104 research outputs found
Expanding Sgr A* dynamical imaging capabilities with an African extension to the Event Horizon Telescope
The Event Horizon Telescope (EHT) has recently published the first images of
the supermassive black hole at the center of our Galaxy, Sagittarius A* (Sgr
A*). Imaging Sgr A* is plagued by two major challenges: variability on short
(approximately minutes) timescales and interstellar scattering along our line
of sight. While the scattering is well studied, the source variability
continues to push the limits of current imaging algorithms. In particular,
movie reconstructions are hindered by the sparse and time-variable coverage of
the array. In this paper, we study the impact of the planned Africa Millimetre
Telescope (AMT, in Namibia) and Canary Islands telescope (CNI) additions to the
time-dependent coverage and imaging fidelity of the EHT array. This African
array addition to the EHT further increases the eastwest (u, v) coverage and
provides a wider time window to perform high-fidelity movie reconstructions of
Sgr A*. We generated synthetic observations of Sgr A*'s accretion flow and used
dynamical imaging techniques to create movie reconstructions of the source. To
test the fidelity of our results, we used one general-relativistic
magneto-hydrodynamic model of the accretion flow and jet to represent the
quiescent state and one semi-analytic model of an orbiting hotspot to represent
the flaring state. We found that the addition of the AMT alone offers a
significant increase in the (u, v) coverage, leading to robust averaged images
during the first hours of the observating track. Moreover, we show that the
combination of two telescopes on the African continent, in Namibia and in the
Canary Islands, produces a very sensitive array to reconstruct the variability
of Sgr A* on horizon scales. We conclude that the African expansion to the EHT
increases the fidelity of high-resolution movie reconstructions of Sgr A* to
study gas dynamics near the event horizon.Comment: Accepted for publication in Astronomy & Astrophysics. Comments: 11
pages, 9 figure
Observational Signatures of Frame Dragging in Strong Gravity
Objects orbiting in the presence of a rotating massive body experience a
gravitomagnetic frame-dragging effect, known as the Lense-Thirring effect, that
has been experimentally confirmed in the weak-field limit. In the strong-field
limit, near the horizon of a rotating black hole, frame dragging becomes so
extreme that all objects must co-rotate with the black hole's angular momentum.
In this work, we perform general relativistic numerical simulations to identify
observable signatures of frame dragging in the strong-field limit that appear
when infalling gas is forced to flip its direction of rotation as it is being
accreted. In total intensity images, infalling streams exhibit "S"-shaped
features due to the switch in the tangential velocity. In linear polarization,
a flip in the handedness of spatially resolved polarization ticks as a function
of radius encodes a transition in the magnetic field geometry that occurs due
to magnetic flux freezing in the dragged plasma. Using a network of telescopes
around the world, the Event Horizon Telescope collaboration has demonstrated
that it is now possible to directly image black holes on event horizon scales.
We show that the phenomena described in this work would be accessible to the
next-generation Event Horizon Telescope (ngEHT) and extensions of the array
into space, which would produce spatially resolved images on event horizon
scales with higher spatial resolution and dynamic range.Comment: Submitted to ApJL. 15 pages, 8 figure
Dynamical Imaging with Interferometry
By linking widely separated radio dishes, the technique of very long baseline
interferometry (VLBI) can greatly enhance angular resolution in radio
astronomy. However, at any given moment, a VLBI array only sparsely samples the
information necessary to form an image. Conventional imaging techniques
partially overcome this limitation by making the assumption that the observed
cosmic source structure does not evolve over the duration of an observation,
which enables VLBI networks to accumulate information as the Earth rotates and
changes the projected array geometry. Although this assumption is appropriate
for nearly all VLBI, it is almost certainly violated for submillimeter
observations of the Galactic Center supermassive black hole, Sagittarius A*
(Sgr A*), which has a gravitational timescale of only ~20 seconds and exhibits
intra-hour variability. To address this challenge, we develop several
techniques to reconstruct dynamical images ("movies") from interferometric
data. Our techniques are applicable to both single-epoch and multi-epoch
variability studies, and they are suitable for exploring many different
physical processes including flaring regions, stable images with small
time-dependent perturbations, steady accretion dynamics, or kinematics of
relativistic jets. Moreover, dynamical imaging can be used to estimate
time-averaged images from time-variable data, eliminating many spurious image
artifacts that arise when using standard imaging methods. We demonstrate the
effectiveness of our techniques using synthetic observations of simulated black
hole systems and 7mm Very Long Baseline Array observations of M87, and we show
that dynamical imaging is feasible for Event Horizon Telescope observations of
Sgr A*.Comment: 16 Pages, 12 Figures, Accepted for publication in Ap
Imaging an Event Horizon: Mitigation of Source Variability of Sagittarius A*
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 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 vari- ability 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 general relativity, 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.Comment: 10 pages, 12figures, accepted for publication in Ap
On the prospects of imaging Sagittarius A* from space
Very Long Baseline Interferometry (VLBI) at sub-millimeter waves has the
potential to image the shadow of the black hole in the Galactic Center,
Sagittarius A* (Sgr A*), and thereby test basic predictions of the theory of
general relativity. We investigate the imaging prospects of a new Space VLBI
mission concept. The setup consists of two satellites in polar or equatorial
circular Medium-Earth Orbits with slightly different radii, resulting in a
dense spiral-shaped uv-coverage with long baselines, allowing for extremely
high-resolution and high-fidelity imaging of radio sources. We simulate
observations of a general relativistic magnetohydrodynamics model of Sgr A* for
this configuration with noise calculated from model system parameters. After
gridding the -plane and averaging visibilities accumulated over multiple
months of integration, images of Sgr A* with a resolution of up to 4 as
could be reconstructed, allowing for stronger tests of general relativity and
accretion models than with ground-based VLBI.Comment: 4 pages, 4 figures, published in Proceedings IAU Symposium No. 342,
201
Black hole parameter estimation with synthetic Very Long Baseline Interferometry data from the ground and from space
The Event Horizon Telescope (EHT) has imaged the shadow of the supermassive
black hole in M87. A library of general relativistic magnetohydrodynamics
(GMRHD) models was fit to the observational data, providing constraints on
black hole parameters. We investigate how much better future experiments can
realistically constrain these parameters and test theories of gravity. We
generate realistic synthetic 230 GHz data from representative input models
taken from a GRMHD image library for M87, using the 2017, 2021, and an expanded
EHT array. The synthetic data are run through a data reduction pipeline used by
the EHT. Additionally, we simulate observations at 230, 557, and 690 GHz with
the Event Horizon Imager (EHI) Space VLBI concept. Using one of the EHT
parameter estimation pipelines, we fit the GRMHD library images to the
synthetic data and investigate how the black hole parameter estimations are
affected by different arrays and repeated observations. Repeated observations
play an important role in constraining black hole and accretion parameters as
the varying source structure is averaged out. A modest expansion of the EHT
already leads to stronger parameter constraints. High-frequency observations
from space rule out all but ~15% of the GRMHD models in our library, strongly
constraining the magnetic flux and black hole spin. The 1 constraints
on the black hole mass improve by a factor of five with repeated high-frequency
space array observations as compared to observations with the current ground
array. If the black hole spin, magnetization, and electron temperature
distribution can be independently constrained, the shadow size for a given
black hole mass can be tested to ~0.5% with the EHI, which allows tests of
deviations from general relativity. High-precision tests of the Kerr metric
become within reach from observations of the Galactic Center black hole
Sagittarius A*.Comment: 21 pages, 18 figures, accepted for publication in Astronomy &
Astrophysic
Polarimetric Geometric Modeling for mm-VLBI Observations of Black Holes
The Event Horizon Telescope (EHT) is a millimeter very long baseline
interferometry (VLBI) array that has imaged the apparent shadows of the
supermassive black holes M87* and Sagittarius A*. Polarimetric data from these
observations contain a wealth of information on the black hole and accretion
flow properties. In this work, we develop polarimetric geometric modeling
methods for mm-VLBI data, focusing on approaches that fit data products with
differing degrees of invariance to broad classes of calibration errors. We
establish a fitting procedure using a polarimetric "m-ring" model to
approximate the image structure near a black hole. By fitting this model to
synthetic EHT data from general relativistic magnetohydrodynamic models, we
show that the linear and circular polarization structure can be successfully
approximated with relatively few model parameters. We then fit this model to
EHT observations of M87* taken in 2017. In total intensity and linear
polarization, the m-ring fits are consistent with previous results from imaging
methods. In circular polarization, the m-ring fits indicate the presence of
event-horizon-scale circular polarization structure, with a persistent dipolar
asymmetry and orientation across several days. The same structure was recovered
independently of observing band, used data products, and model assumptions.
Despite this broad agreement, imaging methods do not produce similarly
consistent results. Our circular polarization results, which imposed additional
assumptions on the source structure, should thus be interpreted with some
caution. Polarimetric geometric modeling provides a useful and powerful method
to constrain the properties of horizon-scale polarized emission, particularly
for sparse arrays like the EHT.Comment: 34 pages, 15 figures, published in The Astrophysical Journal Letter
Reconstructing Video from Interferometric Measurements of Time-Varying Sources
Very long baseline interferometry (VLBI) makes it possible to recover images of astronomical sources with extremely high angular resolution. Most recently, the Event Horizon Telescope (EHT) has extended VLBI to short millimeter wavelengths with a goal of achieving angular resolution sufficient for imaging the event horizons of nearby supermassive black holes. VLBI provides measurements related to the underlying source image through a sparse set spatial frequencies. An image can then be recovered from these measurements by making assumptions about the underlying image. One of the most important assumptions made by conventional imaging methods is that over the course of a night's observation the image is static. However, for quickly evolving sources, such as the galactic center's supermassive black hole (Sgr A*) targeted by the EHT, this assumption is violated and these conventional imaging approaches fail. In this work we propose a new way to model VLBI measurements that allows us to recover both the appearance and dynamics of an evolving source by reconstructing a video rather than a static image. By modeling VLBI measurements using a Gaussian Markov Model, we are able to propagate information across observations in time to reconstruct a video, while simultaneously learning about the dynamics of the source's emission region. We demonstrate our proposed Expectation-Maximization (EM) algorithm, StarWarps, on realistic synthetic observations of black holes, and show how it substantially improves results compared to conventional imaging algorithms. Additionally, we demonstrate StarWarps on real VLBI data of the M87 Jet from the VLBA
Unraveling Twisty Linear Polarization Morphologies in Black Hole Images
We investigate general relativistic magnetohydrodynamic simulations (GRMHD)
to determine the physical origin of the twisty patterns of linear polarization
seen in spatially resolved black hole images and explain their morphological
dependence on black hole spin. By characterising the observed emission with a
simple analytic ring model, we find that the twisty morphology is determined by
the magnetic field structure in the emitting region. Moreover, the dependence
of this twisty pattern on spin can be attributed to changes in the magnetic
field geometry that occur due to the frame dragging. By studying an analytic
ring model, we find that the roles of Doppler boosting and lensing are
subdominant. Faraday rotation may cause a systematic shift in the linear
polarization pattern, but we find that its impact is subdominant for models
with strong magnetic fields and modest ion-to-electron temperature ratios.
Models with weaker magnetic fields are much more strongly affected by Faraday
rotation and have more complicated emission geometries than can be captured by
a ring model. However, these models are currently disfavoured by the recent EHT
observations of M87*. Our results suggest that linear polarization maps can
provide a probe of the underlying magnetic field structure around a black hole,
which may then be usable to indirectly infer black hole spins. The generality
of these results should be tested with alternative codes, initial conditions,
and plasma physics prescriptions.Comment: 25 pages, 19 figure
Reference Array and Design Consideration for the next-generation Event Horizon Telescope
We describe the process to design, architect, and implement a transformative
enhancement of the Event Horizon Telescope (ngEHT). This program - the
next-generation Event Horizon Telescope (ngEHT) - will form a networked global
array of radio dishes capable of making high-fidelity real-time movies of
supermassive black holes (SMBH) and their emanating jets. This builds upon the
EHT principally by deploying additional modest-diameter dishes to optimized
geographic locations to enhance the current global mm/submm wavelength Very
Long Baseline Interferometric (VLBI) array, which has, to date, utilized mostly
pre-existing radio telescopes. The ngEHT program further focuses on observing
at three frequencies simultaneously for increased sensitivity and Fourier
spatial frequency coverage. Here, the concept, science goals, design
considerations, station siting and instrument prototyping are discussed, and a
preliminary reference array to be implemented in phases is described.Comment: Submitted to the journal Galaxie
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