Bulletin of the American Physical Society
APS April Meeting 2016
Volume 61, Number 6
Saturday–Tuesday, April 16–19, 2016; Salt Lake City, Utah
Session K13: Compact Objects |
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Chair: Vicky Kalogera, Northwestern University Room: 250F |
Sunday, April 17, 2016 1:30PM - 1:42PM |
K13.00001: Can magnetic-field windup kill the r-mode instability of neutron stars? John Friedman, Lee Lindblom, Luciano Rezzolla At second order in perturbation theory, the unstable r-mode of a rotating star includes growing differential rotation whose form and growth rate are determined by gravitational radiation reaction. With no magnetic field, the angular velocity of a fluid element grows exponentially until the mode reaches its nonlinear saturation amplitude and remains nonzero after saturation. With a background magnetic field, the differential rotation winds up and amplifies the field, and previous work suggests that the amplification may damp out the instability. A background magnetic field, however, turns the time-independent perturbations corresponding to adding differential rotation into perturbations with characteristic frequencies of order the Alfven frequency. We argue that magnetic field growth stops soon after the mode reaches its saturation amplitude. We show that this is the case for a toy model, where magnetic amplification for small saturation amplitude is too small to damp the r-mode. For a more realistic model of a cold, rotating neutron star, an analogous upper limit depends on the assumption that there are no marginally unstable perturbations. [Preview Abstract] |
Sunday, April 17, 2016 1:42PM - 1:54PM |
K13.00002: Black Hole Battery Janna Levin, Daniel D'Orazio Black holes are dark dead stars. Neutron stars are giant magnets. As the neutron star orbits the black hole, an electronic circuit forms that generates a blast of power just before the black hole absorbs the neutron star whole. The black hole battery conceivably would be observable at cosmological distances. Possible channels for luminosity include synchro-curvature radiation, a blazing fireball, or even an unstable, short-lived black hole pulsar. As suggested by Mingarelli, Levin, and Lazio, some fraction of the battery power could also be reprocessed into coherent radio emission to populate a subclass of fast radio bursts. [Preview Abstract] |
Sunday, April 17, 2016 1:54PM - 2:06PM |
K13.00003: Modeling Neutron Star Stability with a Modified Tolman-Oppenheimer-Volkoff Equation Spasen Chaykov, James O'Brien The Tolman-Oppenheimer-Volkoff (TOV) equation represents the solution to the Einstein field equations where the source of curvature is given by the stress-energy tensor of a perfect fluid. In flat space it has the form $T_{\mu\nu} = (\rho + p)U_\mu U_\nu + p\eta_{\mu\nu}$ and the convention for curved space-time is to just replace the Minkowski metric with $g_{\mu\nu} $. For our research we instead use a modified stress-energy tensor of the form $T_{\mu\nu} = (\rho + p)U_\mu U_\nu + pg_{\mu\nu} + \pi_{\mu\nu} $ where the anisotropic $\pi_{\mu\nu} $ is a symmetric, traceless rank two tensor which obeys $U^\mu\pi_{\mu\nu} = 0$. The motivation is that such a term in the stress-energy tensor can account for effects due to the curvature of space-time and would not be present in the tensor describing flat space.The final revised TOV equation is of the form $-r^2p' = GM\rho[1+\frac{p-2q}{\rho}][1+\frac{4\pi r^3(p-2q)}{M}][1-\frac{2GM}{r}]^{-1}-2r^2q' - 6rq $ where the primes indicate differentiation with respect to the radial coordinate and the q terms arise from the components of $\pi_{\mu\nu} $. The equation was then solved numerically with both a polytropic and a MIT bag model equations of state. The result is a changed prediction for the stability range of neutron stars. [Preview Abstract] |
Sunday, April 17, 2016 2:06PM - 2:18PM |
K13.00004: Axial Symmetric Solutions to Einstein's Field Equations for Deformed Neutron Stars Omair Zubairi, Fridolin Weber Traditional models of neutron stars are constructed under of assumption that they are perfect spheres. This is not correct, however, if the matter inside of neutron stars is described by an non-isotropic model for the equation of state. Examples of such stars are magnetars and neutron stars that would contain color-superconducting quark matter. In this work, we derive the stellar structure equations which describe the properties of non-isotropic neutron stars. The equations are solved numerically in two dimensions. We calculate stellar properties such as masses and radii along with pressure and density profiles and investigate any changes from conventional spherically symmetric neutron stars. [Preview Abstract] |
Sunday, April 17, 2016 2:18PM - 2:30PM |
K13.00005: Hyperons, Delta Baryons, and Deconfined Quarks in Neutron Star Cores William Spinella, Fridolin Weber Many solutions to the so-called "hyperon puzzle" rely on determinations of the meson-hyperon coupling constants that stiffen the neutron star equation of state, pushing the appearance of hyperons to higher densities. In our work we investigate the effect a similarly enhanced set of meson-hyperon couplings has on the appearance of hyperons, delta baryons, and deconfined quarks in the neutron star core. To this end we model hadronic matter using the relativistic mean field approach with density dependent meson-baryon coupling constants, and deconfined quark matter with a three-flavor nonlocal variant of the Nambu-Jona-Lasinio model. The scalar meson-hyperon coupling constants are fit to hypernuclear potentials at saturation density, while the vector meson-hyperon coupling constants are extracted from a recent theoretical study. Our results indicate that delta baryons could be highly favored over hyperons and appear at very low densities, and that hyperons, deltas, and deconfined quarks could all potentially coexist in the cores of $\sim2$ solar mass neutron stars. [Preview Abstract] |
Sunday, April 17, 2016 2:30PM - 2:42PM |
K13.00006: Can we use the Slow-Rotation Approximation in Electromagnetic Observations of Black Holes? Dimitry Ayzenberg, Kent Yagi, Nicolas Yunes Future electromagnetic observations of BHs may allow us to test General Relativity in the strong-field regime. Such tests, however, require knowledge of rotating BH solutions in modified gravity. Several rotating BH solutions in modified theories have only been found in the slow-rotation and small-coupling approximations. This talk presents the results of an investigation into whether the systematic error introduced due to the approximate nature of these BH metrics is small enough relative to the observational error to allow us to use electromagnetic observations to constrain deviations from General Relativity. We address this by considering whether electromagnetic observables constructed from a slow-rotation approximation to the Kerr metric can fit observables constructed from the full Kerr metric with systematic errors smaller than current observational errors. We focus on BH shadow and continuum spectrum observations. Our results suggest that the modified gravity solutions found in the slow-rotation and small-coupling approximations may be sufficient to constrain realistic deviations from General Relativity with continuum spectrum and BH shadow observations. [Preview Abstract] |
Sunday, April 17, 2016 2:42PM - 2:54PM |
K13.00007: Simulations of Double White Dwarf Mergers Patick Motl, Jan Staff, Dominic Marcello, Geoffrey Clayton, Juhan Frank We present numerical simulations of double white dwarf mergers initiated by mass transfer instability. In particular, we are interested in the possible connection between such double degenerate mergers and the peculiar irregular variable R Corona Borealis stars. For the merger of a Carbon-Oxygen white dwarf with a Helium white dwarf, the degree to which Carbon from the accreting star is dredged up plays a crucial role in the appearance of the rejuvenated, merged object. We explore the amount of dredge up in the accreting star and its influence in stellar evolution models initialized from the merged object resulting from dynamical evolutions. [Preview Abstract] |
Sunday, April 17, 2016 2:54PM - 3:06PM |
K13.00008: Quark matter and fermionic dark matter compact stars CHHANDA SAMANTA, Somenath Mukhopadhyay, Devasish Narayan Basu Compact stars, made of quark matter and fermionic dark matter with arbitrary masses and interaction strengths, are studied by solving the Tolman--Oppenheimer--Volkoff equation of general relativity. The mass-radius relation for quark matter compact stars is obtained from the MIT bag model equation of state (EoS) with thin crust for different bag constants. The EoS of non-self-annihilating dark matter for an interacting Fermi gas with dark matter particle of 1-100 GeV mass is studied. For sufficiently strong interactions, the maximum stable mass of compact stars and its radius are controlled by the parameter of the interaction, both increasing linearly with the interaction strength. The mass-radius relation for compact stars made of strongly interacting fermions shows that the radius remains approximately constant for a wide range of compact stars. [Preview Abstract] |
Sunday, April 17, 2016 3:06PM - 3:18PM |
K13.00009: VHE Observations of TeV Gamma-Ray Binaries by VERITAS Payel Kar Among many High Mass X-ray Binary (HMXB) systems, only a few exhibit TeV gamma-ray emission. Contemporaneous multi-wavelength observations of these sources are crucial for understanding their astrophysical properties. LS I $+$61 303 and HESS J0632$+$057 are two such TeV Binaries which have been observed by VERITAS and its multiwavelength partners over years. As previously seen at X-ray wavelengths, ~a TeV flux enhancement for HESS J0632$+$057 near orbital phase 0.75 has now been seen for the first time by VERITAS. This was found using updated analysis techniques implemented on the entire 200 hour data set spanning December 2006 to January 2015. From October 2014 to November 2014, LS I $+$61 303 exhibited its brightest flare ever observed. The flare provided evidence for TeV flux correlations with the emission at X-ray wavelengths. Previous flares of this system observed by VERITAS (2011-2012) had no such correlations. Studies made with multiwavelength observations facilitate our understanding of the gamma ray emission models from these HMXB systems. The results for the above two sources will be presented, along with other new ~results from VERITAS, ~improving our knowledge of this sparsely populated class of sources. [Preview Abstract] |
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