Echo Emission From Dust Scattering and X-Ray Afterglows of Gamma-Ray Bursts

We investigate the effect of X-ray echo emission in gamma-ray bursts (GRBs). We find that the echo emission can provide an alternative way of understanding X-ray shallow decays and jet breaks. In particular, a shallow decay followed by a "normal" decay and a further rapid decay of X-ray afterglows can be together explained as being due to the echo from prompt X-ray emission scattered by dust grains in a massive wind bubble around a GRB progenitor. We also introduce an extra temporal break in the X-ray echo emission. By fitting the afterglow light curves, we can measure the locations of the massive wind bubbles, which will bring us closer to finding the mass loss rate, wind velocity, and the age of the progenitors prior to the GRB explosions.Comment: 25 pages, 3 figures, 2 tables. Accepted for publication in Ap

Doctor of Philosophy

dissertationThe current study investigated the effectiveness of an evidenced-based social skills program, the Superheroes Social Skills program to determine its effectiveness with children who have autism spectrum disorders (ASD) and have been identified by teachers or parents as being highly bullied at school. Three participants with ASD, between the ages of 6 and 10, received socials skills instruction using the Superheroes Social Skills program. Along with the social skills lessons, Superheroes Social Skills includes lessons that specifically address bullying. All participants received instruction three times a week for 12 weeks. There were also eight normally developing students who attended the lessons and served as peer models. Generalization probes of social interaction during free play periods, in both a research and naturalistic setting, were conducted for each participant in order to determine treatment efficacy. After the implementation of the program, effect sizes (ES), Percentage of All Non-Overlapping Data (PAND), and Percentage of Non-Overlapping Data (PND) were calculated to examine differences in the amount of social interaction during the free play periods. The average total social engagement score for the participants showed a moderate ES using PAND (ES=0.34) and the No Assumptions method (ES=0.42). In the naturalistic setting, which was the playground at recess, large ES were found using PAND (ES=0.92) and the No Assumptions method (ES=0.85). In order to assess the program's impact on the victim's response to bullying, the participants with ASD engaged in bullying role-play scenarios during the intervention. The victim's behavioral responses were coded to determine if any changes were made. Increases in appropriate responding to bullying and the use of appropriate body language were observed across participants. Along with the observational data, the participants' responses on pre- and postmeasures of social responsiveness and victimization were compared. The results of the study suggest increases in social skills and decreases in reports of being a victim of bullying

What Produced the Ultraluminous Supernova Remnant in NGC 6946?

The ultraluminous supernova remnant (SNR) in NGC 6946 is the brightest known SNR in X-rays, ~1000 times brighter than Cas A. To probe the nature of this remnant and its progenitor, we have obtained high-dispersion optical echelle spectra. The echelle spectra detect H-alpha, [N II], and [O III] lines, and resolve these lines into a narrow (FWHM ~20--40 km/s) component from un-shocked material and a broad (FWHM ~250 km/s) component from shocked material. Both narrow and broad components have unusually high [N II]/H-alpha ratios, ~1. Using the echelle observation, archival HST images, and archival ROSAT X-ray observations, we conclude that the SNR was produced by a normal supernova, whose progenitor was a massive star, either a WN star or a luminous blue variable. The high luminosity of the remnant is caused by the supernova ejecta expanding into a dense, nitrogen-rich circumstellar nebula created by the progenitor.Comment: 20 pages, 5 figures. To be published in The Astronomical Journal, March 200

The connection between entropy and the absorption spectra of Schwarzschild black holes for light and massless scalar fields

We present heuristic arguments suggesting that if EM waves with wavelengths somewhat larger than the Schwarzschild radius of a black hole were fully absorbed by it, the second law of thermodynamics would be violated, under the Bekenstein interpretation of the area of a black hole as a measure of its entropy. Thus, entropy considerations make the well known fact that large wavelengths are only marginally absorbed by black holes, a natural consequence of thermodynamics. We also study numerically the ingoing radial propagation of a scalar field wave in a Schwarzschild metric, relaxing the standard assumption which leads to the eikonal equation, that the wave has zero spatial extent. We find that if these waves have wavelengths larger that the Schwarzschild radius, they are very substantially reflected, fully to numerical accuracy. Interestingly, this critical wavelength approximately coincides with the one derived from entropy considerations of the EM field, and is consistent with well known limit results of scattering in the Schwarzschild metric. The propagation speed is also calculated and seen to differ from the value $c$, for wavelengths larger than $R_{s}$, in the vicinity of $R_{s}$. As in all classical wave phenomena, whenever the wavelength is larger or comparable to the physical size of elements in the system, in this case changes in the metric, the zero extent 'particle' description fails, and the wave nature becomes apparent.Comment: 14 Pages, 4 figures. Accepted for publication in the Journal Entrop

Colorectal Cancer Chemoprevention: Is This the Future of Colorectal Cancer Prevention?

Colorectal cancer (CRC) is presently one of the most common causes of cancer-related death in our setting and affects a great number of people each year. Screening strategies are commonly used but they do not seem enough to avoid CRC development or prevent completely its mortality. Because of this fact other prevention strategies have gained interest in recent years. Chemoprevention seems to be an attractive option in this setting and several drugs have been studied in this field. This review is focused on salicylates, nonsteroidal anti-inflammatory drugs (NSAIDs) and cycloxygenase-2 inhibitors (COXIBs), whose mechanism of action could be directly related to colon cancer chemoprevention

Stellar wind bubbles around WR and [WR] stars

We study the dynamics of stellar wind bubbles around hydrogen-deficient stars using numerical simulations with time- and ion dependent cooling. We consider two types of hydrogen-deficient stars, massive WR stars, producing Ring Nebulae, and low mass [WR] stars, producing Planetary Nebulae. We show that for the Planetary Nebulae, the different cooling properties of the hydrogen-deficient wind lead to a later transition from momentum- to energy-driven flow, which could explain the observed turbulence of these nebulae. We find that Ring Nebulae should all be energy-driven, and show how comparing the bubble's momentum and kinetic energy to the input wind momentum and kinetic energy, can give misleading information about the dynamics of the bubble.Comment: 9 pages, 3 figures, to be published in A&

Clouds in the atmospheres of extrasolar planets. II. Thermal emission spectra of Earth-like planets influenced by low and high-level clouds

We study the impact of multi-layered clouds (low-level water and high-level ice clouds) on the thermal emission spectra of Earth-like planets orbiting different types of stars. Clouds have an important influence on such planetary emission spectra due to their wavelength dependent absorption and scattering properties. We also investigate the influence of clouds on the ability to derive information about planetary surface temperatures from low-resolution spectra.Comment: accepted for publication in A&

Further developments in stress initialization in geomechanics via FEM and a two-step procedure involving airy functions

The in-situ stress field in rock masses is a key aspect when a numerical analysis of a rock mass is carried out in any area of geo-engineering, such as civil, mining, or Oil & Gas. A method for the numerical generation of the in-situ stress state in the FE context, based on Airy stress functions was previously introduced. It involves two steps: 1) an estimate of the stress state at each Gauss point is generated, and 2) global equilibrium is verified and re-balancing nodal forces are applied as needed. In this paper, new developments towards improving the accuracy of the stress proposal are discussed. A real application example has been used to illustrate the results achieved with the new implementation