| Villain, Perz-Azorin, Pons, Miralles | 3D anisotropic cooling of strongly magnetized NSs |
| Abstract. Anisotropic cooling of strongly magnetized neutron stars is studied through a three-dimensional spectral code. |
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| Randall L. Cooper & Ramesh Narayan | A Two-Zone Model for Type I X-ray Bursts on Accreting Neutron Stars |
| Abstract. We construct a two-zone model to describe H and He burning on the surface of an accreting neutron star and use it to study the triggering of type I X-ray bursts. Although highly simplified, the model reproduces all of the bursting regimes seen in the more complete global linear stability analysis of Narayan & Heyl (2003), including the regime of delayed mixed bursts. The results are also consistent with observations of type I X-ray bursts. At low accretion rates, thermonuclear helium burning via the well-known thin-shell thermal instability triggers bursts. As the accretion rate increases, however, the trigger mechanism evolves from the fast thermal instability to a slowly growing overstability involving both H and He burning. The competition between nuclear
heating via the beta-limited CNO cycle as well as the triple-alpha process and radiative cooling via photon diffusion and emission drives oscillations with a period approximately equal to the H-burning timescale. If these oscillations grow, the gradually rising temperature at the base of the helium layer eventually provokes a thin-shell thermal instability and hence a delayed mixed burst. This overstability closely resembles the delayed mixed bursts of
Narayan & Heyl. For accretion rates greater than ~25% of the Eddington limit,
there is no instability or overstability, and so there are no bursts. |
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| Deborah N. Aguilera (collaborators: J. Pons, J. A. Miralles, D. Blaschke, N. Scoccola, H. Grigorian) | CSL condensates in protoneutron star evolution |
| Abstract. We consider the color spin locking (CSL) phase of two-flavor quark
matter in a nonlocal, separable model. Different form factors are used
to represent the momentum dependence of the quark vertex.
This smoothing of the NJL model formfactor shows advantageous features
for the phenomenology of compact stars: (i) a lowering of the
critical chemical potential for the onset of the chiral phase
transition as a prerequisite for stability of hybrid stars with
extended quark matter cores and (ii) a suppression of the direct
Urca process for all quark quasiparticle modes
with reduced minimum gaps being in the range of values
interesting for phenomenological studies of hybrid star cooling evolution.
The critical temperatures of the CSL condensates and the possibility of finding such a phase in a protoneutron star evolution is discussed.
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| Helber Dussan, Charles Horowitz | Equtaion of state of neutron star crust |
| Abstract. The neutron star crust plays an important role in emissivity, heat transport and in general conductive properties of neutron stars. Using relativistic mean field theory as a frame work to describe the microscopic dynamic, we are looking for the Equation of State of the crust. |
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| A. Chbihi, G. Verde, J.D. Frankland, J. Moisan, J.P. Wieleczko | Exploring the symmetry energy with isospin effects in heavy-ion collisions |
| Abstract. In this contribution we will describe an experiment proposal that we will perform at GANIL. We propose to measure the isotopic distributions produced in 40,48Ca+40,48Ca collisions at E/A=35 MeV using the multidetector INDRA coupled to VAMOS Spectrometer. From a detailed study of these distributions in multifragmentation events we will estimate the relative contribution of surface and volume terms to the symmetry energy in the nuclear EOS. The knowledge of these relative contributions and, especially, the relevance of the surface term are key to explore to what extent one can learn about the density dependence of the symmetry energy in infinite nuclear matter from multifragmentation of finite nuclei and from nuclear reaction dynamics. |
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| Jennifer Marsh, Nicole Wityk, Jan Staff, Rachid Ouyed | Fireballs From CFL Quark Stars: Application to Gamma Ray Bursts |
| Abstract. Recent studies of photon-generation mechanisms in the color-superconducting color-flavor locked (CFL) phase of dense quark matter have found gamma-ray emissivities in excess of 10^50 erg/cm^3/s for temperatures in the 10-30MeV range. It has been suggested that this property can trigger ultra-relativistic fireballs at the surface of hypothetical hot quark stars with an energy release of up to 10^48-10^50 erg within a fraction of a millisecond. This theoretical inner engine provides sufficient energy for the production of observed gamma ray bursts (GRB). If the newly born quark star is surrounded by an accretion disk following its formation the star will undergo a succession of accretion and ejections phases which describe its inner engine activity, and provides a credible justification for observations. We suggest that quark stars in the CFL phase surrounded by an accretion disk following their birth constitute natural candidates for GRB inner engines. |
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| Miss Claire Tarbert, Dr Daniel Watts, Crystal Ball at MAMI Collaboration | Matter Radius Measurement of 208Pb with the Crystal Ball at MAMI |
| Abstract. The charge radius of 208Pb has been well established via elastic electron scattering experiments, however the matter radius has never been pinned down with the same accuracy. A measurement of the “neutron skin” on 208Pb would provide valuable information on the compressibility of neutron rich matter and directly impact the physics of neutron stars. Coherent Neutral Pion Photoproduction provides a novel and elegant way of measuring this quantity with an electromagnetic probe. In April 2005 the newly installed Crystal Ball at MAMI was used to measure the reaction on a range of nuclei up to 208Pb and the uncertainties in the extracted matter radii are expected to be ~0.035fm. In this talk the setup and preliminary results from the experiment will be presented. |
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| Denis Leahy, Sharon Morsink and Coire Cadeau | New methods for modeling pulse shapes of rapidly rotating pulsars |
| Abstract. Pulse shapes of accreting neutron stars can reveal properties of the neutron star and emission region, such as the star\'s mass and radius, and the orientation of the emission region and observer. To properly interpret observations of rapidly rotating X-ray pulsars, one requires theoretical models which account for general relativity, timing effects and stellar oblateness. We have developed a new computer code to calculate pulse shapes, which fully accounts for these effects, and obtain estimates on the accuracy of parameters obtained with a fitting method which uses a Schwarzschild approximation to the calculation of the light curve. Approximations to the full calculations have also been developed which can be applied to fitting routines for modelling observed pulse shapes. As an example, results will be given for modelling the pulse shape of the accreting millisecond pulsar SAX J1808-3654.
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| Denis Leahy, Rachid Ouyed, Jan Staff, Brian Niebergal | Quantifying the quark star hypothesis for AXPs and SGRs |
| Abstract. We provide arguments supporting the quark star model for AXPs
and SGRs. With prompt formation of hybrid stars from supernova in
massive stars, we show how to avoid the problems faced
by the magnetar model raised by Vinke and Kuiper (2006). We neither
require ms spin periods nor 10**16 Gauss magnetic fields.
Fossil magnetic fields, possibly amplified by a color ferromagnetic phase
in the core of the hybrid star (Iwasaki 2005), can account for observed
AXP and SGR fields. After a delay determined by weak interactions in quark
matter, the hybrid star converts by a quark nova into a u-d-s
superconducting quark star and is manifested as an AXP or SGR. The steady
luminosity of an AXP/SGR is provided by field decay in the quark star as
the magnetic flux contained in vortices is expelled during spin-down,
and outburst luminosity is provided by accretion of small amounts of
matter onto the quark star. |
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| Jan E Staff, Rachid Ouyed, Prashanth Jaikumar | Quark deconfinement in neutron star cores: The effects of spin-down |
| Abstract. We study the role of spin-down in driving quark deconfinement in the high
density core of isolated neutron stars. We assume that spin-down is solely
due to magnetic braking. We find that the time to reach deconfinement is
strongly dependent on the magnetic field. For a fiducial critical density of
5 times saturation density for the transition to the quark phase, we find
that neutron stars lighter than 1.5 solar masses cannot reach a deconfined
phase. Depending on the equation of state, neutron stars of more than 1.5
solar masses can enter a quark phase only if they are spinning faster than
about 3 milliseconds, whereas larger spin periods imply that they are either
already quark stars or will never become one.
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| Brian Niebergal, Rachid Ouyed, Denis Leahy, Wolfgang Dobler | Quark Star Bursting and Spin-Down |
| Abstract. We present a new model where the onset of colour superconductivity in quark stars is responsible for Anomolous X-Ray Pulsars (AXPs) and Soft-Gamma Repeaters (SGRs). In this model the bursts are caused by the reorganization of the quark stars magnetosphere following the formation of rotationally aligned vortices in the star\'s interior. These vortices confine the internal magnetic field, through the Meissner effect, into alignment with the rotation axis instigating the exterior field to also align. This results in the cessation of pulsed radio emission. We present 3D numerical (MHD) simulations of the magnetosphere alignment, as well as an analytic treatment of the spin-down due to vortex expulsion. We find that a typical quark star can release energy of order 10^44 erg during the burst, with peak emission at 280 keV assuming synchrotron. Furthermore, for initial conditions representitive of AXPs/SGRs we find that spin-down due to vortex expulsions results in parameters indicative of Radio Quiet Isolated Neutron Stars (RQINSs). |
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| Kelsey Hoffman, Robert Rutledge, Lars Bildsten, Edward Brown, George Pavlov, and Vyacheslav Zavlin | Radius Measurements of Aql X-1 |
| Abstract. Measuring the radius of neutron stars can help constrain the equation of state of nuclear matter. The thermal emission of a neutron star is one method which provides a reliable estimate of the radius. This thermal emission can be treated as resulting from a neutron star with a hydrogen atmosphere. We have measured the quiescent spectrum of 11 Chandra ACIS-S observations of the transient, low-mass X-ray binary, Aql X-1. We will present the radius measurement, assuming a hydrogen atmosphere model, for Aql X-1. |
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| Olga Liliana Caballero | Simulations of Non-Uniform High Dense matter and Neutrino Interactions in Supernovae |
| Abstract. Core collapse supernovae are one of the most energetic explosions known in nature. The collapse is senstive to neutrino interactions in high dense matter. We study the effects of ion correlation on the neutrino mean free path by calculating the structure factor for two different media; one of them is composed by one single ion specie while the other one is a mixture of different ions. Molecular dynamic simulation results show a decrease in the neutrino mena free path for a medium composed of a mixture of ions. |
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| Krishna Rajagopal, Andreas Schmitt | Stressed pairing in conventional color superconductors is unavoidable |
| Abstract. At sufficiently high densities, cold dense three-flavor quark matter
is in the color-flavor locked (CFL) phase, in which all nine quarks pair
in a particularly symmetric fashion.
At smaller densities, the heaviness of the strange quark and the
requirements of electric and color neutrality impose a ``stress\'\' on
the pairing. At sufficiently small densities, this stress is
large enough that the system can lower its energy
by breaking pairs, resulting in some unconventional color superconductor
which includes gapless excitations, spatial inhomogeneity,
counter-propagating currents, or all three.
We investigate whether there is some less symmetric but still
conventional pattern of pairing that can evade the stress. In
other words, is there a pattern of pairing in which, once
electric and color neutrality are imposed by suitable
chemical potentials, pairing only occurs among those quarks whose
Fermi momenta would be equal in the absence of pairing? We use
graph-theoretical methods to classify 511 patterns of
conventional color superconducting pairing, and show that none
of them meet this requirement. All feel a stress, and all
can be expected to become unstable to gapless modes at a density comparable
to that at which the CFL phase becomes unstable.
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| Andrei Kryjevski | Superfluid Current Generation in CFL Quark Matter at Nonzero Strange Quark Mass |
| Abstract. We find that for large enough strange quark mass, kaon condensed CFL phase of asymptotically dense strongly interacting 3 flavor quark matter is unstable with respect to generation of currents of Nambu Goldstone bosons due to spontaneous breaking of baryon number symmetry and hypercharge symmetry in CFLK0 ground state. The total baryon and hypercharge currents vanish in the ground state. We find that CFLK0 and the new state are separated by the first order phase transition.
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| Manjari Bagchi, Rachid Ouyed, Jan Staff, Subharthi Ray, Mira Dey, Jishnu Dey | Temperature dependence of strange stars\' maximum mass and consequences to models of Gamma ray bursts within the ``Quark-Nova\" picture. |
| Abstract. The M-R curve for strange stars with any Equation of State (EoS)
and at any temperature has a maximum value for the star mass.
This point denotes an instability and so if there is accretion
onto a star having this maximum mass, it collapses into a black
hole. Surprisingly, we found that for a given EoS, the value of
maximum mass decreases with increasing temperature which
indicates that black holes with higher temperatures have lower
mass - somehow matching to the famous temperature law for black
holes. This might have interesting consequences for models of
Gamma-Ray Bursts (GRB) within the ``Quark-Nova\" picture where the
normal matter get converted into the strange quark matter with
simultaneous ejection of some of the normal matter which will
later fall back and form an accretion disk. If enough matter is
accreted onto the newly formed strange star, it will collapse and
form a black hole. Continued accretion onto the black hole can
lead to an ultra-relativistic jet with internal shocks. This may
explain the recently observed late re-energization of GRBs and
cause the giant x-ray flare observed in some early afterglows of
GRBs. |
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| Dimitrios Psaltis | Using Neutron Stars to Test General Relativity |
| Abstract. Gravity in the strong-field regime remains largely unconstrained by experiments to date.
Indeed, a large class of gravity theories can be constructed that obey the
Einstein equivalence principle and cannot be rejected by solar-system or double-neutron
star tests, but that diverge from general relativity in the strong-field regime. I show that such
theories predict neutron stars with significantly different properties than their general relativistic counterparts. I then discuss how recent observations with X-ray observatories, such as
Chandra and XMM-Newton, provide new quantitative constraints on strong-field gravity.
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