arXiv subjects

Hiroshi Nagai

Publications and source records attributed to Hiroshi Nagai.

85 records · Page 5Linked to original sources

arXiv2011

Exploring the central sub-pc region of the γ-ray bright radio galaxy 3C 84 with the VLBA at 43 GHz in the period of 2002-2008

Following the discovery of a new radio component right before the GeV γ-ray detection since 2008 August by Fermi Gamma-ray Space Telescope, we present a detailed study of the kinematics and lightcurve on the central sub-pc scale of 3C 84 using the archival VLBA 43-GHz data covering the period between 2002 January to 2008 November. We find that the new component "C3", previously reported by the observations with the VLBI Exploration of Radio Astrometry (VERA), was already formed in 2003. The flux density of C3 increases moderately until 2008, and then it becomes brighter rapidly after 2008. The radio core, C1, also shows a similar trend. The apparent speed of C3 with reference to the core C1 shows moderate acceleration from 0.10c to 0.47c between 2003 November to 2008 November, but is still sub-relativistic. We further try to fit the observed broadband spectrum by the one-zone synchrotron self-Compton (SSC) model using the measured apparent speed of C3. The fit can reproduce the observed γ-ray emission, but does not agree with the observed radio spectral index between 22 and 43 GHz.

astro-ph.HE↗

arXiv2011

Millimeter Radio Continuum Emissions as the Activity of Super Massive Black Holes in Nearby Early-Type Galaxies and Low-Luminosity Active Galactic Nuclei

We conducted millimeter continuum observations for samples of nearby early-type galaxies (21 sources) and nearby low-luminosity active galactic nuclei (LLAGN; 16 sources) at 100 GHz (3 mm) using the Nobeyama Millimeter Array (NMA). In addition, we performed quasi-simultaneous observations at 150 GHz (2 mm) and 100 GHz for five LLAGNs. Compact nuclear emissions showing flat or inverted spectra at centimeter-to-millimeter wavelengths were found in many LLAGNs and several early-type galaxies. Moreover, significant flux variability was detected in three LLAGNs. These radio properties are similar to Sgr A*. The observed radio luminosities are consistent with the fundamental plane of black hole activity that has suggested on the basis of samples with black hole masses ranging from 10 to 10^10 M_sun. This implies nuclear jets powered by radiatively inefficient accretion flows onto black holes.

astro-ph.CO↗

arXiv2011

Very Long Baseline Array Imaging of Parsec-scale Jet Structures in Radio-loud Narrow-line Seyfert 1 Galaxies

We conducted very long baseline interferometry (VLBI) observations of five radio-loud narrow-line Seyfert 1 (NLS1) galaxies in milliarcsecond resolutions at 1.7 GHz (18 cm) using the Very Long Baseline Array (VLBA). Significant parsec-scale structures were revealed for three out of the five sources with high brightness temperature by direct imaging; this is convincing evidence for nonthermal jets. FBQS J1644+2619 with an inverted spectrum showed a prominent one-sided linear structure, indicating Doppler beaming with an intrinsic jet speed of >0.74c. FBQS J1629+4007, also with an inverted spectrum, showed rapid flux variability, indicating Doppler beaming with an intrinsic jet speed of >0.88c. Thus, we found convincing evidence that these two NLS1s can generate at least mildly or highly relativistic jets, which may make them apparently radio loud even if they are intrinsically radio quiet. On the other hand, the other three NLS1s had steep spectra and two of them showed significantly diffuse pc-scale structures, which were unlikely to be strongly beamed. Thus, some NLS1s have the ability to generate jets strong enough to make them intrinsically radio loud without Doppler beaming. NLS1s as a class show a number of extreme properties and radio-loud ones are very rare. We build on these radio results to understand that the central engines of radio-loud NLS1s are essentially the same as that of other radio-loud AGNs in terms of the formation of nonthermal jets.

astro-ph.CO↗

arXiv2010

VLBI Monitoring of 3C 84 (NGC 1275) in Early Phase of the 2005 Outburst

Multi-epoch Very Long Baseline Interferometry (VLBI) study of the sub-pc scale jet of 3C 84 is presented. 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. We confirmed that the outburst was associated with the central ~1 pc core, accompanying the emergence of a new component. This is striking evidence of the recurrence of jet activity. The new component became brighter during 2008, in contrast to the constant gamma-ray emission that was observed with the Fermi Gamma-ray Space Telescope during the same time. We found that the projected speed of the new component is 0.23c from 2007/297 (2007 October 24) to 2009/114 (2009 April 24). The direction of movement of this component differs from that of the pre-existing component by ~40 degree. This is the first measurement of kinematics of a sub-pc jet in a gamma-ray active phase. Possible detection of jet deceleration and the jet kinematics in connection with the gamma-ray emission is discussed.

astro-ph.HE↗

arXiv2009

Suzaku observation of the giant radio galaxy 3C 326

A Suzaku observation of a giant radio galaxy, 3C 326, which has a physical size of about 2 Mpc, was conducted on 2008 January 19 -- 21. In addition to several X-ray sources, diffuse emission was significantly detected associated with its west lobe, but the east lobe was contaminated by an unidentified X-ray source WARP J1552.4+2007. After careful evaluation of the X-ray and Non X-ray background, the 0.4 -- 7 keV X-ray spectrum of the west lobe is described by a power-law model. The photon index and 1 keV flux density was derived as $1.82_{-0.24}^{+0.26}\pm0.04$ and $19.4_{-3.2}^{+3.3}\pm 3.0$ nJy, respectively, where the first and second errors represent the statistical and systematic ones. The diffuse X-rays were attributed to be inverse Compton radiation by the synchrotron radio electrons scattering off the cosmic microwave background photons. This radio galaxy is the largest among those with lobes detected through inverse Compton X-ray emission. A comparison of the radio to X-ray fluxes yields the energy densities of electron and magnetic field as $u_e = (2.3 \pm 0.3 \pm 0.3) \times 10^{-13}$ ergs/cm3 and $u_m = (1.2_{-0.1}^{+0.2}\pm 0.2) \times 10^{-14}$ ergs/cm3, respectively. The galaxy is suggested to host a low luminosity nucleus with an absorption-corrected 2 -- 10 keV luminosity of $<2 \times 10^{42}$ ergs/s, together with a relatively weak radio core. The energetics in the west lobe of 3C 326 were compared with those of moderate radio galaxies with a size of $\sim 100$ kpc. The west lobe of 3C 326 is confirmed to agree with the correlations for the moderate radio galaxies, $u_e \propto D^{-2.2\pm0.4}$ and $u_m \propto D^{-2.4\pm0.4}$, where $D$ is their total physical size. This implies that the lobes of 3C 326 are still being energized by the jet, despite the current weakness of the nucleus.

astro-ph.HE↗

arXiv2009

On the origin of Fanaroff-Riley classification of radio galaxies: Deceleration of supersonic radio lobes

We argue that the origin of "FRI/FRI{-.1em}I dichotomy" -- the division between Fanaroff-Riley class I (FRI) with subsonic lobes and class I{-.1em}I (FRI{-.1em}I) radio sources with supersonic lobes is sharp in the radio-optical luminosity plane (Owen-White diagram) -- can be explained by the deceleration of advancing radio lobes. The deceleration is caused by the growth of the effective cross-sectional area of radio lobes. We derive the condition in which an initially supersonic lobe turns into a subsonic lobe, combining the ram-pressure equilibrium between the hot spots and the ambient medium with the relation between "the hot spot radius" and "the linear size of radio sources" obtained from the radio observations. We find that the dividing line between the supersonic lobes and subsonic ones is determined by the ratio of the jet power $L_{\rm j}$ to the number density of the ambient matter at the core radius of the host galaxy $\bar{n}_{\rm a}$. It is also found that there exists the maximal ratio of $(L_{\rm j}/\bar{n}_{\rm a})$ and its value resides in $(L_{\rm j}/\bar{n}_{\rm a})_{\rm max}\approx 10^{44-47} {\rm erg} {\rm s}^{-1} {\rm cm}^{3}$, taking account of considerable uncertainties. This suggests that the maximal value $(L_{\rm j}/\bar{n}_{\rm a})_{\rm max}$ separates between FRIs and FRI{-.1em}Is.

astro-ph.GA↗

arXiv2008

Fate of baby radio galaxies: Dead or Alive ?

In order to reveal the long-term evolution of relativistic jets in active galactic nuclei (AGNs), we examine the dynamical evolution of variously-sized radio galaxies [i.e., compact symmetric objects (CSOs), medium-size symmetric objects (MSOs), Fanaroff-Riley type II radio galaxies (FRIIs)]. By comparing the observed relation between the hot spot size and the linear size of radio source with a coevolution model of hot spot and cocoon, we find that the advance speed of hot spots and lobes inevitably show the deceleration phase (CSO-MSO phase) and the acceleration phase (MSO-FRII phase). The deceleration is caused by the growth of the cross-sectional area of the cocoon head. Moreover, by comparing the hot spot speed with the sound speed of the ambient medium, we predict that only CSOs whose initial advance speed is higher than 0.3-0.5c can evolve into FRIIs.

astro-ph↗

arXiv2008

The Fate of Young Radio Galaxies: Decelerations Inside Host Galaxies?

We examine the evolution of variously-sized radio galaxies [i.e., compact symmetric objects (CSOs), medium-size symmetric objects (MSOs), Fanaroff-Riley type II radio galaxies (FRIIs)], by comparing the relation between the hot spot size and the projected linear size with a coevolution model of hot spots and a cocoon. We take account of the deceleration effect by the cocoon head growth. We find that the advance speed of hot spots and lobes inevitably show the deceleration phase (CSO-MSO phase) and the acceleration phase (MSO-FRII phase). This is ascribed to the change of the power-law index of ambient density profile in the MSO phase ($\sim $1 kpc). It is also found that the cocoon shape becomes nearly spherical or disrupted for MSOs, while an elongated morphology is predicted for CSOs and FRIIs. This seems to be consistent with the higher fraction of distorted morphology of MSOs than that of CSOs and FRI. Finally, we predict that only CSOs whose initial advance speed is higher than about 0.1c can evolve into FRIIs, comparing the hot spot speed with the sound speed of the ambient medium.

astro-ph↗

arXiv2008

Time Variation of Rotation Measure Gradient in 3C 273 Jet

The existence of a gradient in the Faraday rotation measure (RM) of the quasar 3C 273 jet is confirmed by follow-up observations. A gradient transverse to the jet axis is seen for more than 20 mas in projected distance. Taking account of the viewing angle, we estimate it to be more than 100 pc. Comparing to the distribution of the RM in 1995, we detect a time variation of it at the same distance from the core over 7 yr. We discuss the origin of the Faraday rotation based on this rapid time variation. We rule out foreground media such as a narrow-line region, and suggest a helical magnetic field in the sheath region as the origin of this gradient of the RM.

astro-ph↗

arXiv2008

Multifrequency Polarimetry of the Nrao 140 Jet: Possible Detection of a Helical Magnetic Field and Constraints on its Pitch Angle

We present results from multifrequency polarimetry of NRAO 140 using the Very Long Baseline Array. These observations allow us to reveal the distributions of both the polarization position angle and the Faraday rotation measure (RM). These distributions are powerful tools to discern the projected and line-of-sight components of the magnetic field, respectively. We find a systematic gradient in the RM distribution, with its sign being opposite at either side of the jet with respect to the jet axis. The sign of the RM changes only with the direction of the magnetic field component along the line of sight, so this can be explained by the existence of helical magnetic components associated with the jet itself. We derive two constraints for the pitch angle of the helical magnetic field from the distributions of the RM and the projected magnetic field; the RM distribution indicates that the helical fields are tightly wound, while that of the projected magnetic field suggests they are loosely wound around the jet axis. This inconsistency may be explained if the Faraday rotator is not cospatial with theemitting region. Our results may point toward a physical picture in which an ultra-relativistic jet (spine) with a loosely wound helical magnetic field is surrounded by a sub-relativistic wind layer (sheath) with a tightly wound helical magnetic field.

astro-ph↗

arXiv2007

Japanese VLBI Network observations of radio-loud narrow-line Seyfert 1 galaxies

We performed phase-reference very long baseline interferometry (VLBI) observations on five radio-loud narrow-line Seyfert 1 galaxies (NLS1s) at 8.4 GHz with the Japanese VLBI Network (JVN). Each of the five targets (RXS J08066+7248, RXS J16290+4007, RXS J16333+4718, RXS J16446+2619, and B3 1702+457) in milli-Jansky levels were detected and unresolved in milli-arcsecond resolutions, i.e., with brightness temperatures higher than 10^7 K. The nonthermal processes of active galactic nuclei (AGN) activity, rather than starbursts, are predominantly responsible for the radio emissions from these NLS1s. Out of the nine known radio-loud NLS1s, including the ones chosen for this study, we found that the four most radio-loud objects exclusively have inverted spectra. This suggests a possibility that these NLS1s are radio-loud due to Doppler beaming, which can apparently enhance both the radio power and the spectral frequency.

astro-ph↗

arXiv2006

VLBI observations of the most radio-loud, narrow-line quasar SDSS J094857.3+002225

We observed the narrow-line quasar SDSS J094857.3+002225, which has the highest known radio loudness for a narrow-line Seyfert~1 galaxy (NLS1), at 1.7--15.4 GHz with the Very Long Baseline Array (VLBA). This is the first very-long-baseline interferometry (VLBI) investigation for a radio-loud NLS1. We independently found very high brightness temperatures from (1) its compactness in a VLBA image and (2) flux variation among the VLBA observation, our other observations with the VLBA, and the Very Large Array (VLA). A Doppler factor larger than 2.7--5.5 was required to meet an intrinsic limit of brightness temperature in the rest frame. This is evidence for highly relativistic nonthermal jets in an NLS1. We suggest that the Doppler factor is one of the most crucial parameters determining the radio loudness of NLS1s. The accretion disk of SDSS J094857.3+002225 is probably in the very high state, rather than the high/soft state, by analogy with X-ray binaries with strong radio outbursts and superluminal jets such as GRS 1915+105.

astro-ph↗

arXiv2006

The Kinematic and Spectral Ages of the Compact Radio Source CTD 93

We present a study of the kinematic and spectral ages of the Gigahertz-Peaked Spectrum (GPS) source CTD~93. Measurements of the hot spot separation over 8.5 yr show evidence of an increase. The separation rate along the source axis is 0.34$\pm0.11c$ (H$_{0}$=72 km s$^{-1}$ Mpc$^{-1}$), which results in a kinematic age of 2200$\pm$700 yr. Assuming that two hot spots are moving apart at equal speeds, we derive an advance speed of 0.17$\pm0.06c$. The radio lobe spectra show a high frequency steepening, as expected if energetic electrons lose energy by synchrotron radiation. The spectral break decreases with the distance from the hot spot in the northern component of CTD~93. This tendency is expected from the basic scenario of radio lobe evolution involving particle acceleration at the hot spots, with the radio lobes populated by high energy electrons which have leaked from the hot spots. Although a core-jet morphology for CTD~93 has previously been proposed, these results indicate that the morphology is similar to that of Compact Symmetric Object (CSO). From the spectral fits in the northern component we found a break frequency of 3.7 GHz at the edge of the lobe. The resultant spectral age is $\sim$300 yr assuming the equipartition magnetic field. This requires the advance speed of 0.26$c$, which shows a good agreement of the hot spot motion of 0.17$\pm0.06c$. Our results strongly support the hypothesis that CSOs are young radio sources.

astro-ph↗