Bulletin of the American Physical Society
2018 Annual Meeting of the APS Four Corners Section
Volume 63, Number 16
Friday–Saturday, October 12–13, 2018; University of Utah, Salt Lake City, Utah
Session C07: Astrophysical Processes |
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Chair: Robert Cooper, NMSU Room: JFB B-1 |
Friday, October 12, 2018 10:45AM - 11:09AM |
C07.00001: Radiative cooling and modeling the Crab Synchrotron Nebula Invited Speaker: Joseph Foy Models of the Crab synchrotron nebula include energy losses by radiative and adiabatic expansion, but neglect the effect of radiative losses on the flow itself. The standard approach of all such models to date is to calculate a post-shocked flow and modify only the energy equation while assuming steady state flow dynamics. Even the most recent efforts at simulating the synchrotron emission from the Crab continue to adopt this approach. Typically, these authors argue that since the radiated power is only about 10 percent of the pulsar's spin down luminosity, radiative losses should have only negligible effects on flow dynamics. This argument implicitly assumes a spherically symmetric nebula: the estimate can be as high as 20 to 30 percent for a more realistic wedge geometry, for example. A new, time-independent model of the Crab synchrotron nebula that incorporates radiative cooling for the first time is presented. Specifically, the relativistic magnetohydrodynamic (MHD) flow equations is first derived and then solved with radiative energy losses fully coupled to these equations from the start. The results of model calculations give plausible agreement with observations of the Crab Nebula. It is demonstrated that, for the particle energy and magnetic field values typical of the Crab, synchrotron cooling has too significant an effect upon the flow structure to be ignored. The possibility that the dynamic “wisps" observed in the nebular flow of the Crab are due to synchrotron cooling instabilities is discussed. |
Friday, October 12, 2018 11:09AM - 11:21AM |
C07.00002: Anti-correlation between X-ray luminosity and pulsed fraction in the Small Magellanic Cloud pulsar SXP 1323 Jun Yang We report the evidence for the anti-correlation between pulsed fraction (PF) and luminosity of the X-ray pulsar SXP 1323, found for the first time in a luminosity range $10^{35}$--$10^{37}$ erg s$^{-1}$ from observations spanning 15 years. The phenomenon of a decrease in X-ray PF when the source flux increases has been observed in our pipeline analysis of other X-ray pulsars in the Small Magellanic Cloud (SMC). It is expected that the luminosity under a certain value decreases as the PF decreases due to the propeller effect. Above the propeller region, an anti-correlation between the PF and flux might occur either as a result of an increase in the un-pulsed component of the total emission or a decrease of the pulsed component. Additional modes of accretion may also be possible, such as spherical accretion and a change in emission geometry. At higher mass accretion rates, the accretion disk could also extend closer to the neutron star (NS) surface, where a reduced inner radius leads to hotter inner disk emission. These modes of plasma accretion may affect the change in the beam configuration to fan-beam dominant emission. |
Friday, October 12, 2018 11:21AM - 11:33AM |
C07.00003: Gamma-ray Bursts in Inhomogeneous Interstellar Media Jacob Fields, Matthew Anderson, Eric Winston Hirschmann, David W Neilsen Gamma-ray bursts (GRBs) are some of the most energetic phenomena in the known universe, but some of the physics surrounding these events remains poorly understood. In particular, light curves associated with long GRBs often demonstrate a high degree of variability. While variations in these spectra could result from the dynamics of the central engine, interactions of the blast wave with an inhomogeneous interstellar medium can also play a role. We investigate this latter effect by studying the collision of relativistic blast waves with dense stellar ejecta and estimate the effect on an observed light curve. We solve the relativistic fluid equations for a blast configuration and model the inhomogeneity as shells surrounding the star, such as might occur when an aging star expels its outer layers. We discuss the numerical algorithms and present preliminary results from our simulations. |
Friday, October 12, 2018 11:33AM - 11:45AM |
C07.00004: Using line-depth ratios to determine surface gravity of solar-type stars Jessica Galbraith-Frew, Inese I Ivans Determination of the chemical composition of stars in the Milky Way Galaxy allows us to understand galactic evolution. Large scale spectroscopic surveys are the key to determining the chemical composition of stars and thus, our galaxy’s history. In order to determine the abundance of a particular element, one must know the effective temperature, surface gravity, overall stellar metallicity, and atomic transition parameters for the corresponding absorption line. Of these parameters surface gravity remains the most elusive. Determination of the stars surface gravity is complicated by broadening effects and has degeneracies with effective temperature. By taking the ratio of two line-depths many of the dependencies can be removed. These line-depth ratios (LDR) are a function of the effective temperature, surface gravity, and line abundance, with the temperature being the dominant contributor. I will present the application of the LDR technique to solar-type stars to determine their surface gravity, and explore application to large-scale surveys. |
Friday, October 12, 2018 11:45AM - 11:57AM |
C07.00005: Classification of Isometry Algebras of Exact Solutions of Einstein's Field Equations Eugene Hwang, Charles Torre Exact solutions of Einstein's equations have been classified according to their Lie algebras of isometries and their isotropy subalgebras. Solutions were taken from the Utah State University (USU) electronic library of solutions of Einstein's field equations and the classification used Maple code developed at USU. This classification adds to the data contained in the library of solutions and provides additional tools for addressing the equivalence problem for solutions to the Einstein field equations. |
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