arXiv subjects

H. Nagai

Publications and source records attributed to H. Nagai.

At least 19 recordsLinked to original sources

arXiv2020

Jansky VLA observations of synchrotron emitting optical hotspots of 3C 227 and 3C 445 radio galaxies

We report results on deep Jansky Very Large Array A-configuration observations at 22 GHz of the hotspots of the radio galaxies 3C227 and 3C445. Synchrotron emission in the optical on scales up to a few kpc was reported for the four hotspots. Our VLA observations point out the presence of unresolved regions with upper limit to their linear size of about 100 pc. This is the first time that such compact components in hotspots have been detected in a mini-sample, indicating that they are not a peculiar characteristic of a few individual hotspots. The polarization may reach values up to 70 per cent in compact (about 0.1 kpc scale) regions within the hotspot, indicating a highly ordered magnetic field with size up to a hundred parsecs. On larger scales, the average polarization of the hotspot component is about 30-45 per cent, suggesting the presence of a significant random field component, rather than an ordered magnetic field. This is further supported by the displacement between the peaks in polarized intensity and in total intensity images that is observed in all the four hotspots. The electric vector position angle is not constant, but changes arbitrarily in the central part of the hotspot regions, whereas it is usually perpendicular to the total intensity contours of the outermost edge of the hotspot structure, likely marking the large-scale shock front. The misalignment between X-ray and radio-to-optical emission suggests that the former is tracing the current particle acceleration, whereas the latter marks older shock fronts.

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arXiv2019

First Detection of Radio Linear Polarization in a Gamma Ray Burst Afterglow

We report the first detection of radio polarization of a GRB afterglow with the first intensive combined use of telescopes in the millimeter and submillimeter ranges for GRB171205A. The linear polarization degree in the millimeter band at the sub-percent level ($0.27 \pm 0.04\%$) is lower than those observed in late-time optical afterglows (weighted average of $\sim 1\%$). The Faraday depolarization by non-accelerated, cool electrons in the shocked region is one of possible mechanisms for the low value. In this scenario, larger total energy by a factor of $\sim 10$ than ordinary estimate without considering non-accelerated electrons is required. The polarization position angle varies by at least 20 degrees across the millimeter band, which is not inconsistent with this scenario. This result indicates that polarimetry in the millimeter and submillimeter ranges is a unique tool for investigating GRB energetics, and coincident observations with multiple frequencies or bands would provide more accurate measurements of the non-accelerated electron fraction.

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arXiv2019

The ALMA Discovery of the Rotating Disk and Fast Outflow of Cold Molecular Gas in NGC 1275

We present ALMA Band 6 observations of the CO(2-1), HCN(3-2), and HCO$^{+}$(3-2) lines in the nearby radio galaxy / brightest cluster galaxy (BCG) of NGC 1275 with the spatial resolution of $\sim20$ pc. In the previous observations, CO(2-1) emission was detected as radial filaments lying in the east-west direction. We resolved the inner filament and found that the filament cannot be represented by a simple infalling stream both morphologically and kinematically. The observed complex nature of the filament resembles the cold gas structure predicted by recent numerical simulations of cold chaotic accretion. A crude estimate suggests that the accretion rate of the cold gas can be higher than that of hot gas. Within the central 100 pc, we detected a rotational disk of the molecular gas whose mass is $\sim10^{8} M_{\sun}$. This is the first evidence of the presence of massive cold gas disk on this spatial scale for BCGs. The disk rotation axis is approximately consistent with the axis of the radio jet on subpc scales. This probably suggests that the cold gas disk is physically connected to the innermost accretion disk which is responsible for jet launching. We also detected absorption features in the HCN(3-2) and HCO$^{+}$(3-2) spectra against the radio continuum emission mostly radiated by $\sim1.2$-pc size jet. The absorption features are blue-shifted from the systemic velocity by $\sim$300-600~km~s$^{-1}$, which suggests the presence of outflowing gas from the active galactic nucleus (AGN). We discuss the relation of the AGN feeding with cold accretion, the origin of blue-shifted absorption, and estimate of black hole mass using the molecular gas dynamics.

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arXiv2019

Stochastic modeling of the time variability of ALMA calibrators

Characterizing the variability of the extragalactic sources used for calibration in the Atacama Large Millimeter/Sub-millimeter Array (ALMA) is key to assess the flux scale uncertainty of science observations. To this end, we model the variability of 39 quasars which have been used by ALMA as secondary flux calibrators using continuous time stochastic processes. This formalism is specially adapted to the multi-frequency, quasi-periodic sampling which characterizes the calibration monitoring of ALMA. We find that simple mixtures of Ornstein-Uhlenbeck processes can describe well the flux and spectral index variability of these sources for Bands 3 and 7 (91.5 and 103.5, and 343.5 GHz, respectively). The spectral shape of the calibrators are characterized by negative spectral indices, mostly between $-0.35$ and $-0.80$, and with additional concavity. The model provides forecasts, interpolations, and uncertainty estimations for the observed fluxes that depend on the intrinsic variability of the source. These can be of practical use for the ALMA data calibrator survey and data quality assurance.

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arXiv2019

ALMA Polarimetry of AT2018cow

We present the first radio polarimetric observations of a fast-rising blue optical transient, AT2018cow. Two epochs of polarimetry with additional coincident photometry were performed with the Atacama Large Millimeter/submillimeter Array (ALMA). The overall photometric results based on simultaneous observations in the 100 and 230 GHz bands are consistent with the non-thermal radiation model reported by Ho et al. (2019) and indicate that the spectral peaks ($\sim110$ GHz at the first epoch and $\sim67$ GHz at the second epoch) represent the synchrotron self-absorption frequency. The non-detection of linear polarization with $<$0.15% in the 230 GHz band at the phase when the effect of synchrotron self-absorption was quite small in the band may be explained by internal Faraday depolarization with high circumburst density and strong magnetic field. This result supports the stellar explosion scenario rather than the tidal disruption model. The maximum energy of accelerating particles at the shocks of AT2018cow-like objects is also discussed.

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arXiv2018

Evidence of jet-clump interaction: a flip of the radio jet head of 3C~84

Radio jets in active galaxies have been expected to interact with circumnuclear environments in their early phase evolutions. By performing the multi-epoch monitoring observation with the KVN and VERA Array (KaVA) at 43~GHz, we investigate the kinematics of the notable newborn bright component C3 located at the tip of the recurrent jet of 3C~84. During 2015 August-September, we discover the flip of C3 and the amount of the flip is about 0.4~milli-arcsecond in angular scale, which corresponds to 0.14 parsec in physical scale. After the flip of C3, it wobbled at the same location for a few months and then it restarted to propagate towards the southern direction. The flux density of C3 coherently showed the monotonic increase during the observation period. The flip is in good agreement with hydrodynamical simulations of jets in clumpy ambient medium. We estimate the number density of the putative clump based on the momentum balance between the jet thrust and the ram pressure from the clump and it is about $10^{3-5}~{\rm cm^{-3}}$. We briefly discuss possible origins of the clump.

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arXiv2018

A wide and collimated radio jet in 3C 84 on the scale of a few hundred gravitational radii

Understanding the launching, acceleration, and collimation of jets powered by active galactic nuclei remains an outstanding problem in relativistic astrophysics. This is partly because observational tests of jet formation models suffer from the limited angular resolution of ground-based very long baseline interferometry that has thus far been able to probe the transverse jet structure in the acceleration and collimation zone of only two sources. Here we report radio interferometric observations of 3C 84 (NGC 1275), the central galaxy of the Perseus cluster, made with an array including the orbiting radio telescope of the RadioAstron mission. The obtained image transversely resolves the edge-brightened jet in 3C 84 only 30 microarcseconds from the core, which is ten times closer to the central engine than what has been possible in previous ground-based observations, and it allows us to measure the jet collimation profile from ~ 100 to ~10000 gravitational radii from the black hole. The previously found, almost cylindrical jet profile on scales larger than a few thousand r_g is now seen to continue at least down to a few hundred r_g from the black hole and we find a broad jet with a transverse radius larger than about 250 r_g at only 350 r_g from the core. If the bright outer jet layer is launched by the black hole ergosphere, it has to rapidly expand laterally on scales smaller than 100 r_g. If this is not the case, then this jet sheath is likely launched from the accretion disk.

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arXiv2017

Enhanced Polarized Emission from the One-Parsec-Scale Hotspot of 3C 84 as a Result of the Interaction with Clumpy Ambient Medium

We present Very Long Baseline Array polarimetric observations of the innermost jet of 3C$\sim$84 (NGC$\sim$1275) at 43$\sim$GHz. A significant polarized emission is detected at the hotspot of the innermost re-started jet, which is located $\sim$1 pc south from the radio core. While the previous report presented a hotspot at the southern end of the western limb, the hotspot location has been moved to the southern end of the eastern limb. Faraday rotation is detected within an entire bandwidth of the 43-GHz band. The measured rotation measure (RM) is at most (6.3$\pm$1.9)$\times10^{5}$$\sim$rad$\sim$m$^{-2}$ and might be slightly time variable on the timescale of a month by a factor of a few. Our measured RM and the RM previously reported by the CARMA and SMA observations cannot be consistently explained by the spherical accretion flow with a power-law profile. We propose that a clumpy/inhomogeneous ambient medium is responsible for the observed rotation measure. Using equipartition magnetic field, we derive the electron density of $2\times10^{4}$$\sim$cm$^{-3}$. Such an electron density is consistent with the cloud of narrow line emission region around the central engine. We also discuss the magnetic field configuration from black hole scale to pc scale and the origin of low polarization.

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arXiv2017

Fossil shell in 3C 84 as TeV $γ$-ray emitter and cosmic-ray accelerator

We explore physical properties of the shocked external medium (i.e., a shell) in 3C 84 associated with the recurrent radio lobe born around 1960. In the previous work of Ito et al., we investigated a dynamical and radiative evolution of such a shell after the central engine stops the jet launching and we found that a fossil shell emission overwhelms that of the rapidly fading radio lobe. We apply this model to 3C 84 and find the followings: (i) The fossil shell made of shocked diffuse ambient matter with the number density of 0.3 cm$^{-3}$ radiates bright Inverse-Compton (IC) emission with the seed photons of the radio emission from the central compact region and the IC emission is above the sensitivity threshold of the Cherenkov Telescope Array (CTA). (ii) When the fossil shell is produced in a geometrically thick ionized plasma with the number density of $10^{3}$ cm$^{-3}$ and the field strength in the shell may reach about 17 mG in the presence of magnetic fields amplification and the radio emission becomes comparable to the sensitivity of deep imaging VLBI observations. A possible production of ultra high energy cosmic-rays (UHECRs) in the dense shocked plasma is also argued.

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arXiv2017

ALMA polarization observations of the particle accelerators in the hot spot of the radio galaxy 3C 445

We present Atacama Large Millimeter Array (ALMA) polarization observations at 97.5 GHz of the southern hot spot of the radio galaxy 3C 445. The hot spot structure is dominated by two bright components enshrouded by diffuse emission. Both components show fractional polarization between 30 and 40 per cent, suggesting the presence of shocks. The polarized emission of the western component has a displacement of about 0.5 kpc outward with respect to the total intensity emission, and may trace the surface of a front shock. Strong polarization is observed in a thin strip marking the ridge of the hot spot structure visible from radio to optical. No significant polarization is detected in the diffuse emission between the main components, suggesting a highly disordered magnetic field likely produced by turbulence and instabilities in the downstream region that may be at the origin of the extended optical emission observed in this hot spot. The polarization properties support a scenario in which a combination of both multiple and intermittent shock fronts due to jet dithering, and spatially distributed stochastic second-order Fermi acceleration processes are present in the hot spot complex.

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arXiv2017

White Paper on East Asian Vision for mm/submm VLBI: Toward Black Hole Astrophysics down to Angular Resolution of 1~R$_{S}$

This White Paper details the intentions and plans of the East Asian Very Long Baseline Interferometry (VLBI) community for pushing the frontiers of millimeter/submillimeter VLBI. To this end, we shall endeavor to actively promote coordinated efforts in the East Asia region. Our goal is to establish firm collaborations among the East Asia VLBI community in partnership with related institutes in North America and Europe and to expand existing global mm/submm VLBI arrays for (a) exploring the vicinity of black holes with an ultimate angular resolution down to 1~R$_{S}$ (Schwarzschild radius) and (b) investigating the dynamics of circumstellar gas in star-forming regions and late-type stars, and circumnuclear gas around active galactic nuclei (AGNs). In the first half of this White Paper, we highlight scientific accomplishments of the East Asia (EA) VLBI community. Various VLBI research results on M87, Sgr A*, blazars, narrow-line Seyfert~1 galaxies, and compact symmetric objects are described, and future visions of our VLBI science are briefly presented. Maser science of star formation, stellar evolution, and physics of accretion disks around AGNs are also discussed. A new vision for conducting multi-transition maser studies using mm/submm VLBI together with the Atacama Large Millimeter/Submillimeter Array (ALMA) is described. In the second half of this White Paper, we describe the EA community's vision for using mm/submm VLBI arrays in the framework or extended version of the Event Horizon Telescope (EHT) and the Global Millimeter VLBI Array (GMVA). The accomplishment of the aforementioned goal will maximize the overall scientific outcomes of mm/submm VLBI in the world.

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arXiv2016

ALMA Science Verification Data: Millimeter Continuum Polarimetry of the Bright Radio Quasar 3C 286

We present full-polarization observations of the compact, steep-spectrum radio quasar 3C~286 made with the ALMA at 1.3~mm. These are the first full-polarization ALMA observations, which were obtained in the framework of Science Verification. A bright core and a south-west component are detected in the total intensity image, similar to previous centimeter images. Polarized emission is also detected toward both components. The fractional polarization of the core is about 17\%, this is higher than the fractional polarization at centimeter wavelengths, suggesting that the magnetic field is even more ordered in the millimeter radio core than it is further downstream in the jet. The observed polarization position angle (or EVPA) in the core is $\sim$\,$39^{\circ}$, which confirms the trend that the EVPA slowly increases from centimeter to millimeter wavelengths. With the aid of multi-frequency VLBI observations, we argue that this EVPA change is associated with the frequency-dependent core position. We also report a serendipitous detection of a sub-mJy source in the field of view, which is likely to be a submillimeter galaxy.

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arXiv2016

Discovery of off-axis jet structure of TeV blazar Mrk 501 with mm-VLBI

High-resolution millimeter wave very-long-baseline interferometry (mm-VLBI) is an ideal tool for probing the structure at the base of extragalactic jets in detail. The TeV blazar Mrk 501 is one of the best targets among BL Lac objects for studying the nature of off-axis jet structures because it shows different jet position angles at different scales. The aim of this study is to investigate the properties of the off-axis jet structure through high-resolution mm-VLBI images at the jet base and physical parameters such as kinematics, flux densities, and spectral indices. We performed Very Long Baseline Array (VLBA) observations over six epochs from 2012 February to 2013 February at 43 GHz. Quasi-simultaneous Global Millimeter VLBI Array (GMVA) observations at 86 GHz were performed in May 2012. We discover a new jet component at the northeast direction from the core in all the images at 43 and 86 GHz. The new component shows the off-axis location from the persistent jet extending to the southeast. The 43 GHz images reveal that the scattering of the positions of the NE component is within ~0.2 mas. The 86 GHz data reveals a jet component located 0.75 mas southeast of the radio core. We also discuss the spectral indices between 43 and 86 GHz, where the northeast component has steeper spectral index and the southeast component has comparable or flatter index than the radio core does.

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arXiv2015

Fossil shell emission in dying radio loud AGNs

We investigate shell emission associated with dying radio loud AGNs. First, based on our recent work by Ito et al. (2015), we describe the dynamical and spectral evolutions of shells after stopping the jet energy injection. We find that the shell emission overwhelms that of the radio lobes soon after stopping the jet energy injection because fresh electrons are continuously supplied into the shell via the forward shock while the radio lobes rapidly fade out without jet energy injection. We find that such fossil shells can be a new class of target sources for SKA telescope. Next, we apply the model to the nearby radio source 3C84. Then, we find that the fossil shell emission in 3C84 is less luminous in radio band while it is bright in TeV gamma-ray band and it can be detectable by CTA.

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arXiv2015

Discovery of a wandering radio jet base after a large X-ray flare in the blazar Markarian 421

We investigate the location of the radio jet bases ("radio cores") of blazars in radio images, and their stationarity by means of dense very long baseline interferometry (VLBI) observations. In order to measure the position of a radio core, we conducted 12 epoch astrometric observation of the blazar Markarian 421 with the VLBI Exploration of Radio Astrometry at 22 GHz immediately after a large X-ray flare, which occurred in the middle of 2011 September. For the first time, we find that the radio core is not stationary but rather changes its location toward 0.5 mas downstream. This angular scale corresponds to the de-projected length of a scale of $10^5$ Schwarzschild radii (Rs) at the distance of Markarian~421. This radio-core wandering may be a new type of manifestation associated with the phenomena of large X-ray flares.

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arXiv2015

Probing Very Early Stage of Radio Source Evolution in NGC 1275 with VERA

We report on VLBI observations of NGC 1275 with VLBI Exploration of Radio Astrometry (VERA) and Very Long Baseline Array (VLBA) during 2008 and 2013 at 15, 22 and 43 GHz. Our observations provide long-term variations in the radio flux and structure of the inner jet in NGC 1275 on sub-parsec scales, particularly at C3 which is a new radio component accountable for the recent active state in radio regime since early-2000s. We found the apparent velocity of C3 was $β_{\rm{app}} = 0.267 \pm 0.007 $, which was consistent with typical velocities of the hot spot and/or radio lobes in the young radio sources. Furthermore, the radio flux and size of C3 had simultaneously increased during September 2007 and May 2012 with keeping optically-thin spectra, suggesting that the particle acceleration occurred inside C3 as generally seen in the hot spots of the radio lobes. Our new observations suggest that C3 is a new-born hot spot and/or radio lobe at a very early stage of its evolution. Our results imply the radio lobes in the radio galaxies might be already formed on sub-parsec scales from the central SMBH and might play an important role to produce $\rm γ$-ray emission in the young radio sources.

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arXiv2015

An Overview of the 2014 ALMA Long Baseline Campaign

A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ~15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from September to late November 2014, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ~350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.

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arXiv2015

A strong radio brightening at the jet base of M87 in the period of the elevated TeV gamma-ray state in 2012

The nearby radio galaxy M87 offers a unique opportunity for exploring the connection between gamma-ray production and jet formation at an unprecedented linear resolution. However, the origin and location of the gamma-rays in this source is still elusive. Based on previous radio/TeV correlation events, the unresolved jet base (radio core) and the peculiar knot HST-1 at 120 pc from the nucleus are proposed as candidate site(s) of gamma-ray production. Here we report our intensive, high-resolution radio monitoring observations of the M87 jet with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012, together with contemporaneous high-energy gamma-ray light curves obtained by the Fermi Large Area Telescope. During this period, an elevated level of the M87 flux is reported at TeV with VERITAS. We detected a remarkable flux increase in the radio core with VERA at 22/43 GHz coincident with the VHE activity. Meanwhile, HST-1 remained quiescent in terms of its flux density and structure in the radio band. These results strongly suggest that the TeV gamma-ray activity in 2012 originates in the jet base within 0.03 pc (projected) from the central black hole.

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