181 research outputs found
Unexpected Reconstruction of the alpha-Boron (111) Surface
We report on a novel reconstruction of the alpha-boron (111) surface,
discovered using an ab initio evolution structure search, and reveal that it
has an unexpected neat structure and much lower surface energy than the
recently proposed (111)-I_R,(a) surface. For this reconstruction, every single
interstitial boron atom forms bridges with the unique polar-covalent bonds
between neighboring B_12 icosahedra, which perfectly meet the electron counting
rule and are responsible for the reconstruction-induced metal-semiconductor
transition. The peculiar charge transfer between the interstitial atoms and the
icosahedra plays an important role in stabilizing the surface.Comment: [email protected];[email protected]
Tentative evidence of spatially extended GeV emission from SS433/W50
We analyze 10 years of Fermi-LAT data towards the SS433/W50 region. With the
latest source catalog and diffuse background models, the gamma-ray excess from
SS433/W50 is detected with a significance of 6{\sigma} in the photon energy
range of 500 MeV - 10 GeV. Our analysis indicates that an extended flat disk
morphology is preferred over a point-source description, suggesting that the
GeV emission region is much larger than that of the TeV emission detected by
HAWC. The size of the GeV emission is instead consistent with the extent of the
radio nebula W50, a supernova remnant being distorted by the jets, so we
suggest that the GeV emission may originate from this supernova remnant. The
spectral result of the GeV emission is also consistent with an supernova
remnant origin. We also derive the GeV flux upper limits on the TeV emission
region, which put moderate constrains on the leptonic models to explain the
multiwavelength data.Comment: 7 pages, 4 figures, accepted for publication in A&
Limiting Superluminal Neutrino Velocity and Lorentz Invariance Violation by Neutrino Emission from the Blazar TXS 0506+056
The detection of high-energy neutrino coincident with the blazar TXS 0506+056
provides a unique opportunity to test Lorentz invariance violation (LIV) in the
neutrino sector. Thanks to the precisely measured redshift, i.e., ,
the comoving distance of the neutrino source is determined. In this work, we
obtain and discuss the constraints on the superluminal neutrino velocity
and the LIV by considering the energy loss of superluminal
neutrino during propagation. Given superluminal electron velocity (), a very stringent constraint on superluminal neutrino velocity can be
reached, i.e., , corresponding to the
quantum gravity (QG) scale
and for linear (quadratic)
LIV, which are orders of magnitude tighter for linear LIV and orders tighter for quadratic LIV compared to the time-of-flight constraint
from MeV neutrinos of SN 1987A. While given the subluminal electron velocity, a
weaker constraint on the superluminal neutrino velocity is obtained, i.e.,
, which is consistent with the
conclusions of previous works. We also study the neutrino detection probability
due to the distortion of neutrino spectral shape during propagation, which
gives slightly weaker constraints than above by a factor of .Comment: 4 pages, 1 table, accepted by Phys. Rev.
Detection of gamma-ray emission from the Coma cluster with Fermi Large Area Telescope and tentative evidence for an extended spatial structure
Many galaxy clusters have giant halos of non-thermal radio emission,
indicating the presence of relativistic electrons in the clusters. Relativistic
protons may also be accelerated by merger and/or accretion shocks in galaxy
clusters. These cosmic-ray (CR) electrons and/or protons are expected to
produce gamma-rays through inverse-Compton scatterings or inelastic
collisions respectively. Despite of intense efforts in searching for
high-energy gamma-ray emission from galaxy clusters, conclusive evidence is
still missing so far. Here we report the discovery of MeV gamma-ray
emission from the Coma cluster direction with an unbinned likelihood analysis
of the 9 years of {\it Fermi}-LAT Pass 8 data. The gamma-ray emission shows a
spatial morphology roughly coincident with the giant radio halo, with an
apparent excess at the southwest of the cluster. Using the test statistic
analysis, we further find tentative evidence that the gamma-ray emission at the
Coma center is spatially extended. The extended component has an integral
energy flux of in the
energy range of 0.2 - 300 GeV and the spectrum is soft with a photon index of
. Interpreting the gamma-ray emission as arising from CR proton
interaction, we find that the volume-averaged value of the CR to thermal
pressure ratio in the Coma cluster is about . Our results show that
galaxy clusters are likely a new type of GeV gamma-ray sources, and they are
probably also giant reservoirs of CR protons.Comment: 10 pages, 10 figures, Accepted by Physical Review D, more spatial
models for the gamma-ray emission are used, systematic checks on the results
are adde
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