12,148 research outputs found
Polarization of kilonova emission from a black hole-neutron star merger
A multi-messenger, black hole (BH) - neutron star (NS) merger event still
remains to be detected. The tidal (dynamical) ejecta from such an event,
thought to produce a kinonova, is concentrated in the equatorial plane and
occupies only part of the whole azimuthal angle. In addition, recent
simulations suggest that the outflow or wind from the post-merger remnant disk,
presumably anisotropic, can be a major ejecta component responsible for a
kilonova. For any ejecta whose photosphere shape deviates from the spherical
symmetry, the electron scattering at the photosphere causes a net polarization
in the kilonova light. Recent observational and theoretical polarization
studies have been focused to the NS-NS merger kilonova AT2017gfo. We extend
those work to the case of a BH-NS merger kilonova. We show that the degree of
polarization at the first hr can be up to 3\% if a small amount
() of free neutrons have survived in the fastest component
of the dynamical ejecta, whose beta-decay causes a precursor in the kilonova
light. The polarization degree can be 0.6\% if free neutrons survived in
the fastest component of the disk wind. Future polarization detection of a
kilonova will constrain the morphology and composition of the dominant ejecta
component, therefore help to identify the nature of the merger.Comment: 10 pages, 5 figures. Accepted for publication in Ap
The Laplacian Eigenvalues and Invariants of Graphs
In this paper, we investigate some relations between the invariants
(including vertex and edge connectivity and forwarding indices) of a graph and
its Laplacian eigenvalues. In addition, we present a sufficient condition for
the existence of Hamiltonicity in a graph involving its Laplacian eigenvalues.Comment: 10 pages,Filomat, 201
Transport Coefficients from Extremal Gauss-Bonnet Black Holes
We calculate the shear viscosity of strongly coupled field theories dual to
Gauss-Bonnet gravity at zero temperature with nonzero chemical potential. We
find that the ratio of the shear viscosity over the entropy density is
, which is in accordance with the zero temperature limit of the ratio
at nonzero temperatures. We also calculate the DC conductivity for this system
at zero temperature and find that the real part of the DC conductivity vanishes
up to a delta function, which is similar to the result in Einstein gravity. We
show that at zero temperature, we can still have the conclusion that the shear
viscosity is fully determined by the effective coupling of transverse gravitons
in a kind of theories that the effective action of transverse gravitons can be
written into a form of minimally coupled scalars with a deformed effective
coupling.Comment: 23 pages, no figure; v2, refs added; v3, more refs added; v4, version
to appear in JHE
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