256 research outputs found
Measuring the Ellipticity of M 87* Images
The Event Horizon Telescope (EHT) images of the supermassive black hole at
the center of the galaxy M 87 provided the first image of the accretion
environment on horizon scales. General relativity predicts that the image of
the shadow should be nearly circular, given the inclination angle of the black
hole M 87*. A robust detection of ellipticity in the image reconstructions of M
87* could signal new gravitational physics on horizon scales. Here we analyze
whether the imaging parameters used in EHT analyses are sensitive to ring
ellipticity and measure the constraints on the ellipticity of M 87*. We find
that the top set is unable to recover ellipticity. Even for simple geometric
models, the true ellipticity is biased low, preferring circular rings.
Therefore, to place a constraint on the ellipticity of M 87*, we measure the
ellipticity of 550 simulated data sets of GRMHD simulations. We find that
images with intrinsic axis ratios of 2:1 are consistent with the ellipticity
seen from the EHT image reconstructions.Comment: accepted for publication to Ap
Variable X-ray absorption in the mini-BAL QSO PG 1126-041
X-ray studies of active galactic nuclei (AGN) with powerful nuclear winds are
important for constraining the physics of the inner accretion/ejection flow
around supermassive black holes (SMBHs) and for understanding the impact of
such winds on the AGN environment. Our main scientific goal is to constrain the
properties of the circum-nuclear matter close to the SMBH in the mini-broad
absorption line quasar (mini-BAL QSO) PG 1126-041 using a multi-epoch
observational campaign with XMM-Newton. We performed temporally resolved X-ray
spectroscopy and simultaneous UV and X-ray photometry on the most complete set
of observations and on the deepest X-ray exposure of a mini-BAL QSO ever. We
found complex X-ray spectral variability on time scales of both months and
hours, which is best reproduced by means of variable massive ionized absorbers
along the line of sight. As a consequence, the observed optical-to-X-ray
spectral index is found to be variable with time. In the highest
signal-to-noise observation we detected highly ionized X-ray absorbing material
outflowing much faster (v ~ 16500 km/s) than the UV absorbing one (v ~ 5000
km/s). This highly ionized absorber is found to be variable on very short (a
few kiloseconds) time scales. Our findings are qualitatively consistent with
line-driven accretion disk winds scenarios. Our observations have opened the
time-resolved X-ray spectral analysis field for mini-BAL QSOs. Only with future
deep studies will we be able to map the dynamics of the inner flow and
understand the physics of AGN winds and their impact on the environment.Comment: Replaced to match the published versio
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
Bayesian Black Hole Photogrammetry
We propose a simple, analytic dual-cone accretion model for horizon scale
images of the cores of Low-Luminosity Active Galactic Nuclei (LLAGN), including
those observed by the Event Horizon Telescope (EHT). Our underlying model is of
synchrotron emission from an axisymmetric, magnetized plasma, which is
constrained to flow within two oppositely oriented cones that are aligned with
the black hole's spin axis. We show that this model can accurately reproduce
images for a variety of time-averaged general relativistic magnetohydrodynamic
(GRMHD) simulations, that it accurately recovers both the black hole and
emission parameters from these simulations, and that it is sufficiently
efficient to be used to measure these parameters in a Bayesian inference
framework with radio interferometric data. We show that non-trivial topologies
in the source image can result in non-trivial multi-modal solutions when
applied to observations from a sparse array, such as the EHT 2017 observations
of M87. The presence of these degeneracies underscores the importance of
employing Bayesian techniques that adequately sample the posterior space for
the interpretation of EHT measurements. We fit our model to the EHT
observations of M87 and find a 95% Highest Posterior Density Interval
(HPDI) for the mass-to-distance ratio of ,
and give an inclination of . These new
measurements are consistent with mass measurements from the EHT and stellar
dynamical estimates (e.g., Gebhardt et al. 2011; EHTC et al. 2019a,b; Liepold
et al. 2023), and with the spin axis inclination inferred from properties of
the M87 jet (e.g., Walker et al. 2018)
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
How Spatially Resolved Polarimetry Informs Black Hole Accretion Flow Models
The Event Horizon Telescope (EHT) Collaboration has successfully produced
images of two supermassive black holes, enabling novel tests of black holes and
their accretion flows on horizon scales. The EHT has so far published total
intensity and linear polarization images, while upcoming images may include
circular polarization, rotation measure, and spectral index, each of which
reveals different aspects of the plasma and space-time. The next-generation EHT
(ngEHT) will greatly enhance these studies through wider recorded bandwidths
and additional stations, leading to greater signal-to-noise, orders of
magnitude improvement in dynamic range, multi-frequency observations, and
horizon-scale movies. In this paper, we review how each of these different
observables informs us about the underlying properties of the plasma and the
spacetime, and we discuss why polarimetric studies are well-suited to
measurements with sparse, long-baseline coverage.Comment: Submitted for Galaxies Special Issue "From Vision to Instrument:
Creating a Next-Generation Event Horizon Telescope for a New Era of Black
Hole Science
Researching for better instructional methods using AB experiments in MOOCs: results and challenges
We conducted two AB experiments (treatment vs. control) in a massive open online course. The first experiment evaluates deliberate practice activities (DPAs) for developing problem solving expertise as measured by traditional physics problems. We find that a more interactive drag-and-drop format of DPA generates quicker learning than a multiple choice format but DPAs do not improve performance on solving traditional physics problems more than normal homework practice. The second experiment shows that a different video shooting setting can improve the fluency of the instructor which in turn improves the engagement of the students although it has no significant impact on the learning outcomes. These two cases demonstrate the potential of MOOC AB experiments as an open-ended research tool but also reveal limitations. We discuss the three most important challenges: wide student distribution, “open-book” nature of assessments, and large quantity and variety of data. We suggest possible methods to cope with those.Google (Firm)Massachusetts Institute of Technolog
Bayesian Accretion Modeling: Axisymmetric Equatorial Emission in the Kerr Spacetime
The Event Horizon Telescope (EHT) has produced images of two supermassive
black holes, Messier~87* (M 87*) and Sagittarius~A* (Sgr A*). The EHT
collaboration used these images to indirectly constrain black hole parameters
by calibrating measurements of the sky-plane emission morphology to images of
general relativistic magnetohydrodynamic (GRMHD) simulations. Here, we develop
a model for directly constraining the black hole mass, spin, and inclination
through signatures of lensing, redshift, and frame dragging, while
simultaneously marginalizing over the unknown accretion and emission
properties. By assuming optically thin, axisymmetric, equatorial emission near
the black hole, our model gains orders of magnitude in speed over similar
approaches that require radiative transfer. Using 2017 EHT M 87* baseline
coverage, we use fits of the model to itself to show that the data are
insufficient to demonstrate existence of the photon ring. We then survey
time-averaged GRMHD simulations fitting EHT-like data, and find that our model
is best-suited to fitting magnetically arrested disks, which are the favored
class of simulations for both M 87* and Sgr A*. For these simulations, the
best-fit model parameters are within of the true mass and within
for inclination. With 2017 EHT coverage and 1\% fractional
uncertainty on amplitudes, spin is unconstrained. Accurate inference of spin
axis position angle depends strongly on spin and electron temperature. Our
results show the promise of directly constraining black hole spacetimes with
interferometric data, but they also show that nearly identical images permit
large differences in black hole properties, highlighting degeneracies between
the plasma properties, spacetime, and most crucially, the unknown emission
geometry when studying lensed accretion flow images at a single frequency.Comment: Accepted to ApJ, 16 pages, 10 figure
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