Keiichi Asada

The Structure of the M87 Jet: A Transition from Parabolic to Conical Streamlines

The structure of the M87 jet, from milliarcsecond to arcsecond scales, is extensively investigated, utilizing the images taken with the European VLBI Network, MERLIN, and Very Long Baseline Array. We discover that the jet maintains a parabolic streamline over a range in size scale equal to 105 times the Schwarzschild radius. The jet then transitions into a conical shape farther downstream. This suggests that the magnetohydrodynamic jet is initially subjected to the confinement by the external gas which is dominated by the gravitational influence of the supermassive black hole. Afterward the jet then freely expands with a conical shape. This geometrical transition indicates that the origin of the HST-1 complex may be a consequence of the overcollimation of the jet. Our result suggests that when even higher angular resolution is provided by a future submillimeter very long baseline interferometry experiments, we will be able to explore the origin of active galactic nucleus jets.

Resolved magnetic-field structure and variability near the event horizon of Sagittarius A∗

Magnetic fields near the event horizon Astronomers have long sought to examine a black holes event horizon—the boundary around the black hole within which nothing can escape. Johnson et al. used sophisticated interferometry techniques to combine data from millimeter-wavelength telescopes around the world. They measured polarization just outside the event horizon of Sgr A*, the supermassive black hole at the center of our galaxy, the Milky Way. The polarization is a signature of ordered magnetic fields generated in the accretion disk around the black hole. The results help to explain how black holes accrete gas and launch jets of material into their surroundings. Science, this issue p. 1242 Magnetic fields around the event horizon of a supermassive black hole have been probed. 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 magnetic fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intrahour variability associated with these fields.

THE PARABOLIC JET STRUCTURE IN M87 AS A MAGNETOHYDRODYNAMIC NOZZLE

The structure and dynamics of the M87 jet from sub-milliarcsec to arcsecond scales are continuously examined. We analyzed the Very Long Baseline Array archival data taken at 43 and 86 GHz to measure the size of very long baseline interferometry (VLBI) cores. Millimeter/sub-millimeter VLBI cores are considered as innermost jet emissions, which has been originally suggested by Blandford & Konigl. Those components fairly follow an extrapolated parabolic streamline in our previous study so that the jet has a single power-law structure with nearly 5 orders of magnitude in the distance starting from the vicinity of the supermassive black hole (SMBH), less than 10 Schwarzschild radius (r s). We further inspect the jet parabolic structure as a counterpart of the magnetohydrodynamic (MHD) nozzle in order to identify the property of a bulk acceleration. We interpret that the parabolic jet consists of Poynting-flux dominated flows, powered by large-amplitude, nonlinear torsional Alfven waves. We examine the non-relativistic MHD nozzle equation in a parabolic shape. The nature of trans-fast magnetosonic flow is similar to the one of transonic solution of Parkers hydrodynamic solar wind; the jet becomes super-escape as well as super-fast magnetosonic at around ~103 r s, while the upstream trans-Alfvenic flow speed increases linearly as a function of the distance at ~102-103 r s. We here point out that this is the first evidence to identify these features in astrophysical jets. We propose that the M87 jet is magnetically accelerated, but thermally confined by the stratified interstellar medium inside the sphere of gravitational influence of the SMBH potential, which may be a norm in active galactic nucleus jets.

230 GHz VLBI Observations of M87: Event-horizon-scale Structure during an Enhanced Very-high-energy 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 ◦ 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 � 1 × 10 10 K derived from the compact flux density of � 1 Jy and the angular size of � 40 µas � 5.5 Rs, which is broadly consistent with the peak brightness of the radio cores at 1-86 GHz located within � 10 2 Rs. Our observations occurred in the middle of an enhancement in very-high-energy (VHE) -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 �20-60 Rs. Subject headings: galaxies: active —galaxies: individual (M87) —galaxies: jets —radio continuum: galaxies —techniques: high angular resolution —techniques: interferometric

Time Variation of the Rotation Measure Gradient in the 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.

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.

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 ± 0.11)c (H0 = 72 km s-1 Mpc-1), which results in a kinematic age of 2200 ± 700 yr. Assuming that two hot spots are moving apart at equal speeds, we derive an advance speed of (0.17 ± 0.06)c. 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 that 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 ~300 yr, assuming the equipartition magnetic field. This requires the advance speed of 0.26c, which shows a good agreement of the hot-spot motion of (0.17 ± 0.06)c. Our results strongly support the hypothesis that CSOs are young radio sources.

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 is known to have the highest radio loudnessin thenarrow-line Seyfert 1 galaxies(NLS1s), at 1.7–15.4GHz withthe VeryLong BaselineArray (VLBA). This is the first very-long-baseline interferometry (VLBI) investigation of a radio-loud NLS1. We independently discovered (1) very high brightness temperatures from its compactness in a VLBA image and (2) flux variation among the VLBA observation and 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 important parameters in the determination of the radio loudness of NLS1s. The accretion disk of SDSS J094857.3+002225 is probably in a 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.

VERY LONG BASELINE ARRAY IMAGING OF PARSEC-SCALE RADIO EMISSIONS IN NEARBY RADIO-QUIET NARROW-LINE SEYFERT 1 GALAXIES

We conducted Very Long Baseline Array (VLBA) observations of seven nearby narrow-line Seyfert 1 (NLS1) galaxies at 1.7 GHz (λ18 cm) with milliarcsecond resolution. This is the first systematic very long baseline interferometry study focusing on the central parsec-scale regions of radio-quiet NLS1s. Five of the seven were detected at a brightness temperature of 5 × 10 6 K and contain radio cores with high brightness temperatures of >6 × 10 7 K, indicating a nonthermal process driven by jet-producing central engines as in radio-loud NLS1s and other active galactic nucleus classes. VLBA images of MRK 1239, MRK 705, and MRK 766 exhibit parsec-scale jets with clear linear structures. A large portion of the radio power comes from diffuse emission components that are distributed within the nuclear regions (300 pc), which is a common characteristic throughout the observed NLS1s. Jet kinetic powers limited by the Eddington limit may be insufficient to allow the jets to escape to kiloparsec scales for these radio-quiet NLS1s with low-mass black holes of 10 7 M� .

MULTI-FREQUENCY POLARIMETRY TOWARD S5 0836+710: A POSSIBLE SPINE-SHEATH STRUCTURE FOR THE JET

We perform multi-frequency polarimetry toward 0836+710 using data from the Very Long Baseline Array. These observations allow us to measure both the distributions of the polarization position angle and the Faraday rotation measure (RM). We find a systematic gradient in the RM distribution as has been reported in several observations of relativistic jets emanating from active galactic nuclei. The RM corresponds to the line-of-sight component of the magnetic field. Thus, a systematic gradient of the RM along the transverse direction of the jet implies the existence of helical magnetic components associated with the jet itself. We derive the pitch angle of the helical magnetic field independently from the distribution of the projected magnetic field and from the RM data. Their discrepancies can be understood in a spine/sheath structure of the jet.