121 research outputs found
First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass
In this paper we quantify the temporal variability and image morphology of
the horizon-scale emission from Sgr A*, as observed by the EHT in 2017 April at
a wavelength of 1.3 mm. We find that the Sgr A* data exhibit variability that
exceeds what can be explained by the uncertainties in the data or by the
effects of interstellar scattering. The magnitude of this variability can be a
substantial fraction of the correlated flux density, reaching 100\% on
some baselines. Through an exploration of simple geometric source models, we
demonstrate that ring-like morphologies provide better fits to the Sgr A* data
than do other morphologies with comparable complexity. We develop two
strategies for fitting static geometric ring models to the time-variable Sgr A*
data; one strategy fits models to short segments of data over which the source
is static and averages these independent fits, while the other fits models to
the full dataset using a parametric model for the structural variability power
spectrum around the average source structure. Both geometric modeling and
image-domain feature extraction techniques determine the ring diameter to be
as (68\% credible intervals), with the ring thickness
constrained to have an FWHM between 30\% and 50\% of the ring diameter.
To bring the diameter measurements to a common physical scale, we calibrate
them using synthetic data generated from GRMHD simulations. This calibration
constrains the angular size of the gravitational radius to be
\mathrm{\mu as}, which we combine with an independent
distance measurement from maser parallaxes to determine the mass of Sgr A* to
be M.Comment: 65 pages, 35 figures, published in The Astrophysical Journal Letters
on May 12, 2022. See the published paper for the full authors lis
First M87 Event Horizon Telescope Results. IX. Detection of Near-horizon Circular Polarization
Event Horizon Telescope (EHT) observations have revealed a bright ring of
emission around the supermassive black hole at the center of the M87 galaxy.
EHT images in linear polarization have further identified a coherent spiral
pattern around the black hole, produced from ordered magnetic fields threading
the emitting plasma. Here, we present the first analysis of circular
polarization using EHT data, acquired in 2017, which can potentially provide
additional insights into the magnetic fields and plasma composition near the
black hole. Interferometric closure quantities provide convincing evidence for
the presence of circularly polarized emission on event-horizon scales. We
produce images of the circular polarization using both traditional and newly
developed methods. All methods find a moderate level of resolved circular
polarization across the image (), consistent with
the low image-integrated circular polarization fraction measured by the ALMA
array (). Despite this broad agreement, the methods show
substantial variation in the morphology of the circularly polarized emission,
indicating that our conclusions are strongly dependent upon the imaging
assumptions because of the limited baseline coverage, uncertain telescope gain
calibration, and weakly polarized signal. We include this upper limit in an
updated comparison to general relativistic magnetohydrodynamic (GRMHD)
simulation models. This analysis reinforces the previously reported preference
for magnetically arrested accretion flow models. We find that most simulations
naturally produce a low level of circular polarization consistent with our
upper limit, and that Faraday conversion is likely the dominant production
mechanism for circular polarization at 230 GHz in M87*.Comment: 47 pages, 31 figures, published in The Astrophysical Journal Letters
on Nov. 8, 2023. See the published paper for the full authors lis
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
Quantitative symmetry breaking of groundstates for a class of weighted Emden–Fowler equations
We prove that symmetrybreaking occurs in dimensions N ≥ 3 for the groundstate solutions to a class of Emden-Fowler equa-tions on the unit ball, with Dirichlet boundary conditions. We show that this phenomenon occurs forlarge values of a certain exponent for a radial weight function appearing in the equation. The problemreads as a possibly large perturbation of the classical H ́enon equation. In particular we consider aweight function having a spherical shell of zeroes centred at the origin and of radius R. A quantitativecondition on R for this phenomenon to occur is given by means of universal constants, such as thebest constant for the subcritical Sobolev embedding
Observing---and Imaging---Active Galactic Nuclei with the Event Horizon Telescope
Originally developed to image the shadow region of the central black hole in
Sagittarius A* and in the nearby galaxy M87, the Event Horizon Telescope (EHT)
provides deep, very high angular resolution data on other AGN sources too. The
challenges of working with EHT data have spurred the development of new image
reconstruction algorithms. This work briefly reviews the status of the EHT and
its utility for observing AGN sources, with emphasis on novel imaging
techniques that offer the promise of better reconstructions at 1.3 mm and other
wavelengths.Comment: 10 pages, proceedings contribution for Blazars through Sharp
Multi-Wavelength Eyes, submitted to Galaxie
Resolving the inner parsec of the blazar J1924-2914 with the Event Horizon Telescope
The blazar J1924-2914 is a primary Event Horizon Telescope (EHT) calibrator
for the Galactic Center's black hole Sagittarius A*. Here we present the first
total and linearly polarized intensity images of this source obtained with the
unprecedented 20 as resolution of the EHT. J1924-2914 is a very compact
flat-spectrum radio source with strong optical variability and polarization. In
April 2017 the source was observed quasi-simultaneously with the EHT (April
5-11), the Global Millimeter VLBI Array (April 3), and the Very Long Baseline
Array (April 28), giving a novel view of the source at four observing
frequencies, 230, 86, 8.7, and 2.3 GHz. These observations probe jet properties
from the subparsec to 100-parsec scales. We combine the multi-frequency images
of J1924-2914 to study the source morphology. We find that the jet exhibits a
characteristic bending, with a gradual clockwise rotation of the jet projected
position angle of about 90 degrees between 2.3 and 230 GHz. Linearly polarized
intensity images of J1924-2914 with the extremely fine resolution of the EHT
provide evidence for ordered toroidal magnetic fields in the blazar compact
core
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