ABSTRACT M87 is arguably the best supermassive black hole (BH) to explore jet and/or accretion physics, due to its proximity and fruitful high-resolution multi-waveband observations. We model the ...multi-wavelength spectral energy distribution (SED) of the M87 core that observed at a scale of 0.4 arcsec (∼105Rg, Rg is gravitational radius), as recently presented by Prieto et al. Similar to Sgr A*, we find that the millimeter bump as observed by the Atacama Large Millimeter/submillimeter Array can be modeled by the synchrotron emission of the thermal electrons in an advection-dominated accretion flow (ADAF), while the low-frequency radio emission and X-ray emission may predominantly come from the jet. The millimeter radiation from ADAF predominantly comes from the region within 10Rg, which is roughly consistent with the recent very long baseline interferometry observations at 230 GHz. We further calculate the Faraday rotation measure (RM) from both ADAF and jet models, and find that the RM predicted from the ADAF is roughly consistent with the measured value, while the RM predicted from the jet is much higher if jet velocity close to the BH is low or moderate (e.g., vjet 0.6 c). With the constraints from the SED modeling and RM, we find that the accretion rate close to the BH horizon is ( is Bondi accretion rate), where the electron density profile, ne ∝ r∼−1, in the accretion flow, is consistent with that determined from X-ray observation inside the Bondi radius and recent numerical simulations.
Abstract
The (sub)millimeter wavelengths (86–690 GHz) very long baseline interferometry will provide ∼5–40
μ
as angular resolution, ∼10 mJy baseline sensitivity, and ∼1
μ
as yr
−1
proper-motion ...precision, which can directly detect supermassive black hole binary (SMBHB) systems by imaging the two visible sources and tracking their relative motions. Such a way exhibits an advantage compared to indirect detect methods of observing periodic signals in motion and light curves, which are difficult to confirm from competing models. Moreover, tracking relative motion at (sub)millimeter wavelengths is more reliable, as there is a negligible offset between the emission region and the black hole center. In this way, it is unnecessary to correct the black hole location by a prior of jet morphology as it would be required at longer wavelengths. We extend the formalism developed in D’Orazio & Loeb (2018) to link the observations with the orbital evolution of SMBHBs from the ≲10 kpc dynamical friction stages to the ≲0.01 pc gravitational radiation stages, and estimate the detectable numbers of SMBHBs. By assuming 5% of active galactic nuclei holding SMBHBs, we find that the number of detectable SMBHBs with redshift
z
≤ 0.5 and mass
M
≤ 10
11
M
⊙
is about 20. Such a detection relies heavily on proper-motion precision and sensitivity. Furthermore, we propose that the simultaneous multifrequency technique plays a key role in meeting the observational requirements.
Abstract
We report multifrequency VLBI studies of the subparsec scale structure of the two-sided jets in the nearby radio galaxy NGC 4261. Our analyses include new observations using the source ...frequency phase referencing technique with the Very Long Baseline Array at 44 and 88 GHz, as well as archival data at 15 and 43 GHz. Our results show an extended double-sided structure at 43/44 GHz and provide a clear image of the nuclear region at 88 GHz, showing a core size of ∼0.09 mas and a brightness temperature of ∼1.3 × 10
9
K. Proper motions are measured for the first time in the two-sided jets, with apparent speeds ranging from 0.31 ± 0.14 c to 0.59 ± 0.40 c in the approaching jet and 0.32 ± 0.14 c in the receding jet. The jet-to-counterjet brightness ratio allows us to constrain the viewing angle to between ∼54° and 84° and the intrinsic speed to between ∼0.30 c and 0.55 c. We confirm the parabolic shape of the upstream jet on both sides of the central engine, with a power-law index of 0.56 ± 0.07. Notably, the jet collimation is found to be already completed at subparsec scales, with a transition location of about 0.61 pc, which is significantly smaller than the Bondi radius of 99.2 pc. This behavior can be interpreted as the initial confinement of the jet by external pressure from either the geometrically thick, optically thin advection-dominated accretion flows or the disk wind launched from it. Alternatively, the shape transition may also be explained by the internal flow transition from a magnetically dominated to a particle-dominated regime.
Abstract We report multifrequency and multiepoch very long baseline interferometry studies of the subparsec jet in the Sombrero galaxy (M104, NGC 4594). Using Very Long Baseline Array data at 12, 22, ...44, and 88 GHz, we study the kinematics of the jet and the properties of the compact core. The subparsec jet is clearly detected at 12 and 22 GHz, and the inner jet base is resolved down to ∼70 Schwarzschild radii ( R s ) at 44 GHz. The proper motions of the jet are measured with apparent subrelativistic speeds of 0.20 ± 0.08 c and 0.05 ± 0.02 c for the approaching and the receding jet, respectively. Based on the apparent speed and jet-to-counterjet brightness ratio, we estimate the jet viewing angle to be larger than ∼37°, and the intrinsic speed to be between ∼0.10 c and 0.40 c . Their joint probability distribution suggests the most probable values of the viewing angle and intrinsic speed to be 66 ° − 6 ° + 4 ° and 0.19 ± 0.04 c , respectively. We also find that the measured brightness temperatures of the core at 12, 22, and 44 GHz are close to the equipartition brightness temperature, indicating that the energy density of the radiating particles is comparable to the energy density of the magnetic field in the subparsec jet region. Interestingly, the measured core size at 88 GHz (∼25 ± 5 R s ) deviates from the expected frequency dependence seen at lower frequencies. This may indicate a different origin for the millimeter emission, which can be explained by an advection-dominated accretion flow (ADAF) model. This model further predicts that at 230 and 340 GHz, the ADAF may dominate the radio emission over the jet.
Abstract
We report on the first pulsar and transient survey of the Galactic Center (GC) with the Atacama Large Millimeter/submillimeter Array (ALMA). The observations were conducted during the Global ...Millimeter VLBI Array campaign in 2017 and 2018. We carry out searches using time series of both total intensity and other polarization components in the form of Stokes parameters. We incorporate acceleration and its derivative in the pulsar search, and also search in segments of the entire observation to compensate for potential orbital motion of the pulsar. While no new pulsar is found, our observations yield the polarization profile of the GC magnetar PSR J1745−2900 at millimeter wavelength for the first time, which turns out to be nearly 100% linearly polarized. Additionally, we estimate the survey sensitivity placed by both system and red noise, and evaluate its capability of finding pulsars in orbital motion with either Sgr A* or a binary companion. We show that the survey is sensitive to only the most luminous pulsars in the known population and future observations with ALMA in Band-1 will deliver significantly deeper survey sensitivity on the GC pulsar population.
Abstract
Observational studies of the innermost regions of the edge-on jets in nearby active galactic nuclei (AGN) are crucial to understanding their kinematics and morphology. For the inner jet of ...the nearby low-luminosity AGN in M84, we present new high-sensitivity observations with very long baseline Interferometry since 2019, as well as archival Very Long Baseline Array observations in 2014. We find that the compact core in M84 has an inverted-to-flat spectrum from 1.5–88 GHz. Based on the turnover frequency of 4.2 ± 0.2 GHz in the spectrum, we estimated a magnetic field strength of 1 ∼ 10 mG and an electron number density of ∼10
5
cm
−3
in the core region. Three inner jet components within ∼3 mas of the core are identified and traced in the images at 22 GHz, whose apparent speeds are 0.11
c
, 0.27
c
, and 0.32
c
, respectively. We calculate the viewing angle of
58
−
18
+
17
°
for the inner jet based on the proper motion and the flux ratio of jet to counterjet. A propagating sinusoidal model with a wavelength of ∼3.4 mas is used to fit the helical morphology of the jet extended to 20 mas (∼2.2 × 10
4
Schwarzschild radii).
ABSTRACT
The Event Horizon Telescope (EHT), with ∼20 $\mu$ as high angular resolution, recently resolved the millimetre image of the suppermassive black hole in the Galaxy, Sagittarius A∗. This opens ...a new window to study the plasma on horizon scales. The accreting disc probably contains a small fraction of non-thermal electrons and their emissions should contribute to the observed image. We study if such contributions are sufficient to cause structural differences detectable by current and future observational capabilities. We introduce non-thermal electrons in a semi-analytical accretion disc, which considers viscosity-leading heating processes, and adopt a continued hybrid electron energy distribution of thermal distribution and power-law tail. We generate the black hole images and extract the structural features as crescent parameters. We find the existence of non-thermal electron radiation makes the crescent much brighter, slightly larger, moderately thicker, and much more symmetric. When the non-thermal connecting Lorentz factor γc = 65, which is equivalent to the non-thermal electrons accounting for ∼1.5 per cent of the totals, non-thermal effects cause ∼2 per cent size difference at 230 GHz. Comparing with the structural changes caused by other physical factors, including inclination between the system and the observer, black hole spin, and interstellar medium scattering effects, we find that although non-thermal electron radiation takes the most unimportant role at 230 GHz, it becomes more significant at 345 GHz.
ABSTRACT The black hole in the center of the Galaxy, associated with the compact source Sagittarius A* (Sgr A*), is predicted to cast a shadow upon the emission of the surrounding plasma flow, which ...encodes the influence of general relativity (GR) in the strong-field regime. The Event Horizon Telescope (EHT) is a Very Long Baseline Interferometry (VLBI) network with a goal of imaging nearby supermassive black holes (in particular Sgr A* and M87) with angular resolution sufficient to observe strong gravity effects near the event horizon. General relativistic magnetohydrodynamic (GRMHD) simulations show that radio emission from Sgr A* exhibits variability on timescales of minutes, much shorter than the duration of a typical VLBI imaging experiment, which usually takes several hours. A changing source structure during the observations, however, violates one of the basic assumptions needed for aperture synthesis in radio interferometry imaging to work. By simulating realistic EHT observations of a model movie of Sgr A*, we demonstrate that an image of the average quiescent emission, featuring the characteristic black hole shadow and photon ring predicted by GR, can nonetheless be obtained by observing over multiple days and subsequent processing of the visibilities (scaling, averaging, and smoothing) before imaging. Moreover, it is shown that this procedure can be combined with an existing method to mitigate the effects of interstellar scattering. Taken together, these techniques allow the black hole shadow in the Galactic center to be recovered on the reconstructed image.
The Event Horizon Telescope (EHT) is a project to assemble a Very Long Baseline Interferometry (VLBI) network of millimeter wavelength dishes that can resolve strong field general relativistic ...signatures near a supermassive black hole. As planned, the EHT will include enough dishes to enable imaging of the predicted black hole "shadow," a feature caused by severe light bending at the black hole boundary. The center of M87, a giant elliptical galaxy, presents one of the most interesting EHT targets as it exhibits a relativistic jet, offering the additional possibility of studying jet genesis on Schwarzschild radius scales. Fully relativistic models of the M87 jet that fit all existing observational constraints now allow horizon-scale images to be generated. We perform realistic VLBI simulations of M87 model images to examine the detectability of the black shadow with the EHT, focusing on a sequence of model images with a changing jet mass load radius. When the jet is launched close to the black hole, the shadow is clearly visible both at 230 and 345 GHz. The EHT array with a resolution of 20-30 mu as resolution (~2-4 Schwarzschild radii) is able to image this feature independent of any theoretical models and we show that imaging methods used to process data from optical interferometers are applicable and effective for EHT data sets. We demonstrate that the EHT is also capable of tracing real-time structural changes on a few Schwarzschild radii scales, such as those implicated by very high-energy flaring activity of M87. While inclusion of ALMA in the EHT is critical for shadow imaging, the array is generally robust against loss of a station.
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