Kazunori Akiyama

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.

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.

MAGNETIZATION DEGREE AT THE JET BASE OF M87 DERIVED FROM THE EVENT HORIZON TELESCOPE DATA: TESTING THE MAGNETICALLY DRIVEN JET PARADIGM

We explore the degree of magnetization at the jet base of M87 by using the observational data of the event horizon telescope (EHT) at 230 GHz obtained by Doeleman et al. By utilizing the method in Kino et al., we derive the energy densities of magnetic fields (UB) and electrons and positrons (U±) in the compact region detected by EHT (EHT-region) with its full-width-half-maximum size 40 µas. First, we assume that an optically-thick region for synchrotron self absorption (SSA) exists in the EHT-region. Then, we find that the SSA-thick region should not be too large not to overproduce the Poynting power at the EHT-region. The allowed ranges of the angular size and the magnetic field strength of the SSA-thick region are 21 µas ≤ θthick ≤ 26.3 µas and 50 G ≤ Btot ≤ 124 G, respectively. Correspondingly UB ≫ U± is realized in this case. We further examine the composition of plasma and energy density of protons by utilizing the Faraday rotation measurement (RM) at 230 GHz obtained by Kuo et al. Then, we find that UB ≫ U± + Up still holds in the SSA-thick region. Second, we examine the case when EHT-region is fully SSA-thin. Then we find that UB ≫ U± still holds unless protons are relativistic. Thus, we conclude that magnetically driven jet scenario in M87 is viable in terms of energetics close to ISCO scale unless the EHT-region is fully SSA-thin and relativistic protons dominated. Subject headings: galaxies: active — galaxies: jets — radio continuum: galaxies —black hole physics —radiation mechanisms: non-thermal

MODELING SEVEN YEARS OF EVENT HORIZON TELESCOPE OBSERVATIONS WITH RADIATIVELY INEFFICIENT ACCRETION FLOW MODELS

An initial three-station version of the Event Horizon Telescope, a millimeter-wavelength very-long baseline interferometer, has observed Sagittarius A* (Sgr A*) repeatedly from 2007 to 2013, resulting in the measurement of a variety of interferometric quantities. Of particular importance, there is now a large set of closure phases, measured over a number of independent observing epochs. We analyze these observations within the context of a realization of semi-analytic radiatively inefficient disk models, implicated by the low luminosity of Sgr A*. We find a broad consistency among the various observing epochs and between different interferometric data types, with the latter providing significant support for this class of models of Sgr A*. The new data significantly tighten existing constraints on the spin magnitude and its orientation within this model context, finding a spin magnitude of

INTERSTELLAR SCINTILLATION AND THE RADIO COUNTERPART OF THE FAST RADIO BURST FRB 150418

a=0.10^{+0.30+0.56}_{-0.10-0.10}

Pilot KaVA monitoring on the M 87 jet: Confirming the inner jet structure and superluminal motions at sub-pc scales

, an inclination with respect to the line of sight of

PRECL: A new method for interferometry imaging from closure phase

\theta={60^\circ}^{+5^\circ+10^\circ}_{-8^\circ-13^\circ}

Detection of Intrinsic Source Structure at ~3 Schwarzschild Radii with Millimeter-VLBI Observations of SAGITTARIUS A*

, and a position angle of

The ALMA Phasing System: A Beamforming Capability for Ultra-high-resolution Science at (Sub)Millimeter Wavelengths

\xi={156^\circ}^{+10^\circ+14^\circ}_{-17^\circ-27^\circ}

Superresolution full-polarimetric imaging for radio interferometry with sparse modeling

east of north. These are in good agreement with previous analyses. Notably, the previous