHz and a flux density at the turnover frequency, , ranging from 3 to 6 Jy. The derived SSA magnetic fields, , are in a range from 0.157 ± 0.104 to 0.255 ± 0.146 mG. We estimated the equipartition magnetic field strengths to be in a range from 0.95 ± 0.15 to 1.93 ± 0.30 mG. The equipartition magnetic field strengths are up to a factor of 10 higher than the values of . We conclude that the downstream jet may be more particle energy dominated.

We present the results of multi-epoch monitoring of a blazar J1159+2914, one of the targets of a Very Long Baseline Interferometry (VLBI) monitoring program : Interferometric MOnitoring of GAmma-ray Bright AGNs (iMOGABA), as a Korean VLBI Network (KVN) Key Science Program (KSP). The observations were conducted simultaneously at 22, 43, 86, and 129 GHz, during 4 years from December 2012 to December 2016. Obtained total fluxes ranged from 0.26 to 2.88 Jy at all frequencies with a mean rms noise of 26 mJy. We also used the 15 and 230 GHz data observed by Owens Valley Radio Observatory (OVRO) and Sub-Millimeter Array (SMA), respectively. In order to analyze the characteristics of variabilities, we estimated variability time scale from 15 GHz data, using three different functions, such as structure function, Gaussian function, and exponential function. Also, in order to study the multi-frequency correlations, we compared the light curve of 15 GHz with that of 22, 43, and 86 GHz, using cross-correlation analysis. Moreover, we estimated B-field strength using core sizes from VLBA 43 GHz data, turnover frequencies and maximum total fluxes from KVN data, and variability time scales from OVRO data, in order to study the variability of B-field strength nearby the radio emission region.

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 ≈0.3 c at ≈0.5 mas from the jet base to ≈2.7 c at ≈20 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 ≈5.8 c 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 (Γ ∝ z 0.16±0.01 ) 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 nonnegligible 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.

A long standing problem in the study of Active Galactic Nuclei (AGNs) is that the observed VLBI core is in fact a blending of the actual AGN core (classically defined by the tau = 1 surface) and the upstream regions of the jet or optically thin flows. This blending may cause some biases in the observables of the core, such as its flux density, size or brightness temperature, which may lead to misleading interpretation of the derived quantities and physics. We study the effects of such blending under the view of the Korean VLBI Network (KVN) for a sample of AGNs at 43 GHz by comparing their observed properties with observations obtained using the Very Large Baseline Array (VLBA). Our results suggest that the observed core sizes are a factor similar to 11 larger than these of VLBA, which is similar to the factor expected by considering the different resolutions of the two facilities. We suggest the use of this factor to consider blending effects in KVN measurements. Other parameters, such as flux density or brightness temperature, seem to possess a more complicated dependence.

The KVN(Korean VLBI Network)-style simultaneous multi-frequency receiving mode is demonstrated to be promising for mm-VLBI observations. Recently, other Very long baseline interferometry (VLBI) facilities all over the globe start to implement compatible optics systems. Simultaneous dual/multi-frequency VLBI observations at mm wavelengths with international baselines are thus possible. In this paper, we present the results from the first successful simultaneous 22/43 GHz dual-frequency observation with KaVA(KVN and VERA array), including images and astrometric results. Our analysis shows that the newly implemented simultaneous receiving system has brought a significant extension of the coherence time of the 43 GHz visibility phases along the international baselines. The astrometric results obtained with KaVA are consistent with those obtained with the independent analysis of the KVN data. Our results thus confirm the good performance of the simultaneous receiving systems for the non-KVN stations. Future simultaneous observations with more global stations bring even higher sensitivity and micro-arcsecond level astrometric measurements of the targets.

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 (<3 mas) components with speeds from (3.5+/-1.4)c to (6.8+/-1.5)c. Jet component speeds tend to be higher with increasing distances from the core. We compared our data with archival Very Long Baseline Array (VLBA) data from the Boston University (BU) 43 GHz and the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) 15.4 GHz monitoring programs. Whereas MOJAVE data and our data are in good agreement, jet speeds obtained from the BU Program data in the same time period are about twice as high as the ones we obtain from the KaVA data. The discrepancy at 43 GHz indicates that radio arrays with different angular resolution identify and trace different jet features even when the data are obtained at the same frequency and at the same time. The flux densities of jet components decay exponentially, in agreement with a synchrotron cooling time scale of about 1 year. Using known electron Lorentz factor values (about 9,000), we estimate the magnetic field strength to be around 1-3 micro-Tesla. When adopting a jet viewing angle of 5 degrees, the intrinsic jet speed is of order 0.99c.

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 Cerenkov telescopes detected variable very high energy (photon energies >100 GeV) emission. We present long-term multifrequency 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.

AbstractWe report preliminary results of long-term multi-frequency monitoring observations of S5 0716+714. We conducted observations at 22 and 43 GHz using Korean VLBI Network (KVN) radio telescopes and combined 8 (UMRAO), 15 (OVRO), 95 (CARMA), and 230 GHz (SMA) data to investigate characteristics of radio flares. We identified six flares (P1-P6) from 2010 November to 2014 June. The magnetic field strengths by assuming a synchrotron self-absorption model are in the range of 9 mG - 11 G in P2, 26 mG - 3G in P3, 3 mG - 38 G in P4, and 1 mG - 8 G in P6.

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.

We present the catalog of the KVN Calibrator Survey (KVNCS). This first part of the KVNCS is a single-dish radio survey simultaneously conducted at 22 ( K band) and 43 GHz ( Q band) using the Korean VLBI Network (KVN) from 2009 to 2011. A total of 2045 sources are selected from the VLBA Calibrator Survey with an extrapolated flux density limit of 100 mJy at the K band. The KVNCS contains 1533 sources in the K band with a flux density limit of 70 mJy and 553 sources in the Q band with a flux density limit of 120 mJy; it covers the whole sky down to −32.°5 in decl. We detected 513 sources simultaneously in the K and Q bands; ∼76% of them are flat-spectrum sources (−0.5 ≤ α ≤ 0.5). From the flux–flux relationship, we anticipated that most of the radiation of many of the sources comes from the compact components. The sources listed in the KVNCS therefore are strong candidates for high-frequency VLBI calibrators.

We present the results of very long baseline interferometry (VLBI) observations of gamma-ray bright blazar S5 0716+714 using the Korean VLBI Network (KVN) at the 22, 43, 86, and 129 GHz bands, as part of the Interferometric Monitoring of Gamma-ray Bright active galactic nuclei (iMOGABA) KVN key science program. Observations were conducted in 29 sessions from 2013 January 16 to 2016 March 1, with the source being detected and imaged at all available frequencies. In all epochs, the source was compact on the milliarcsecond scale, yielding a compact VLBI core dominating the synchrotron emission on these scales. Based on the multiwavelength data between 15 GHz (Owens Valley Radio Observatory) and 230 GHz (Submillimeter Array), we found that the source shows multiple prominent enhancements of the flux density at the centimeter (cm) and millimeter (mm) wavelengths, with mm enhancements leading cm enhancements by -16 +/- 8 days. The turnover frequency was found to vary between 21 and 69 GHz during our observations. By assuming a synchrotron self-absorption model for the relativistic jet emission in S5 0716+714, we found the magnetic field strength in the mas emission region to be <= 5 mG during the observing period, yielding a weighted mean of 1.0 +/- 0.6 mG for higher turnover frequencies (e.g., > 45 GHz).