arXiv subjects

K. Asada

Publications and source records attributed to K. Asada.

26 records · Page 2Linked to original sources

arXiv2014

Greenland Telescope Project --- Direct Confirmation of Black Hole with Sub-millimeter VLBI

A 12-m diameter radio telescope will be deployed to the Summit Station in Greenland to provide direct confirmation of a Super Massive Black Hole (SMBH) by observing its shadow image in the active galaxy M87. The telescope (Greenland Telescope: GLT) is to become one of the Very Long Baseline Interferometry (VLBI) stations at sub-millimeter (submm) regime, providing the longest baseline > 9,000 km to achieve an exceptional angular resolution of 20 micro arc sec at 350 GHz, which will enable us to resolve the shadow size of ~40 micro arc sec. The triangle with the longest baselines formed by the GLT, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and the Submillimeter Array (SMA) in Hawaii will play a key role for the M87 observations. We have been working on the image simulations based on realistic conditions for a better understanding of the possible observed images. In parallel, retrofitting of the telescope and the site developments are in progress. Based on three years of opacity monitoring at 225 GHz, our measurements indicate that the site is excellent for submm observations, comparable to the ALMA site. The GLT is also expected to make single-dish observations up to 1.5 THz.

astro-ph.IM↗

arXiv2014

Limb-Brightened Jet of 3C 84 Revealed by the 43-GHz Very-Long-Baseline-Array Observation

We present a study of sub-pc scale radio structure of the radio galaxy 3C 84/NGC 1275 based on the Very Long Baseline Array (VLBA) data at 43 GHz. We discover a limb-brightening in the "restarted" jet associated with the 2005 radio outburst. In the 1990s, the jet structure was ridge-brightening rather than limb-brightening, despite the observations being done with similar angular resolution. This indicates that the transverse jet structure has changed recently. This change in the morphology shows an interesting agreement with the $γ$-ray flux increase, i.e., the $γ$-ray flux in 1990s was at least seven times lower than the current one. One plausible explanation for the limb-brightening is the velocity structure of the jet in the context of the stratified jet, which is a successful scenario to explain the $γ$-ray emission in some active galactic nuclei (AGNs). If this is the case, the change in apparent transverse structure might be caused by the change in the transverse velocity structure. We argue the possibility that the transition from ridge-brightening to limb-brightening is related to the $γ$-ray time variability on the timescale of decades. We also discuss the collimation profile of the jet.

astro-ph.HE↗

arXiv2014

Measuring Mass Accretion Rate onto the Supermassive Black Hole in M 87 Using Faraday Rotation Measure with the Submillimeter Array

We present the first constraint on Faraday rotation measure (RM) at submillimeter wavelengths for the nucleus of M 87. By fitting the polarization position angles ($χ$) observed with the SMA at four independent frequencies around $\sim$230 GHz and interpreting the change in $χ$ as a result of \emph{external} Faraday rotation associated with accretion flow, we determine the rotation measure of the M 87 core to be between $-$7.5$\times$10$^{5}$ and 3.4$\times$10$^{5}$ rad/m$^{2}$. Assuming a density profile of the accretion flow that follows a power-law distribution and a magnetic field that is ordered, radial, and has equipartition strength, the limit on the rotation measure constrains the mass accretion rate $\dot{M}$ to be below 9.2$\times$10$^{-4}$ M$_{\odot}$~yr$^{-1}$ at a distance of 21 Schwarzchild radii from the central black hole. This value is at least two orders of magnitude smaller than the Bondi accretion rate, suggesting significant suppression of the accretion rate in the inner region of the accretion flow. Consequently, our result disfavors the classical \emph{advection dominated accretion flow} (ADAF) and prefers the \emph{adiabatic inflow-outflow solution} (ADIOS) or \emph{convection-dominated accretion flow} (CDAF) for the hot accretion flow in M 87.

astro-ph.GA↗

arXiv2013

Greenland Telescope (GLT) Project: "A Direct Confirmation of Black Hole with Submillimeter VLBI"

The GLT project is deploying a new submillimeter (submm) VLBI station in Greenland. Our primary scientific goal is to image a shadow of the supermassive black hole (SMBH) of six billion solar masses in M87 at the center of the Virgo cluster of galaxies. The expected SMBH shadow size of 40-50 $μ$as requires superbly high angular resolution, suggesting that the submm VLBI would be the only way to obtain the shadow image. The Summit station in Greenland enables us to establish baselines longer than 9,000 km with ALMA in Chile and SMA in Hawaii as well as providing a unique $u$--$v$ coverage for imaging M87. Our VLBI network will achieve a superior angular resolution of about 20 $μ$as at 350 GHz, corresponding to $\sim2.5$ times of the Schwarzschild radius of the supermassive black hole in M87. We have been monitoring the atmospheric opacity at 230 GHz since August. 2011; we have confirmed the value on site during the winter season is comparable to the ALMA site thanks to high altitude of 3,200 m and low temperature of $-50\degr$C. We will report current status and future plan of the GLT project towards our expected first light on 2015--2016.

astro-ph.IM↗

arXiv2012

225 GHz Atmospheric Opacity Measurements from Two Arctic Sites

We report the latest results of 225 GHz atmospheric opacity measurements from two arctic sites; one on high coastal terrain near the Eureka weather station, on Ellesmere Island, Canada, and the other at the Summit Station near the peak of the Greenland icecap. This is a campaign to search for a site to deploy a new telescope for submillimeter Very Long Baseline Interferometry and THz astronomy in the northern hemisphere. Since 2011, we have obtained 3 months of winter data near Eureka, and about one year of data at the Summit Station. The results indicate that these sites offer a highly transparent atmosphere for observations in submillimeter wavelengths. The Summit Station is particularly excellent, and its zenith opacity at 225 GHz is statistically similar to the Atacama Large Milllimeter/submillimeter Array in Chile. In winter, the opacity at the Summit Station is even comparable to that observed at the South Pole.

astro-ph.IM↗

arXiv2012

Possible Detection of Apparent Superluminal inward motion in Markarian 421 after the Giant X-ray flare in February, 2010

We report on the VLBI follow-up observations using the Japanese VLBI Network (JVN) array at 22 GHz for the largest X-ray flare of TeV blazar Mrk 421 that occurred in mid-February, 2010. The total of five epochs of observations were performed at intervals of about 20 days between March 7 and May 31, 2010. No new-born component associated with the flare was seen directly in the total intensity images obtained by our multi-epoch VLBI observations. However, one jet component located at ~1 mas north-west from the core was able to be identified, and its proper motion can be measured as -1.66+/-0.46 mas yr^-1, which corresponds to an apparent velocity of -3.48+/-0.97 c. Here, this negative velocity indicates that the jet component was apparently moving toward the core. As the most plausible explanation, we discuss that the apparent negative velocity was possibly caused by the ejection of a new component, which could not be resolved with our observations. In this case, the obtained Doppler factor of the new component is around 10 to 20, which is consistent with the ones typically estimated by model fittings of spectral energy distribution for this source.

astro-ph.HE↗

arXiv2010

VLBI Monitoring of 3C 84 in Gamma-ray Active Phase

We report multi-epoch Very Long Baseline Interferometry (VLBI) study of the innermost jet of 3C 84. We carried out 14-epoch VLBI observations during 2006-2009 with the Japanese VLBI Network (JVN) and the VLBI Exploration of Radio Astrometry (VERA), immediately following the radio outburst that began in 2005. Comparison between VLBI lightcurve and single-dish lightcurve indicates that the outburst was associated with the central ~1 pc core. We found that this outburst accompanied the emergence of a new component, and the projected speed of this new component was 0:23c from 2007/297 (2007 October 24) to 2009/114 (2009 April 24). We argue the site of gamma-ray emission detected by Fermi/LAT and jet kinematics in connection with gamma-ray emission mechanism.

astro-ph.CO↗

arXiv2010

Evidence of non-thermal X-ray emission from radio lobes of Cygnus A

Using deep Chandra ACIS observation data for Cygnus A, we report evidence of non-thermal X-ray emission from radio lobes surrounded by a rich intra-cluster medium (ICM). The diffuse X-ray emission, which are associated with the eastern and western radio lobes, were observed in a 0.7--7 keV Chandra$ ACIS image. The lobe spectra are reproduced with not only a single-temperature Mekal model, such as that of the surrounding ICM component, but also an additional power-law (PL) model. The X-ray flux densities of PL components for the eastern and western lobes at 1 keV are derived as 77.7^{+28.9}_{-31.9} nJy and 52.4^{+42.9}_{-42.4} nJy, respectively, and the photon indices are 1.69^{+0.07}_{-0.13} and 1.84^{+2.90}_{-0.12}, respectively. The non-thermal component is considered to be produced via the inverse Compton (IC) process, as is often seen in the X-ray emission from radio lobes. From a re-analysis of radio observation data, the multiwavelength spectra strongly suggest that the seed photon source of the IC X-rays includes both cosmic microwave background radiation and synchrotron radiation from the lobes. The derived parameters indicate significant dominance of the electron energy density over the magnetic field energy density in the Cygnus A lobes under the rich ICM environment.

astro-ph.CO↗