Article

Astronomy - A neutron star in F-sharp

Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
Science (Impact Factor: 31.48). 04/2006; 311(5769):1876-7. DOI: 10.1126/science.1125125
Source: arXiv

ABSTRACT Discovery of the fastest spinning pulsar gives new constraints on the size of a neutron star and matter under extreme conditions
and decreases the need for gravitational waves to impose a limiting maximum spin.

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    ABSTRACT: Rotating neutron stars have modes that are driven unstable by gravitational radiation reaction, principally the r-mode, a Rossby wave with n = 3, m = 2, and hence large gravitational radiation reaction. Here n and m label the Legendre functions associated with the mode. The r-mode instability is active when gravitational driving dominates viscous dissipation. It has been suggested that this instability can (1) set the largest angular frequency of rotation of accreting neutron stars and (2) significantly spin down newborn neutron stars preventing them from reaching millisecond periods. Both the maximum frequency that neutron stars can reach and the frequency to which newborn stars can be spun down to in the first few years after formation depend on the neutron star composition via viscous dissipation and neutrino cooling. The nonlinear development of the instability plays a very important role in determining how the saturation process works, and also illustrates how instabilities can saturate at low amplitudes as a consequence of nearly resonant excitation of other modes. We model the nonlinear interactions between modes together with basic neutron star physics including viscous heating, cooling and spin evolution of the star. The nonlinear effects are included via three-mode couplings. We show that in most scenarios one triplet of modes is sufficient to stop the growth of the instability. To explore possible nonlinear behaviors we parameterize uncertain properties of neutron stars such as the superfluid transition temperature and the rate at which the star cools via neutrino emission. The average evolution of the mode amplitudes can usually be approximated by quasi-stationary states that change slowly with spin frequency and temperature and can be determined algebraically. The spin and temperature evolution follow or oscillate around trajectories along sequences of quasi-stationary states. In the Low Mass X-ray Binary (LMXB) case (Chapter 2), after some brief initial oscillations, the modes settle into their quasi-stationary states and the quasi-steady approximation is almost exact. The star heats via viscous dissipation from the three modes and, if this heating is balanced by neutrino cooling, then the evolution will either be stable or enter a slow thermogravitational runaway on a very long timescale of [approximate] 10^6 years. The stable evolutions can be (1) cyclic--with a small cycle size and a typical frequency change of at most 10%, or (2) the star can evolve toward a full equilibrium state in which the accretion torque balances the gravitational radiation emission. Alternatively, if the cooling cannot balance the heating, a faster runaway occurs, the r-mode crosses several parametric instability thresholds, and more modes need to be included. In the young neutron star case (Chapter 3), the pulsar is hot T ~ 10^10 K and cools fast. The evolution depends on whether the neutrino cooling can be stopped by viscous heating from the three modes. In this case the evolution is more dynamic. After a short precursor, the modes oscillate around quasi- stationary states and the spin and temperature of the star oscillate around thermal equilibrium. There are three possible outcomes: the neutron star can spin down on different sides of or along the r-mode stability curve. If the viscosity is too low to stop the cooling a runaway occurs.
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    ABSTRACT: The three-flavor crystalline color-superconducting (CCS) phase of quantum chromodynamics (QCD) is a candidate phase for the ground state of cold matter at moderate densities above the density of the deconfinement phase transition. Apart from being a superfluid, the CCS phase has properties of a solid, such as a lattice structure and a shear modulus, and hence the ability to sustain multipolar deformations in gravitational equilibrium. We construct equilibrium configurations of hybrid stars composed of nuclear matter at low, and CCS quark matter at high, densities. Phase equilibrium between these phases is possible only for rather stiff equations of state of nuclear matter and large couplings in the effective Nambu--Jona-Lasinio Lagrangian describing the CCS state. We identify a new branch of stable CCS hybrid stars within a broad range of central densities which, depending on the details of the equations of state, either bifurcate from the nuclear sequence of stars when the central density exceeds that of the deconfinement phase transition or form a new family of configurations separated from the purely nuclear sequence by an instability region. The maximum masses of our nonrotating hybrid configurations are consistent with the presently available astronomical bounds. The sequences of hybrid configurations that rotate near the mass-shedding limit are found to be more compact and thus support substantially larger spins than their same mass nuclear counterparts.
    Physical review D: Particles and fields 01/2008; 77(2). DOI:10.1103/PHYSREVD.77.023004 · 4.86 Impact Factor
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    ABSTRACT: Globular clusters are preferred for the study of millisecond pulsars (MSPs), given the 蠙100 increase in their number per unit stellar mass than in the Galaxy at large. X-ray observations of globulars with imaging grazing incidence telescopes have proven to be at least as sensitive as radio telescopes for MSP detection and spectral classification, but not (yet) for period discovery due to the relatively low count rates. However, for known periods, pulse-phase spectroscopy studies are remarkably effective. We provide an initial overview of the current X-ray studies of MSPs in globular clusters as well as in the Galaxy field. Early X-ray studies of MSPs with ROSAT, ASCA, and RXTE are reviewed briefly and put into the context of current results. Globular clusters observed with the Chandra X-ray Observatory, given its exceptional angular resolution, have clarified the range of MSP types (thermal vs. non-thermal) and overall populations. Observations of several nearby field MSPs with XMM-Newton, with its temporal-spectral resolution, have given new measurements of the M/R (compactness) of neutron stars from precise measures of their soft X-ray pulse profiles as a function of energy. X-ray spectral-timing of MSPs can then best constrain the equation of state of neutron stars when future broadband (0.1-10keV) X-ray telescopes with very high throughput (蠙10xChandra or XMM-Newton) are in operation.
    02/2009: pages 165-179;

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