2,881 research outputs found
General relativistic magnetohydrodynamical simulations of the jet in M87
(abridged) The connection between black hole, accretion disk, and radio jet
can be best constrained by fitting models to observations of nearby low
luminosity galactic nuclei, in particular the well studied sources Sgr~A* and
M87. There has been considerable progress in modeling the central engine of
active galactic nuclei by an accreting supermassive black hole coupled to a
relativistic plasma jet. However, can a single model be applied to a range of
black hole masses and accretion rates? Here we want to compare the latest
three-dimensional numerical model, originally developed for Sgr A* in the
center of the Milky Way, to radio observations of the much more powerful and
more massive black hole in M87. We postprocess three-dimensional GRMHD models
of a jet-producing radiatively inefficient accretion flow around a spinning
black hole using relativistic radiative transfer and ray-tracing to produce
model spectra and images. As a key new ingredient to these models, we allow the
proton-electron coupling in these simulations depend on the magnetic properties
of the plasma. We find that the radio emission in M87 is well described by a
combination of a two-temperature accretion flow and a hot single-temperature
jet. The model fits the basic observed characteristics of the M87 radio core.
The best fit model has a mass-accretion rate of Mdot approx 9x10^{-3} MSUN/YR
and a total jet power of P_j \sim 10^{43} erg/s. Emission at 1.3mm is produced
by the counter jet close to the event horizon. Its characteristic crescent
shape surrounding the black hole shadow could be resolved by future
millimeter-wave VLBI experiments. The model was successfully derived from one
for the supermassive black hole in center of the Milky Way by appropriately
scaling mass and accretion rate. This suggests the possibility that this model
could also apply to a larger range of low-luminosity black holes.Comment: 15 pages, 14 figures, accepted to Astronomy and Astrophysics, after
language proofs, with correct titl
Insights into the nature of northwest-to-southeast aligned ionospheric wavefronts from contemporaneous Very Large Array and ionosondes observations
The results of contemporaneous summer nighttime observations of midlatitude
medium scale traveling ionospheric disturbances (MSTIDs) with the Very Large
Array (VLA) in New Mexico and nearby ionosondes in Texas and Colorado are
presented. Using 132, 20-minute observations, several instances of MSTIDs were
detected, all having wavefronts aligned northwest to southeast and mostly
propagating toward the southwest, consistent with previous studies of MSTIDs.
However, some were also found to move toward the northeast. It was found that
both classes of MSTIDs were only found when sporadic-E (Es) layers of moderate
peak density (1.5<foEs<3 MHz) were present. Limited fbEs data from one
ionosonde suggests that there was a significant amount of structure with the Es
layers during observations when foEs>3 MHz that was not present when 1.5<foEs<3
MHz. No MSTIDs were observed either before midnight or when the F-region height
was increasing at a relatively high rate, even when these Es layers were
observed. Combining this result with AE indices which were relatively high at
the time (an average of about 300 nT and maximum of nearly 700 nT), it is
inferred that both the lack of MSTIDs and the increase in F-region height are
due to substorm-induced electric fields. The northeastward-directed MSTIDs were
strongest post-midnight during times when the F-region was observed to be
collapsing relatively quickly. This implies that these two occurrences are
related and likely both caused by rare shifts in F-region neutral wind
direction from southwest to northwest.Comment: Accepted for publication in the Journal of Geophysical Researc
Observational appearance of inefficient accretion flows and jets in 3D GRMHD simulations: Application to Sgr~A*
Radiatively inefficient accretion flows (RIAFs) are believed to power
supermassive black holes (SMBH) in the underluminous cores of galaxies. Such
black holes are typically accompanied by flat-spectrum radio cores indicating
the presence of moderately relativistic jets. One of the best constrained RIAFs
is associated with the SMBH in the Galactic center, Sgr A*. Since the plasma in
RIAFs is only weakly collisional, the dynamics and the radiative properties of
these systems are very uncertain. Here we want to study the impact of varying
electron temperature on the appearance of accretion flows and jets. Using 3-D
GRMHD accretion flow simulations, we use ray tracing methods to predict spectra
and radio images of RIAFs allowing for different electron heating mechanisms in
the in- and outflowing parts of the simulations. We find that small changes in
the electron temperature can result in dramatic differences in the relative
dominance of jets and accretion flows. Application to Sgr A* shows that radio
spectrum and size of this source can be well reproduced with a model where
electrons are more efficiently heated in the jet. The X-ray emission is
sensitive to the electron heating mechanism in the jets and disk and therefore
X-ray observations put strong constraints on electron temperatures and geometry
of the accretion flow and jet. For Sgr A*, the jet model also predicts a
significant frequency-dependent core shift which could place independent
constraints on the model once measured accurately. We conclude that more
sophisticated models for electron distribution functions are crucial for
constraining GRMHD simulations with actual observations. For Sgr A*, the radio
appearance may well be dominated by the outflowing plasma. Nonetheless, at the
highest radio frequencies, the shadow of the event horizon should still be
detectable with future Very Long Baseline Interferometric observations.Comment: A&A accepted, 11 figures, 1 tabl
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