arXiv subjects

Mareki Honma

Publications and source records attributed to Mareki Honma.

At least 55 records · Page 3Linked to original sources

arXiv2020

Gravitational Test Beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole

The 2017 Event Horizon Telescope (EHT) observations of the central source in M87 have led to the first measurement of the size of a black-hole shadow. This observation offers a new and clean gravitational test of the black-hole metric in the strong-field regime. We show analytically that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the predicted black-hole shadows that are inconsistent with even the current EHT measurements. We use numerical calculations of regular, parametric, non-Kerr metrics to identify the common characteristic among these different parametrizations that control the predicted shadow size. We show that the shadow-size measurements place significant constraints on deviation parameters that control the second post-Newtonian and higher orders of each metric and are, therefore, inaccessible to weak-field tests. The new constraints are complementary to those imposed by observations of gravitational waves from stellar-mass sources.

gr-qc↗

arXiv2020

Magnetic Field Structure of Orion Source I

We observed polarization of the SiO rotational transitions from Orion Source I (SrcI) to probe the magnetic field in bipolar outflows from this high mass protostar. Both 43 GHz $J$=1-0 and 86 GHz $J$=2-1 lines were mapped with $\sim$20 AU resolution, using the Very Large Array (VLA) and Atacama Large Millimeter/Submillimeter Array (ALMA), respectively. The $^{28}$SiO transitions in the ground vibrational state are a mixture of thermal and maser emission. Comparison of the polarization position angles in the $J$=1-0 and $J$=2-1 transitions allows us to set an upper limit on possible Faraday rotation of $10^{4}$ radians m$^{-2}$, which would twist the $J$=2-1 position angles typically by less than 10 degrees. The smooth, systematic polarization structure in the outflow lobes suggests a well ordered magnetic field on scales of a few hundred AU. The uniformity of the polarization suggests a field strength of $\sim$30 milli-Gauss. It is strong enough to shape the bipolar outflow and possibly lead to sub-Keplerian rotation of gas at the base of the outflow. The strikingly high fractional linear polarizations of 80-90% in the $^{28}$SiO $v$=0 masers require anisotropic pumping. We measured circular polarizations of 60% toward the strongest maser feature in the $v$=0 $J$=1-0 peak. Anisotropic resonant scattering (ARS) is likely to be responsible for this circular polarization. We also present maps of the $^{29}$SiO $v$=0 $J$=2-1 maser and several other SiO transitions at higher vibrational levels and isotopologues.

astro-ph.SR↗

arXiv2020

Multiple outflows in the high-mass cluster forming region, G25.82-0.17

We present results of continuum and spectral line observations with ALMA and 22 GHz water (H$_2$O) maser observations using KaVA and VERA toward a high-mass star-forming region, G25.82-0.17. Multiple 1.3 mm continuum sources are revealed, indicating the presence of young stellar objects (YSOs) at different evolutionary stages, namely an ultra-compact HII region, G25.82-E, a high-mass young stellar object (HM-YSO), G25.82-W1, and starless cores, G25.82-W2 and G25.82-W3. Two SiO outflows, at N-S and SE-NW orientations, are identified. The CH$_3$OH 8$_{-1}$-7$_{0}$ E line, known to be a class I CH$_3$OH maser at 229 GHz is also detected showing a mixture of thermal and maser emission. Moreover, the H$_2$O masers are distributed in a region ~0.25" shifted from G25.82-W1. The CH$_3$OH 22$_{4}$-21$_{5}$ E line shows a compact ring-like structure at the position of G25.82-W1 with a velocity gradient, indicating a rotating disk or envelope. Assuming Keplerian rotation, the dynamical mass of G25.82-W1 is estimated to be $>$25 M$_{\odot}$ and the total mass of 20 M$_\odot$-84 M$_\odot$ is derived from the 1.3 mm continuum emission. The driving source of the N-S SiO outflow is G25.82-W1 while that of the SE-NW SiO outflow is uncertain. Detection of multiple high-mass starless$/$protostellar cores and candidates without low-mass cores implies that HM-YSOs could form in individual high-mass cores as predicted by the turbulent core accretion model. If this is the case, the high-mass star formation process in G25.82 would be consistent with a scaled-up version of low-mass star formation.

astro-ph.GA↗

arXiv2020

Super-resolution Imaging of the Protoplanetary Disk HD 142527 Using Sparse Modeling

With an emphasis on improving the fidelity even in super-resolution regimes, new imaging techniques have been intensively developed over the last several years, which may provide substantial improvements to the interferometric observation of protoplanetary disks. In this study, sparse modeling (SpM) is applied for the first time to observational data sets taken by the Atacama Large Millimeter/submillimeter Array (ALMA). The two data sets used in this study were taken independently using different array configurations at Band 7 (330 GHz), targeting the protoplanetary disk around HD 142527; one in the shorter-baseline array configuration (~ 430 m), and the other in the longer-baseline array configuration (~ 1570 m). The image resolutions reconstructed from the two data sets are different by a factor of ~ 3. We confirm that the previously known disk structures appear on the images produced by both SpM and CLEAN at the standard beam size. The image reconstructed from the shorter-baseline data using the SpM matches that obtained with the longer-baseline data using CLEAN, achieving a super-resolution image from which a structure finer than the beam size can be reproduced. Our results demonstrate that on-going intensive development in the SpM imaging technique is beneficial to imaging with ALMA.

astro-ph.EP↗

arXiv2020

Annual parallax measurement of a Mira variable star BX Cam with VERA

We report results of astrometric VLBI observations toward a Mira variable star BX~Cam using the VLBI array "VERA". The observations were performed from February 2012 to November 2014. Obtained parallax is 1.73$\pm$0.03 mas corresponding to a distance of 0.58$\pm$0.01 kpc. Parallax of this source was also reported in Gaia DR2 as 4.13$\pm$0.25 mas, and there is a 240 \% difference between these two measurements. Astrometric results from our VLBI observations show that we exactly traced angular motions of the seven maser spots in BX~Cam. We calculated stellar luminosities using both parallaxes, and obtained luminosities of $L_{\ast}^{\mathrm{VERA}} = 4950\pm170 L_{\odot}$ and $L_{\ast}^{\mathrm{Gaia}} = 870\pm110 L_{\odot}$. Deduced luminosities also support a validity of the parallax that we determined with VERA. Evaluating the two parallaxes, we concluded that the parallax of 1.73$\pm$0.03 mas from the VERA observations is correct for BX~Cam. We obtained a systemic motion of BX~Cam as (${μ_α}\cosδ^{\mathrm{sys}}$, $μ_δ^{\mathrm{sys}}$) $=$ (13.48$\pm$0.14, $-$34.30$\pm$0.18) mas\,yr$^{-1}$. A total of 73 H$_2$O maser spots detected from our VLBI observations show a spatial distribution of 30 au $\times$ 80 au with a strong elongation along north-south direction. They show outflows with a three-dimensional velocity of 14.79$\pm$1.40 km\,s$^{-1}$. From a comparison between time variations of $V$-band magnitudes and H$_2$O maser, we found that variation of the H$_2$O maser is relevant to that seen in $V$-band even though the H$_2$O maser does not recover its maximum flux in each cycle.

astro-ph.SR↗

arXiv2020

The First VERA Astrometry Catalog

We present the first astrometry catalog from the Japanese VLBI (very long baseline interferometer) project VERA (VLBI Exploration of Radio Astrometry). We have compiled all the astrometry results from VERA, providing accurate trigonometric annual parallax and proper motion measurements. In total, 99 maser sources are listed in the VERA catalog. Among them, 21 maser sources are newly reported while the rest of 78 sources are referred to previously published results or those in preparation for forthcoming papers. The accuracy in the VERA astrometry are revisited and compared with those from the other VLBI astrometry projects such as BeSSeL (The Bar and Spiral Structure Legacy) Survey and GOBELINS (the Gould's Belt Distances Survey) with the VLBA (Very Long Baseline Array). We have confirmed that most of the astrometry results are consistent with each other, and the largest error sources are due to source structure of the maser features and their rapid variation, along with the systematic calibration errors and different analysis methods. Combined with the BeSSeL results, we estimate the up-to-date fundamental Galactic parameter of $R_{0}=7.92\pm0.16_{\rm{stat.}}\pm0.3_{\rm{sys.}}$~kpc and $Ω_{\odot}=30.17\pm0.27_{\rm{stat.}}\pm0.3_{\rm{sys.}}$~km~s$^{-1}$~kpc$^{-1}$, where $R_{0}$ and $Ω_{\odot}$ are the distance from the Sun to the Galactic center and the Sun's angular velocity of the Galactic circular rotation, respectively.

astro-ph.GA↗

arXiv2019

Kinematics of the M87 jet in the collimation zone: gradual acceleration and velocity stratification

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 ($Γ\propto z^{0.16\pm0.01}$) 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.

astro-ph.HE↗

arXiv2019

Black hole Spin Measurement Based on Time-domain VLBI Observations of Infalling Gas Cloud

The black hole spacetime is described by general relativity and characterized by two quantities: the black hole mass and spin. Black hole spin measurement requires information from the vicinity of the event horizon, which is spatially resolved for the Galactic center SagittariusA* (SgrA*) and nearby radio galaxy M87 by means of very long baseline interferometry (VLBI) observations with the Event Horizon Telescope (EHT). In this paper, we simulate EHT observations for a gas cloud intermittently falling onto a black hole, and construct a method for spin measurement based on its relativistic flux variation. The light curve of the infalling gas cloud is composed of peaks formed by photons which directly reach a distant observer and by secondary ones reaching the observer after more than one rotation around the black hole. The time interval between the peaks is determined by a period of photon rotation near the photon circular orbit which uniquely depends on the spin. We perform synthetic EHT observations for SgrA* under a more realistic situation that a number of gas clouds intermittently fall towards the black hole with various initial parameters. Even for this case, the black hole spin dependence is detectable in correlated flux densities which are accurately calibrated by baselines between sites with redundant stations. The synthetic observations indicate that our methodology can be applied to EHT observations of Sgr A* since April 2017.

astro-ph.HE↗

arXiv2019

Vertical Structure and Kinematics of the Galactic Outer Disk

We report measurements of parallax and proper motion for four 22 GHz water maser sources as part of VERA Outer Rotation Curve project. All sources show Galactic latitudes of $>$ 2$^{\circ}$ and Galactocentric distances of $>$ 11 kpc at the Galactic longitude range of 95$^{\circ}$ $< l <$ 126$^{\circ}$. The sources trace the Galactic warp reaching to 200$\sim$400 pc, and indicate the signature of the warp to 600 pc toward the north Galactic pole. The new results along with previous results in the literature show the maximum height of the Galactic warp is increased with Galactocentric distance. Also, we examined velocities perpendicular to the disk for the sample, and found an oscillatory behavior between the vertical velocities and Galactic heights. This behavior suggests the existence of the bending (vertical density) waves, possibly induced by a perturbing satellite (e.g. passage of the Sagittarius dwarf galaxy).

astro-ph.GA↗

arXiv2019

Outer rotation curve of the Galaxy with VERA IV: Astrometry of IRAS 01123+6430 and the possibility of cloud-cloud collision

As part our investigation into the Galactic rotation curve, we carried out Very Long Baseline Interferometry (VLBI) observations towards the star-forming region IRAS 01123+6430 using VLBI Exploration of Radio Astrometry (VERA) to measure its annual parallax and proper motion. The annual parallax was measured to be 0.151+/-0.042 mas, which corresponds to a distance of D=6.61^{+2.55}_{-1.44} kpc, and the obtained proper motion components were (μ_αcosδ, μ_δ)=(-1.44+/-0.15, -0.27+/-0.16) mas yr^{-1} in equatorial coordinates. Assuming Galactic constants of (R_0, Θ_0)=(8.05+/-0.45 kpc, 238+/-14 km s^{-1}), the Galactocentric distance and rotation velocity were measured to be (R, Θ)=(13.04+/-2.24 kpc, 239+/-22 km s^{-1}), which are consistent with a flat Galactic rotation curve. The newly estimated distance provides a more accurate bolometric luminosity of the central young stellar object, L_Bol=(3.11+/-2.86)\times 10^3 L_solar, which corresponds to a spectral type of B1-B2. The analysis of 12CO(J=1-0) survey data obtained with the Five College Radio Astronomical Observatory (FCRAO) 14 m telescope shows that the molecular cloud associated with IRAS 01123+6430 consists of arc-like and linear components, which well matches a structure predicted by numerical simulation of the cloud-cloud collision (CCC) phenomenon. The coexistence of arc-like and linear components implies that the relative velocity of initial two clouds was as slow as 3-5 km s^{-1}, which meets the expected criteria of massive star formation where the core mass is effectively increased in the presence of low relative velocity (~3-5 km s^{-1}), as suggested by Takahira et al.(2014).

astro-ph.GA↗

arXiv2019

Studying black holes on horizon scales with space-VLBI

The Event Horizon Telescope (EHT) recently produced the first horizon-scale image of a supermassive black hole. Expanding the array to include a 3-meter space telescope operating at >200 GHz enables mass measurements of many black holes, movies of black hole accretion flows, and new tests of general relativity that are impossible from the ground.

astro-ph.IM↗

arXiv2019

Stellar Overdensity in the Local Arm in Gaia DR2

Using the cross-matched data of Gaia DR2 and 2MASS Point Source Catalog, we investigated the surface density distribution of stars aged ~1 Gyr in the thin disk in the range of 90° <= l <= 270°. We selected 4,654 stars above the turnoff corresponding to the age ~1 Gyr, that fall within a small box region in the color-magnitude diagram, (J-Ks)0 versus M(Ks), for which the distance and reddening are corrected. The selected sample shows an arm-like overdensity at 90° <= l <= 190°.This overdensity is located close to the Local arm traced by high-mass star forming regions (HMSFRs), but its pitch angle is slightly larger than that of the HMSFR-defined arm. Although the significance of the overdensity we report is marginal, its structure poses questions concerning both of the competing scenarios of spiral arms, the density-wave theory and the dynamic spiral arm model. The offset between the arms traced by stars and HMSFRs, i.e., gas, is difficult to be explained by the dynamic arm scenario. On the other hand, the pitch angle of the stellar Local arm, if confirmed, larger than that of the Perseus arm is difficult to be explained with the classical density-wave scenario. The dynamic arm scenario can explain it if the Local arm is in a growing up phase, while the Perseus arm is in a disrupting phase. Our result provides a new and complex picture of the Galactic spiral arms, and encourages further studies.

astro-ph.GA↗

arXiv2019

The first bird's-eye view of a gravitationally unstable accretion disk in high-mass star formation

We report on the first bird's-eye view of the innermost accretion disk around the high-mass protostellar object G353.273+0.641, taken by Atacama Large Millimter/submillimeter Array long-baselines. The disk traced by dust continuum emission has a radius of 250 au, surrounded by the infalling rotating envelope traced by thermal CH$_3$OH lines. This disk radius is consistent with the centrifugal radius estimated from the specific angular momentum in the envelope. The lower-limit envelope mass is $\sim$5-7 M$_{\odot}$ and accretion rate onto the stellar surface is 3 $\times$ 10$^{-3}$ M$_{\odot}$ yr$^{-1}$ or higher. The expected stellar age is well younger than 10$^{4}$ yr, indicating that the host object is one of the youngest high-mass objects at present. The disk mass is 2-7 M$_{\odot}$, depending on the dust opacity index. The estimated Toomre's $Q$ parameter is typically 1-2 and can reach 0.4 at the minimum. These $Q$ values clearly satisfy the classical criteria for the gravitational instability, and are consistent with the recent numerical studies. Observed asymmetric and clumpy structures could trace a spiral arm and/or disk fragmentation. We found that 70$\%$ of the angular momentum in the accretion flow could be removed via the gravitational torque in the disk. Our study has indicated that the dynamical nature of a self-gravitating disk could dominate the early phase of high-mass star formation. This is remarkably consistent with the early evolutionary scenario of a low-mass protostar.

astro-ph.SR↗

arXiv2019

Jet Kinematics of the Quasar 4C +21.35 from Observations with the KaVA Very Long Baseline Interferometry Array

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.

astro-ph.GA↗

arXiv2019

Source-Frequency Phase-Referencing Observation of AGNs with KaVA Using Simultaneous Dual-Frequency Receiving

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.

astro-ph.IM↗

arXiv2019

The Synergy between VLBI and Gaia astrometry

With the publication of Gaia DR2, 1.3 billion stars now have public parallax and proper motion measurements. In this contribution, we compare the results for sources that have both optical and radio measurements, focusing on circumstellar masers. For these large, variable and bright AGB stars, the VLBI astrometry results can be more robust. Moreover, there are a number of applications where VLBI astrometry provides unique data for studying stellar populations and Galactic structure. The BeSSel project not only provides parallax and proper motions at much larger distances than Gaia can reach, but it also uniquely samples the spiral arms of the Galaxy. The evolved stars in the BAaDE sample can potentially constrain the dynamics and stellar content of the inner bulge and bar of the Milky Way, not reachable in the optical.

astro-ph.SR↗

arXiv2019

Extremely high excitation SiO lines in disk-outflow system in Orion Source I

We present high-resolution images of the submillimeter SiO line emissions of a massive young stellar object Orion Source I using the Atacama Large Millimeter/ Submillimeter Array (ALMA) at band 8. We detected the 464 GHz SiO v=4 J=11-10 line in Source I, which is the first detection of the SiO v=4 line in star-forming regions, together with the 465 GHz 29SiO v=2 J=11-10 and the 428 GHz SiO v=2 J=10-9 lines with a resolution of 50 AU. The 29SiO v=2 J=11-10 and SiO v=4 J=11-10 lines have compact structures with the diameter of <80 AU. The spatial and velocity distribution suggest that the line emissions are associated with the base of the outflow and the surface of the edge-on disk. In contrast, SiO v=2 J=10-9 emission shows a bipolar structure in the direction of northeast-southwest low-velocity outflow with ~200 AU scale. The emission line exhibits a velocity gradient along the direction of the disk elongation. With the assumption of the ring structure with Keplerian rotation, we estimated the lower limit of the central mass to be 7 solar mass and the radius of 12 AU< r <26 AU.

astro-ph.GA↗

arXiv2018

A Balloon-Borne Very Long Baseline Interferometry Experiment in the Stratosphere: Systems Design and Developments

The balloon-borne very long baseline interferometry (VLBI) experiment is a technical feasibility study for performing radio interferometry in the stratosphere. The flight model has been developed. A balloon-borne VLBI station will be launched to establish interferometric fringes with ground-based VLBI stations distributed over the Japanese islands at an observing frequency of approximately 20 GHz as the first step. This paper describes the system design and development of a series of observing instruments and bus systems. In addition to the advantages of avoiding the atmospheric effects of absorption and fluctuation in high frequency radio observation, the mobility of a station can improve the sampling coverage (`uv-coverage') by increasing the number of baselines by the number of ground-based counterparts for each observation day. This benefit cannot be obtained with conventional arrays that solely comprise ground-based stations. The balloon-borne VLBI can contribute to a future progress of research fields such as black holes by direct imaging.

astro-ph.IM↗