117 research outputs found
General relativistic magnetohydrodynamical -jet models for Sgr A*
The observed spectral energy distribution of an accreting supermassive black
hole typically forms a power-law spectrum in the Near Infrared (NIR) and
optical wavelengths, that may be interpreted as a signature of accelerated
electrons along the jet. However, the details of acceleration remain uncertain.
In this paper, we study the radiative properties of jets produced in
axisymmetric GRMHD simulations of hot accretion flows onto underluminous
supermassive black holes both numerically and semi-analytically, with the aim
of investigating the differences between models with and without accelerated
electrons inside the jet. We assume that electrons are accelerated in the jet
regions of our GRMHD simulation. To model them, we modify the electrons'
distribution function in the jet regions from a purely relativistic thermal
distribution to a combination of a relativistic thermal distribution and the
-distribution function. Inside the disk, we assume a thermal
distribution for the electrons. We calculate jet spectra and synchrotron maps
by using the ray tracing code {\tt RAPTOR}, and compare the synthetic
observations to observations of Sgr~A*. Finally, we compare numerical models of
jets to semi-analytical ones. We find that in the -jet models, the
radio-emitting region size, radio flux, and spectral index in NIR/optical bands
increase for decreasing values of the parameter, which corresponds to
a larger amount of accelerated electrons. The model with ,
(the percentage of electrons that are accelerated), and
observing angle fits the observed Sgr~A* emission in the
flaring state from the radio to the NIR/optical regimes, while ,
, and observing angle fit the upper
limits in quiescence.Comment: 17 pages, 16 figures, 1 tabl
RAPTOR II: Polarized radiative transfer in curved spacetime
Accreting supermassive black holes are sources of polarized radiation that
propagates through highly curved spacetime before reaching the observer. In
order to help interpret observations of such polarized emission, accurate and
efficient numerical schemes for polarized radiative transfer in curved
spacetime are needed. In this manuscript we extend our publicly available
radiative transfer code RAPTOR to include polarization. We provide a brief
review of different codes and methods for covariant polarized radiative
transfer available in the literature and existing codes, and present an
efficient new scheme. For the spacetime-propagation aspect of the computation,
we develop a compact, Lorentz-invariant representation of a polarized ray. For
the plasma-propagation aspect of the computation, we perform a formal analysis
of the stiffness of the polarized radiative-transfer equation with respect to
our explicit integrator, and develop a hybrid integration scheme that switches
to an implicit integrator in case of stiffness, in order to solve the equation
with optimal speed and accuracy for all possible values of the local
optical/Faraday thickness of the plasma. We perform a comprehensive code
verification by solving a number of well-known test problems using RAPTOR and
comparing its output to exact solutions. We also demonstrate convergence with
existing polarized radiative-transfer codes in the context of complex
astrophysical problems. RAPTOR is capable of performing polarized radiative
transfer in arbitrary, highly curved spacetimes. This capability is crucial for
interpreting polarized observations of accreting black holes, which can yield
information about the magnetic-field configuration in such accretion flows. The
efficient formalism implemented in RAPTOR is computationally light and
conceptually simple. The code is publicly available
Observing supermassive black holes in virtual reality
We present a full 360 degree (i.e., 4 steradian) general-relativistic
ray-tracing and radiative transfer calculations of accreting supermassive black
holes. We perform state-of-the-art three-dimensional general relativistic
magnetohydrodynamical simulations using the BHAC code, subsequently
post-processing this data with the radiative transfer code RAPTOR. All
relativistic and general-relativistic effects, such as Doppler boosting and
gravitational redshift, as well as geometrical effects due to the local
gravitational field and the observer's changing position and state of motion,
are therefore calculated self-consistently. Synthetic images at four
astronomically-relevant observing frequencies are generated from the
perspective of an observer with a full 360-degree view inside the accretion
flow, who is advected with the flow as it evolves. As an example, we calculated
images based on recent best-fit models of observations of Sagittarius A*. These
images are combined to generate a complete 360-degree Virtual Reality movie of
the surrounding environment of the black hole and its event horizon. Our
approach also enables the calculation of the local luminosity received at a
given fluid element in the accretion flow, providing important applications in,
e.g., radiation feedback calculations onto black hole accretion flows. In
addition to scientific applications, the 360-degree Virtual Reality movies we
present also represent a new medium through which to communicate black hole
physics to a wider audience, serving as a powerful educational tool.Comment: 25 pages, 11 figures, 1 movie;
https://www.youtube.com/watch?v=SXN4hpv977s&t=57
RAPTOR I: Time-dependent radiative transfer in arbitrary spacetimes
Observational efforts to image the immediate environment of a black hole at
the scale of the event horizon benefit from the development of efficient
imaging codes that are capable of producing synthetic data, which may be
compared with observational data. We aim to present RAPTOR, a new public code
that produces accurate images, animations, and spectra of relativistic plasmas
in strong gravity by numerically integrating the equations of motion of light
rays and performing time-dependent radiative transfer calculations along the
rays. The code is compatible with any analytical or numerical spacetime. It is
hardware-agnostic and may be compiled and run both on GPUs and CPUs. We
describe the algorithms used in RAPTOR and test the code's performance. We have
performed a detailed comparison of RAPTOR output with that of other
radiative-transfer codes and demonstrate convergence of the results. We then
applied RAPTOR to study accretion models of supermassive black holes,
performing time-dependent radiative transfer through general relativistic
magneto-hydrodynamical (GRMHD) simulations and investigating the expected
observational differences between the so-called fast-light and slow-light
paradigms. Using RAPTOR to produce synthetic images and light curves of a GRMHD
model of an accreting black hole, we find that the relative difference between
fast-light and slow-light light curves is less than 5%. Using two distinct
radiative-transfer codes to process the same data, we find integrated flux
densities with a relative difference less than 0.01%. For two-dimensional GRMHD
models, such as those examined in this paper, the fast-light approximation
suffices as long as errors of a few percent are acceptable. The convergence of
the results of two different codes demonstrates that they are, at a minimum,
consistent.Comment: 18 pages, 14 figures, 5 table
monty: a Monte Carlo Compton Scattering code including non-thermal electrons
Low-luminosity active galactic nuclei are strong sources of X-ray emission
produced by Compton scattering originating from the accretion flows surrounding
their supermassive black holes. The shape and energy of the resulting spectrum
depend on the shape of the underlying electron distribution function (DF). In
this work, we present an extended version of the grmonty code, called
monty. The grmonty code previously only included a thermal Maxwell
J\"utner electron distribution function. We extend the gromty code with
non-thermal electron DFs, namely the and power-law DFs, implement
Cartesian Kerr-Schild coordinates, accelerate the code with MPI, and couple the
code to the non-uniform AMR grid data from the GRMHD code BHAC. For the Compton
scattering process, we derive two sampling kernels for both distribution
functions. Finally, we present a series of code tests to verify the accuracy of
our schemes. The implementation of non-thermal DFs opens the possibility of
studying the effect of non-thermal emission on previously developed black hole
accretion models.Comment: 12 pages, 9 figures, submitted to journa
Synchrotron polarization signatures of surface waves in supermassive black hole jets
Supermassive black holes in active galactic nuclei (AGN) are known to launch
relativistic jets, which are observed across the entire electromagnetic
spectrum and are thought to be efficient particle accelerators. Their primary
radiation mechanism for radio emission is polarized synchrotron emission
produced by a population of non-thermal electrons. In this Letter, we present a
global general relativistic magnetohydrodynamical (GRMHD) simulation of a
magnetically arrested disk (MAD). After the simulation reaches the MAD state,
we show that waves are continuously launched from the vicinity of the black
hole and propagate along the interface between the jet and the wind. At this
interface, a steep gradient in velocity is present between the mildly
relativistic wind and the highly relativistic jet. The interface is, therefore,
a shear layer, and due to the shear, the waves generate roll-ups that alter the
magnetic field configuration and the shear layer geometry. We then perform
polarized radiation transfer calculations of our GRMHD simulation and find
signatures of the waves in both total intensity and linear polarization,
effectively lowering the fully resolved polarization fraction. The tell-tale
polarization signatures of the waves could be observable by future Very Long
Baseline Interferometric observations, e.g., by the next-generation Event
Horizon Telescope.Comment: 20 pages, 17 figures, accepted for publication in ApJ
First M87 Event Horizon Telescope Results and the Role of ALMA
In April 2019, the Event Horizon Telescope (EHT) collaboration revealed the
first image of the candidate super-massive black hole (SMBH) at the centre of
the giant elliptical galaxy Messier 87 (M87). This event-horizon-scale image
shows a ring of glowing plasma with a dark patch at the centre, which is
interpreted as the shadow of the black hole. This breakthrough result, which
represents a powerful confirmation of Einstein's theory of gravity, or general
relativity, was made possible by assembling a global network of radio
telescopes operating at millimetre wavelengths that for the first time included
the Atacama Large Millimeter/ submillimeter Array (ALMA). The addition of ALMA
as an anchor station has enabled a giant leap forward by increasing the
sensitivity limits of the EHT by an order of magnitude, effectively turning it
into an imaging array. The published image demonstrates that it is now possible
to directly study the event horizon shadows of SMBHs via electromagnetic
radiation, thereby transforming this elusive frontier from a mathematical
concept into an astrophysical reality. The expansion of the array over the next
few years will include new stations on different continents - and eventually
satellites in space. This will provide progressively sharper and
higher-fidelity images of SMBH candidates, and potentially even movies of the
hot plasma orbiting around SMBHs. These improvements will shed light on the
processes of black hole accretion and jet formation on event-horizon scales,
thereby enabling more precise tests of general relativity in the truly strong
field regime.Comment: 11 pages + cover page, 6 figure
Monitoring the Morphology of M87* in 2009-2017 with the Event Horizon Telescope
The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature—a ring with azimuthal brightness asymmetry—and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009-2017 to be consistent with a persistent asymmetric ring of ∼40 μas diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin
Aortic valve calcification volumes and chronic brain infarctions in patients undergoing transcatheter aortic valve implantation
Chronic silent brain infarctions, detected as new white matter hyperintensities on magnetic resonance imaging (MRI) following transcatheter aortic valve implantation (TAVI), are associated with long-term cognitive deterioration. This is the first study to investigate to which extent the calcification volume of the native aortic valve (AV) measured with cardiac computed tomography angiography (CTA) predicts the increase in chronic white matter hyperintensity volume after TAVI. A total of 36 patients (79 ± 5 years, median EuroSCORE II 1.9%, Q1–Q3 1.5–3.4%) with severe AV stenosis underwent fluid attenuation inversion recovery (FLAIR) MRI < 24 h prior to TAVI and at 3 months follow-up for assessment of cerebral white matter hyperintensity volume (mL). Calcification volumes (mm3) of the AV, aortic arch, landing zone and left ventricle were measured on the CTA pre-TAVI. The largest calcification volumes were found in the AV (median 692 mm3) and aortic arch (median 633 mm3), with a large variation between patients (Q1–Q3 482–1297 mm3 and 213–1727 mm3, respectively). The white matter hyperintensity volume increased in 72% of the patients. In these patients the median volume increase was of 1.1 mL (Q1–Q3 0.3–4.6 mL), corresponding with a 27% increase from baseline (Q1–Q3 7–104%). The calcification volume in the AV predicted the increase of white matter hyperintensity volume (Δ%), with a 35% increase of white matter hyperintensity volume, per 100 mm3 of AV calcification volume (SE 8.5, p < 0.001). The calcification volumes in the aortic arch, landing zone and left ventricle were not associated with the increase in white matter hyperintensity volume. In 72% of the patients new chronic white matter hyperintensities developed 3 months after TAVI, with a median increase of 27%. A higher calcification volume in the AV was associated with a larger increase in the white matter hyperintensity volume. These findings show the potential for automated AV calcium screening as an imaging biomarker to predict chronic silent brain infarctions
Antibiotic Selection Pressure and Macrolide Resistance in Nasopharyngeal Streptococcus pneumoniae: A Cluster-Randomized Clinical Trial
Jeremy Keenan and colleagues report that during a cluster-randomized clinical trial in Ethiopia, nasopharyngeal pneumococcal resistance to macrolides was significantly higher in communities randomized to receive azithromycin compared with untreated control communities
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