Imaging the immediate vicinity of supermassive black holes (SMBHs) and extracting a BH-spin signature is one of the grand challenges in astrophysics. M87 is known as one of the best targets for ...imaging the BH shadow and it can be partially thick against synchrotron self-absorption (SSA), particularly in a flaring state with a high mass accretion rate. However, little is known about influences of the SSA-thick region on BH shadow images. Here we investigate BH shadow images of M87 at 230 GHz properly taking into account the SSA-thick region. When the BH has a high spin value, the corresponding BH shadow image shows the positional offset between the center of the photon ring and that of the SSA-thick ring at the innermost stable circular orbit (ISCO) due to the frame-dragging effect in the Kerr spacetime. As a result, we find that a dark-crescent structure is generally produced between the photon ring and the SSA-thick ISCO ring in the BH shadow image. The scale size of the dark crescent increases with BH spin: its width reaches up to ∼2 gravitational radius when the BH spin is 99.8% of its maximum value. The dark crescent is regarded as a new signature of a highly spinning BH. This feature is expected to appear in flaring states with relatively high mass accretion rate rather than the quiescent states. We have simulated the image reconstruction of our theoretical image by assuming the current and future Event Horizon Telescope (EHT) array, and have found that the future EHT including space-very long baseline interferometry in 2020s can detect the dark crescent.
We study Faraday rotation in the jet of M87 inside the Bondi radius using eight Very Long Baseline Array data sets, one at 8 GHz, four at 5 GHz, and three at 2 GHz. We obtain Faraday rotation ...measures (RMs) measured across the bandwidth of each data set. We find that the magnitude of RM systematically decreases with increasing distance from 5000 to 200,000 Schwarzschild radii. The data, showing predominantly negative RM signs without significant difference of the RMs on the northern and southern jet edges, suggest that the spatial extent of the Faraday screen is much larger than the jet. We apply models of hot accretion flows, thought to be prevalent in active galactic nuclei with a relatively low luminosity such as M87, and find that the decrease of RM is described well by a gas density profile . This behavior matches the theoretically expected signature of substantial winds, nonrelativistic un-collimated gas outflows from hot accretion flows, which is consistent with the results of various numerical simulations. The pressure profile inferred from the density profile is flat enough to collimate the jet, which can result in gradual acceleration of the jet in a magneto-hydrodynamical process. This picture is in good agreement with the observed gradual collimation and acceleration of the M87 jet inside the Bondi radius. The dominance of negative RMs suggests that the jet and wind axis are misaligned such that the jet emission exposes only one side of the toroidal magnetic fields permeating the winds.
Abstract
We study the collimation and acceleration of jets in the nearby giant radio galaxy NGC 315, using multifrequency Very Long Baseline Array observations and archival High Sensitivity Array and ...Very Large Array data. We find that the jet geometry transitions from a semi-parabolic shape into a conical/hyperbolic shape at a distance of ≈10
5
gravitational radii. We constrain the frequency-dependent position of the core, from which we locate the jet base. The jet collimation profile in the parabolic region is in good agreement with the steady axisymmetric force-free electrodynamic solution for the outermost poloidal magnetic field line anchored to the black hole event horizon on the equatorial plane, similar to the nearby radio galaxies M87 and NGC 6251. The velocity field derived from the asymmetry in brightness between the jet and counterjet shows gradual acceleration up to the bulk Lorentz factor of Γ ∼ 3 in the region where the jet collimation occurs, confirming the presence of the jet acceleration and collimation zone. These results suggest that the jets are collimated by the pressure of the surrounding medium and accelerated by converting Poynting flux to kinetic energy flux. We discover limb brightening of the jet in a limited distance range where the angular resolution of our data is sufficient to resolve the jet transverse structure. This indicates that either the jet has a stratified velocity field of fast-inner and slow-outer layers or the particle acceleration process is more efficient in the outer layer owing to the interaction with the surroundings on parsec scales.
This paper theoretically investigates the relations between the structure of relativistic jets and produced synchrotron images, by using a steady, axisymmetric force-free jet model. We especially ...focus on the limb-brightened jets that are largely symmetric to the jet axes and observed in some active galactic nuclei, such as M87, Mrk 501, Cyg A, and 3C84. We find that symmetrically limb-brightened images can be produced when magnetic field lines of the jet penetrate a fast-spinning black hole (BH), as motivated by the Blandford-Znajek mechanism. On the other hand, jets with magnetic field lines that pass through a slowly spinning BH or the Keplerian accretion disk produce highly asymmetric radio images. In addition, the edge of a counterjet tends to be luminous in the accretion-disk model even for rather small viewing angles, which may be problematic for some observed jets. We also suggest that the site of particle accelerations in relativistic jets can be constrained by fitting the radio images to observations. This kind of study focusing on the jet images far away from the central engine is complementary to those concentrating directly on the innermost region with upcoming data from the Event Horizon Telescope.
ABSTRACT
The supermassive black holes (SMBHs) of narrow-line Seyfert 1 galaxies (NLS1s) are at the lower end of the mass function of active galactic nuclei (AGNs) and reside preferentially in ...late-type host galaxies with pseudobulges, which are thought to be formed by internal secular evolution. On the other hand, the population of radio-loud NLS1s presents a challenge for the relativistic jet paradigm, which states that powerful radio jets are associated exclusively with very high mass SMBHs in elliptical hosts, which are built up through galaxy mergers. We investigated distorted radio structures associated with the nearest gamma-ray-emitting, radio-loud NLS1, 1H 0323+342. This provides supporting evidence for the merger hypothesis based on past optical/near-infrared observations of its host galaxy. The anomalous radio morphology consists of two different structures: the inner curved structure of the currently active jet and an outer linear structure of low-brightness relics. Such coexistence might be indicative of the stage of an established black hole binary with precession before the black holes coalesce in the galaxy merger process. 1H 0323+342 and other radio-loud NLS1s under galaxy interactions may be extreme objects on the evolutionary path from radio-quiet NLS1s to normal Seyfert galaxies with larger SMBHs in classical bulges through mergers and merger-induced jet phases.
We investigated the detailed inner jet structure of M87 using Very Long Baseline Array data at 2, 5, 8.4, 15, 23.8, 43, and 86 GHz, especially focusing on the multi-frequency properties of the radio ...core at the jet base. First, we measured the size of the core region transverse to the jet axis, defined as W sub(c), at each frequency nu, and found a relation between W sub(c) and nu: W sub(c)(nu) alpha nu super(-0.71 + or - 0.05). Then, by combining W sub(c)(nu) and the frequency dependence of the core position r sub(c)(nu), which was obtained in our previous study, we constructed a collimation profile of the innermost jet W sub(c)(r) down to ~10 Schwarzschild radii (R sub(s)) from the central black hole. We found that W sub(c)(r) smoothly connects with the width profile of the outer edge-brightened, parabolic jet and then follows a similar radial dependence down to several tens of R sub(s). Closer to the black hole, the measured radial profile suggests a possible change in the jet collimation shape from the outer parabolic one, where the jet shape tends to become more radially oriented. This result could be related to a magnetic collimation process or/and interactions with surrounding materials at the jet base. The present results shed light on the importance of higher-sensitivity/resolution imaging studies of M87 at 86, 43, and 22 GHz; these studies should be examined more rigorously.
ABSTRACT We report on results from new high-sensitivity, high-resolution 86 GHz (3.5 mm) observations of the jet base in the nearby radio galaxy M87, obtained by the Very Long Baseline Array in ...conjunction with the Green Bank Telescope. The resulting image has a dynamic range exceeding 1500 to 1, the highest ever achieved for this jet at this frequency, resolving and imaging a detailed jet formation/collimation structure down to ∼10 Schwarzschild radii ( ). The obtained 86 GHz image clearly confirms some important jet features known at lower frequencies, i.e., a jet base with a wide opening angle, a limb-brightened intensity profile, a parabola-shape collimation profile and a counter jet. The limb-brightened structure is already well developed at mas ( , projected) from the core, where the corresponding apparent opening angle becomes as wide as ∼100°. The subsequent jet collimation near the black hole evolves in a complicated manner; there is a "constricted" structure at tens of from the core, where the jet cross section is locally shrinking. We suggest that external pressure support from the inner part of the radiatively inefficient accretion flow may be dynamically important in shaping/confining the footprint of the magnetized jet. We also present the first 86 GHz polarimetric experiment using very long baseline interferometry for this source, where a highly polarized (∼20%) feature is detected near the jet base, indicating the presence of a well-ordered magnetic field. As a by-product, we additionally report a 43/86 GHz polarimetric result for our calibrator 3C 273, suggesting an extreme rotation measure near the core.
The M87 jet is extensively examined by utilizing general relativistic magnetohydrodynamic (GRMHD) simulations, as well as the steady axisymmetric force-free electrodynamic (FFE) solution. ...Quasi-steady funnel jets are obtained in GRMHD simulations up to the scale of ∼100 gravitational radii (rg) for various black hole (BH) spins. As is known, the funnel edge is approximately determined by the following equipartitions: (i) the magnetic and rest-mass energy densities and (ii) the gas and magnetic pressures. Our numerical results give an additional factor that they follow the outermost parabolic streamline of the FFE solution, which is anchored to the event horizon on the equatorial plane. We also show that the matter-dominated, nonrelativistic corona/wind plays a dynamical role in shaping the funnel jet into the parabolic geometry. We confirm a quantitative overlap between the outermost parabolic streamline of the FFE jet and the edge of the jet sheath in very long baseline interferometry (VLBI) observations at ∼(101-105)rg, suggesting that the M87 jet is likely powered by the spinning BH. Our GRMHD simulations also indicate a lateral stratification of the bulk acceleration (i.e., the spine-sheath structure), as well as an emergence of knotty superluminal features. The spin characterizes the location of the jet stagnation surface inside the funnel. We suggest that the limb-brightened feature could be associated with the nature of the BH-driven jet, if the Doppler beaming is a dominant factor. Our findings can be examined with (sub)millimeter VLBI observations, giving a clue for the origin of the M87 jet.
Abstract
We carry out general relativistic ray-tracing radiative-transfer calculations to study whether a localized emission from plasma rings created at the stagnation surface in the jet funnel, to ...which we refer as stagnation rings, can explain the ring image of M87* observed by the Event Horizon Telescope (EHT) 2017. The resultant images consist of the direct image of the stagnation rings and the ring images formed via the strong deflection by the black hole (BH) gravity, to which we refer as quasi photon-ring. For the model with the BH spin
a
*
= 0.99, the direct image of the counter-jet and quasi photon-ring are almost coincident to the photon ring with diameter ∼ 40
μ
as, while the approaching jet shows the small ring image inside them. The synthetic observation image assumes that the EHT2017 array is consistent with that observed in M87*, because the array is a bit sparse to detect the inner ring image. This indicates that the ring image in M87* may contain the important feature of the jet bases in addition to the photon ring. We find that forthcoming EHT observations can resolve the stagnation ring image and may enable us to explore the plasma-injection mechanism in the jet funnel.
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