arXiv subjects

Mareki Honma

Publications and source records attributed to Mareki Honma.

At least 91 records · Page 5Linked to original sources

arXiv2016

Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales

The Galactic Center black hole Sagittarius A* (Sgr A*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A* obtained with the EHT on a total of 13 observing nights over 4 years. Closure phases, the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180-degree rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A* that is not obscured by refraction due to interstellar electrons along the line of sight.

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arXiv2016

ALMA Observation of the 658 GHz Vibrationally Excited H$_{2}$O Maser in Orion KL Source I

We present an observational study of the vibrationally excited H$_{2}$O line at 658 GHz ($ν_{2}$=1, 1$_{1, 0}$-1$_{0, 1}$) toward Orion KL using the Atacama Large Millimeter/Submillimeter Array (ALMA). This line is clearly detected at the position of the massive protostar candidate, the Source I. The spatial structure is compact with a size of about 100 AU and is elongated along the northeast-southwest low-velocity (18 km s$^{-1}$) bipolar outflow traced by 22 GHz H$_{2}$O masers, SiO masers, and thermal SiO lines. A velocity gradient can be seen perpendicular to the bipolar outflow. Overall spatial and velocity structure seems analogous to that of the 321 GHz H$_{2}$O maser line previously detected with ALMA and vibrationally excited SiO maser emission. The brightness temperature of the 658 GHz H$_{2}$O line is estimated to be higher than 2$\times$10$^{4}$ K, implying that it is emitted via maser action. Our results suggest that the 658 GHz H$_{2}$O maser line is emitted from the base of the outflow from a rotating and expanding accretion disk as observed for the SiO masers and the 321 GHz H$_{2}$O maser. We also search for two other H$_{2}$O lines at 646 GHz (9$_{7, 3}$-8$_{8, 0}$ and 9$_{7, 2}$-8$_{8, 1}$), but they are not detected in Orion KL.

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arXiv2015

High-Sensitivity 86GHz (3.5mm) VLBI Observations of M87: Deep Imaging of the Jet Base at a 10 Schwarzschild-Radius Resolution

We report on results from new high-sensitivity, high-resolution 86GHz (3.5 millimeter) observations of the jet base in the nearby radio galaxy M87, obtained by the Very Long Baseline Array in conjunction with the Green Bank Telescope. The resulting image has a dynamic range exceeding 1500 to 1, the highest ever achieved for this jet at this frequency, resolving and imaging a detailed jet formation/collimation structure down to ~10 Schwarzschild radii (Rs). The obtained 86GHz image clearly confirms some important jet features known at lower frequencies, i.e., a wide-opening angle jet base, a limb-brightened intensity profile, a parabola-shape collimation profile and a counter jet. The limb-brightened structure is already well developed at < 0.2mas (< 28Rs, projected) from the core, where the corresponding apparent opening angle becomes as wide as ~100 degrees. The subsequent jet collimation near the black hole evolves in a complicated manner; there is a "constricted" structure at tens Rs from the core, where the jet cross section is locally shrinking. We suggest that an external pressure support from the inner part of radiatively-inefficient accretion flow may be dynamically important in shaping/confining the footprint of the magnetized jet. We also present the first VLBI 86GHz polarimetric experiment for this source, where a highly polarized (~20%) feature is detected near the jet base, indicating the presence of a well-ordered magnetic field. As a by-product, we additionally report a 43/86 GHz polarimetric result for our calibrator 3C 273 suggesting an extreme rotation measure near the core.

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arXiv2015

Resolved Magnetic-Field Structure and Variability Near the Event Horizon of Sagittarius A*

Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intra-hour variability associated with these fields.

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arXiv2015

230 GHz VLBI observations of M87: event-horizon-scale structure at the enhanced very-high-energy $\rm γ$-ray state in 2012

We report on 230 GHz (1.3 mm) VLBI observations of M87 with the Event Horizon Telescope using antennas on Mauna Kea in Hawaii, Mt. Graham in Arizona and Cedar Flat in California. For the first time, we have acquired 230 GHz VLBI interferometric phase information on M87 through measurement of closure phase on the triangle of long baselines. Most of the measured closure phases are consistent with 0$^{\circ}$ as expected by physically-motivated models for 230 GHz structure such as jet models and accretion disk models. The brightness temperature of the event-horizon-scale structure is $\sim 1 \times 10^{10}$ K derived from the compact flux density of $\sim 1$ Jy and the angular size of $\sim 40 $ $\rm μ$as $\sim$ 5.5 $R_{\rm s}$, which is broadly consistent with the peak brightness of the radio cores at 1-86 GHz located within $\sim 10^2$ $R_{\rm s}$. Our observations occurred in the middle of an enhancement in very-high-energy (VHE) $\rm γ$-ray flux, presumably originating in the vicinity of the central black hole. Our measurements, combined with results of multi-wavelength observations, favor a scenario in which the VHE region has an extended size of $\sim$20-60 $R_{\rm s}$.

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arXiv2015

Outer Rotation Curve of the Galaxy with VERA III: Astrometry of IRAS 07427-2400 and Test of the Density-Wave Theory

We report the trigonometric parallax of IRAS 07427-2400 with VERA to be 0.185 $\pm$ 0.027 mas, corresponding to a distance of 5.41$^{+0.92}_{-0.69}$ kpc. The result is consistent with the previous result of 5.32$^{+0.49}_{-0.42}$ kpc obtained by Choi et al. (2014) within error. To remove the effect of internal maser motions (e.g., random motions), we observed six maser features associated with IRAS 07427-2400 and determined systematic proper motions of the source by averaging proper motions of the six maser features. The obtained proper motions are ($μ_α$cos$δ$, $μ_δ$) = ($-$1.79 $\pm$ 0.32, 2.60 $\pm$ 0.17) mas yr$^{-1}$ in equatorial coordinates, while Choi et al. (2014) showed ($μ_α$cos$δ$, $μ_δ$) = ($-$2.43 $\pm$ 0.02, 2.49 $\pm$ 0.09) mas yr$^{-1}$ with one maser feature. Our astrometry results place the source in the Perseus arm, the nearest main arm in the Milky Way. Using our result with previous astrometry results obtained from observations of the Perseus arm, we conducted direct (quantitative) comparisons between 27 astrometry results and an analytic gas dynamics model based on the density-wave theory and obtained two results. First is the pitch angle of the Perseus arm determined by VLBI astrometry, 11.1 $\pm$ 1.4 deg, differing from what is determined by the spiral potential model (probably traced by stars), $\sim$ 20 deg. The second is an offset between a dense gas region and the bottom of the spiral potential model. The dense gas region traced by VLBI astrometry is located downstream of the spiral potential model, which was previously confirmed in the nearby grand-design spiral galaxy M51 in Egusa et al. (2011).

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arXiv2015

The Nuclear Structure of 3C84 with Space VLBI (RadioAstron) Observations

The radio galaxy 3C84 is a representative of gamma-ray-bright misaligned active galactic nuclei (AGN) and one of the best laboratories to study the radio properties of subparsec scale jets. We discuss here the past and present activity of the nuclear region within the central 1pc and the properties of subparsec-sized components C1, C2 and C3. We compare these results with the high resolution space-VLBI image at 5GHz obtained with the RadioAstron satellite and we shortly discuss the possible correlation of radio emission with the gamma-ray emission.

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arXiv2015

Outer Rotation Curve of the Galaxy with VERA II: Annual Parallax and Proper Motion of the Star-Forming Region IRAS21379+5106

We conducted astrometric VLBI observations of water-vapor maser emission in the massive star forming region IRAS 21379+5106 to measure the annual parallax and proper motion, using VERA. The annual parallax was measured to be $0.262 \pm 0.031$ mas corresponding to a trigonometric distance of $3.82^{+0.51}_{-0.41}$ kpc. The proper motion was $(μ_α\cosδ, μ_δ)=(-2.74 \pm 0.08, -2.87 \pm 0.18)$ mas yr$^{-1}$. Using this result, the Galactic rotational velocity was estimated to be $V_θ=218\pm 19$ km s$^{-1}$ at the Galactocentric distance $R=9.22\pm0.43$ kpc, when we adopted the Galactic constants $R_0=8.05\pm 0.45$ kpc and $V_0=238\pm 14$ km s$^{-1}$. With newly determined distance, {the bolometric luminosity of the central young stellar object was re-evaluated to $(2.15\pm 0.54)\times 10^3 L_\odot$, which corresponds to spectral type of} B2--B3. Maser features were found to be distributed along a straight line from south-west to north-east. In addition, a vector map of the internal motions constructed from the residual proper motions implies that maser features trace a bipolar flow and that it cannot be explained by simple ballistic motion.

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arXiv2015

Accelerating a water maser face-on jet from a high mass young stellar object

We report on a long-term single-dish and VLBI monitoring for intermittent flare activities of a Dominant Blue-Shifted H$_{2}$O Maser (DBSM) associated with a southern high mass young stellar object, G353.273+0.641. Bi-weekly single-dish monitoring using Hokkaido University Tomakomai 11-m radio telescope has shown that a systematic acceleration continues over four years beyond a lifetime of individual maser features. This fact suggests that the H$_{2}$O maser traces a region where molecular gas is steadily accelerated. There were five maser flares during five-years monitoring, and maser distributions in four of them were densely monitored by the VLBI Exploration of Radio Astrometry (VERA). The overall distribution of the maser features suggests the presence of a bipolar jet, with the 3D kinematics indicating that it is almost face-on (inclination angle of $\sim$ 8$^{\fdg}$--17$^{\fdg}$ from the line-of-sight). Most of maser features were recurrently excited within a region of 100$\times$100 AU$^{2}$ around the radio continuum peak, while their spatial distributions significantly varied between each flare. This confirms that episodic propagations of outflow shocks recurrently invoke intermittent flare activities. We also measured annual parallax, deriving the source distance of 1.70 $^{+0.19}_{-0.16}$ kpc that is consistent with the commonly-used photometric distance.

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arXiv2015

ALMA Imaging of Millimeter/Submillimeter Continuum Emission in Orion KL

We have carried out high resolution observations with Atacama Large Millimeter/Submillimeter Array (ALMA) of continuum emission from Orion KL region. We identify 11 compact sources at ALMA band 6 (245 GHz) and band 7 (339 GHz), including Hot Core, Compact Ridge, SMA1, IRc4, IRc7, and a radio source I (Source I). Spectral energy distribution (SED) of each source is determined by using previous 3 mm continuum emission data. Physical properties such as size, mass, hydrogen number density and column density are discussed based on the dust graybody SED. Among 11 identified sources, Source I, a massive protostar candidate, is a dominant energy source in Orion KL. We extensively investigate its SED from centimeter to submillimeter wavelengths. The SED of Source I can be fitted with a single power-law index of 1.97 suggesting an optically thick emission. We employ the H$^{-}$ free-free emission as an opacity source of this optically thick emission. The temperature, density, and mass of the circumstellar disk associated with Source I are constrained by the SED of H$^{-}$ free-free emission. Still the fitting result shows a significant deviation from the observed flux densities. Combined with the thermal dust graybody SED to explain excess emission at higher frequency, a smaller power-law index of 1.60 for the H$^{-}$ free-free emission is obtained in the SED fitting. The power-law index smaller than 2 would suggest a compact source size or a clumpy structure unresolved with the present study. Future higher resolution observations with ALMA are essential to reveal more detailed spatial structure and physical properties of Source I.

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arXiv2014

VERA and ALMA Observations of the H2O Supermaser Burst in Orion KL

The 22 GHz H2O maser in Orion KL has shown extraordinary burst events in 1979-1985 and 1998-1999, sometimes called supermaser. We have conducted monitoring observations of the supermaser in Orion KL using VERA, VLBI Exploration of Radio Astrometry, in the current third burst since 2011 March. Three flux maxima are detected in 2011 and 2012 with rising and falling timescales of 2-7 months. Time variations of the supermaser seem symmetric for all of the active phases. The maximum total flux density of 135000 Jy is observed in 2012 June while it is still one order of magnitude lower than those in previous bursts. The supermaser consists of two spatially different components at different velocities. They are elongated along a northwest-southeast direction perpendicular to the low-velocity outflow driven by Source I. Proper motions of the supermaser features with respect to Source I are measured toward west and southwest directions, almost parallel to the low-velocity outflow. The flux density and linewidth show an anti-correlation as expected for an unsaturated maser emission. The supermaser is located close to the methylformate (HCOOCH3) line and continuum emission peaks in the Orion Compact Ridge detected by ALMA. The broader velocity range of the weak HCOOCH3 emission at the supermaser position would be an evidence of a shock front. On the other hand, the 321 GHz H2O line is not detected at the position of the supermaser. It can be explained qualitatively by one of the theoretical H2O excitation models without extraordinary conditions. Our results support a scenario that the supermaser is excited in the dense gas interacting with the low-velocity outflow in the Compact Ridge. The extremely high flux density and its symmetric time variation for rising and falling phases could be explained by a beaming effect during the amplification process rather than changes in physical conditions.

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arXiv2014

Super-resolution imaging with radio interferometer using sparse modeling

We propose a new technique to obtain super-resolution images with radio interferometer using sparse modeling. In standard radio interferometry, sampling of ($u$, $v$) is quite often incomplete and thus obtaining an image from observed visibilities becomes an underdetermined problem, and a technique so-called "zero-padding" is often used to fill up unsampled grids in ($u$, $v$) plane, resulting in image degradation by finite beam size as well as numerous side-lobes. In this paper we show that directly solving such an underdetermined problem based on sparse modeling (in this paper LASSO) avoids the above problems introduced by zero-padding, leading to super-resolution images in which structure finer than the standard beam size (diffraction limit) can be reproduced. We present results of one-dimensional and two-dimensional simulations of interferometric imaging, and discuss its implications to super-resolution imaging, particularly focusing on imaging of black hole shadows with millimeter VLBI.

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arXiv2014

VLBI observations of bright AGN jets with KVN and VERA Array (KaVA): Evaluation of Imaging Capability

The Korean very-long-baseline interferometry (VLBI) network (KVN) and VLBI Exploration of Radio Astrometry (VERA) Array (KaVA) is the first international VLBI array dedicated to high-frequency (23 and 43 GHz bands) observations in East Asia. Here, we report the first imaging observations of three bright active galactic nuclei (AGNs) known for their complex morphologies: 4C 39.25, 3C 273, and M 87. This is one of the initial result of KaVA early science. Our KaVA images reveal extended outflows with complex substructure such as knots and limb brightening, in agreement with previous Very Long Baseline Array (VLBA) observations. Angular resolutions are better than 1.4 and 0.8 milliarcsecond at 23 GHz and 43 GHz, respectively. KaVA achieves a high dynamic range of ~1000, more than three times the value achieved by VERA. We conclude that KaVA is a powerful array with a great potential for the study of AGN outflows, at least comparable to the best existing radio interferometric arrays.

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arXiv2014

The First Very Long Baseline Interferometry Image of 44 GHz Methanol Maser with the KVN and VERA Array (KaVA)

We have carried out the first very long baseline interferometry (VLBI) imaging of 44 GHz class I methanol maser (7_{0}-6_{1}A^{+}) associated with a millimeter core MM2 in a massive star-forming region IRAS 18151-1208 with KaVA (KVN and VERA Array), which is a newly combined array of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We have succeeded in imaging compact maser features with a synthesized beam size of 2.7 milliarcseconds x 1.5 milliarcseconds (mas). These features are detected at a limited number of baselines within the length of shorter than approximately 650 km corresponding to 100 Mlambda in the uv-coverage. The central velocity and the velocity width of the 44 GHz methanol maser are consistent with those of the quiescent gas rather than the outflow traced by the SiO thermal line. The minimum component size among the maser features is ~ 5 mas x 2 mas, which corresponds to the linear size of ~ 15 AU x 6 AU assuming a distance of 3 kpc. The brightness temperatures of these features range from ~ 3.5 x 10^{8} to 1.0 x 10^{10} K, which are higher than estimated lower limit from a previous Very Large Array observation with the highest spatial resolution of ~ 50 mas. The 44 GHz class I methanol maser in IRAS 18151-1208 is found to be associated with the MM2 core, which is thought to be less evolved than another millimeter core MM1 associated with the 6.7 GHz class II methanol maser.

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arXiv2014

Trigonometric Parallax of IRAS 22555+6213 with VERA: 3-Dimensional View of Sources Along the Same Line of Sight

We report the results of the measurement of the trigonometric parallax of an H$_2$O maser source in \r22 with the VLBI Exploration of Radio Astrometry (VERA). The annual parallax was determined to be 0.314$\pm$0.070~mas, corresponding to a distance of 3.18$^{+0.90}_{-0.66}$~kpc. Our results confirm \r22 to be located in the Perseus arm. We computed the peculiar motion of \r22 to be $(U_{src}, V_{src}, W_{src}) = (4\pm4, -32\pm6, 8\pm6)$~\kms, where $U_{src}$, $V_{src}$, and $W_{src}$ are directed toward the Galactic center, in the direction of Galactic rotation and toward the Galactic north pole, respectively. \r22, NGC\,7538 and Cepheus A lie along the same line of sight, and are within 2$^{\circ}$ on the sky. Their parallax distances with which we derived their absolute position in the Milky Way show that \r22 and \ngc~are associated with the Perseus Arm, while \cepa~is located in the Local Arm. We compared the kinematic distances of \iras, \ga, \gb, \gc, \gd, \cepa, \ngc, \ge, \gf~and \r22~derived with flat and non-flat rotation curve with its parallax distance and found the kinematic distance derived from the non-flat rotation assumption ($-$5 to $-$39 \kms~lag) to be consistent with the parallax distance.

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arXiv2014

VLBI Astrometry of AGB Variables with VERA -- A Mira Type Variable T Lepus

We conducted phase referencing VLBI observations of the Mira variable T~Lepus (T~Lep) using VERA, from 2003 to 2006. The distance to the source was determined from its annual parallax which was measured to be 3.06$\pm$0.04 mas, corresponding to a distance of 327$\pm$4\,pc. Our observations revealed the distribution and internal kinematics of H$_2$O masers in T~Lep, and we derived a source systemic motions of 14.60$\pm$0.50 mas yr$^{-1}$ and $-$35.43$\pm$0.79 mas yr$^{-1}$ in right ascension and declination, respectively. We also determined a LSR velocity of $V_\mathrm{LSR}^{\ast} = -$27.63 km s$^{-1}$. Comparison of our result with an image recently obtained from the VLTI infrared interferometer reveals a linear scale picture of the circumstellar structure of T~Lep. Analysis of the source systemic motion in the Galacto-centric coordinate frame indicates a large peculiar motion, which is consistent with the general characteristics of AGB stars. This source makes a contribution to the calibration of the period-luminosity relation of Galactic Mira variables. From the compilation of data for nearby Mira variables found in the literature, whose distances were derived from astrometric VLBI observations, we have calibrated the Galactic Mira period-luminosity relation to a high degree of accuracy.

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arXiv2014

Hot Molecular Circumstellar Disk around Massive Protostar Orion Source I

We report new Atacama Large Millimeter/Submillimeter Array (ALMA) observations of a circumstellar disk around Source I in Orion KL, an archetype of massive protostar candidate. We detected two ortho-H$_{2}$O lines at 321 GHz ($10_{2,9}$-$9_{3,6}$) and 336 GHz ($ν_{2}=1, 5_{2,3}$-$6_{1,6}$) for the first time in Source I. The latter one is in vibrationally excited state at the lower state energy of 2939 K, suggesting an evidence of hot molecular gas close to Source I. The integrated intensity map of the 321 GHz line is elongated along the bipolar outflow while the 336 GHz line map is unresolved with a beam size of 0.4\arcsec. Both of these maps show velocity gradient perpendicular to the bipolar outflow. The velocity centroid map of the 321 GHz line implies spatial and velocity structure similar to that of vibrationally-excited SiO masers tracing the root of the outflow emanating from the disk surface. In contrast, the 336 GHz line is most likely emitted from the disk midplane with a diameter of 0.2\arcsec (84 AU) as traced by a radio continuum emission and a dark lane devoid of the vibrationally-excited SiO maser emission. The observed velocity gradient and the spectral profile of the 336 GHz H$_{2}$O line can be reconciled with a model of an edge-on ring-like structure with an enclosed mass of $>$7$M_{\odot}$ and an excitation temperature of $>$3000 K. The present results provide a further evidence of hot and neutral circumstellar disk rotating around Source I with a diameter of $\sim$100 AU scale.

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arXiv2014

Accurate Parallax Measurement toward the Symbiotic Star R Aquarii

Multi-epoch phase-referencing VLBI (Very Long Baseline Interferometry) observations with VERA (VLBI Exploration of Radio Astrometry) were performed for the symbiotic star R Aquarii (R Aqr) from September 2005 to Oct 2006. Tracing one of the $v=2$, $J=1-0$ SiO maser spots, we measured an annual parallax of $π= 4.59\pm0.24$ mas, corresponding to a distance of $218_{-11}^{+12}$ pc. Our result is consistent with earlier distance measurements, but yields the highest accuracy of about $5\%$ level. Applying our distance, we derived an absolute K-band magnitude of $M_{\mathrm{K}} = -7.71 \pm 0.11$, which is consistent with the recent Period-Luminosity relation by VLBI parallax measurements for 5 OH-Mira variables. In addition, the expansion age of an inner nebulae around R Aqr is found to be about 240 years, corresponds to about the year 1773.

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