14 research outputs found
GRRMHD Simulations of MAD Accretion Disks Declining from Super-Eddington to Sub-Eddington Accretion Rates
We present two general relativistic radiation magnetohydrodynamics (GRRMHD)
simulations of magnetically arrested disks (MADs) around non-spinning ()
and spinning () supermassive black holes (BHs). In each simulation,
the mass accretion rate is decreased with time such that we sample
Eddington-scaled rates over the range . For the non-spinning BH model, the
total and radiative efficiencies increase as the accretion rate decreases,
varying over the range and
, respectively. This model shows very little jet
activity. In contrast, the spinning BH model has a strong relativistic jet
powered by spin energy extracted from the BH. The jet power declines with
accretion rate such that while the total and
radiative efficiencies are and
, respectively. We confirm that mildly
sub-Eddington disks can extract substantial power from a spinning BH, provided
they are in the MAD state. The jet profile out to is roughly
parabolic with a power-law index of during the
sub-Eddington evolution. Both models show significant variability in the
outgoing radiation which is likely associated with episodes of magnetic flux
eruptions. The model shows semi-regular variations with a period of
over the final of the simulation,
which suggests that magnetic flux eruptions may be an important source of
quasi-periodic variability. For the simulated accretion rates, the
model is spinning up while the model is spinning down.
Spinup-spindown equilibrium of the BH will likely be achieved at , assuming continuous accretion in the MAD state.Comment: 23 pages, 18 figure
Strongly Magnetized Tidal Disruption Event Disks via Stream Injection in GRMHD
Magnetically arrested accretion disks (MADs) around a rapidly rotating black
hole (BH) have been proposed as a model for jetted tidal disruption events
(TDEs). However, the dynamics of strongly magnetized disks in a more realistic
simulation which can mimic the chaotic dynamics during a TDE have previously
been unexplored. Here we employ global GRMHD simulations of a pre-existing MAD
disk interacting with an injected TDE stream with impact parameter to investigate how strongly magnetized TDEs differ from the
standard MAD picture. We demonstrate for the first time that a MAD or semi-MAD
state can be sustained and jets powered by the BH spin are produced in a TDE.
We also demonstrate that the strength of the self-intersection shock depends on
how dense the disk is relative to the stream, or the density contrast
. The jet or funnel can become significantly tilted (by
) due to the self-intersection outflow when . In
models with a powerful jet and , the tilted jet interacts with
and ultimately tilts the disk by as much as 23 degrees from the incoming
stream. We illustrate that as increases, the tilt of the jet and disk
is expected to realign with the BH spin once . We illustrate
how the tilt can rapidly realign if increases rapidly and apply this
to TDEs which have shown X-ray evolution on timescales of days-weeks.Comment: 21 pages, 22 figures, videos available at
https://www.youtube.com/playlist?list=PL6Na55ZD3RmoJl7Rjhn6gCeAE0HWYCI0
Jets from SANE Super-Eddington Accretion Disks: Morphology, Spectra, and Their Potential as Targets for ngEHT
We present general relativistic radiation magnetohydrodynamics (GRRMHD)
simulations of super-Eddington accretion flows around supermassive black holes
(SMBHs) which may apply to tidal disruption events (TDEs). We perform long
duration () simulations which achieve mass accretion
rates times the Eddington rate and produce thermal synchrotron
spectra and images of their jets. The jet reaches a maximum velocity of , but the density weighted outflow velocity is .
Gas flowing beyond the funnel wall expands conically and drives a strong shock
at the jet head while variable mass ejection along the jet axis results in
internal shocks and dissipation. For a model, the
radio/submillimeter spectra peak at GHz and the luminosity increases
with BH spin, exceeding in the brightest
models. The emission is extremely sensitive to as some models show an
order of magnitude decrease in the peak frequency and up to four orders of
magnitude decline in their radio/submillimeter luminosity as
approaches 20. Assuming a maximum VLBI baseline distance of $10 \
{\rm{G}}\lambdaT_i/T_e=1D\lesssim1105\sigmaD\lesssim45\lesssim10^{36} \
{\rm{erg\, s^{-1}}}>230$ GHz
radio/submillimeter followup of future TDEs
A Dark Spot on a Massive White Dwarf
We present the serendipitous discovery of eclipse-like events around the
massive white dwarf SDSS J152934.98+292801.9 (hereafter J1529+2928). We
selected J1529+2928 for time-series photometry based on its spectroscopic
temperature and surface gravity, which place it near the ZZ Ceti instability
strip. Instead of pulsations, we detect photometric dips from this white dwarf
every 38 minutes. Follow-up optical spectroscopy observations with Gemini
reveal no significant radial velocity variations, ruling out stellar and brown
dwarf companions. A disintegrating planet around this white dwarf cannot
explain the observed light curves in different filters. Given the short period,
the source of the photometric dips must be a dark spot that comes into view
every 38 min due to the rotation of the white dwarf. Our optical spectroscopy
does not show any evidence of Zeeman splitting of the Balmer lines, limiting
the magnetic field strength to B<70 kG. Since up to 15% of white dwarfs display
kG magnetic fields, such eclipse-like events should be common around white
dwarfs. We discuss the potential implications of this discovery on transient
surveys targeting white dwarfs, like the K2 mission and the Large Synoptic
Survey Telescope.Comment: ApJ Letters, in pres
On the Comparison of AGN with GRMHD Simulations: II. M87
Horizon-scale observations of the jetted active galactic nucleus M87 are
compared with simulations spanning a broad range of dissipation mechanisms and
plasma content in three-dimensional general relativistic flows around spinning
black holes. Observations of synchrotron radiation from radio to X-ray
frequencies can be compared with simulations by adding prescriptions specifying
the relativistic electron-plus-positron distribution function and associated
radiative transfer coefficients. A suite of time-varying simulations with
various spins and plasma magnetizations is chosen to represent distinct
possibilities for the M87 jet/accretion flow/black hole (JAB) system. We then
input turbulent heating and equipartition-based emission prescriptions (and
piecewise combinations thereof) in the time-dependent 3D simulations, in which
jet morphology, polarization and variation are "observed" and compared with
real observations so as to try to infer the rules that govern the polarized
emissivity. The models in this paper support a magnetically arrested disk (MAD)
with several possible spin/emission model combinations supplying the jet in
M87, whose inner jet and black hole shadow have been observed down to the
photon ring at 230 GHz by the Event Horizon Telescope (EHT). We also show that
some MAD cases that are dominated by intrinsic circular polarization have
near-linear V/I dependence on unpaired electron or positron content while SANE
polarization exhibits markedly greater positron-dependent Faraday effects --
future probes of the SANE/MAD dichotomy and plasma content with the EHT. This
is the second work in a series also applying the "observing" simulations
methodology to near-horizon regions of supermassive black holes in Sgr A* and
3C 279.Comment: 25 pages, 27 figures, submitted to MNRA
Four new massive pulsating white dwarfs including an ultramassive DAV
We report the discovery of four massive (M > 0.8M ) ZZ Ceti white dwarfs, including an ultramassive 1.16M star. We obtained ground-based, time series photometry for 13 white dwarfs from the Sloan Digital Sky Survey Data Release 7 and Data Release 10 whose atmospheric parameters place them within the ZZCeti instability strip.We detect monoperiodic pulsations in three of our targets (J1015, J1554 and J2038) and identify three periods of pulsation in J0840 (173, 327 and 797 s). Fourier analysis of the remaining nine objects does not indicate variability above the 4 A detection threshold. Our preliminary asteroseismic analysis of J0840 yields a stellar mass M = 1.14 ± 0.01M , hydrogen and helium envelope masses of MH = 5.8 × 10−7M and MHe = 4.5 × 10−4M and an expected core crystallized mass ratio of 50–70 per cent. J1015, J1554 and J2038 have masses in the range 0.84–0.91M and are expected to have a CO core; however, the core of J0840 could consist of highly crystallized CO or ONeMg given its high mass. These newly discovered massive pulsators represent a significant increase in the number of known ZZ Ceti white dwarfs with mass M > 0.85M , and detailed asteroseismic modelling of J0840 will allow for significant tests of crystallization theory in CO and ONeMg core white dwarfs
Modeling Reconstructed Images of Jets Launched by SANE Super-Eddington Accretion Flows around SMBHs with the ngEHT
Tidal disruption events (TDEs) around supermassive black holes (SMBHs) are a potential laboratory to study super-Eddington accretion disks and sometimes result in powerful jets or outflows which may shine in the radio and sub-millimeter bands. In this work, we modeled the thermal synchrotron emission of jets by general relativistic radiation magneto-hydrodynamics (GRRMHD) simulations of a BH accretion disk/jet system which assumed the TDE resulted in a magnetized accretion disk around a BH accreting at ∼12–25 times the Eddington accretion rate. Through synthetic observations with the Next Generation Event Horizon Telescope (ngEHT) and an image reconstruction analysis, we demonstrate that TDE jets may provide compelling targets within the context of the models explored in this work. In particular, we found that jets launched by a SANE super-Eddington disk around a spin a*=0.9 reach the ngEHT detection threshold at large distances (up to 100 Mpc in this work). A two-temperature plasma in the jet or weaker jets, such as a spin a*=0 model, requires a much closer distance, as we demonstrate detection at 10 Mpc for limiting cases of a*=0,R=1 or a*=0.9,R=20. We also demonstrate that TDE jets may appear as superluminal sources if the BH is rapidly rotating and the jet is viewed nearly face on