Bulletin of the American Physical Society
APS April Meeting 2015
Volume 60, Number 4
Saturday–Tuesday, April 11–14, 2015; Baltimore, Maryland
Session U2: Pulsars, Neutron Stars and Black Holes |
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Sponsoring Units: DAP Room: Holiday 1 |
Monday, April 13, 2015 3:30PM - 3:42PM |
U2.00001: Weighing ULX Black Holes Using X-ray Reverberation Mapping Tod Strohmayer, Dheeraj Pasham Ultra-luminous X-ray sources (ULXs) are bright, variable, non-AGN point sources in nearby galaxies. Their high luminosities and X-ray timing properties suggest some of the brightest ULXs may be accreting intermediate-mass black holes (mass range of a few$\times$(100-1000) solar masses), but definitive, dynamical mass measurements have not yet been achieved. Here we present initial results of the first simultaneous X-ray - optical reverberation mapping study of a ULX in order to weigh its black hole. If the optical flux is driven by X-ray irradiation, then the time lag between the two bands gives the size of the accretion disk. This time lag can then be combined with measured line widths from the disk to obtain a mass estimate in a manner analogous to AGN reverberation mapping. As a pilot study, we observed ULX NGC 5408 X-1 simultaneously in the optical (using FORS2 and the 8m Very Large Telescope) and the X-ray (XMM-Newton). We detect the optical counterpart and obtain a photometric light curve with time resolution of a few minutes. We also detect X-ray variability and present a cross-correlation analysis for this object. We describe the implications of these results for NGC 5408 X-1's mass and discuss future prospects for extending this method to weigh ULX black holes in general [Preview Abstract] |
Monday, April 13, 2015 3:42PM - 3:54PM |
U2.00002: A 400 solar mass black hole in the galaxy M82 Dheeraj Pasham, Tod Strohmayer, Richard Mushotzky M82's brightest X-ray source has been thought to be an intermediate-mass black hole (100-10000 $M_{\odot}$) because of its extremely high X-ray luminosity and variability characteristics, although some models suggest that its mass may be only of the order of 20 $M_{\odot}$. The previous mass estimates were based on scaling relations which used X-ray low-frequency characteristic timescales which have large intrinsic uncertainties. In stellar-mass black holes, we know that the high frequency quasi-periodic oscillations that occur in a 3:2 frequency ratio (100-450 Hz) are stable and scale inversely with black hole mass with a reasonably small dispersion. The discovery of such stable oscillations thus potentially offers an alternative and less ambiguous mass determination for intermediate-mass black holes, but has hitherto not been realized. I will discuss the discovery of stable, twin-peak (3:2 frequency ratio) X-ray oscillations from M82 X-1 at the frequencies of 3.32 Hz and 5.07 Hz and how this helps overcome the systematic uncertainties present in previous studies. Assuming we can extend the stellar-mass relationship, we estimate its black hole mass to be 428$\pm$105 $M_{\odot}$. I will also discuss future prospects of extending this method to weigh intermediate-mass black holes. [Preview Abstract] |
Monday, April 13, 2015 3:54PM - 4:06PM |
U2.00003: Impact of Deformation on the Structure of Non-Rotating Neutron Stars Omair Zubairi, Fridolin Weber, Efrain Ferrer, Vivian Incera Conventional models of compact objects such as neutron stars assume they are perfect spheres. However, due to high magnetic fields, certain classes of neutron stars such as magnetars and neutron stars containing color-superconducting quark matter cores are expected to be deformed (non-spherical). In this work, we seek to examine the stellar structure of such objects in the framework of general relativity. We derive the stellar structures equations of non-spherical neutron stars and calculate stellar properties such as masses, radii, along with pressure and density profiles and investigate any changes from standard spherical models. [Preview Abstract] |
Monday, April 13, 2015 4:06PM - 4:18PM |
U2.00004: Limiting the Accretion-fed Growth of Neutron Stars during Common Envelope Morgan MacLeod, Enrico Ramirez-Ruiz This talk focuses on the orbital inspiral of a neutron star (NS) through the envelope of its giant-branch companion during a common envelope (CE) episode. These CE episodes are necessary to produce close pairs of NSs that can inspiral and merge due to gravitational wave losses in less than a Hubble time. Yet, as an embedded NS spirals to tighter separations within the CE, it can also accrete from the surrounding material. Standard theories for the hydrodynamics of CE events predict that embedded NSs may gain enough mass to force their collapse to black holes. We argue that CE structure, and in particular, the density gradient across the accretion radius of the NS leads to flow morphologies that prevent the NS from gaining much mass during its CE inspiral. The modest mass gains we predict can reconcile theories of neutrino-cooled accretion onto NSs with the observed masses of NSs in close double-NS binaries. [Preview Abstract] |
Monday, April 13, 2015 4:18PM - 4:30PM |
U2.00005: ABSTRACT WITHDRAWN |
Monday, April 13, 2015 4:30PM - 4:42PM |
U2.00006: Fanning the Flames: X-ray Burst Probes of Nuclear Burning Simin Mahmoodifar, Tod Strohmayer Type I X-ray bursts are thermonuclear explosions observed in many accreting neutron stars (NSs) that result from rapid unstable burning of hydrogen and helium accreted onto the surface of the star. During an X-ray burst the X-ray flux rapidly rises by a factor of ~10-20 in a couple of seconds and then decays on a longer timescale as the surface of the star cools. Oscillations have been detected during the rise and/or decay of some of these X-ray bursts that have frequencies within a few Hz of the stellar spin frequency and must be due to nonuniform emission from the stellar surface. Here I discuss the results of simulations of the rise and decay of a typical X-ray burst light curve and the evolution of their fractional oscillation amplitudes. We generate light curves using a physical model for a spreading hot spot, taking into account the effect of the Coriolis force (latitude-dependent flame spreading speed), as well as relativistic effects. I will explain how the combination of the light curve and fractional amplitude evolution can constrain the properties of the flame spreading, such as ignition latitude, which would be important for measuring NSs masses and radii using X-ray burst oscillations. I discuss the prospects for future X-ray missions such as ESA's LOFT in this area. [Preview Abstract] |
Monday, April 13, 2015 4:42PM - 4:54PM |
U2.00007: Neutron Star Universality Near the Maximum Mass Sharon Morsink, Jason Fedorowich Rotating neutron stars have relationships between some properties (such as moment of inertia, I, Love number and quadrupole moment, Q) that are approximately independent of the equation of state (EOS). These universal I-Love-Q relations and other similar universal properties have a number of potentially useful astrophysical applications. It is also important to investigate the underlying causes for the existence of these relations. Some of a neutron star's properties (such as the star's rotational deformation) have a dependence on how ``close'' the star is to the maximum allowed mass. In this talk we will illustrate how the rotating neutron star's properties for different EOS approach the same similar solution as the maximum mass star is approached. [Preview Abstract] |
Monday, April 13, 2015 4:54PM - 5:06PM |
U2.00008: Neutrino emissivity in the quark-hadron mixed phase of neutron stars William Spinella, Fridolin Weber Neutrino-pair bremsstrahlung due to interactions between electrons and the crystalline lattice in the quark-hadron mixed phase of high mass neutron stars has been previously studied by Na et al. 2012. We extend this study by first replacing the MIT bag model with the nonlocal three-flavor Nambu-Jona-Lasinio model to describe the quark matter phase. We then include rod and slab rare-phase geometries in addition to spherical blobs. Finally we compare contributions due to Bragg diffraction and electron-phonon scattering. We find that the neutrino emissivity due to electron-lattice interactions in the mixed phase may be substantial at low temperature and quark fraction. [Preview Abstract] |
Monday, April 13, 2015 5:06PM - 5:18PM |
U2.00009: Braking Index of Isolated Pulsars Oliver Hamil, Jirina Stone, Martin Urbanec, Gabriela Urbancova Isolated pulsars are rotating neutron stars with accurately measured angular velocities $\Omega$, and their time derivatives which show unambiguously that the pulsars are slowing down. The exact mechanism of the spin-down is a question of debate in detail, but the commonly accepted view is that it arises through emission of magnetic dipole radiation (MDR). The energy loss by a rotating pulsar is proportional to a model dependent power of $\Omega$. This relation leads to the power law $\dot{\Omega}$ = -K $\Omega^{\rm n}$ where $n$ is called the braking index, equal to the ratio ($\Omega \ddot{\Omega}$)/ ${\dot{\Omega}}^2$. The simple MDR model predicts the value of n = 3, but observations of isolated pulsars provide rather precise values of $n$, individually accurate to a few percent or better, in the range 1$ <$ n $ < $ 2.8, which is consistently less than the predictions of the MDR model. In this work, we study the dynamical limits of the MDR model as a function of angular velocity. The effects of variation in the rest mass, the moment of inertia, and the dependence on a realistic Equation of State of the rotating star are considered. Furthermore, we introduce a simulated superfluid effect by which the angular momentum of the core is eliminated from the calculation. [Preview Abstract] |
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