6,697 research outputs found
Twistor and Polytope Interpretations for Subleading Color One-Loop Amplitudes
We use the relation of the one-loop subleading-color amplitudes to the
one-loop -point leading color amplitudes in SYM, to derive a
polytope interpretation for the former in the case, and a representation
in momentum twistor space for the general case. These techniques are
explored in detail for the 5-point and 6-point amplitudes. We briefly discuss
the implications for IR divergences.Comment: 30 pages, 2 figures; typos corrected, citations added; fig2 modified,
fig. 1 and explanations added, mostly around eq. 2.11; version accepted in
Nucl.Phys.
Spectroscopic confirmation of an ultra-faint galaxy at the epoch of reionization
Within one billion years of the Big Bang, intergalactic hydrogen was ionized
by sources emitting ultraviolet and higher energy photons. This was the final
phenomenon to globally affect all the baryons (visible matter) in the Universe.
It is referred to as cosmic reionization and is an integral component of
cosmology. It is broadly expected that intrinsically faint galaxies were the
primary ionizing sources due to their abundance in this epoch. However, at the
highest redshifts (; lookback time 13.1 Gyr), all galaxies with
spectroscopic confirmations to date are intrinsically bright and, therefore,
not necessarily representative of the general population. Here, we report the
unequivocal spectroscopic detection of a low luminosity galaxy at . We
detected the Lyman- emission line at {\AA} in two separate
observations with MOSFIRE on the Keck I Telescope and independently with the
Hubble Space Telescope's slit-less grism spectrograph, implying a source
redshift of . The galaxy is gravitationally magnified by
the massive galaxy cluster MACS J1423.8+2404 (), with an estimated
intrinsic luminosity of mag and a stellar mass of
solar masses. Both are an order of
magnitude lower than the four other Lyman- emitters currently known at
, making it probably the most distant representative source of
reionization found to date
Slipping magnetic reconnection during an X-Class solar flare observed by SDO/AIA
We present SDO/AIA observations of an eruptive X-class flare of July 12,
2012, and compare its evolution with the predictions of a 3D numerical
simulation. We focus on the dynamics of flare loops that are seen to undergo
slipping reconnection during the flare. In the AIA 131A observations, lower
parts of 10 MK flare loops exhibit an apparent motion with velocities of
several tens of km/s along the developing flare ribbons. In the early stages of
the flare, flare ribbons consist of compact, localized bright transition-region
emission from the footpoints of the flare loops. A DEM analysis shows that the
flare loops have temperatures up to the formation of Fe XXIV. A series of very
long, S-shaped loops erupt, leading to a CME observed by STEREO. The observed
dynamics are compared with the evolution of magnetic structures in the
"standard solar flare model in 3D". This model matches the observations well,
reproducing both the apparently slipping flare loops, S-shaped erupting loops,
and the evolution of flare ribbons. All of these processes are explained via 3D
reconnection mechanisms resulting from the expansion of a torus-unstable flux
rope. The AIA observations and the numerical model are complemented by radio
observations showing a noise storm in the metric range. Dm-drifting pulsation
structures occurring during the eruption indicate plasmoid ejection and
enhancement of reconnection rate. The bursty nature of radio emission shows
that the slipping reconnection is still intermittent, although it is observed
to persist for more than an hour
On All-loop Integrands of Scattering Amplitudes in Planar N=4 SYM
We study the relationship between the momentum twistor MHV vertex expansion
of planar amplitudes in N=4 super-Yang-Mills and the all-loop generalization of
the BCFW recursion relations. We demonstrate explicitly in several examples
that the MHV vertex expressions for tree-level amplitudes and loop integrands
satisfy the recursion relations. Furthermore, we introduce a rewriting of the
MHV expansion in terms of sums over non-crossing partitions and show that this
cyclically invariant formula satisfies the recursion relations for all numbers
of legs and all loop orders.Comment: 34 pages, 17 figures; v2: Minor improvements to exposition and
discussion, updated references, typos fixe
Variability of Broad and Blueshifted Component of [OIII]5007 in IZWI
Although the existence of asymmetrical profile of [OIII]5007 has
been discovered for ages, its filiation and physics are poorly understood. Two
new spectra of I ZWI taken on Nov 16, 2001 and on Dec 3, 2002 were compared
with the spectra taken by BG92. Following results are obtained. 1)The certain
variations of broad [OIII] during about 10 years separating the observations
are identified. The inferred length scale of broad [OIII] emitting region
ranges from 0.3pc to 3pc. By assuming a Keplerian motion in emitting region,
the material emitting broad [OIII] is likely to be located at transient
emission line region, between BLR and NLR. 2)We find a positive relation
between the FeII emission and flux of H(or continuum). On the other
hand, the parameter RFe decreases with ionizing continuum marginally. 3)We
detect a low ionized NLR in I ZWI, because of the low flux ratios
().Comment: 14 pages, 3 figures, NewA in pres
A Universal Model of Global Civil Unrest
Civil unrest is a powerful form of collective human dynamics, which has led
to major transitions of societies in modern history. The study of collective
human dynamics, including collective aggression, has been the focus of much
discussion in the context of modeling and identification of universal patterns
of behavior. In contrast, the possibility that civil unrest activities, across
countries and over long time periods, are governed by universal mechanisms has
not been explored. Here, we analyze records of civil unrest of 170 countries
during the period 1919-2008. We demonstrate that the distributions of the
number of unrest events per year are robustly reproduced by a nonlinear,
spatially extended dynamical model, which reflects the spread of civil disorder
between geographic regions connected through social and communication networks.
The results also expose the similarity between global social instability and
the dynamics of natural hazards and epidemics.Comment: 8 pages, 3 figure
Radial Velocity Studies of Close Binary Stars. XV
Radial-velocity measurements and sine-curve fits to the orbital radial
velocity variations are presented for the last eight close binary systems
analyzed the same way as in the previous papers of this series: QX And, DY Cet,
MR Del, HI Dra, DD Mon, V868 Mon, ER Ori, and Y Sex. For another seven systems
(TT Cet, AA Cet, CW Lyn, V563 Lyr, CW Sge, LV Vir and MW Vir) phase coverage is
insufficient to provide reliable orbits but radial velocities of individual
components were measured. Observations of a few complicated systems observed
throughout the DDO close-binary program are also presented; among them an
especially interesting is the multiple system V857 Her which - in addition to
the contact binary - very probably contains one or more sub-dwarf components of
much earlier spectral type. All suspected binaries which were found to be most
probably pulsating stars are briefly discussed in terms of mean radial
velocities and projected rotation velocities (v sin i) as well as spectral type
estimates. In two of them, CU CVn and V752 Mon, the broadening functions show a
clear presence of non-radial pulsations. The previously missing spectral types
for the DDO I paper are given here in addition to such estimates for most of
the program stars of this paper.Comment: submitted to A
Slipping magnetic reconnection, chromospheric evaporation, implosion, and precursors in the 2014 September 10 X1.6-class solar flare
© 2016. The American Astronomical Society. All rights reserved.. We investigate the occurrence of slipping magnetic reconnection, chromospheric evaporation, and coronal loop dynamics in the 2014 September 10 X-class flare. Slipping reconnection is found to be present throughout the flare from its early phase. Flare loops are seen to slip in opposite directions toward both ends of the ribbons. Velocities of 20-40 km s-1 are found within time windows where the slipping is well resolved. The warm coronal loops exhibit expanding and contracting motions that are interpreted as displacements due to the growing flux rope that subsequently erupts. This flux rope existed and erupted before the onset of apparent coronal implosion. This indicates that the energy release proceeds by slipping reconnection and not via coronal implosion. The slipping reconnection leads to changes in the geometry of the observed structures at the Interface Region Imaging Spectrograph slit position, from flare loop top to the footpoints in the ribbons. This results in variations of the observed velocities of chromospheric evaporation in the early flare phase. Finally, it is found that the precursor signatures, including localized EUV brightenings as well as nonthermal X-ray emission, are signatures of the flare itself, progressing from the early phase toward the impulsive phase, with the tether-cutting being provided by the slipping reconnection. The dynamics of both the flare and outlying coronal loops is found to be consistent with the predictions of the standard solar flare model in three dimensions
Fractal assessment analysis of China's air-HSR network integration
High-speed rail (HSR) has emerged as a significant mode for intercity transport in several countries, particularly China, setting an environment that may promote integration between air and HSR networks. To better measure the current level of integration of China's air-HSR intermodal network and identify implementation issues, this paper establishes a novel assessment framework that considers three primary areas: service capability, network connectivity and transfer potential. The framework is based on a comprehensive literature review of network measurement and assessment methodologies. Then, fractal theory is used to establish an assessment model that associates the fractal dimension to the level of intermodal integration, which can serve as an important complement to traditional weighting methods. The model and framework are applied to the 10 cities in China with the potential for air-HSR integration. The results show that international hub airports, together with their closest HSR station, do not necessarily perform at a higher integration level than regional hubs. The paper also proposes policy and practical recommendations to enhance air-HSR network integration levels from service supply, network coordination and transfer design perspectives
- …