Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session T09: Progress in Understanding Neutron Stars and Pulsars |
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Sponsoring Units: DAP Chair: Bernard Kelly, NASA GSFC Room: Sheraton Governor's Square 11 |
Monday, April 15, 2019 3:30PM - 3:42PM |
T09.00001: The shape of rotating neutron stars and systematic errors in pulse profile observations parameter estimation Hector O. Silva, George Pappas, Nicolás Yunes, Kent Yagi The Neutron star Interior Composition Explorer (NICER) is currently observing the pulse profiles produced by hotspots on the surface of rotating neutron stars (NSs), allowing us to measure their radii with unprecedented precision in the very near-future, helping solve the long standing problem of determining the equation of state (EOS) at supranuclear densities found in the interior of these stars. In the standard approach to model these pulse profiles, photons are assumed to propagate in a Schwarzschild background despite being emitted by an oblate surface. This oblateness (due to rotation) is described by analytic formulas, obtained by fitting catalogs of rotating NS models (covering a large sample of EOSs and spin frequencies) to some prescribed shape function. However, this smearing over EOSs, introduces a systematic error when estimating e.g. the star's radius, as the star's actual shape is determined by a single, underlying EOS. The question then arises: are the shape functions currently in use by NICER accurate enough to keep this error under control? How large it? Can we improve these formulas to tame this error? We present a first step in tackling this issue and present new, accurate formulas to model the shape of rotating NSs. |
Monday, April 15, 2019 3:42PM - 3:54PM |
T09.00002: A larger-than-2-solar-mass pulsar in NANOGrav: leveraging Shapiro delay to constrain the neutron star equation of state H. Thankful Cromartie, Emmanuel Fonseca, Scott Ransom, Paul Demorest Despite its importance to our understanding of physics at supranuclear densities, the neutron star equation of state (EoS) remains poorly understood. Millisecond pulsar (MSP) timing — a technique which involves very accurately predicting the arrival time of every pulse from an MSP — continues to place some of the most stringent constraints on the neutron star interior EoS. In highly inclined MSP binary systems, the gravitational potential of the pulsar’s companion induces a discrepancy between expected and measured pulse times of arrival. Measurement of this general relativistic phenomenon, called Shapiro Delay, can yield the mass of both the MSP and its companion. By combining data from the NANOGrav 12.5-year data set with recent orbital-phase-specific observations using the Green Bank Telescope, we have measured the mass of the MSP J0740+6620 to be 2.10 ± 0.11 solar masses. It may therefore be the most massive neutron star ever observed, and would serve as a strong constraint on the neutron star EoS. |
Monday, April 15, 2019 3:54PM - 4:06PM |
T09.00003: Improved Analytic Modeling of Neutron Star Interiors Nan Jiang, Kent Yagi Studies of neutron star (NS) observations are extremely timely given the recent detection of gravitational waves from a binary NS merger GW170817, and an International Space Station payload NICER currently in operation aiming to determine radii of NS to a precision better than 5%. In many cases, NS solutions are constructed numerically since realistic equations of state are given in tables. However, to relate observables like the NS mass and radius to interior quantities like central density and pressure, it would be useful to provide an analytic model of NS interior. One such solution for static and isolated NS is the Tolman VII solution characterized only by two parameters (e.g. mass and radius), though its agreement with numerical solutions is not perfect. We here introduce an improved model based on Tolman VII solution by introducing an additional parameter (related to the central density) to make the analytic density profile agree better with the numerical results. The pressure, gravitational potential and interior mass profiles improve typically by a factor of 3 compared to the original one for typical NS. Our results are first-step calculations towards constructing more realistic NS under rotation or tidal deformation. |
Monday, April 15, 2019 4:06PM - 4:18PM |
T09.00004: Quasi-universal relations for the masses and radii of rotating neutron stars Sharon M Morsink Over the next few years, it is expected that present (eg NICER, LIGO, CHANDRA, XMM) and planned (eg ATHENA, STROBE-X, eXTP) telescopes will be providing precise measurements of the masses and radii of many neutron stars, allowing for a determination of the equation of state of cold supranuclear density matter. The neutron stars' properties will be determined using a variety of methods, all with systematic errors. The different methods and telescopes target neutron stars with different spin rates which introduces another complication, since the rotation of a star increases its mass and radius. Many properties of rotating neutron stars have been shown to obey universal relationships. In this work we explore the extent to which the increase and mass and radius due to rotation follows a universal type of relation and how this can be used to compare equation of state constraints resulting from different methodologies. |
Monday, April 15, 2019 4:18PM - 4:30PM |
T09.00005: Inferring neutron star properties from GW170817 with universal relations Philippe Landry, Bharat Kumar Because all neutron stars share a common equation of state, the tidal deformability measurement from the binary merger GW170817 has implications for the properties of other neutron-star systems. Using equation-of-state insensitive relations between observables like moment of inertia, tidal deformability and stellar compactness, we derive constraints on these properties as a function of neutron-star mass. The constraints allow us to make virtually model-independent inferences for the double pulsar moment of inertia, the radii of X-ray bursters, and millisecond pulsar spins. The gravitational-wave based estimates can be compared or combined with electromagnetic measurements, where available, to test the universality of the equation of state and enhance probes of neutron-star structure. |
Monday, April 15, 2019 4:30PM - 4:42PM |
T09.00006: On the Dynamical Stability of Uber-massive Neutron Stars Pedro Luis Espino, Vasileios Paschalidis The joint detection of gravitational wave (GW) and electromagnetic (EM) signals resulting from the merger of two neutron stars (NS) was a breakthrough in multi-messenger astronomy. A possible remnant of NS-NS mergers is a hyper-massive neutron star (HMNS): a transient configuration that is supported from gravitational collapse by additional centrifugal support due to differential rotation. The space of differentially rotating stars is rich, exhibiting different types of solutions among which there exist configurations that can support more than 2 times the maximum mass of a TOV star. We call these configurations uber-massive. The dynamical stability of uber-massive neutron stars (UMNS) and of the different types of HMNSs has not been studied, yet. We present results from dynamical simulations in general relativity addressing this issue. The primary focus is on UMNSs with the largest possible rest mass and on different types of HMNSs with smaller rest mass. A diverse set of outcomes is explored including gravitational collapse, long lifetimes, and migration to dynamically stable configurations. |
Monday, April 15, 2019 4:42PM - 4:54PM |
T09.00007: Inhomogeneity in the Mass Distribution of Deformed Neutron Stars Omair Zubairi, Fridolin Weber Non-rotating neutron stars with high magnetic fields and/or neutron stars containing color-superconducting quark matters cores break traditional spherical symmetry, thus resulting these objects to be oblong spheroids with distinct polar and equatorial directions. The mass distribution is then expected to be non-homogeneous thus resulting in a non-zero mass quadrupole moment. Recent publications on the stellar structure of deformed neutron stars suggest the mass could significantly change depending on the type of deformation. In this work, we investigate the mass distribution of deformed neutron stars in the framework of general relativity by numerically calculating the gravitational mass quadrupole moment and study its effects on the stellar structure of these deformed objects. |
Monday, April 15, 2019 4:54PM - 5:06PM |
T09.00008: Searches for Galactic Center Pulsars and Radio Observations of the Galactic Center Magnetar PSR J1745–2900 Aaron B. Pearlman, Walid A. Majid, Thomas A. Prince, Jonathon Kocz, Shinji Horiuchi The discovery of new pulsars in tight orbits around the supermassive black hole at the Galactic Center (GC) would allow for unprecedented tests of fundamental physics and theories of gravity, including General Relativity, in an unexplored strong-field regime of gravity. To date, six pulsars have been detected within the central 0.5°, which include an ultra-rare magnetar, PSR J1745–2900, with a projected separation of ~0.1 pc from Sgr A*. The other five known GC pulsars are believed to lie in the foreground, despite their close angular separation from Sgr A*. Although the GC magnetar is the closest known pulsar to Sgr A*, its orbit with the central black hole is not sufficiently short to observe relativistic effects.
We are currently carrying out high frequency radio observations of the GC using the NASA Deep Space Network 70-m radio antenna, DSS-43, in Canberra, Australia. We will discuss new results from a study of radio pulses from the GC magnetar. The magnetar's pulses consist of multiple emission components, some of which show changes on very fast timescales and significant evidence of pulse broadening. Frequency structure is also observed in many of the magnetar's pulse components. Lastly, we will give an overview of our current work towards detecting new GC pulsars. |
Monday, April 15, 2019 5:06PM - 5:18PM |
T09.00009: First Results from the Hard X-Ray Polarimetric Observations of the Accreting X-Ray Pulsar GX 301-2 with the X-Calibur Mission Quincy Abarr, Richard Bose, Gianluigi de Geronimo, Paul Dowkontt, Manel Errando, Thomas Gadson, Victor Guarino, Scott Heatwole, Nirmal Kumar Iyer, Fabian Kislat, Mózsi Kiss, Takao Kitaguchi, Henric S Krawczynski, Rakhee Kushwah, James Lanzi, Shaorui Li, Lindsey Lisalda, Takashi Okajima, Mark Pearce, Zachary Peterson, Brian F Rauch, David Stuchlik, Hiromitsu Takahashi, Nagomi Uchida, Andrew West On December 29, 2018, X-Calibur was launched from McMurdo in Antarctica for a stratospheric balloon flight. Though the flight was short, terminating on January 1, 2019, it was scientifically successful. The observations caught GX 301-2 close to the apastron at elevated flux levels. Here, we present the results; these include the 15-60 keV light cure, the hard X-ray energy spectrum, and the first constraints on the polarization of the hard X-ray emission. We also show results from simultaneous observations with the Neil Gehrels Swift and NICER X-ray telescopes. Finally, we include an outlook of what physical constraints could be obtained by observing X-ray pulsars with future X-ray spectropolarimetric observations. |
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