arXiv subjects

Doosoo Yoon

Publications and source records attributed to Doosoo Yoon.

12 recordsLinked to original sources

arXiv2024

Two-temperature treatments in magnetically arrested disk GRMHD simulations more accurately predict light curves of Sagittarius A*

The Event Horizon Telescope Collaboration (EHTC) observed the Galactic centre source Sagittarius A* (Sgr A*) and used emission models primarily based on single ion temperature (1T) general relativistic magnetohydrodynamic (GRMHD) simulations. This predicted emission is strongly dependent on a modelled prescription of the ion-to-electron temperature ratio. The most promising models are magnetically arrested disk (MAD) states. However, nearly all MAD models exhibit larger temporal fluctuations in radiative 230 GHz emission compared to observations. This limitation possibly stems from the fact that the actual temperature ratio depends on microphysical dissipation, radiative processes and other effects not captured in ideal fluid simulations. Therefore, we investigate the effects of two-temperature (2T) thermodynamics in MAD GRMHD simulations of Sgr A*, where the temperatures of both species are evolved. We find that the 230 GHz synchrotron flux variability more closely matches historical observations when we include the 2T treatment compared to 1T simulations. For the low accretion rates of Sgr A*, a common assumption is to neglect radiative cooling. However, we find that the radiative cooling of electrons-via synchrotron, inverse Compton, and bremsstrahlung processes-reduces the electron temperature in the inner disk, where the EHT observes, by about 10%, which, in turn, decreases both the (sub)millimetre synchrotron flux and its temporal fluctuations compared to uncooled simulations.

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arXiv2020

General relativistic MHD simulations of non-thermal flaring in Sagittarius A*

Sagittarius A* exhibits regular variability in its multiwavelength emission, including daily X-ray flares and roughly continuous near-infrared (NIR) flickering. The origin of this variability is still ambiguous since both inverse Compton and synchrotron emission are possible radiative mechanisms. The underlying particle distributions are also not well constrained, particularly the non-thermal contribution. In this work, we employ the GPU-accelerated general relativistic magnetohydrodynamics (GRMHD) code H-AMR perform a study of flare flux distributions, including the effect of particle acceleration for the first time in high-resolution 3D simulations of Sgr A*. For the particle acceleration, we use the general relativistic ray-tracing (GRRT) code BHOSS to perform the radiative transfer, assuming a hybrid thermal+non-thermal electron energy distribution. We extract ~60 h lightcurves in the sub-millimetre, NIR and X-ray wavebands, and compare the power spectra and the cumulative flux distributions of the lightcurves to statistical descriptions for Sgr A* flares. Our results indicate that non-thermal populations of electrons arising from turbulence-driven reconnection in weakly magnetised accretion flows lead to moderate NIR and X-ray flares and reasonably describe the X-ray flux distribution while fulfilling multiwavelength flux constraints. These models exhibit high rms% amplitudes, >~150% both in the NIR and the X-rays, with changes in the accretion rate driving the 230~GHz flux variability, in agreement with Sgr A* observations.

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arXiv2020

Spectral and Imaging properties of Sgr A* from High-Resolution 3D GRMHD Simulations with Radiative Cooling

The candidate supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*), is known to be fed by a radiatively inefficient accretion flow (RIAF), inferred by its low accretion rate. Consequently, radiative cooling has in general been overlooked in the study of Sgr A*. However, the radiative properties of the plasma in RIAFs are poorly understood. In this work, using full 3D general-relativistic magneto-hydrodynamical simulations, we study the impact of radiative cooling on the dynamical evolution of the accreting plasma, presenting spectral energy distributions and synthetic sub-millimeter images generated from the accretion flow around Sgr A*. These simulations solve the approximated equations for radiative cooling processes self-consistently, including synchrotron, bremsstrahlung, and inverse Compton processes. We find that radiative cooling plays an increasingly important role in the dynamics of the accretion flow as the accretion rate increases: the mid-plane density grows and the infalling gas is less turbulent as cooling becomes stronger. The changes in the dynamical evolution become important when the accretion rate is larger than $10^{-8}\,M_{\odot}~{\rm yr}^{-1}$ ($\gtrsim 10^{-7} \dot{M}_{\rm Edd}$, where $\dot{M}_{\rm Edd}$ is the Eddington accretion rate). The resulting spectra in the cooled models also differ from those in the non-cooled models: the overall flux, including the peak values at the sub-mm and the far-UV, is slightly lower as a consequence of a decrease in the electron temperature. Our results suggest that radiative cooling should be carefully taken into account in modelling Sgr A* and other low-luminosity active galactic nuclei that have a mass accretion rate of $\dot{M} > 10^{-7}\,\dot{M}_{\rm Edd}$.

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arXiv2020

Observational signatures of disk and jet misalignment in images of accreting black holes

Black hole accretion is one of nature's most efficient energy extraction processes. When gas falls in, a significant fraction of its gravitational binding energy is either converted into radiation or flows outwards in the form of black hole-driven jets and disk-driven winds. Recently, the Event Horizon Telescope (EHT), an Earth-size sub-millimetre radio interferometer, captured the first images of M87's black hole. These images were analysed and interpreted using general-relativistic magnetohydrodynamics (GRMHD) models of accretion disks with rotation axes aligned with the black hole spin axis. However, since infalling gas is often insensitive to the black hole spin direction, misalignment between accretion disk and black hole spin may be a common occurrence in nature. In this work, we use the general-relativistic radiative transfer (GRRT) code \texttt{BHOSS} to calculate the first synthetic radio images of (highly) tilted disk/jet models generated by our GPU-accelerated GRMHD code \texttt{HAMR}. While the tilt does not have a noticeable effect on the system dynamics beyond a few tens of gravitational radii from the black hole, the warping of the disk and jet can imprint observable signatures in EHT images on smaller scales. Comparing the images from our GRMHD models to the 43 GHz and 230 GHz EHT images of M87, we find that M87 may feature a tilted disk/jet system. Further, tilted disks and jets display significant time variability in the 230 GHz flux that can be further tested by longer-duration EHT observations of M87.

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arXiv2019

On the Role of Hot Feedback Mode in Active Galactic Nuclei Feedback in an Elliptical Galaxy

Depending on the value of the accretion rate, black hole accretion is divided into cold and hot modes. The two modes have distinctly different physics and correspond to two feedback modes. Most previous feedback works either focus only on one mode, or the accretion physics is not always properly adopted. Here, by performing hydrodynamical numerical simulations of AGN feedback in an elliptical galaxy, we show that including both is important, and gives different results from including just one or the other. We specifically focus on the wind and radiation (but neglecting the jet) feedback in the hot mode, to explore whether this particular mode of feedback can play any important feedback role. For this aim, we have run two test models. In one model, we always adopt the cold mode no matter what value the accretion rate is; in another model, we turn off the AGN once it enters into the hot mode. We have calculated the AGN light curves, black hole growth, star formation, and AGN energy duty-cycle; and compared the results with the model in which the two modes are correctly included. Important differences are found. For example, if we were to adopt only the cold mode, the total mass of newly formed stars would become two orders of magnitude smaller, and the fraction of energy ejected within the high accretion regime (i.e., $L_{\rm BH} > 2\% L_{\rm Edd}$) would be too small to be consistent with observations. We have also investigated the respective roles of wind and radiation in the hot mode.

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arXiv2018

Stellar and AGN feedback in isolated early-type galaxies: the role in regulating star formation and ISM properties

Understanding how galaxies maintain the inefficiency of star formation with physically self-consistent models is a central problem for galaxy evolution. Although numerous theoretical models have been proposed in recent decades, the debate still exists. By means of high-resolution two-dimensional hydrodynamical simulations, we study the three feedback effects (the stellar wind heating, SNe feedback, and AGN feedback) in suppressing star formation activities on the evolution of early-type galaxies with different stellar masses. AGN feedback models are updated based on \citet{Yuan2018}. The gas sources comes exclusively from the mass losses of dying low-mass stars for most of our models. We find that SNe feedback can keep star formation at a significantly low level for low mass elliptical galaxies for a cosmological evolution time. For the high mass galaxies, AGN feedback can efficiently offset the radiative cooling and thus regulate the star formation activities. Such a suppression of star formation is extremely efficient in the inner region of the galaxies. AGB heating cannot account for this suppression for low and high mass galaxies. The X-ray temperature $T_{\rm X}$ and luminosity $L_{\rm X}$ of hot plasma can be in agreement with the observed data with the inclusion of effective feedback processes. These results thus suggest that we can use $T_{\rm X}$ and $L_{\rm X}$ to probe the role of different feedback processes. The inclusion of additional gas sources can make the mass scale between SNe and AGN feedback dominating in suppressing star formation decrease to an observationally inferred value of a few $10^{10}~M_{\odot}$.

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arXiv2018

Active Galactic Nuclei Feedback in an Elliptical Galaxy with the Most Updated AGN Physics (II): High-Angular Momentum Case

This is the second paper of our series of works of studying the effects of active galactic nuclei (AGN) feedback on the cosmological evolution of an isolated elliptical galaxy by performing two-dimensional high-resolution hydrodynamical numerical simulations. In these simulations, the inner boundary is chosen so that the Bondi radius is resolved. Physical processes like star formation, SNe Ia and II are taken into account. Compared to previous works, the main improvements is that we adopt the most updated AGN physics, which is described in detail in the first paper of this series (Yuan et al. 2018, Paper I). These improvements include the discrimination of the two accretion modes of the central AGN and the most updated descriptions of the wind and radiation in the two modes. In Paper I, we consider the case that the specific angular momentum of the gas in the galaxy is very low. In this paper, we consider the case that the specific angular momentum of the gas is high. In the galactic scale, we adopt the gravitational torques raised due to non-axisymmetric structure in the galaxy as the mechanism of the transfer of angular momentum of gas, as proposed in some recent works. Since our simulations are axisymmetric, we make use of a parameterized prescription to mimic this mechanism. Same as Paper I, we investigate the AGN light curve, typical AGN lifetime, growth of the black hole mass, AGN duty-cycle, star formation, and the X-ray surface brightness of the galaxy. Special attention is paid to the effects of specific angular momentum of the galaxy on these properties. We find that some results are qualitatively similar to those shown in Paper I, while some results such as star formation and black hole growth do show a significant difference due to the mass concentration in the galactic disk as a consequence of galactic rotation.

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arXiv2018

Active Galactic Nuclei Feedback in an Elliptical Galaxy with the Most Updated AGN Physics (I): Low-angular Momentum Case

We investigate the effects of AGN feedback on the cosmological evolution of an isolated elliptical galaxy by performing two-dimensional high-resolution hydrodynamical numerical simulations. The inner boundary of the simulation is chosen so that the Bondi radius is resolved. Compared to previous works, the two accretion modes, namely hot and cold, which correspond to different accretion rates and have different radiation and wind outputs, are carefully discriminated and the feedback effects by radiation and wind in each mode are taken into account. The most updated AGN physics, including the descriptions of radiation and wind from the hot accretion flows and wind from cold accretion disks, are adopted. Physical processes like star formation, Type Ia and Type II supernovae are taken into account. We study the AGN light curve, typical AGN lifetime, growth of the black hole mass, AGN duty-cycle, star formation, and the X-ray surface brightness of the galaxy. We compare our simulation results with observations and find general consistency. Comparisons with previous simulation works find significant differences, indicating the importance of AGN physics. The respective roles of radiation and wind feedbacks are examined and it is found that they are different for different problems of interest such as AGN luminosity and star formation. We find that it is hard to neglect any of them, so we suggest to use the names of "cold feedback mode" and "hot feedback mode" to replace the currently used ones.

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arXiv2016

Bow-shock Pulsar Wind Nebulae Passing Through Density Discontinuities

Bow-shock pulsar wind nebulae are a subset of pulsar wind nebulae that form when the pulsar has high velocity due to the natal kick during the supernova explosion. The interaction between the relativistic wind from the fast-moving pulsar and the interstellar medium produces a bow-shock and a trail, which are detectable in H$_α$ emission. Among such bow-shock pulsar wind nebulae, the Guitar Nebula stands out for its peculiar morphology, which consists of a prominent bow-shock head and a series of bubbles further behind. We present a scenario in which multiple bubbles can be produced when the pulsar encounters a series of density discontinuities in the ISM. We tested the scenario using 2-D and 3-D hydrodynamic simulations. The shape of the guitar nebula can be reproduced if the pulsar traversed a region of declining low density. We also show that if a pulsar encounters an inclined density discontinuity, it produces an asymmetric bow-shock head, consistent with observations of the bow-shock of the millisecond pulsar J2124-3358.

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arXiv2015

Global Simulations of the Interaction of Microquasar Jets with a Stellar Wind in High-Mass X-ray Binaries

Jets powered by high-mass X-ray binaries must traverse the powerful wind of the companion star. We present the first global 3D simulations of jet-wind interaction in high-mass X-ray binaries. We show that the wind momentum flux intercepted by the jet can lead to significant bending of the jet and that jets propagating through a spherical wind will be bent to an asymptotic angle $ψ_{\infty}$. We derive simple expressions for $ψ_{\infty}$ as a function of jet power and wind thrust. For known wind parameters, measurements of $ψ_{\infty}$ can be used to constrain the jet power. In the case of Cygnus X-1, the lack of jet precession as a function of orbital phase observed by the VLBA can be used to put a lower limit on the jet power of $L_{\rm jet} \gtrsim 10^{36}\,{\rm ergs\,s^{-1}}$. We further discuss the case where the initial jet is inclined relative to the binary orbital axis. We also analyze the case of Cygnus X-3 and show that jet bending is likely negligible unless the jet is significantly less powerful or much wider than currently thought. Our numerical investigation is limited to isotropic stellar winds. We discuss the possible effect of wind clumping on jet-wind interaction, which are likely significant, but argue that our limits on jet power for Cygnus X-1 are likely unaffected by clumping unless the global wind mass loss rate is orders of magnitude below the commonly assumed range for Cyg X-1.

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arXiv2011

Central Regions of Barred Galaxies: Two-Dimensional Non-self-gravitating Hydrodynamic Simulations

The inner regions of barred galaxies contain substructures such as off-axis shocks, nuclear rings, and nuclear spirals. These substructure may affect star formation, and control the activity of a central black hole (BH) by determining the mass inflow rate. We investigate the formation and properties of such substructures using high-resolution, grid-based hydrodynamic simulations. The gaseous medium is assumed to be infinitesimally-thin, isothermal, and non-self-gravitating. The stars and dark matter are represented by a static gravitational potential with four components: a stellar disk, the bulge, a central BH, and the bar. To investigate various galactic environments, we vary the gas sound speed c_s as well as the mass of the central BH M_BH. Once the flow has reached a quasi-steady state, off-axis shocks tend to move closer to the bar major axis as c_s increases. Nuclear rings shrink in size with increasing c_s, but are independent of M_BH, suggesting that ring position is not determined by the Lindblad resonances. Rings in low-c_s models are narrow since they are occupied largely by gas on x2-orbits and well decoupled from nuclear spirals, while they become broad because of large thermal perturbations in high-c_s models. Nuclear spirals persist only when either c_s is small or M_BH is large; they would otherwise be destroyed completely by the ring material on eccentric orbits. The shape and strength of nuclear spirals depend sensitively on c_s and M_BH such that they are leading if both c_s and M_BH are small, weak trailing if c_s is small and M_BH is large, and strong trailing if both c_s and M_BH are large. While the mass inflow rate toward the nucleus is quite small in low-c_s models because of the presence of a narrow nuclear ring, it becomes larger than 0.01 Msun/yr when c_s is large, providing a potential explanation of nuclear activity in Seyfert galaxies.

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arXiv2011

Jet trails and Mach cones: The interaction of microquasars with the ISM

A sub-set of microquasars exhibit high peculiar velocity with respect to the local standard of rest due to the kicks they receive when being born in supernovae. The interaction between the radio plasma released by microquasar jets from such high-velocity binaries with the ISM must lead to the production of trails and bow shocks similar to what is observed in narrow-angle tailed radio galaxies and pulsar wind nebulae. We present a set of numerical simulations of this interaction that illuminate the long term dynamical evolution and the observational properties of these microquasar bow shock nebulae and trails. We find that this interaction always produces a structure that consists of a bow shock, a trailing neck, and an expanding bubble. Using our simulations to model emission, we predict that the shock surrounding the bubble and the neck should be visible in Hα emission, the interior of the bubble should be visible in synchrotron radio emission, and only the bow shock is likely to be detectable in X-ray emission. We construct an analytic model for the evolution of the neck and bubble shape and compare this model with observations of X-ray binary SAX J1712.6-3739.

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