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We report sub-parsec-scale observations of the 321 GHz H2O emission line in the radio galaxy NGC 1052. The H2O line emitter size is constrained in <0.6 mas distributed on the continuum core component. The brightness temperature exceeding 106 K and the intensity variation indicate certain evidence for maser emission. The maser spectrum consists of redshifted and blueshifted velocity components spanning ∼400 km s−1, separated by a local minimum around the systemic velocity of the galaxy. The spatial distribution of maser components shows a velocity gradient along the jet direction, implying that the population-inverted gas is driven by the jets interacting with the molecular torus. We identified a significant change of the maser spectra between two sessions separated by 14 days. The maser profile showed a radial velocity drift of 127 ± 13 km s−1 yr−1 implying inward gravitational acceleration at 5000 Schwarzschild radii. The results demonstrate the feasibility of future very long baseline interferometry observations to resolve the jet–torus interacting region.

 

The Atacama Large Millimeter/submillimeter Array (ALMA) serendipitously detected H2O $J_{K_{\rm a}, K_{\rm c}} = 10_{2,9}$–93, 6 emission at 321 GHz in NGC 1052. This is the first submillimeter maser detection in a radio galaxy and the most luminous 321 GHz H2O maser known to-date with the isotropic luminosity of $1090\, L_{\odot }$. The line profile consists of a broad velocity component with FWHM = 208 ± 12 km s−1 straddling the systemic velocity and a narrow component with FWHM = 44 ± 3 km s−1 blueshifted by 160 km s−1. The profile is significantly different from the known 22 GHz 61, 6–52, 3 maser which shows a broad profile redshifted by 193 km s−1. The submillimeter maser is spatially unresolved with a synthesized beam of ${0{^{\prime \prime}_{.}}68} \times {0{^{\prime \prime}_{.}}56}$ and coincides with the continuum core position within 12 pc. These results indicate amplification of the continuum emission through high-temperature (>1000 K) and dense [n(H2O) > 104 cm−3] molecular gas in front of the core.

 

Context. Because of its proximity and the large size of its black hole, M 87 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 M 87 jet are limited to the innermost part of the jet (≲100 r s ) or to HST-1 (∼10 5   r s ). No attempt has yet been made to measure the magnetic field strength in between. Aims. We aim to infer the magnetic field strength of the M 87 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. Methods. 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 Very Long Baseline Array (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. Results. 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. In the KaVA observations, the spectral index remains unchanged until ∼10 mas, but this trend is unclear in the VLBA observations. 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 /2mas) −0.73 . Extrapolating to the Event Horizon Telescope 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. 

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. 

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 ± 102μas (axial ratio ∼${1.19}_{-0.19}^{+0.24}$) and ∼300 ± 25μas (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.