996 research outputs found
A probable Milli-Parsec Supermassive Binary Black Hole in the Nearest Quasar Mrk 231
Supermassive binary black holes (BBHs) are unavoidable products of galaxy
mergers and are expected to exist in the cores of many quasars. Great effort
has been made during the past several decades to search for BBHs among quasars;
however, observational evidence for BBHs remains elusive and ambiguous, which
is difficult to reconcile with theoretical expectations. In this paper, we show
that the distinct optical-to-UV spectrum of Mrk 231 can be well interpreted as
emission from accretion flows onto a BBH, with a semimajor axis of ~590AU and
an orbital period of ~1.2 year. The flat optical and UV continua are mainly
emitted from a circumbinary disk and a mini-disk around the secondary black
hole (BH), respectively; and the observed sharp drop off and flux deficit at
wavelength lambda ~ 4000-2500 Angstrom is due to a gap (or hole) opened by the
secondary BH migrating within the circumbinary disk. If confirmed by future
observations, this BBH will provide a unique laboratory to study the interplay
between BBHs and accretion flows onto them. Our result also demonstrates a new
method to find sub-parsec scale BBHs by searching for deficits in the
optical-to-UV continuum among the spectra of quasars.Comment: a typo in equation (2) and also in equation (5) of the Appendix is
fixed; 9 pages, 7 figure
Free radical-scavenging activity and flavonoid contents of Polygonum orientale leaf, stem, and seed extracts
The present study was designed to explore the total flavonoid and taxifolin contents and the radical-scavenging activity of 50% ethanol extracts of Polygonum orientale leaves, stems, and seeds by 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. The extract with higher total flavonoid content has higher radical scavenging activity. Taxifolin (IC50 = 2.83 μmol/L) has antioxidant activity stronger than that of rutin (IC50 = 3.08 μmol/L). The free radical-scavenging potentials of chloroform, ethyl acetate, water, ethanol, and methanol extracts of Polygonum orientale seeds were also investigated. The free radical-scavenging abilities of various extracts were determined as: methanol > ethanol > water > ethyl acetate > chloroform
Frequentist Model Averaging for Global Fr\'{e}chet Regression
To consider model uncertainty in global Fr\'{e}chet regression and improve
density response prediction, we propose a frequentist model averaging method.
The weights are chosen by minimizing a cross-validation criterion based on
Wasserstein distance. In the cases where all candidate models are misspecified,
we prove that the corresponding model averaging estimator has asymptotic
optimality, achieving the lowest possible Wasserstein distance. When there are
correctly specified candidate models, we prove that our method asymptotically
assigns all weights to the correctly specified models. Numerical results of
extensive simulations and a real data analysis on intracerebral hemorrhage data
strongly favour our method
Tetrahedron instantons
We introduce and study tetrahedron instantons, which can be realized in
string theory by D-branes probing a configuration of intersecting
D-branes in flat spacetime with a proper constant -field. Physically they
capture instantons on in the presence of the most general
intersecting real codimension-two supersymmetric defects. Moreover, we
construct the tetrahedron instantons as particular solutions of general
instanton equations in noncommutative field theory. We analyze the moduli space
of tetrahedron instantons and discuss the geometric interpretations. We compute
the instanton partition function both via the equivariant localization on the
moduli space of tetrahedron instantons and via the elliptic genus of the
worldvolume theory on the D-branes probing the intersecting D-branes,
obtaining the same result. The instanton partition function of the tetrahedron
instantons lies between the higher-rank Donaldson-Thomas invariants on
and the partition function of the magnificent four model,
which is conjectured to be the mother of all instanton partition functions.
Finally, we show that the instanton partition function admits a free field
representation, suggesting the existence of a novel kind of symmetry which acts
on the cohomology of the moduli spaces of tetrahedron instantons.Comment: 62 pages; v2: additional references and minor corrections; v3:
numerous minor improvement
Probing M-theory with tetrahedron instantons
The duality between type IIA superstring theory and M-theory enables us to
lift bound states of D-branes and parallel D-branes to M-theory
compactified on an -centered multi-Taub-NUT space . As a
consequence, the rank K-theoretic Donaldson-Thomas invariants of
are connected with the index of M-theory on
. In this paper, we extend the
correspondence by considering intersecting D-branes. In the presence of a
suitable Neveu-Schwarz -field, the system preserves two supercharges. This
system is T-dual to the configuration of tetrahedron instantons which we
introduced in \cite{Pomoni:2021hkn}. We conjecture a closed-form expression for
the K-theoretic tetrahedron instanton partition function, which is the
generating function of the D-D-brane partition functions. We find that
the tetrahedron instanton partition function coincides with the partition
function of the magnificent four model for special values of the parameters,
leading us to conjecture that our system of intersecting D-branes can be
obtained from the annihilation of D-branes and anti-D-branes. Remarkably,
the K-theoretic tetrahedron instanton partition function allows an
interpretation in terms of the index of M-theory on a noncompact Calabi-Yau
fivefold which is related to a superposition of Kaluza-Klein monopoles. The
dimensional reduction of the system allows us to express the cohomological
tetrahedron instanton partition function in terms of the MacMahon function,
generalizing the correspondence between Gromov-Witten invariants and
Donaldson-Thomas invariants for Calabi-Yau threefolds.Comment: 24 page
Exploring customer satisfaction in cold chain logistics using a text mining approach
PurposeWith the fierce competition in the cold chain logistics market, achieving and maintaining excellent customer satisfaction is the key to an enterprise's ability to stand out. This research aims to determine the factors that affect customer satisfaction in cold chain logistics, which helps cold chain logistics enterprises identify the main aspects of the problem. Further, the suggestions are provided for cold chain logistics enterprises to improve customer satisfaction.Design/methodology/approachThis research uses the text mining approach, including topic modeling and sentiment analysis, to analyze the information implicit in customer-generated reviews. First, latent Dirichlet allocation (LDA) model is used to identify the topics that customers focus on. Furthermore, to explore the sentiment polarity of different topics, bi-directional long short-term memory (Bi-LSTM), a type of deep learning model, is adopted to quantify the sentiment score. Last, regression analysis is performed to identify the significant factors that affect positive, neutral and negative sentiment.FindingsThe results show that eight topics that customer focus are determined, namely, speed, price, cold chain transportation, package, quality, error handling, service staff and logistics information. Among them, speed, price, transportation and product quality significantly affect customer positive sentiment, and error handling and service staff are significant factors affecting customer neutral and negative sentiment, respectively.Research limitations/implicationsThe data of the customer-generated reviews in this research are in Chinese. In the future, multi-lingual research can be conducted to obtain more comprehensive insights.Originality/valuePrior studies on customer satisfaction in cold chain logistics predominantly used questionnaire method, and the disadvantage of which is that interviewees may fill out the questionnaire arbitrarily, which leads to inaccurate data. For this reason, it is more scientific to discover customer satisfaction from real behavioral data. In response, customer-generated reviews that reflect true emotions are used as the data source for this research.<br/
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