5,666 research outputs found
Effects of Hydrogen vs. Helium on Electromagnetic Black Hole Observables
The centers of our galaxy and the nearby Messier 87 are known to contain
supermassive black holes, which support accretion flows that radiate across the
electromagnetic spectrum. Although the composition of the accreting gas is
unknown, it is likely a mix of ionized hydrogen and helium. We use a simple
analytic model and a suite of numerical general relativistic
magnetohydrodynamic accretion simulations to study how polarimetric images and
spectral energy distributions of the source are influenced by the
hydrogen/helium content of the accreting matter. We aim to identify general
trends rather than make quantitatively precise predictions, since it is not
possible to fully explore the parameter space of accretion models. If the
ion-to-electron temperature ratio is fixed, then increasing the helium fraction
increases the gas temperature; to match the observational flux density
constraints, the number density of electrons and magnetic field strengths must
therefore decrease. In our numerical simulations, emission shifts from regions
of low to high plasma beta -- both altering the morphology of the image and
decreasing the variability of the light curve -- especially in strongly
magnetized models with emission close to the midplane. In polarized images, we
find that the model gas composition influences the degree to which linear
polarization is (de)scrambled and therefore affects estimates for the resolved
linear polarization fraction. We also find that the spectra of
helium-composition flows peak at higher frequencies and exhibit higher
luminosities. We conclude that gas composition may play an important role in
predictive models for black hole accretion.Comment: 17 pages, 8 figures, accepted for publication in Ap
Black Hole Polarimetry I: A Signature of Electromagnetic Energy Extraction
In 1977, Blandford and Znajek showed that the electromagnetic field
surrounding a rotating black hole can harvest its spin energy and use it to
power a collimated astrophysical jet, such as the one launched from the center
of the elliptical galaxy M87. Today, interferometric observations with the
Event Horizon Telescope (EHT) are delivering high-resolution,
event-horizon-scale, polarimetric images of the supermassive black hole M87* at
the jet launching point. These polarimetric images offer an unprecedented
window into the electromagnetic field structure around a black hole. In this
paper, we show that a simple polarimetric observable that quantifies the
magnetic field helicity -- the sign of in a near-horizon image
-- depends on the sign of the electromagnetic energy flux and therefore
provides a direct probe of black hole energy extraction. In Boyer-Lindquist
coordinates, the Poynting flux for axisymmetric electromagnetic fields is
proportional to the product . The polarimetric observable
likewise depends on the ratio , thereby enabling an
observer to experimentally determine the direction of electromagnetic energy
flow in the near-horizon environment. Data from the 2017 EHT observations of
M87* are consistent with electromagnetic energy outflow. Currently envisioned
multi-frequency observations of M87* will achieve higher dynamic range and
angular resolution, and hence deliver measurements of closer to
the event horizon as well as better constraints on Faraday rotation. Such
observations will enable a definitive test for energy extraction from the black
hole M87*.Comment: 34 pages, 5 figures. Submitted to ApJ
Photon Ring Autocorrelations
In the presence of a black hole, light sources connect to observers along
multiple paths. As a result, observed brightness fluctuations must be
correlated across different times and positions in black hole images. Photons
that execute multiple orbits around the black hole appear near a critical curve
in the observer sky, giving rise to the photon ring. In this paper, a novel
observable is proposed: the two-point correlation function of intensity
fluctuations on the photon ring. This correlation function is analytically
computed for a Kerr black hole surrounded by stochastic equatorial emission,
with source statistics motivated by simulations of a turbulent accretion flow.
It is shown that this two-point function exhibits a universal, self-similar
structure consisting of multiple peaks of identical shape: while the profile of
each peak encodes statistical properties of fluctuations in the source, the
locations and heights of the peaks are determined purely by the black hole
parameters. Measuring these peaks would demonstrate the existence of the photon
ring without resolving its thickness, and would provide estimates of black hole
mass and spin. With regular monitoring over sufficiently long timescales, this
measurement could be possible via interferometric imaging with modest
improvements to the Event Horizon Telescope.Comment: 31 pages, 3 figure
Demonstrating Photon Ring Existence with Single-Baseline Polarimetry
Images of supermassive black hole accretion flows contain features of both
curved spacetime and plasma structure. Inferring properties of the spacetime
from images requires modeling the plasma properties, and vice versa. The Event
Horizon Telescope Collaboration has imaged near-horizon millimeter emission
from both Messier 87* (M87*) and Sagittarius A* (Sgr A*) with
very-long-baseline interferometry (VLBI) and has found a preference for
magnetically arrested disk (MAD) accretion in each case. MAD accretion enables
spacetime measurements through future observations of the photon ring, the
image feature composed of near-orbiting photons. The ordered fields and
relatively weak Faraday rotation of MADs yield rotationally symmetric
polarization when viewed at modest inclination. In this letter, we utilize this
symmetry along with parallel transport symmetries to construct a gain-robust
interferometric quantity that detects the transition between the weakly lensed
accretion flow image and the strongly lensed photon ring. We predict a shift in
polarimetric phases on long baselines and demonstrate that the photon rings in
M87* and Sgr A* can be unambiguously detected {with sensitive, long-baseline
measurements. For M87* we find that photon ring detection in snapshot
observations requires mJy sensitivity on G baselines at
230 GHz and above, which could be achieved with space-VLBI or higher-frequency
ground-based VLBI. For Sgr A*, we find that interstellar scattering inhibits
photon ring detectability at 230 GHz, but mJy sensitivity on
G baselines at 345 GHz is sufficient, which is accessible from the
ground. For both sources, these sensitivity requirements may be relaxed by
repeated observations and averaging.Comment: 14 pages, 7 figures, Accepted to ApJ
Using Machine Learning to Link Black Hole Accretion Flows with Spatially Resolved Polarimetric Observables
We introduce a new library of 535,194 model images of the supermassive black
holes and Event Horizon Telescope (EHT) targets Sgr A* and M87*, computed by
performing general relativistic radiative transfer calculations on general
relativistic magnetohydrodynamics simulations. Then, to infer underlying black
hole and accretion flow parameters (spin, inclination, ion-to-electron
temperature ratio, and magnetic field polarity), we train a random forest
machine learning model on various hand-picked polarimetric observables computed
from each image. Our random forest is capable of making meaningful predictions
of spin, inclination, and the ion-to-electron temperature ratio, but has more
difficulty inferring magnetic field polarity. To disentangle how physical
parameters are encoded in different observables, we apply two different metrics
to rank the importance of each observable at inferring each physical parameter.
Details of the spatially resolved linear polarization morphology stand out as
important discriminators between models. Bearing in mind the theoretical
limitations and incompleteness of our image library, for the real M87* data,
our machinery favours high-spin retrograde models with large ion-to-electron
temperature ratios. Due to the time-variable nature of these targets, repeated
polarimetric imaging will further improve model inference as the EHT and
next-generation (EHT) continue to develop and monitor their targets.Comment: 24 pages, 27 figure
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