arXiv subjects

Siming Liu

Publications and source records attributed to Siming Liu.

At least 73 records · Page 4Linked to original sources

arXiv2018

Global Constraints on Diffusive Particle Acceleration by Strong Nonrelativistic Shocks

Estimating the cosmic-ray acceleration efficiency $ ε$ in supernova remnants (SNRs) through observations is a challenging task in general. Based on the Rankine-Hugoniot shock conditions, we find an anticorrelation between $ ε$ and the power-law spectral index $ α$ of relativistic particle distribution produced via diffusive particle acceleration by nonrelativistic shocks, implying more efficient acceleration in older SNRs with harder radio spectra. Then $ ε$ may be estimated from some hard radio spectral index measurements. Assuming the particle distribution in downstream of strong shocks to be a nonrelativistic Maxwellian plus a relativistic power law with a high-energy cutoff, we also find that the injection rate for relativistic particles $ η$ needs to $ \gtrsim 10^{-6} $ for a prominent decrease of the adiabatic index in SNRs, which implies higher compression ratio and lower values of $ α$. This threshold of $ η$ increases with the shock speed $ u_1 $, which may explain the relatively harder radio spectra of older SNRs with lower $ u_1 $. We show that $ η$ and/or the relativistic cutoff momentum $ p_\text{m} $ need to be low for old SNRs, and expect a gradual increase of $ ε$ as SNR evolves with gradually decreasing $ η$ and $ p_\text{m} $.

astro-ph.HE↗

arXiv2018

Evolution of high-energy particle distribution in Supernova Remnants

Supernova remnants (SNRs) have been considered as the dominant contributors to Galactic cosmic rays. However, the relation between high-energy particles trapped in SNRs and cosmic rays observed at the Earth remains obscure. In this paper, we fit the spectral energy distributions of 35 SNRs with a simple one-zone emission model and analyze correlations of model parameters to uncover the evolution of high-energy particle distribution in SNRs. We find that (1) the particle distribution in general can be described by a broken power-law function with a high-energy cutoff for all SNRs; (2) the low-energy spectrum becomes harder and the break energy decreases with aging of SNRs, (3) for most middle-age SNRs, the energy loss timescale of electrons at the high-energy cutoff is approximately equal to the age of the corresponding remnant implying quenching of very high-energy electron acceleration; for young SNRs, this energy loss timescale is shorter than the age of SNRs implying continuous electon acceleration at the cutoff energy; and for a few old age SNRs, the energy loss timescale is longer than the corresponding age which may suggest escaping of higher energy particles from SNRs. Finally, we comment on the implications of these results on the SNR origin of Galactic cosmic rays.

astro-ph.HE↗

arXiv2018

Neuroevolution for RTS Micro

This paper uses neuroevolution of augmenting topologies to evolve control tactics for groups of units in real-time strategy games. In such games, players build economies to generate armies composed of multiple types of units with different attack and movement characteristics to combat each other. This paper evolves neural networks to control movement and attack commands, also called micro, for a group of ranged units skirmishing with a group of melee units. Our results show that neuroevolution of augmenting topologies can effectively generate neural networks capable of good micro for our ranged units against a group of hand-coded melee units. The evolved neural networks lead to kiting behavior for the ranged units which is a common tactic used by professional players in ranged versus melee skirmishes in popular real-time strategy games like Starcraft. The evolved neural networks also generalized well to other starting positions and numbers of units. We believe these results indicate the potential of neuroevolution for generating effective micro in real-time strategy games.

cs.AI↗

arXiv2018

Co-evolving Real-Time Strategy Game Micro

We investigate competitive co-evolution of unit micromanagement in real-time strategy games. Although good long-term macro-strategy and good short-term unit micromanagement both impact real-time strategy games performance, this paper focuses on generating quality micro. Better micro, for example, can help players win skirmishes and battles even when outnumbered. Prior work has shown that we can evolve micro to beat a given opponent. We remove the need for a good opponent to evolve against by using competitive co-evolution to evolve high-quality micro for both sides from scratch. We first co-evolve micro to control a group of ranged units versus a group of melee units. We then move to co-evolve micro for a group of ranged and melee units versus a group of ranged and melee units. Results show that competitive co-evolution produces good quality micro and when combined with the well-known techniques of fitness sharing, shared sampling, and a hall of fame takes less time to produce better quality micro than simple co-evolution. We believe these results indicate the viability of co-evolutionary approaches for generating good unit micro-management.

cs.NE↗

arXiv2018

Evolutionary Multi-objective Optimization of Real-Time Strategy Micro

We investigate an evolutionary multi-objective approach to good micro for real-time strategy games. Good micro helps a player win skirmishes and is one of the keys to developing better real-time strategy game play. In prior work, the same multi-objective approach of maximizing damage done while minimizing damage received was used to evolve micro for a group of ranged units versus a group of melee units. We extend this work to consider groups composed from two types of units. Specifically, this paper uses evolutionary multi-objective optimization to generate micro for one group composed from both ranged and melee units versus another group of ranged and melee units. Our micro behavior representation uses influence maps to represent enemy spatial information and potential fields generated from distance, health, and weapons cool down to guide unit movement. Experimental results indicate that our multi-objective approach leads to a Pareto front of diverse high-quality micro encapsulating multiple possible tactics. This range of micro provided by the Pareto front enables a human or AI player to trade-off among short term tactics that better suit the player's longer term strategy - for example, choosing to minimize friendly unit damage at the cost of only lightly damaging the enemy versus maximizing damage to the enemy units at the cost of increased damage to friendly units. We believe that our results indicate the usefulness of potential fields as a representation, and of evolutionary multi-objective optimization as an approach, for generating good micro.

cs.NE↗

arXiv2018

Multi-objective evolution for 3D RTS Micro

We attack the problem of controlling teams of autonomous units during skirmishes in real-time strategy games. Earlier work had shown promise in evolving control algorithm parameters that lead to high performance team behaviors similar to those favored by good human players in real-time strategy games like Starcraft. This algorithm specifically encoded parameterized kiting and fleeing behaviors and the genetic algorithm evolved these parameter values. In this paper we investigate using influence maps and potential fields alone to compactly represent and control real-time team behavior for entities that can maneuver in three dimensions. A two-objective fitness function that maximizes damage done and minimizes damage taken guides our multi-objective evolutionary algorithm. Preliminary results indicate that evolving friend and enemy unit potential field parameters for distance, weapon characteristics, and entity health suffice to produce complex, high performing, three-dimensional, team tactics.

cs.NE↗

arXiv2017

Development of a Swarm UAV Simulator Integrating Realistic Motion Control Models For Disaster Operations

Simulation environments for Unmanned Aerial Vehicles (UAVs) can be very useful for prototyping user interfaces and training personnel that will operate UAVs in the real world. The realistic operation of such simulations will only enhance the value of such training. In this paper, we present the integration of a model-based waypoint navigation controller into the Reno Rescue Simulator for the purposes of providing a more realistic user interface in simulated environments. We also present potential uses for such simulations, even for real-world operation of UAVs.

cs.RO↗

arXiv2017

A Systematic Chandra study of Sgr A$^{\star}$: II. X-ray flare statistics

The routinely flaring events from \sgras\ trace dynamic, high-energy processes in the immediate vicinity of the supermassive black hole. We statistically study temporal and spectral properties, as well as fluence and duration distributions, of the flares detected by the \chandra\ X-ray Observatory from 1999 to 2012. The detection incompleteness and bias are carefully accounted for in determining these distributions. We find that the fluence distribution can be well characterized by a power-law with a slope of $1.73^{+0.20}_{-0.19}$, while the durations ($τ$ in seconds) by a log-normal function with a mean $\log(τ)=3.39^{+0.27}_{-0.24}$ and an intrinsic dispersion $σ=0.28^{+0.08}_{-0.06}$. No significant correlation between the fluence and duration is detected. The apparent positive correlation, as reported previously, is mainly due to the detection bias (i.e., weak flares can be detected only when their durations are short). These results indicate that the simple self-organized criticality model has difficulties in explaining these flares. We further find that bright flares usually have asymmetric lightcurves with no statistically evident difference/preference between the rising and decaying phases in terms of their spectral/timing properties. Our spectral analysis shows that although a power-law model with a photon index of $2.0\pm0.4$ gives a satisfactory fit to the joint spectra of strong and weak flares, there is weak evidence for a softer spectrum of weaker flares. This work demonstrates the potential to use statistical properties of X-ray flares to probe their trigger and emission mechanisms, as well as the radiation propagation around the black hole.

astro-ph.HE↗

arXiv2017

Anomalous distributions of primary cosmic rays as evidence for time-dependent particle acceleration in Supernova remnants

Recent precise measurements of cosmic ray (CR) spectra show that the energy distribution of protons is softer than those of heavier nuclei, and there are spectral hardenings for all nuclear compositions above $\sim$200 GV. Models proposed for these anomalies generally assume steady-state solutions of the particle acceleration process. We show that, if the diffusion coefficient has a weak dependence on the particle rigidity near shock fronts of supernova remnants (SNRs), time-dependent solutions of the linear diffusive shock acceleration at two stages of SNR evolution can naturally account for these anomalies. The high-energy component of CRs is dominated by acceleration in the free expansion and adiabatic phases with enriched heavy elements and a high shock speed. The low energy component may be attributed to acceleration by slow shocks propagating in dense molecular clouds with low metallicity in the radiative phase. Instead of a single power-law distribution, the spectra of time-dependent solutions soften gradually with the increase of energy, which may beresponsible for the "knee" of CRs.

astro-ph.HE↗

arXiv2016

Measure the Propagation of a halo CME and Its Driven Shock with the Observations from a Single Perspective at Earth

We present a detailed study of an earth-directed coronal mass ejection (Full halo CME) event happened on 2011 February 15 making use of white light observations by three coronagraphs and radio observations by Wind/WAVES. We applied three different methods to reconstruct the propagation direction and traveling distance of the CME and its driven shock. We measured the kinematics of the CME leading edge from white light images observed by STEREO A and B, tracked the CME-driven shock using the frequency drift observed by Wind/WAVES together with an interplanetary density model, and obtained the equivalent scattering centers of the CME by Polarization Ratio(PR) method. For the first time, we applied PR method to different features distinguished from LASCO/C2 polarimetric observations and calculated their projections onto white light images observed by STEREO A and B. By combining the graduated cylindrical shell (GCS) forward modeling with the PR method, we proposed a new GCS-PR method to derive 3D parameters of a CME observed from a single perspective at Earth. Comparisons between different methods show a good degree of consistence in the derived 3D results.

astro-ph.SR↗

arXiv2016

Evolution of High-Energy Particle Distribution in Mature Shell-Type Supernova Remnants

Multi-wavelength observations of mature supernova remnants (SNRs), especially with recent advances in gamma-ray astronomy, make it possible to constrain energy distribution of energetic particles within these remnants. In consideration of the SNR origin of Galactic cosmic rays and physics related to particle acceleration and radiative processes, we use a simple one-zone model to fit the nonthermal emission spectra of three shell-type SNRs located within 2 degrees on the sky: RX J1713.7-3946, CTB 37B, and CTB 37A. Although radio images of these three sources all show a shell (or half-shell) structure, their radio, X-ray, and gamma-ray spectra are quite different, offering an ideal case to explore evolution of energetic particle distribution in SNRs. Our spectral fitting shows that 1) the particle distribution becomes harder with aging of these SNRs, implying a continuous acceleration process, and the particle distributions of CTB 37A and CTB 37B in the GeV range are harder than the hardest distribution that can be produced at a shock via the linear diffusive shock particle acceleration process, so spatial transport may play a role; 2) the energy loss timescale of electrons at the high-energy cutoff due to synchrotron radiation appears to be always a bit (within a factor of a few) shorter than the age of the corresponding remnant, which also requires continuous particle acceleration; 3) double power-law distributions are needed to fit the spectra of CTB 37B and CTB 37A, which may be attributed to shock interaction with molecular clouds.

astro-ph.HE↗

arXiv2016

On the Power-Law Distributions of X-ray Fluxes from Solar Flares Observed with GOES

Power-law frequency distributions of the peak flux of solar flare X-ray emission have been studied extensively and attributed to a system of self-organized criticality (SOC). In this paper, we first show that, so long as the shape of the normalized light curve is not correlated with the peak flux, the flux histogram of solar flares also follows a power-law distribution with the same spectral index as the power-law frequency distribution of the peak flux, which may partially explain why power-law distributions are ubiquitous in the Universe. We then show that the spectral indexes of the histograms of soft X-ray fluxes observed by GOES satellites in two different energy channels are different: the higher energy channel has a harder distribution than the lower energy channel, which challenges the universal power-law distribution predicted by SOC models and implies a very soft distribution of thermal energy content of plasmas probed by the GOES. The temperature ($T$) distribution, on the other hand, approaches a power-law distribution with an index of 2 for high values of $T$. Application of SOC models to statistical properties of solar flares needs to be revisited.

astro-ph.SR↗

arXiv2016

Energy Flows in the Jupiter-Io System

With the laws of mass conservation, momentum conservation and energy conservation, incorporating the processes of neutral gas ionization and ion diffusion, we develop a self-consistent model for the bright ribbon --- the most prominent feature in Io's plasma torus. The model parameters are well constrained by earlier {\it in situ} observations with the Galileo and Voyager spacescrafts. Our model calculation indicates that the total power dissipated inside the torus is 3.6 times bigger than the total power transported to Jovian ionosphere via Birkeland current. The power dissipation inside the torus is relatively uniform. Most of the power transportation associated with the Birkeland current, however, is localized near the flux tube of Io. With a height-intergrated conductivity of 0.15$\,$mho in Jovian ionosphere, consistent with earlier aeronomy models, the model gives a reasonable fit to the recent observations of the FUV Io tail on Jupiter. Extra mass loading near Io is required in the model. This excess of mass injection into the plasma torus may explain the Io-correlated energy source revealed by earlier EUV observations with the Voyager spacecrafts. Further investigation of the model implications is warranted.

astro-ph.EP↗

arXiv2015

Limitation of the Least Square Method in the Evaluation of Dimension of Fractal Brownian Motions

With the standard deviation for the logarithm of the re-scaled range $\langle |F(t+τ)-F(t)|\rangle$ of simulated fractal Brownian motions $F(t)$ given in a previous paper \cite{q14}, the method of least squares is adopted to determine the slope, $S$, and intercept, $I$, of the log$(\langle |F(t+τ)-F(t)|\rangle)$ vs $\rm{log}(τ)$ plot to investigate the limitation of this procedure. It is found that the reduced $χ^2$ of the fitting decreases with the increase of the Hurst index, $H$ (the expectation value of $S$), which may be attributed to the correlation among the re-scaled ranges. Similarly, it is found that the errors of the fitting parameters $S$ and $I$ are usually smaller than their corresponding standard deviations. These results show the limitation of using the simple least square method to determine the dimension of a fractal time series. Nevertheless, they may be used to reinterpret the fitting results of the least square method to determine the dimension of fractal Brownian motions more self-consistently. The currency exchange rate between Euro and Dollar is used as an example to demonstrate this procedure and a fractal dimension of 1.511 is obtained for spans greater than 30 transactions.

physics.data-an↗

arXiv2014

The structure of TeV-bright shell-type supernova remnants

Aims. Two-dimensional MHD simulations are used to model the emission properties of TeV-bright shell-type supernova remnants (SNRs) and to explore their nature. Methods. In the leptonic scenario for the TeV emission, the $γ$-ray emission is produced via Inverse Compton scattering of background soft photons by high-energy electrons accelerated by the shocks of the SNRs. The TeV emissivity is proportional to the magnetic field energy density and MHD simulations can be used to model the TeV structure of such remnants directly. 2D MHD simulations for SNRs are then performed under the assumption that the ambient interstellar medium is turbulent with the magnetic field and density fluctuations following a Kolmogorov-like power-law spectrum. Results. (1) As expected, these simulations confirm early 1D and 2D modelings of these sources, namely the hydrodynamical evolution of the shock waves and amplification of magnetic field by Rayleigh-Taylor convective flows and by shocks propagating in a turbulent medium; (2) We reproduce rather complex morphological structure for $γ$-rays, suggesting intrinsic variations of the source morphology not related to the structure of the progenitor and environment; (3)The observed radial profile of several remnants are well reproduced with an ambient medium density of $0.1-1$ cm$^{-3}$. An even lower ambient density leads to a sharper drop of the TeV brightness with radius than what is observed near the outer edge of these remnants. Conclusions. In a turbulent background medium, we can reproduce the observed characteristics of several shell-type TeV SNRs with reasonable parameters except for a higher ambient density than that inferred from X-ray observations.

astro-ph.HE↗

arXiv2014

Fermi Large Area Telescope Detection of Supernova Remnant RCW 86

Using $5.4$ year Fermi-LAT data, we report the detection of GeV $γ$-ray emission from the shell-type supernova remnant RCW 86 (G315.4-2.3) with a significance of $\sim5.1σ$. The data slightly favors an extended emission of this supernova remnant. The spectral index of RCW 86 is found to be very hard, $Γ\sim1.4$, in the $0.4$ to $300$ GeV range. A one zone leptonic model can well fit the multi-wavelength data from radio to very high energy $γ$-rays. The very hard GeV $γ$-ray spectrum and the inferred low gas density seem to disfavor the hadronic origin of the $γ$-rays. The $γ$-ray behavior of RCW 86 is very similar to several other TeV shell-type supernova remnants, e.g., RX J1713.7-3946, RX J0852.0-4622, SN 1006 and HESS J1731-347.

astro-ph.HE↗

arXiv2014

Fermi Large Area Telescope observations of the supernova remnant HESS J1731-347

Context: HESS J1731-347 has been identified as one of the few TeV-bright shell-type supernova remnants (SNRs). These remnants are dominated by nonthermal emission, and the nature of TeV emission has been continuously debated for nearly a decade. Aims: We carry out the detailed modeling of the radio to gamma-ray spectrum of HESS J1731-347 to constrain the magnetic field and energetic particles sources, which we compare with those of the other TeV-bright shell-type SNRs explored before. Methods: Four years of data from Fermi Large Area Telescope (LAT) observations for regions around this remnant are analyzed, leading to no detection correlated with the source discovered in the TeV band. The Markov Chain Monte Carlo method is used to constrain parameters of one-zone models for the overall emission spectrum. Results: Based on the 99.9% upper limits of fluxes in the GeV range, one-zone hadronic models with an energetic proton spectral slope greater than 1.8 can be ruled out, which favors a leptonic origin for the gamma-ray emission, making this remnant a sibling of the brightest TeV SNR RX J1713.7-3946, the Vela Junior SNR RX J0852.0-4622, and RCW 86. The best-fit leptonic model has an electron spectral slope of 1.8 and a magnetic field of about 30 muG, which is at least a factor of 2 higher than those of RX J1713.7-3946 and RX J0852.0-4622, posing a challenge to the distance estimate and/or the energy equipartition between energetic electrons and the magnetic field of this source. A measurement of the shock speed will address this challenge and has implications on the magnetic field evolution and electron acceleration driven by shocks of SNRs.

astro-ph.HE↗

arXiv2013

Error Assessment in Modeling with Fractal Brownian Motions

To model a given time series $F(t)$ with fractal Brownian motions (fBms), it is necessary to have appropriate error assessment for related quantities. Usually the fractal dimension $D$ is derived from the Hurst exponent $H$ via the relation $D=2-H$, and the Hurst exponent can be evaluated by analyzing the dependence of the rescaled range $\langle|F(t+τ)-F(t)|\rangle$ on the time span $τ$. For fBms, the error of the rescaled range not only depends on data sampling but also varies with $H$ due to the presence of long term memory. This error for a given time series then can not be assessed without knowing the fractal dimension. We carry out extensive numerical simulations to explore the error of rescaled range of fBms and find that for $0<H<0.5$, $|F(t+τ)-F(t)|$ can be treated as independent for time spans without overlap; for $0.5<H<1$, the long term memory makes $|F(t+τ)-F(t)|$ correlated and an approximate method is given to evaluate the error of $\langle|F(t+τ)-F(t)|\rangle$. The error and fractal dimension can then be determined self-consistently in the modeling of a time series with fBms.

physics.data-an↗