Powerful radio jets from active galactic nuclei are thought to be powered by the accretion of material onto the supermassive black hole (the 'central engine'). M87 is one of the closest examples of this phenomenon, and the structure of its jet has been probed on a scale of about 100 Schwarzschild radii (R(s), the radius of the event horizon). However, the location of the central black hole relative to the jet base (a bright compact radio 'core') remains elusive. Observations of other jets indicate that the central engines are located about 10(4)-10(6)R(s) upstream from the radio core. Here we report radio observations of M87 at six frequencies that allow us to achieve a positional accuracy of about 20 microarcseconds. As the jet base becomes more transparent at higher frequencies, the multifrequency position measurements of the radio core enable us to determine the upstream end of the jet. The data reveal that the central engine of M87 is located within 14-23R(s) of the radio core at 43 GHz. This implies that the site of material infall onto the black hole and the eventual origin of the jet reside in the bright compact region seen on the image at 43 GHz.

We investigated the detailed inner jet structure of M87 using the Very Long Baseline Array data at 2, 5, 8.4, 15, 23.8, 43, and 86 GHz, especially focusing on the multi-frequency properties of the radio core at the jet base. First, we measured a size of the core region transverse to the jet axis, defined as W c , at each frequency ν, and found a relation between W c and ν as W c (ν) ∝ ν −0.71±0.05 . Then, by combining W c (ν) and the frequency dependence of the core position r c (ν), which was obtained by our previous study, we have constructed a collimation profile of the innermost jet W c (r) down to ∼10 Schwarzschild radii (R s ) from the central black hole. We found that W c (r) smoothly connects with the width profile of the outer edge-brightened, parabolic jet, and then follows a similar radial dependence down to several tens of R s . Closer to the black hole, the measured radial profile suggests a possible change of the jet collimation shape from the outer parabolic one, in which the jet shape tends to become more radially-oriented. This could be related to a magnetic collimation process or/and interections with surrounding materials at the jet base. The present results shed light on the importance of higher-sensitivity/resolution imaging studies for M87 at 86, 43 and also 22 GHz, and should be examined more rigorously.

High angular resolution images of extragalactic radio sources are being made with the Highly Advanced Laboratory for Communications and Astronomy (HALCA) satellite and ground-based radio telescopes as part of the Very Long Baseline Interferometry (VLBI) Space Observatory Programme (VSOP). VSOP observations at 1.6 and 5 gigahertz of the milli–arc-second–scale structure of radio quasars enable the quasar core size and the corresponding brightness temperature to be determined, and they enable the motions of jet components that are close to the core to be studied. Here, VSOP images of the gamma-ray source 1156+295, the quasar 1548+056, the ultraluminous quasar 0014+813, and the superluminal quasar 0212+735 are presented and discussed.

We have carried out multi-epoch VLBI observations with VERA (VLBI Exploration of Radio Astrometry) of the 22 GHz H 2 O masers associated with a Class 0 protostar L1448C in the Perseus molecular cloud. The maser features trace the base of collimated bipolar jet driven by one of the infrared counter parts of L1448C named as L1448C(N) or L1448-mm A. We detected possible evidences for apparent acceleration and precession of the jet according to the three-dimensional velocity structure. Based on the phase-referencing VLBI astrometry, we have successfully detected an annual parallax of the H 2 O maser in L1448C to be 4.31±0.33 milliarcseconds (mas) which corresponds to a distance of 232±18 pc from the Sun. The present result is in good agreement with that of another H 2 O maser source NGC 1333 SVS13A in the Perseus molecular cloud, 235 pc. It is also consistent with the photometric distance, 220 pc. Thus, the distance to the western part of the Perseus molecular cloud complex would be constrained to be about 235 pc rather than the larger value, 300 pc, previously reported.

We present analyses to determine the fundamental parameters of the Galaxy based on VLBI astrometry of 52 Galactic maser sources obtained with VERA, VLBA and EVN. We model the Galaxy's structure with a set of parameters including the Galaxy center distance R 0 , the angular rotation velocity at the LSR Ω 0 , mean peculiar motion of the sources with respect to Galactic rotation (U src , V src , W src ), rotation-curve shape index, and the V component of the Solar peculiar motions V ⊙ . Based on a Markov chain Monte Carlo method, we find that the Galaxy center distance is constrained at a 5% level to be R 0 = 8.05 ± 0.45 kpc, where the error bar includes both statistical and systematic errors. We also find that the two components of the source peculiar motion U src and W src are fairly small compared to the Galactic rotation velocity, being U src = 1.0 ± 1.5 km s −1 and W src = −1.4 ± 1.2 km s −1 . Also, the rotation curve shape is found to be basically flat between Galacto-centric radii of 4 and 13 kpc. On the other hand, we find a linear relation between V src and V ⊙ as V src = V ⊙ − 19 (±2) km s −1 , suggesting that the value of V src is fully dependent on the adopted value of V ⊙ . Regarding the rotation speed in the vicinity of the Sun, we also find a strong correlation between Ω 0 and V ⊙ . We find that the angular velocity of the Sun, Ω ⊙ , which is defined as Ω ⊙ ≡ Ω 0 + V ⊙ /R 0 , can be well constrained with the best estimate of Ω ⊙ = 31.09 ± 0.78 km s −1 kpc −1 . This corresponds to Θ 0 = 238 ± 14 km s −1 if one adopts the above value of R 0 and recent determination of V ⊙ ∼12 km s −1 .

We present the initial results of multi-epoch VLBI observations of the 22 GHz H 2 O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry). With the VERA dual-beam receiving system, we have carried out phase-referencing VLBI astrometry and successfully detected an annual parallax of Orion KL to be 2.29±0.10 mas, corresponding to the distance of 437±19 pc from the Sun. The distance to Orion KL is determined for the first time with the annual parallax method in these observations. Although this value is consistent with that of the previously reported, 480±80 pc, which is estimated from the statistical parallax method using proper motions and radial velocities of the H 2 O maser features, our new results provide the much more accurate value with an uncertainty of only 4%. In addition to the annual parallax, we have detected an absolute proper motion of the maser feature, suggesting an outflow motion powered by the radio source I along with the systematic motion of source I itself.

We report on results of multi-epoch VLBI observations of H$_2$O masers associated with a low-mass young stellar object, IRAS 16293$-$2422 in $\rho$ Oph East, and a fringe-phase and position reference source, ICRF J162546.8$-$252738, using the VLBI Exploration of Radio Astrometry (VERA) for high-precision astrometry. We obtained an annual parallax of a maser feature to be $\pi=$ 5.6$^{+1.5}_{-0.5}$ mas, corresponding to a distance of $D=178^{+18}_{-37}$ pc. We also found 10 relative proper motions of maser features with respect to the maser feature mentioned above. The motion of the accompanying young stellar object (YSO) has already been found in thermal continuum emission previously observed with the Very Large Array. The intrinsic motions of masers have been estimated from the relative proper motions after the YSO’s motion is subtracted from, and a systemic secular motion of the position reference feature is added to the proper motions originally measured. The intrinsic maser kinematical structure may trace a bipolar outflow.

In February 2011, a burst event of the H 2 O maser in Orion KL (Kleinmann-Low object) has started after 13-year silence. This is the third time to detect such phenomena in Orion KL, followed by those in 1979-1985 and 1998. We have carried out astrometric observations of the bursting H 2 O maser features in Orion KL with VERA (VLBI Exploration of Radio Astrometry), a Japanese VLBI network dedicated for astrometry. The total flux of the bursting feature at the LSR velocity of 7.58 km s −1 reaches 4.4×10 4 Jy in March 2011. The intensity of the bursting feature is three orders of magnitudes larger than that of the same velocity feature in the quiescent phase in 2006. Two months later, another new feature appears at the LSR velocity of 6.95 km s −1 in May 2011, separated by 12 mas north of the 7.58 km s −1 feature. Thus, the current burst occurs at two spatially different features. The bursting masers are elongated along the northwest-southeast direction as reported in the previous burst in 1998. We