474 research outputs found
Jet-torus connection in radio galaxies: Relativistic hydrodynamics and synthetic emission
High-resolution Very-Long-Baseline Interferometry observations of active
galactic nuclei have revealed asymmetric structures in the jets of radio
galaxies. These asymmetric structures may be due to internal asymmetries in the
jet, could be induced by the different conditions in the surrounding ambient
medium including the obscuring torus, or a combination of the two. In this
paper we investigate the influence of the ambient medium (including the
obscuring torus) on the observed properties of jets from radio galaxies. We
performed special-relativistic hydrodynamic (RHD) simulations of over-pressured
and pressure-matched jets using the special-relativistic hydrodynamics code
\texttt{Ratpenat}, which is based on a second-order accurate finite-volume
method and an approximate Riemann solver. Using a newly developed emission code
to compute the electromagnetic emission, we have investigated the influence of
different ambient medium and torus configurations on the jet structure and
subsequently computed the non-thermal emission produced by the jet and the
thermal absorption due to the torus. To better compare the emission simulations
with observations we produced synthetic radio maps, taking into account the
properties of the observatory. The detailed analysis of our simulations shows
that the observed asymmetries can be produced by the interaction of the jet
with the ambient medium and by the absorption properties of the obscuring
torus.Comment: 14 pages, 17 figures, submitted to A&
Polychaetes of commercial interest from the Mediterranean East Coast of Algeria
Three species of polychaetous annelids are commercially collected, as baits, from natural populations along the coast of Algeria. They are collected by semi-professional bait harvesters supplying a variety of local outlets and are used as bait by local fishermen. Bait harvesters commonly use bleaching liquid (10% in sea water) or a KMnO4 (0.5 to 1% in sea water) solution to force Perinereis cultrifera (Nereididae) individuals out of their algal mat. Hediste diversicolor (Nereididae) and Scolelepis squamata(Spionidae) are dug from intertidal mudflats and sandy beaches. Commercial prices and ways of utilization are given for each species. The necessity for the legislative establishment of a regulatory management plan for worm angling is demonstrated
The Current Ability to Test Theories of Gravity with Black Hole Shadows
Our Galactic Center, Sagittarius A* (Sgr A*), is believed to harbour a
supermassive black hole (BH), as suggested by observations tracking individual
orbiting stars. Upcoming sub-millimetre very-long-baseline-interferometry
(VLBI) images of Sgr A* carried out by the Event-Horizon-Telescope
Collaboration (EHTC) are expected to provide critical evidence for the
existence of this supermassive BH. We assess our present ability to use EHTC
images to determine if they correspond to a Kerr BH as predicted by Einstein's
theory of general relativity (GR) or to a BH in alternative theories of
gravity. To this end, we perform general-relativistic magnetohydrodynamical
(GRMHD) simulations and use general-relativistic radiative transfer (GRRT)
calculations to generate synthetic shadow images of a magnetised accretion flow
onto a Kerr BH. In addition, and for the first time, we perform GRMHD
simulations and GRRT calculations for a dilaton BH, which we take as a
representative solution of an alternative theory of gravity. Adopting the VLBI
configuration from the 2017 EHTC campaign, we find that it could be extremely
difficult to distinguish between BHs from different theories of gravity, thus
highlighting that great caution is needed when interpreting BH images as tests
of GR.Comment: Published in Nature Astronomy on 16.04.18 (including supplementary
information); simulations at https://blackholecam.org/telling_bhs_apart
How to tell an accreting boson star from a black hole
The capability of the Event Horizon Telescope (EHT) to image the nearest
supermassive black hole candidates at horizon-scale resolutions offers a novel
means to study gravity in its strongest regimes and to test different models
for these objects. Here, we study the observational appearance at 230 GHz of a
surfaceless black hole mimicker, namely a non-rotating boson star, in a
scenario consistent with the properties of the accretion flow onto Sgr A*. To
this end, we perform general relativistic magnetohydrodynamic simulations
followed by general relativistic radiative transfer calculations in the boson
star space-time. Synthetic reconstructed images considering realistic
astronomical observing conditions show that, despite qualitative similarities,
the differences in the appearance of a black hole -- either rotating or not --
and a boson star of the type considered here are large enough to be detectable.
These differences arise from dynamical effects directly related to the absence
of an event horizon, in particular, the accumulation of matter in the form of a
small torus or a spheroidal cloud in the interior of the boson star, and the
absence of an evacuated high-magnetization funnel in the polar regions. The
mechanism behind these effects is general enough to apply to other horizonless
and surfaceless black hole mimickers, strengthening confidence in the ability
of the EHT to identify such objects via radio observations.Comment: 16 pages, 12 figures. Published in MNRAS. Adding more information in
the form of appendices, and a new simulation of a different boson star model.
The conclusions do not chang
Aberrational Effects for Shadows of Black Holes
In this paper, we discuss how the shadow of a Kerr black hole depends on the
motion of the observer. In particular, we derive an analytical formula for the
boundary curve of the shadow for an observer moving with given four-velocity at
given Boyer--Lindquist coordinates. We visualize the shadow for various values
of parameters.Comment: 12 pages, 3 figures; Proceedings of the 524. WE-Heraeus-Seminar held
at the Physikzentrum, Bad Honnef, Germany, 17.--23.2.201
Probing spacetime and accretion model for the Galactic Center: Comparison of Kerr and dilaton black hole shadows
In the vicinity of black holes, the influence of strong gravity, plasma
physics, and emission processes govern the behavior of the system. Since
observations such as those carried out by the EHT are not yet able to
unambiguously constrain models for astrophysical and gravitational properties,
it is imperative to explore the accretion models, particle distribution
function, and description of the spacetime geometry. Our current understanding
of these properties is often based on the assumption that the spacetime is
well-described by by the Kerr solution to general relativity, combined with
basic emission and accretion models. We explore alternative models for each
property performing general relativistic magnetohydrodynamic and radiative
transfer simulations. By choosing a Kerr solution to general relativity and a
dilaton solution to Einstein-Maxwell-dilaton-axion gravity as exemplary black
hole background spacetimes, we aim to investigate the influence of accretion
and emission models on the ability to distinguish black holes in two theories
of gravity.
We carry out three-dimensional general relativistic magnetohydrodynamics
simulations of both black holes, matched at their innermost stable circular
orbit, in two distinct accretion scenarios. Using general-relativistic
radiative transfer calculations, we model the thermal synchrotron emission and
in the next step apply a non-thermal electron distribution function, exploring
representative parameters to compare with multiwavelength observations. We
further consider Kerr and dilaton black holes matched at their unstable
circular photon orbits, as well as their event horizons.
From multiwavelength emission and spectral index analysis, we find that
accretion model and spacetime have only a small impact on the spectra compared
to the choice of emission model
Comparison of the ion-to-electron temperature ratio prescription: GRMHD simulations with electron thermodynamics
The Event Horizon Telescope (EHT) collaboration, an Earth-size sub-millimetre
radio interferometer, recently captured the first images of the central
supermassive black hole in M87. These images were interpreted as
gravitationally-lensed synchrotron emission from hot plasma orbiting around the
black hole. In the accretion flows around low-luminosity active galactic nuclei
such as M87, electrons and ions are not in thermal equilibrium. Therefore, the
electron temperature, which is important for the thermal synchrotron radiation
at EHT frequencies of 230 GHz, is not independently determined. In this work,
we investigate the commonly used parameterised ion-to-electron temperature
ratio prescription, the so-called R- model, considering images at 230
GHz by comparing with electron-heating prescriptions obtained from
general-relativistic magnetohydrodynamical (GRMHD) simulations of magnetised
accretion flows in a Magnetically Arrested Disc (MAD) regime with different
recipes for the electron thermodynamics. When comparing images at 230 GHz, we
find a very good match between images produced with the R- prescription
and those produced with the turbulent- and magnetic reconnection- heating
prescriptions. Indeed, this match is on average even better than that obtained
when comparing the set of images built with the R- prescription with
either a randomly chosen image or with a time-averaged one. From this
comparative study of different physical aspects, which include the image,
visibilities, broadband spectra, and light curves, we conclude that, within the
context of images at 230 GHz relative to MAD accretion flows around
supermassive black holes, the commonly-used and simple R- model is able
to reproduce well the various and more complex electron-heating prescriptions
considered here.Comment: 18 pages, 22 figures, accepted for publication in MNRA
Using space-VLBI to probe gravity around Sgr A*
The Event Horizon Telescope (EHT) will soon provide the first high-resolution
images of the Galactic Centre supermassive black hole (SMBH) candidate
Sagittarius A* (Sgr A*), enabling us to probe gravity in the strong-field
regime. Besides studying the accretion process in extreme environments, the
obtained data and reconstructed images could be used to investigate the
underlying spacetime structure. In its current configuration, the EHT is able
to distinguish between a rotating Kerr black hole and a horizon-less object
like a boson star. Future developments can increase the ability of the EHT to
tell different spacetimes apart. We investigate the capability of an advanced
EHT concept, including an orbiting space antenna, to image and distinguish
different spacetimes around Sgr A*. We use GRMHD simulations of accreting
compact objects (Kerr and dilaton black holes, as well as boson stars) and
compute their radiative signatures via general relativistic radiative transfer
calculations. To facilitate comparison with upcoming and future EHT
observations we produce realistic synthetic data including the source
variability, diffractive and refractive scattering while incorporating the
observing array, including a space antenna. From the generated synthetic
observations we dynamically reconstructed black hole shadow images using
regularised Maximum Entropy methods. We employ a genetic algorithm to optimise
the orbit of the space antenna with respect to improved imaging capabilities
and u-v-plane coverage of the combined array (ground array and space antenna
and developed a new method to probe the source variability in Fourier space.
The inclusion of an orbiting space antenna improves the capability of the EHT
to distinguish the spin of Kerr black holes and dilaton black holes based on
reconstructed radio images and complex visibilities.Comment: 15 pages, 14 figures, A&A accepte
Visibility of black hole shadows in low-luminosity AGN
Accreting black holes tend to display a characteristic dark central region called the black hole shadow, which depends only on space–time/observer geometry and which conveys information about the black hole’s mass and spin. Conversely, the observed central brightness depression, or image shadow, additionally depends on the morphology of the emission region. In this paper, we investigate the astrophysical requirements for observing a meaningful black hole shadow in GRMHD-based models of accreting black holes. In particular, we identify two processes by which the image shadow can differ from the black hole shadow: evacuation of the innermost region of the accretion flow, which can render the image shadow larger than the black hole shadow, and obscuration of the black hole shadow by optically thick regions of the accretion flow, which can render the image shadow smaller than the black hole shadow, or eliminate it altogether. We investigate in which models the image shadows of our models match their corresponding black hole shadows, and in which models the two deviate from each other. We find that, given a compact and optically thin emission region, our models allow for measurement of the black hole shadow size to an accuracy of 5 per cent. We show that these conditions are generally met for all MAD simulations we considered, as well as some of the SANE simulations
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