arXiv subjects

Siming Liu

Publications and source records attributed to Siming Liu.

At least 109 records · Page 6Linked to original sources

arXiv2008

Relative Spectra and Distributions of Fluences of 3He and 4He in Solar Energetic Particles

Solar Energetic Particles (SEPs) show a rich variety of spectra and relative abundances of many ionic species and their isotopes. A long standing puzzle has been the extreme enrichments of 3He ions. The most extreme enrichments are observed in low fluence, the so-called impulsive, events which are believed to be produced at the flare site in the solar corona with little scattering and acceleration during transport to the Earth. In two earlier papers (Liu et al. 2004 and 2006) we showed how such extreme enrichments can result in the model developed by Petrosian and Liu (2004), where ions are accelerated stochastically by plasma waves or turbulence. In this paper we address the relative distributions of the fluences of 3He and 4He ions presented by Ho et al. (2005) which show that while the distribution of 4He fluence like many other extensive characteristics of solar flare, is fairly broad, the 3He fluence is limited to a narrow range. Moreover, the ratio of the fluences shows a strong correlation with the 4He fluence. One of the predictions of our model was presence of steep variation of the fluence ratio with the level of turbulence or the rate of acceleration. We show here that this feature of the model can reproduce the observed distribution of the fluences with very few free parameters. The primary reason for the success of the model in both fronts is because fully ionized 3He ion, with its unique charge to mass ratio, can resonantly interact with more plasma modes and accelerate more readily than 4He. Essentially in most flares, all background 3He ions are accelerated to few MeV/nucleon range, while this happens for 4He ions only in very strong events. A much smaller fraction of 4He ions reach such energies in weaker events.

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arXiv2008

Stochastic Electron Acceleration in Shell-Type Supernova Remnants

We study the stochastic electron acceleration by fast mode waves in the turbulent downstream of weakly magnetized collisionless astrophysical shocks. The acceleration is most efficient in a dissipative layer, and the model characteristics are determined by the shock speed, density, magnetic field, and turbulence decay length. The model explains observations of shell-type supernova remnants RX J1713.7-3946 and J0852.0-4622 and can be tested by observations in hard X-rays with the HXMT and NuSTAR or gamma-rays with the GLAST.

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arXiv2008

Cascade and Damping of Alfvén-Cyclotron Fluctuations: Application to Solar Wind Turbulence Spectrum

With the diffusion approximation, we study the cascade and damping of Alfvén-cyclotron fluctuations in solar plasmas numerically. Motivated by wave-wave couplings and nonlinear effects, we test several forms of the diffusion tensor. For a general locally anisotropic and inhomogeneous diffusion tensor in the wave vector space, the turbulence spectrum in the inertial range can be fitted with power-laws with the power-law index varying with the wave propagation direction. For several locally isotropic but inhomogeneous diffusion coefficients, the steady-state turbulence spectra are nearly isotropic in the absence of damping and can be fitted by a single power-law function. However, the energy flux is strongly polarized due to the inhomogeneity that leads to an anisotropic cascade. Including the anisotropic thermal damping, the turbulence spectrum cuts off at the wave numbers, where the damping rates become comparable to the cascade rates. The combined anisotropic effects of cascade and damping make this cutoff wave number dependent on the wave propagation direction, and the propagation direction integrated turbulence spectrum resembles a broken power-law, which cuts off at the maximum of the cutoff wave numbers or the $^4$He cyclotron frequency. Taking into account the Doppler effects, the model can naturally reproduce the broken power-law wave spectra observed in the solar wind and predicts that a higher break frequency is aways accompanied with a greater spectral index change that may be caused by the increase of the Alfvén Mach number, the reciprocal of the plasma beta, and/or the angle between the solar wind velocity and the mean magnetic field. These predictions can be tested by future observations.

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arXiv2007

Stochastic Acceleration in the Western Hotspot of Pictor A

Chandra's high resolution observations of radio galaxies require a revisit of the relevant electron acceleration processes. Although the diffusive shock particle acceleration model may explain spectra of spatially unresolved sources, it encounters difficulties in explaining the structure and spectral properties of recently discovered Chandra X-ray features in several low-power radio sources. We argue that these observations strongly suggest stochastic electron acceleration by magnetized turbulence, and show that the simplest stochastic particle acceleration model with energy independent acceleration and escape timescales can overcome most of these difficulties. We use the bright core of the western hotspot of Pictor A as an example to demonstrate the model characteristics, which may be tested with high energy observations.

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arXiv2007

The Nature of Linearly Polarized Millimeter and Sub-millimeter Emission in Sagittarius A*

The linearly polarized millimeter and sub-millimeter emission in Sagittarius A* is produced within 10 Schwarzschild radii of the supermassive black hole at the Galactic Center and may originate from a hot magnetized accretion disk, where electrons are heated efficiently by turbulent plasma waves. In such a scenario, the flux density and polarization are very sensitive to the electron heating rate and the inclination angle of disk, respectively, and the major axis of the sub-millimeter intrinsic polarization, which is aligned with the rotation axis of the disk, is perpendicular to the major axis of the polarized near-infrared emission. In combination with MHD simulations, which study the properties of the magnetic field and viscous stresses, the current spectral and polarization measurements give tight constraints on the model parameters. Simultaneous observations will be able to test the model.

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arXiv2007

Black Hole Accretion in Low States: Electron Heating

Plasmas in an accretion flow are heated by MHD turbulence generated through the magneto-rotational instability. The viscous stress driving the accretion is intimately connected to the microscopic processes of turbulence dissipation. We show that, in a few well-observed black hole accretion systems, there is compelling observational evidence of efficient electron heating by turbulence or collective plasma effects in low accretion states, when Coulomb collisions are not efficient enough to establish a thermal equilibrium between electrons and ions at small radii. However, charged particles reach a thermal equilibrium with their kind much faster than with others through Coulomb collisions, a two-temperature accretion flow is expected. We consider a Keplerian accretion flow with a constant mass accretion rate in the pseudo-Newtonian gravitational potential and take into account the bremsstrahlung, synchrotron, and inverse Comptonization cooling processes. The critical mass accretion rate, below which the two-temperature solution may exist, is determined by the cooling processes and the collisional energy exchanges between electrons and ions and has very weak dependence on the collision-less heating of electrons by turbulence, which becomes more important at lower accretion rates. Collision-less heating of electrons by MHD turbulence can no longer be ignored in quantitative investigations of these systems.

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arXiv2007

A Possible Link Between the Galactic Center HESS Source and Sgr A*

Recently, HESS and other air Cerenkov telescopes have detected a source of TeV gamma-rays coincident with the Galactic center. It is not yet clear whether the gamma-rays are produced via leptonic or hadronic processes, so it is important to consider possible acceleration sites for the charged particles which produce the gamma-rays. One exciting possibility for the origin of these particles is the central black hole, Sgr A*, where the turbulent magnetic fields close to the event horizon can accelerate protons to TeV energies. Using a realistic model of the density distribution in a 6 pc x 6 pc x 6pc cube at the Galactic center, we here calculate the trajectories followed by these TeV protons as they gyrate through the turbulent medium surrounding Sgr A*. Diffusing out from the black hole, the protons produce TeV gamma-rays via pi^0 decay following a collision with a proton in the surrounding medium. After following over 222,000 such trajectories, we find that the circumnuclear ring around Sgr A* can reproduce the observed 0.1-100 TeV HESS spectrum and flux if the protons are injected into this medium with an effective power-law index of 0.75, significantly harder than the observed photon index of 2.25. The total energy in the steady-state 1-40 TeV proton population surrounding Sgr A* is inferred to be approx 5x10^{45} ergs. Only 31% of the emitted 1-100 TeV protons encounter the circumnuclear torus, leaving a large flux of protons that diffuse outward to contribute to the Galactic ridge emission observed by HESS on scales of >~ 1 degree.

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arXiv2006

MHD Simulations of Accretion onto Sgr A*: Quiescent Fluctuations, Outbursts, and Quasi-Periodicity

High resolution observations of Sgr A* have revealed a wide variety of phenomena, ranging from intense rapid flares to quasi-periodic oscillations, making this object an ideal system to study the properties of low luminosity accreting black holes. In this paper, we use a pseudo-spectral algorithm to construct and evolve a three-dimensional magnetohydrodynamic model of the accretion disk in Sgr A*. Assuming a hybrid thermal-nonthermal emission scheme, we show that the MHD turbulence can by itself only produce factor of two fluctuations in luminosity. These amplitudes in variation cannot explain the magnitude of flares observed in this system. However, we also demonstrate that density perturbations in the disk do produce outbursts qualitatively similar to those observed by XMM-Newton in X-rays and ground-based facilities in the near infrared. Quasi-periodic oscillations emerge naturally in the simulated lightcurves. We attribute these to non-axisymmetric density perturbations that emerge as the disk evolves back toward its quiescent state.

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arXiv2006

Probing the Density in the Galactic Center Region: Wind-Blown Bubbles and High-Energy Proton Constraints

Recent observations of the Galactic center in high-energy gamma-rays (above 0.1TeV) have opened up new ways to study this region, from understanding the emission source of these high-energy photons to constraining the environment in which they are formed. We present a revised theoretical density model of the inner 5pc surrounding Sgr A* based on the fact that the underlying structure of this region is dominated by the winds from the Wolf-Rayet stars orbiting Sgr A*. An ideal probe and application of this density structure is this high energy gamma-ray emission. We assume a proton-scattering model for the production of these gamma-rays and then determine first whether such a model is consistent with the observations and second whether we can use these observations to further constrain the density distribution in the Galactic center.

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arXiv2006

Stochastic Acceleration in the Galactic Center HESS Source

Stochastic acceleration of electrons interacting resonantly with a turbulent magnetic field in a small accretion torus appears to be the likely mechanism responsible for much of Sagittarius A*'s millimeter and shorter wavelength spectrum. The longer wavelength radiation is produced at larger radii by electrons either diffusing from smaller scales or accelerated in situ. An important prediction of this model is the ejection of a significant flux of relativistic protons from a magnetic-field-dominated acceleration site into the wind-shocked medium surrounding the black hole. Recently, several air Cerenkov telescopes, notably HESS, have detected TeV emission from the Galactic center, with characteristics hinting at a p-p-induced pion decay process for the γ-ray emission. Given (1) the size of this acceleration region measured in the radio band and (2) the wind-injected ISM mapped with Chandra using the diffuse X-rays, it is feasible to test the idea that protons accelerated within \~20 Schwarzschild radii of the black hole produce the TeV emission farther out. We show a fraction of TeV protons scattering about once within ~3 pc of Sagittarius A* and the proton power (~10^37 erg s^-1) produced in concert with the 7 mm radio emission matches the TeV luminosity well. This model explains why the TeV source does not vary on a timescale of a year or less. The particle cascade generated by the p-p scatterings also produces bremsstrahlung, inverse Compton, and synchrotron emission at longer wavelengths from secondary particles. We compare these with current measurements and demonstrate that GLAST will detect this source during its one-year all-sky survey.

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arXiv2006

A Testable the Stochastic Acceleration Model for Flares in Sagittarius A*

The near-IR and X-ray flares in Sagittarius A* are believed to be produced by relativistic electrons via synchrotron and synchrotron self-Comptonization, respectively. These electrons are likely energized by turbulent plasma waves through second-order Fermi acceleration that, in combination with the radiative cooling processes, produces a relativistic Maxwellian distribution in the steady state. This model has four principal parameters, namely the magnetic field B, the electron density n and temperature γ_c mec2, and the size of the flare region R. In the context of stochastic acceleration, the quantities Rn^1/2B and γ_cRn should remain nearly constant in time. Therefore, simultaneous spectroscopic observations in the NIR and X-ray bands can readily test the model, which, if proven to be valid, may be used to determine the evolution of the plasma properties during an eruptive event with spectroscopic observations in either band or simultaneous flux density measurements in both bands. The formulae can be applied to other isolated or confined systems, where electrons are accelerated to relativistic energies by plasma wave turbulence and produce most of the emission via synchrotron processes.

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arXiv2006

Correlation between Flux and Spectral Index during Flares in Sagittarius A*

Flares in Sagittarius A* are produced by hot plasmas within a few Schwarzschild radii of the supermassive black hole at the Galactic center. The recent detection of a correlation between the spectral index and flux during a near infrared (NIR) flare provides a means to conduct detailed investigations of the plasma heating and radiation processes. We study the evolution of the electron distribution function under the influence of a turbulent magnetic field in a hot collisionless plasma. The magnetic field, presumably generated through instabilities in the accretion flow, can both heat the plasma via resonant wave-particle coupling and cool the electrons via radiation. The electron distribution can generally be approximated as relativistic Maxwellian. To account for the observed correlation, we find that the magnetic field needs to be anti-correlated with the electron ''temperature''. NIR and X-ray light curves are produced for a cooling and a heating phase. The model predicts simultaneous flare activity in the NIR and X-ray bands, which can be compared with observations. These results can be applied to MHD simulations to study the radiative characteristics of collisionless plasmas, especially accretion flows in low-luminosity AGNs.

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arXiv2006

RHESSI Observation of Chromospheric Evaporation

We present analyses of the spatial and spectral evolution of hard X-ray emission observed by {\it RHESSI} during the impulsive phase of an M1.7 flare on 2003 November 13. In general, as expected, the loop top (LT) source dominates at low energies while the footpoint (FP) sources dominate the high energy emission. At intermediate energies, both the LT and FPs may be seen, but during certain intervals emission from the legs of the loop dominates, in contrast to the commonly observed LT and FP emission. The hard X-ray emission tends to rise above the FPs and eventually merge into a single LT source. This evolution starts first at low energies and proceeds to higher energies. The spectrum of the resultant LT source becomes more and more dominated by a thermal component with an increasing emission measure as the flare proceeds. The soft and hard X-rays show a Neupert-type behavior. With a non-thermal bremsstrahlung model the brightness profile along the loop is used to determine the density profile and its evolution, which reveals a gradual increase of the gas density in the loop. These results are evidence for chromospheric evaporation and are consistent with the qualitative features of hydrodynamic simulations of this phenomenon. However, some observed source morphology and its evolution cannot be accounted for by previous simulations. Therefore simulations with more realistic physical conditions are required to explain the results and the particle acceleration and plasma heating processes.

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arXiv2005

Evolution of the Loop-Top Source of Solar Flares--Heating and Cooling Processes

We present a study of the spatial and spectral evolution of the loop-top (LT) sources in a sample of 6 flares near the solar limb observed by {\it RHESSI}. A distinct coronal source, which we identify as the LT source, was seen in each of these flares from the early ``pre-heating'' phase through the late decay phase. Spectral analyses reveal an evident steep power-law component in the pre-heating and impulsive phases, suggesting that the particle acceleration starts upon the onset of the flares. In the late decay phase the LT source has a thermal spectrum and appears to be confined within a small region near the top of the flare loop, and does not spread throughout the loop, as is observed at lower energies. The total energy of this source decreases usually faster than expected from the radiative cooling but much slower than that due to the classical Spitzer conductive cooling along the flare loop. These results indicate the presence of a distinct LT region, where the thermal conductivity is suppressed significantly and/or there is a continuous energy input. We suggest that plasma wave turbulence could play important roles in both heating the plasma and suppressing the conduction during the decay phase of solar flares. With a simple quasi-steady loop model we show that the energy input in the gradual phase can be comparable to that in the impulsive phase and demonstrate how the observed cooling and confinement of the LT source can be used to constrain the wave-particle interaction.

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arXiv2005

Stochastic Electron Acceleration During the NIR and X-ray Flares in Sagittarius A*

Recent near-IR (NIR) and X-ray observations of Sagittarius A*'s spectrum have yielded several strong constraints on the transient energization mechanism, justifying a re-examination of the stochastic acceleration model proposed previously for these events. We here demonstrate that the new results are fully consistent with the acceleration of electrons via the transit-time damping process. But more importantly, these new NIR and X-ray flares now can constrain the source size, the gas density, the magnetic field, and the wave energy density in the turbulent plasma. Future simultaneous multi-wavelength observations with good spectral information will, in addition, allow us to study their temporal evolution, which will eventually lead to an accurate determination of the behavior of the plasma just minutes prior to its absorption by the black hole.

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arXiv2005

Stochastic Acceleration of $^3$He and $^4$He in Solar Flares by Parallel Propagating Plasma Waves: General Results

We study the acceleration in solar flares of $^3$He and $^4$He from a thermal background by parallel propagating plasma waves with a general broken power-law spectrum that takes into account the turbulence generation processes at large scales and the thermal damping effects at small scales. The exact dispersion relation for a cold plasma is used to describe the relevant wave modes. Because low-energy $α$-particles only interact with small scale waves in the $^4$He-cyclotron branch, where the wave frequencies are below the $α$-particle gyro-frequency, their pitch angle averaged acceleration time is at least one order of magnitude longer than that of $^3$He ions, which mostly resonate with relatively higher frequency waves in the proton-cyclotron (PC) branch. The $α$-particle acceleration rate starts to approach that of $^3$He beyond a few tens of keV nucleon$^{-1}$, where $α$-particles can also interact with long wavelength waves in the PC branch. However, the $^4$He acceleration rate is always smaller than that of $^3$He. Consequently, the acceleration of $^4$He is suppressed significantly at low energies, and the spectrum of the accelerated $α$-particles is always softer than that of $^3$He. The model gives reasonable account of the observed low-energy $^3$He and $^4$He fluxes and spectra in the impulsive solar energetic particle events observed with the {\it Advanced Composition Explorer}. We explore the model parameter space to show how observations may be used to constrain the model.

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arXiv2004

RHESSI Observations of a Simple Large X-ray Flare on 11-03-2003

We present data analysis and interpretation of a simple X-class flare observed with RHESSI on November 3, 2003. In contrast to other X-class flares observed previously, this flare shows a very simple morphology with well defined looptop (LT) and footpoint (FP) sources. The almost monotonic upward motion of the LT source and increase in separation of the two FP sources are consistent with magnetic reconnection models proposed for solar flares. In addition, we find that the source motions are relatively slower during the more active phases of hard X-ray emission; the emission centroid of the LT source shifts toward higher altitudes with the increase of energy; the separation between the LT emission centroids at two different photon energies is anti-correlated with the FP flux. Non-uniformity of the reconnecting magnetic fields could be a possible explanation of these features.

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arXiv2004

Stochastic Acceleration of Electrons and Protons. I. Acceleration by Parallel Propagating Waves

Stochastic acceleration of electrons and protons by waves propagating parallel to the large scale magnetic fields of magnetized plasmas is studied with emphasis on the feasibility of accelerating particles from a thermal background to relativistic energies and with the aim of determining the relative acceleration of the two species in one source. In general, the stochastic acceleration by these waves results in two distinct components in the particle distributions, a quasi-thermal and a hard nonthermal, with the nonthermal one being more prominent in hotter plasmas and/or with higher level turbulence. This can explain many of the observed features of solar flares. Regarding the proton to electron ratio, we include the effects of $^4$He in the dispersion relation and the damping of the turbulent waves by the thermal background plasma. The relative acceleration of protons and electrons is very sensitive to the ratio of the electron plasma frequency to its gyro-frequency. Protons are preferentially accelerated in weakly magnetized plasmas. The formalism developed here is applicable to the acceleration of other ion species and to other astrophysical systems.

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