118 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
Flares in the Galactic Centre II: polarisation signatures of flares at mm-wavelengths
Recent polarimetric mm-observations of the galactic centre by Wielgus et al.
(2022a) showed sinusoidal loops in the Q-U plane with a duration of one hour.
The loops coincide with a quasi-simultaneous X-ray flare. A promising mechanism
to explain the flaring events are magnetic flux eruptions in magnetically
arrested accretion flows (MAD). In our previous work (Porth et al. 2021), we
studied the accretion flow dynamics during flux eruptions. Here, we extend our
previous study by investigating whether polarization loops can be a signature
produced by magnetic flux eruptions. We find that loops in the Q-U plane are
robustly produced in MAD models as they lead to enhanced emissivity of
compressed disk material due to orbiting flux bundles. A timing analysis of the
synthetic polarized lightcurves demonstrate a polarized excess variability at
timescales of ~ 1 hr. The polarization loops are also clearly imprinted on the
cross-correlation of the Stokes parameters which allows to extract a typical
periodicity of 30 min to 1 hr with some evidence for a spin dependence. These
results are intrinsic to the MAD state and should thus hold for a wide range of
astrophysical objects. A subset of GRMHD simulations without saturated magnetic
flux (single temperature SANE models) also produces Q-U loops. However, in
disagreement with the findings of Wielgus et al. (2022a), loops in these
simulations are quasi-continuous with a low polarization excessComment: submitted to MNRA
Magnetic flux eruptions at the root of time-lags in low-luminosity AGN
Sagittarius A is a compact radio source at the center of the Milky Way
that has not conclusively shown evidence for the presence of a relativistic
jet. Nevertheless, indirect methods at radio frequencies do indicate consistent
outflow signatures. Brinkerink et al. (2015) found temporal shifts between
frequency bands, called time-lags, which are associated with flares and/or
outflows of the accretion system. It is possible to gain information on the
emission and potential outflow mechanics by interpreting these time-lags. By
means of combined general-relativistic magnetrohydrodynamical and radiative
transfer modeling, we study the origin of the time-lags for magnetically
arrested disc models at three black hole spins ( = 0.9375, 0, -0.9375).
The study also includes a targeted `slow light' study for one of the
best-fitting `fast light' windows. We were able to recover the time-lags found
by Brinkerink et al. (2015) in various windows of our simulated lightcurves.
The theoretical interpretation of these most-promising time-lag windows is
threefold; i) a magnetic flux eruption perturbs the jet-disc boundary and
creates a flux tube, ii) the flux tube orbits and creates a clear emission
feature, and iii) the flux tube interacts with the jet-disc boundary. The
best-fitting windows have an intermediate (i=30/50) inclination
and zero-BH-spin. The targeted `slow light' study did not yield better-fitting
time-lag results, which indicates that the fast vs. slow light paradign is
often not intuitively understood and is likely influential in timing-sensitive
studies.Comment: 17 pages, 11 figure
Kink instability: evolution and energy dissipation in Relativistic Force-Free Non-Rotating Jets
We study the evolution of kink instability in a force-free, non-rotating
plasma column of high magnetization. The main dissipation mechanism is
identified as reconnection of magnetic field-lines with various intersection
angles, driven by the compression of the growing kink lobes. We measure
dissipation rates ,
where is the linear growth time of the kink instability. This value is
consistent with the expansion velocity of the kink mode, which drives the
reconnection. The relaxed state is close to a force-free Taylor state. We
constraint the energy of that state using considerations from linear stability
analysis. Our results are important for understanding magnetic field
dissipation in various extreme astrophysical objects, most notably in
relativistic jets. We outline the evolution of the kink instability in such
jets and derive constrains on the conditions that allow for the kink
instability to grow in these systems.Comment: 18 pages, 13 figure
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
Linear analysis of the Kelvin-Helmholtz instability in relativistic magnetized symmetric flows
We study the linear stability of a planar interface separating two fluids in
relative motion, focusing on the symmetric configuration where the two fluids
have the same properties (density, temperature, magnetic field strength, and
direction). We consider the most general case with arbitrary sound speed
, Alfv\'en speed , and magnetic field orientation. For
the instability associated with the fast mode, we find that the lower bound of
unstable shear velocities is set by the requirement that the projection of the
velocity onto the fluid-frame wavevector is larger than the projection of the
Alfv\'en speed onto the same direction, i.e., shear should overcome the effect
of magnetic tension. In the frame where the two fluids move in opposite
directions with equal speed , the upper bound of unstable velocities
corresponds to an effective relativistic Mach number , where is the fast speed assuming a
magnetic field perpendicular to the wavevector (here, all velocities are in
units of the speed of light), and is the laboratory-frame angle
between the flow velocity and the wavevector projection onto the shear
interface. Our results have implications for shear flows in the magnetospheres
of neutron stars and black holes -- both for single objects and for merging
binaries -- where the Alfv\'en speed may approach the speed of light.Comment: 11 pages, 7 figures, 1 table, Accepted for publication in Monthly
Notices of the Royal Astronomical Societ
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
Disappearing thermal X-ray emission as a tell-tale signature of merging massive black hole binaries
The upcoming Laser Interferometer Space Antenna (LISA) is expected to detect
gravitational waves (GWs) from massive black hole binaries (MBHB). Finding the
electromagnetic (EM) counterparts for these GW events will be crucial for
understanding how and where MBHBs merge, measuring their redshifts,
constraining the Hubble constant and the graviton mass, and for other novel
science applications. However, due to poor GW sky localisation,
multi-wavelength, time-dependent electromagnetic (EM) models are needed to
identify the right host galaxy among many candidates. We studied merging MBHBs
embedded in a circumbinary disc using high-resolution two-dimensional
simulations, with a -law equation of state, incorporating viscous
heating, shock heating, and radiative cooling. We simulate the binary from
large separation until after merger, allowing us to model the decoupling of the
binary from the circumbinary disc (CBD). We compute the EM signatures and
identify distinct features before, during, and after the merger. Our main
result is a multi-band EM signature: we find that the MBHB produces strong
thermal X-ray emission until 1-2 days prior to the merger. However, as the
binary decouples from the CBD, the X-ray-bright minidiscs rapidly shrink in
size, become disrupted, and the accretion rate drops precipitously. As a
result, the thermal X-ray luminosity drops by orders of magnitude, and the
source remains X-ray dark for several days after the merger, regardless of any
post-merger effects such as GW recoil or mass loss. Looking for the abrupt
spectral change where the thermal X-ray disappears is a tell-tale EM signature
of LISA mergers that does not require extensive pre-merger monitoring.Comment: 14 pages, 16 figures, 1 table, submitted to journa
Self-lensing flares from black hole binaries III: general-relativistic ray tracing of circumbinary accretion simulations
Self-lensing flares (SLFs) are expected to be produced once or twice per
orbit by an accreting massive black hole binary (MBHB), if the eclipsing MBHBs
are observed close to edge-on. SLFs can provide valuable electromagnetic (EM)
signatures to accompany the gravitational waves (GWs) detectable by the
upcoming Laser Interferometer Space Antenna (LISA). EM follow-ups are crucial
for, e.g., sky-localization, and constraining the Hubble constant and the
graviton mass. We use high-resolution two-dimensional viscous hydrodynamical
simulations of a circumbinary disk (CBD) embedding a MBHB. We then use very
high-cadence output of these hydrodynamical simulation inputs for a
general-relativistic ray-tracing code to produce synthetic spectra and
phase-folded light curves. Our main results show a significant periodic
amplification of the flux with the characteristic shape of a sharp flare with a
central dip, as the foreground black hole (BH) transits across the minidisk and
shadow of the background BH, respectively. These corroborate previous
conclusions based on the microlensing approximation and analytical toy models
of the emission geometry. We also find that at lower inclinations, without some
occlusion of the minidisk emission by the CBD, shocks from quasi-periodic
mass-trading between the minidisks can produce bright flares which can mimic
SLFs and could hinder their identification.Comment: 14 pages, 11 figures, submitted to journal, split Fig. 1 by
frequency, fixed some typo
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