R-3 Repository :: Browsing by Author "Liang, Edison P."

Browsing by Author "Liang, Edison P."

Now showing 1 - 12 of 12

Analysis and interpretation of gamma-ray burst continuum spectral evolution with BATSE data
(1999) Crider, Anthony Wayne; Liang, Edison P.
Once a day, a flash of gamma-rays erupts somewhere in space and is detected by an international fleet of satellites. Since their first detection over a quarter century ago, these gamma-ray bursts have puzzled researchers who could not determine their distance, emission mechanism, or progenitor. Much of this confusion arose as theorists attempted to create a single model to explain what we now believe are at least two, and probably more, populations of gamma-ray transients. Within the past two years, thanks largely to the Dutch-Italian satellite BeppoSAX, astronomers discovered that bursts have multiwavelength fading afterglows. This helped them determine that most gamma-ray bursts are from distant galaxies. However, it did not answer the questions regarding the emission mechanism or the progenitor. We place constraints on the emission mechanism by closely examining the spectral evolution of gamma-ray bursts observed by the American instrument BATSE. From a sample of 41 distinct pulses in 26 bright gamma-ray bursts, we have determined that the pulses appear to be radiatively cooling. We also studied the evolving spectral shape in 79 bursts. In particular, we found that both the range and evolution of the spectral index below the spectral break conflict with the predictions of a popular synchrotron shock model. They instead suggest inverse Comptonization in a hybrid thermal plus nonthermal plasma as the emission mechanism. With our Monte Carlo codes, we have begun the generation of a library of inverse Compton spectra. Using them, we have made preliminary fits to two bursts with prompt multiwavelength data. The characteristic "terrace-shaped" Compton spectrum is evident in both using BATSE data alone. This shape appears to be confirmed for the January 1, 1997 burst using BeppoSAX X-ray data and for the January 23, 1999 burst using optical data from the ground-based robotic telescope, ROTSE. Both bursts appear to be enshrouded in a material with a high initial Thomson scattering depth tT≳20 . Fitting with a larger, more organized library of Monte Carlo simulations will be required before precise limits can be placed on physical parameters such as the energies, masses, and densities of bursters.
Continuum spectral evolution of gamma-ray bursts
(1996) Kargatis, Vincent Emanuel; Liang, Edison P.
Gamma-ray bursts (GRBs) remain one of the most inexplicable astrophysical phenomena observed today. While counterparts at other wavelengths would provide the best clues as to the nature of GRBs, none have been observed. To supplement studies on GRB distribution and population statistics, temporal morphologies, and spectral line searches, we focus on the analysis of GRB continuum spectral evolution. Previous spectral evolution studies have shown a variety of patterns: most individual pulses show a hard-to-soft evolution, but studies of both the SIGNE and BATSE GRB databases reveal several other patterns, including hardness-intensity tracking, soft-to-hard, static, and chaotic spectral evolution. This type of analysis attempts to identify spectral evolution signatures that can discriminate between different physical scenarios or different GRB subpopulations based on temporal profile, duration, intensity, or spatial distribution. Contrary to most studies that use only one model and one parameter to characterize spectral evolution, several models are used here. Statistically equivalent models are shown to give consistent physical results. I verify the variety of spectral evolution patterns present in GRBs, and investigate how the actual shape of the spectrum evolves, following multi-parameter spectral fits in time. Different spectral evolution patterns exist simultaneously in multiple parameters. Hardness-intensity correlations in pulse and over burst decay phases are quantitatively examined: correlation is often significant, but the relation between hardness and intensity is non-unique. Hardness-intensity lag-times are found to correlate to the rise-time of the hardness profile. Comparisons of double-pulse GRBs reveals a variety of results, including the implication that late-emitting pulses are less affected by early emission.
Modeling of laser-generated radiative blast waves, with applications to late-term supernova remnants
(2003) Keilty, Katherine Anne; Liang, Edison P.
The goal of laser astrophysics is to provide a means by which aspects of specific astrophysical phenomena can be reproduced in the laboratory. Although the hydrodynamic instabilities of early supernova remnants have already been studied using this method, the role of significant radiative losses in shock propagation (for example, in late-term remnants) has only been imperfectly modeled. This thesis introduces an improved self-similar analytic approach to radiative blast-wave evolution where the total amount of energy loss remains constant in proportion to the energy flux entering the shock front. The approximation is solved for the cases in which both energy loss from the shock front and heating of the shock (due to the presence of ionization precursors) are significant. Because this solution is independent of the exact method of cooling, it is appropriate for both the laboratory and astrophysical regimes. In addition, this thesis applies the analytic approximation to laboratory-produced radiative blast waves as well as to numerical models of these experimental blast waves. These results will allow for better design of laser-based experiments with further applications to astrophysical phenomena, as well as for an increase in the understanding of the challenges involved in scaling radiative phenomena between laboratory experiments and astrophysical theory.
Monte-Carlo Simulation and Measurements of Electrons, Positrons, and Gamma-rays Generated by Laser-Solid Interactions
(2015-04-20) Henderson, Alexander Hastings; Liang, Edison P.; Yepes, Pablo; Cox, Dennis
Lasers have grown more powerful in recent years, opening up new frontiers in physics. From early intensities of less than 1010 W/cm2, lasers can now achieve intensities over 1021 W/cm2. Ultraintense laser can become powerful new tools to produce relativistic electrons, positron-electron pairs, and gamma-rays. The pair production efficiency is equal to or greater than that of linear accelerators, the most common method of antimatter generation in the past. The gamma-rays and electrons produced can be highly collimated, making these interactions of interest for beam generation. Monte-Carlo particle transport simulation has long been used in physics for simulating various particle and radiation processes, and is well-suited to simulating both electromagnetic cascades resulting from laser-solid interactions and the response of electron/positron spectrometers and gamma-ray detectors. We have used GEANT4 Monte-Carlo particle transport simulation to design and calibrate charged-particle spectrometers using permanent magnets as well as a Forward Compton Electron Spectrometer to measure gamma-rays of higher energies than have previously been achieved. We have had some success simulating and measuring high positron and gamma-rays yields from laser-solid interactions using gold target at the Texas Petawatt Laser (TPW). While similar spectrometers have been developed in the past, we are to our knowledge the first to successfully use permanent magnet spectrometers to detect positrons originating from laser-solid interactions in this energy range. We believe we are also the first to successfully detect multi-MeV gamma rays using a permanent magnet Forward Compton Electron Spectrometer. Monte-Carlo particle transport simulation has been used by other groups to model positron production from laser-solid ineraction, but at the time that we began we were, as far as we know, the first to have a significant amount of empirical data to work with. We were thus at liberty to estimate the initial conditions, compare simulation results to data, and adjust as needed to obtain a better estimate of the actual initial conditions. We have also developed a new method for measuring the yield and angular distribution of gamma-rays using a two-dimensional dosimeter array. In this work, we examine the experimental and simulation results as well as the physical processes behind them. In addition, the gamma-rays produced by our experiments could be useful for photo-nuclear reactors and homeland security purposes. In our experiments, we measured narrow energy-band positrons and electrons which have potential medical uses.
Particle acceleration near astrophysical compact objects--Several problems in high energy astrophysics
(1995) Li, Hui; Liang, Edison P.
Particle acceleration in astrophysical plasmas has been a longstanding and challenging problem and it has been both intensively and extensively highlighted by the recent observations from Compton Gamma Ray Observatory on various astrophysical objects, including Gamma Ray Bursts (GRBs), Active Galactic Nuclei (AGNs) and Galactic Black Hole Candidates (gBHCs). In this work, I study the stochastic particle acceleration due to the resonant interactions between the turbulent plasma waves and particles. I employ the particle orbital theory approach by treating the effects of various waves as perturbations to particle's zeroth-order Hamiltonian. The particle's momentum and pitch angle diffusion coefficients ($D\sb{pp},\ D\sb{\mu\mu})$ are derived for interactions of proton/Alfven-wave, electron/fast-magnetosonic-wave and electron /whistler-wave, though the formalism can be generalized to other type of waves. Based on the Monte Carlo code I have developed, which solves the coupled time-dependent wave, particle, and photon kinetic equations, these results have been or are being applied to the central region of both AGNs and gBHCs, with applications for gamma-ray production and energetic particle outflows. By calculating the particle trajectories under the influence of gravity and radiation pressure near the center of galactic black hole accretion disk, I show that the recent discoveries of relativistic outflows from several X-ray binaries (e.g., GRS1915+105 and GROJ1655$-$40) can be well explained by radiation acceleration. The calculated final jet velocity is in good agreement with the observations and further constraints can be put on the composition and the power of those jets. The isotropic but inhomogeneous distribution of GRBs over the whole sky apparently requires more exotic explanations. A novel model for GRBs from high velocity neutron stars, which escape into our Galactic halo, has been developed. I show, in detail, the difficulties the halo models are facing and propose several scenarios to overcome them. This model predicts that bright burst distribution should deviate from isotropy and it still awaits confrontation with the observations.
Radial-zoned thermal accretion disk model around black holes
(1997) Luo, Chuan; Liang, Edison P.
High energy astrophysics is a relatively new discipline that has witnessed tremendous advancement in the past 25 years or so with the advent of space technology, which provides the field with an indispensable observational tool in the X-ray and $\gamma$-ray range previously unreachable by ground-based observations. Ongoing discoveries in this field demand new theories to explain new phenomena. In this thesis, I summarize my work on one of these topics in high energy astrophysics, the theory of accretion disks around black holes. Accretion disk models are applied to the dynamics of compact objects surrounded by material with organized angular momentum. The disk forms due to the process by which differentially rotating fluid layers interact with each other and transport angular momentum outwards with the gas spiraling into the central object, a black hole in this case. This process, known as accretion, is a very effective way to tap the energy of the gravitational field. In fact, for a non-rotating black hole, the conversion rate from mass to energy is about six percent. This is about ten times as effective as the sun, while for an extreme rotating black hole, as much as forty percent of the rest mass of the accreted gas is converted into energy. Based on the X-ray spectra and time variability of black hole candidates, I have developed a so-called radial-zoned thermal accretion disk model by dividing the disk radially into three zones. The outer zone is optically thick and the inner and middle zones are optically thin, with the latter being irradiated by the outer part of the disk. This model can produce a composite spectrum of black body radiation from the outer zone, optically thin comptonized thermal bremsstrahlung radiation from the inner zone, and inverse comptonized soft photon radiation from the middle. In this work, I have studied both the linear and nonlinear dynamic evolution and stability of this accretion disk model by solving the whole set of partial differential equations numerically. Spectral fitting and time series analysis to high energy satellite data have been done based on the steady disk model to fix parameters and to cross-examine the results from other methods.
Resonant Compton Scattering in Highly-Magnetized Pulsars
(2014-04-24) Wadiasingh, Zorawar; Baring, Matthew G.; Liang, Edison P.; Wolf, Michael
Soft gamma repeaters and anomalous X-ray pulsars are subset of slow-rotating neutron stars, known as magnetars, that have extremely high inferred surface magnetic fields, of the order 100-1000 TeraGauss. Hard, non-thermal and pulsed persistent X-ray emission extending between 10 keV and 230 keV has been seen in a number of magnetars by RXTE, INTEGRAL, and Suzaku. In this thesis, the author considers inner magnetospheric models of such persistent hard X-ray emission where resonant Compton upscattering of soft thermal photons is anticipated to be the most efficient radiative process. This high efficiency is due to the relative proximity of the surface thermal photons, and also because the scattering becomes resonant at the cyclotron frequency. At the cyclotron resonance, the effective cross section exceeds the classical Thomson one by over two orders of magnitude, thereby enhancing the efficiency of continuum production and cooling of relativistic electrons. In this thesis, a new Sokolov and Ternov formulation of the QED Compton scattering cross section for strong magnetic fields is employed in electron cooling and emission spectra calculations. This formalism is formally correct for treating spin-dependent effects and decay rates that are important at the cyclotron resonance. The author presents electron cooling rates at arbitrary interaction points in a magnetosphere using the QED cross sections. The QED effects reduce the rates below high-field extrapolations of older magnetic Thomson results. The author also computes angle-dependent upscattering model spectra, formed using collisional integrals, for uncooled monoenergetic relativistic electrons injected in inner regions of pulsar magnetospheres. These spectra are integrated over closed field lines and obtained for different observing perspectives. The spectral cut-off energies are critically dependent on the observer viewing angles and electron Lorentz factor. It is found that electrons with energies less than around 15 MeV will emit most of their radiation below 250 keV, consistent with the observed turnovers in magnetar hard X-ray tails. Moreover, electrons of higher energy still emit most of the radiation below 1 MeV, except for very select viewing perspectives that sample tangents to field lines. This small parameter space makes it difficult to observe signals extending into the Fermi-LAT band. Polarization dependence in spectra is illustrated, offering potential constraints for models of magnetar emission in anticipation of a future hard X-ray polarimetry missions.
Studies of Low Luminosity Active Galactic Nuclei with Monte Carlo and Magnetohydrodynamic Simulations
(2012-09-05) Hilburn, Guy; Liang, Edison P.; Baring, Matthew G.; Mellor-Crummey, John
Results from several studies are presented which detail explorations of the physical and spectral properties of low luminosity active galactic nuclei. An initial Sagittarius A* general relativistic magnetohydrodynamic simulation and Monte Carlo radiation transport model suggests accretion rate changes as the dominant flaring method. A similar study on M87 introduces new methods to the Monte Carlo model for increased consistency in highly energetic sources. Again, accretion rate variation seems most appropriate to explain spectral transients. To more closely resolve the methods of particle energization in active galactic nuclei accretion disks, a series of localized shearing box simulations explores the effect of numerical resolution on the development of current sheets. A particular focus on numerically describing converged current sheet formation will provide new methods for consideration of turbulence in accretion disks.
The investigation of intrinsic spectral and temporal properties of gamma-ray bursts
(2005) Kocevski, Daniel; Liang, Edison P.
Gamma-ray bursts (GRBs) have become a truly unique puzzle in Astronomy. Unlike quasars and pulsars which were explained within years of their discovery, the origins of GRBs remain a mystery more then thirty years after their discovery. Even the most basic information about these events, such as a concrete estimate of their distance, wasn't known until only a few years ago. The difficulty in our ability to obtain information about the nature of GRBs reflects their transient nature, which occur in random location in the sky with a rate of roughly one a day. Obtaining multi-wavelength observations of such brief events is a situation that astronomy has rarely been faced with. It has only been recently, with the advent of rapid arcminute x-ray localization, that we have started to decern information regarding the intrinsic properties of these events. In this thesis, I present a compilation of research describing our efforts to investigate the intrinsic properties of GRBs. The thesis is broken down into chapters including an introduction to the field of GRB astronomy, three chapters covering a range of spectral and temporal investigation, two of which have been published in the Astrophysics Journal with plans to submit the third and a conclusion outlining current and future work on the promises of using GRBs for cosmological research.
Understanding the Nature of Blazars High Energy Emission with Time Dependent Multi-zone Modeling
(2012-09-05) Chen, Xuhui; Fossati, Giovanni; Liang, Edison P.; Raphael, Robert M.
In this thesis we present a time-dependent multi-zone radiative transfer code and its applications to study the multiwavelength emission of blazars. The multiwavelength variability of blazars is widely believed to be a direct manifestation of the formation and propagation of relativistic jets, and hence the related physics of the black hole - accretion disk - jet system. However, the understanding of these variability demands highly sophisticated theoretical analysis and numerical simulations. Especially, the inclusion of the light travel time eﬀects(LTTEs) in these calculations has long been realized important, but very diﬃcult. The code we use couples Fokker-Planck and Monte Carlo methods, in a 2 dimensional (cylindrical) geometry. For the ﬁrst time all the LTTEs are fully considered, along with a proper, full, self-consistent treatment of Compton cooling, which depends on the LTTEs. Using this code, we studied a set of physical processes that are relevant to the variability of blazars, including electron injection and escape, radiative cooling, and stochastic particle acceleration. Our comparison of the observational data and the simulation results revealed that a combination of all those processes is needed to reproduce the observed behaviors of the emission of blue blazars. The simulation favors that the high energy emission at quiet and ﬂare stages comes from the same location. We have further modeled red blazars PKS 1510-089. External radiation, which comes from the broad line region (BLR) or infrared torus, is included in the model. The results conﬁrm that external Compton model can adequately describe the emission from red blazars. The emission from BLR is favored as the source of Inverse Compton seed photons, compared to synchrotron and IR torus radiation.
Vortices in the co-orbital region of embedded protoplanets
(2004) Koller, Josef; Liang, Edison P.
This thesis presents two-dimensional hydrodynamic disk simulations with embedded protoplanets, emphasizing the non-linear dynamics in the co-orbital region. In particular, it demonstrates how a protoplanetary disk responds to embedded low mass planets at the inviscid limit. Since the potential vorticity (PV) flow is not conserved, due to the spiral shocks and possibly boundary layer effects emanating from the planet, the PV profile develops inflection points which eventually render the flow unstable. Vortices are produced in association with the potential vorticity minima. Born in the separatrix region, these vortices experience close encounters with the planet, consequently exerting strong torques on the planet. The existence of these vortices, if confirmed, have important implications on planetary migration rates. The formation of vortices is discussed in more detail and a key parameter is found which depends solely on planet mass and sound speed. With this key parameter, one can predict the disk evolution, PV growth rates, and threshold conditions for forming vortices in the co-orbital region. An analytical estimate for the change of PV due to shocks is compared to the actual change in PV in the hydrodynamic simulations. They match well except in the inner region where vortices form. In addition, extensive resolution tests were carried out but uncertainties remain about the physics of this particular region.
X-ray and gamma-ray emissions from galactic black hole candidates: Observations and analysis
(2000) Lin, Dechun; Liang, Edison P.
Though it has been more than 30 years since the first discovery of the classic Galactic black hole candidate (GBHC) Cygnus X-1, the X-ray emission mechanisms of GBHCs are still not well understood. It is generally agreed that black holes accrete materials from nearby objects or media to form accretion disks around them. The accretion disks can be heated up to a temperature above 1 keV. Such a hot disk emits X- and Gamma-rays, observations of which are essential for understanding the accreting and heating processes. Our multi-wavelength observations of GRS 1758-258 (a GBHC) in August 1997 revealed several properties that are important for constraining the emission model and the geometry of the accretion disk. (1) Its spectrum does not have a significant soft component. This implies that the cold optically thick disk must be small or be mostly covered by a optically thin hot corona. (2) The spectrum has an exponential cutoff around 200 keV, which means that the hot corona is largely thermal. (3) No significant iron lines are detected. This suggests that the reprocessing of the X-ray by the cold disk is negligible. We systematically analyzed RXTE archival data to study the energy dependency of the variability of X-ray flux from four hard X-ray sources: Cygnus X-1, GX 339-4, GRS 1758-258 and 1E 1740.7-2942. Cygnus X-1 was found to have flatter power density spectrum (PDS) shapes at higher energies, while the other three have energy independent PDS shapes. No current models can fully explain these results. A general trend was found among the four sources that the variability anti-correlates with the X-ray flux. We found that the 0.5--10 Hz quasi-periodic oscillations (QPOs) observed in GRS 1915+105 (a GBHC) has peculiar phase lag behaviors. When the QPO fundamental frequency is low (0.5--2.0 Hz), positive phase lags, which mean that hard photons arrive later than soft photons, were observed in both fundamental and first harmonic frequencies. The phase lags have opposite signs at the two frequencies when the fundamental frequency is high (2.0--4.5 Hz). Such strange behaviors can not be explained by current models. We found the flat radio spectrum observed in most GBHCs can be explained by a hybrid thermal/non-thermal plasma, which can also reproduce the power-law tail that have been observed in many GBHCs beyond the thermal X-ray spectrum.

Browsing by Author "Liang, Edison P."

Results Per Page

Sort Options