12,831 research outputs found

### The vertical composition of neutrino-dominated accretion disks in gamma-ray bursts

We investigate the vertical structure and elements distribution of
neutrino-dominated accretion flows around black holes in spherical coordinates
with the reasonable nuclear statistical equilibrium. According our
calculations, heavy nuclei tend to be produced in a thin region near the disk
surface, whose mass fractions are primarily determined by the accretion rate
and the vertical distribution of temperature and density. In this thin region,
we find that $^{56}\rm Ni$ is dominant for the flow with low accretion rate
(e.g., $0.05$$M_{\odot}$ $\rm s^{-1}$) but $^{56}\rm Fe$ is dominant for the
high counterpart (e.g., $1 M_{\odot}$ $\rm s^{-1}$). The dominant $^{56}\rm Ni$
in the special region may provide a clue to understand the bumps in the optical
light curve of core-collapse supernovae.Comment: 15 pages, 2 figures, accepted for publication in Ap

### Maxima of the $Q$-index of non-bipartite $C_{3}$-free graphs

A classic result in extremal graph theory, known as Mantel's theorem, states
that every non-bipartite graph of order $n$ with size $m>\lfloor
\frac{n^{2}}{4}\rfloor$ contains a triangle. Lin, Ning and Wu [Comb. Probab.
Comput. 30 (2021) 258-270] proved a spectral version of Mantel's theorem for
given order $n.$ Zhai and Shu [Discrete Math. 345 (2022) 112630] investigated a
spectral version for fixed size $m.$ In this paper, we prove $Q$-spectral
versions of Mantel's theorem.Comment: 14 pages, 4 figure

### Millicharged Atomic Dark Matter

We present a simplified version of the atomic dark matter scenario, in which
charged dark constituents are bound into atoms analogous to hydrogen by a
massless hidden sector U(1) gauge interaction. Previous studies have assumed
that interactions between the dark sector and the standard model are mediated
by a second, massive Z' gauge boson, but here we consider the case where only a
massless gamma' kinetically mixes with the standard model hypercharge and
thereby mediates direct detection. This is therefore the simplest atomic dark
matter model that has direct interactions with the standard model, arising from
the small electric charge for the dark constituents induced by the kinetic
mixing. We map out the parameter space that is consistent with cosmological
constraints and direct searches, assuming that some unspecified mechanism
creates the asymmetry that gives the right abundance, since the dark matter
cannot be a thermal relic in this scenario. In the special case where the dark
"electron" and "proton" are degenerate in mass, inelastic hyperfine transitions
can explain the CoGeNT excess events. In the more general case, elastic
transitions dominate, and can be close to current direct detection limits over
a wide range of masses.Comment: 5 pages, 2 figures; v2: added references, and formula for dark
ionization fraction; published versio

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