8,296 research outputs found
Expectile Matrix Factorization for Skewed Data Analysis
Matrix factorization is a popular approach to solving matrix estimation
problems based on partial observations. Existing matrix factorization is based
on least squares and aims to yield a low-rank matrix to interpret the
conditional sample means given the observations. However, in many real
applications with skewed and extreme data, least squares cannot explain their
central tendency or tail distributions, yielding undesired estimates. In this
paper, we propose \emph{expectile matrix factorization} by introducing
asymmetric least squares, a key concept in expectile regression analysis, into
the matrix factorization framework. We propose an efficient algorithm to solve
the new problem based on alternating minimization and quadratic programming. We
prove that our algorithm converges to a global optimum and exactly recovers the
true underlying low-rank matrices when noise is zero. For synthetic data with
skewed noise and a real-world dataset containing web service response times,
the proposed scheme achieves lower recovery errors than the existing matrix
factorization method based on least squares in a wide range of settings.Comment: 8 page main text with 5 page supplementary documents, published in
AAAI 201
Long-term X-ray Variability Study of IC342 from XMM-Newton Observations
We presented the results of an analysis of four XMM-Newton observations of
the starburst galaxy IC342 taken over a four-year span from 2001 to 2005, with
an emphasis on investigating the long-term flux and spectral variability of the
X-ray point sources. We detected a total of 61 X-ray sources within 35'
30' of the galaxy down to a luminosity of (1-2)1037 erg s-1
depending on the local background. We found that 39 of the 61 detected sources
showed long-term variability, in which 26 of them were classified as X-ray
transients. We also found 19 sources exhibiting variations in hardness ratios
or undergoing spectral transitions among observations, and were identified as
spectral variables. In particular, 8 of the identified X-ray transients showed
spectral variability in addition to flux variability. The diverse patterns of
variability observed is indicative of a population of X-ray binaries. We used
X-ray colors, flux and spectral variability, and in some cases the optical or
radio counterparts to classify the detected X-ray sources into several stellar
populations. We identified a total of 11 foreground stars, 1 supersoft sources
(SSS), 3 quasisoft sources (QSS), and 2 supernova remnants (SNR). The
identified SSS/QSS are located near or on the spiral arms, associate with young
stellar populations; the 2 SNR are very close to the starburst nucleus where
current star formation activities are dominated. We also discovered a spectral
change in the nuclear source of IC342 for the first time by a series of X-ray
spectrum analysis.Comment: 45 pages, 6 figures accepted by Ap
The Progenitors of Type Ia Supernovae: Are They Supersoft Sources?
In a canonical model, the progenitors of Type Ia supernovae (SNe Ia) are
accreting, nuclear-burning white dwarfs (NBWDs), which explode when the white
dwarf reaches the Chandrasekhar mass, M_C. Such massive NBWDs are hot (kT ~100
eV), luminous (L ~ 10^{38} erg/s), and are potentially observable as luminous
supersoft X-ray sources (SSSs). During the past several years, surveys for soft
X-ray sources in external galaxies have been conducted. This paper shows that
the results falsify the hypothesis that a large fraction of progenitors are
NBWDs which are presently observable as SSSs. The data also place limits on
sub-M_C models. While Type Ia supernova progenitors may pass through one or
more phases of SSS activity, these phases are far shorter than the time needed
to accrete most of the matter that brings them close to M_C.Comment: submitted to ApJ 18 November 2009; 17 pages, 2 figure
Populations of Supersoft X-ray Sources: Novae, tidal disruption, Type Ia supernovae, accretion-induced collapse, ionization, and intermediate-mass black holes?
Observations of hundreds of supersoft x-ray sources (SSSs) in external
galaxies have shed light on the diversity of the class and on the natures of
the sources. SSSs are linked to the physics of Type Ia supernovae and
accretion-induced collapse, ultraluminous x-ray sources and black holes, the
ionization of the interstellar medium, and tidal disruption by supermassive
black holes. The class of SSSs has an extension to higher luminosities:
ultraluminous SSSs have luminosities above 10^39 erg/s. There is also an
extension to higher energies: quasisoft x-ray sources (QSSs) emit photons with
energies above 1 eV, but few or none with energies above 2 keV. Finally, a
significant fraction of the SSSs found in external galaxies switch states
between observations, becoming either quasisoft or hard. For many systems
``supersoft'' refers to a temporary state; SSSs are sources, possibly including
a variety of fundamentally different system types, that pass through such a
state. We review those results derived from extragalactic data and related
theoretical work that are most surprising and that suggest directions for
future research.Comment: submitted to Astron.Nachr.; latex, 6 figure
Growing Story Forest Online from Massive Breaking News
We describe our experience of implementing a news content organization system
at Tencent that discovers events from vast streams of breaking news and evolves
news story structures in an online fashion. Our real-world system has distinct
requirements in contrast to previous studies on topic detection and tracking
(TDT) and event timeline or graph generation, in that we 1) need to accurately
and quickly extract distinguishable events from massive streams of long text
documents that cover diverse topics and contain highly redundant information,
and 2) must develop the structures of event stories in an online manner,
without repeatedly restructuring previously formed stories, in order to
guarantee a consistent user viewing experience. In solving these challenges, we
propose Story Forest, a set of online schemes that automatically clusters
streaming documents into events, while connecting related events in growing
trees to tell evolving stories. We conducted extensive evaluation based on 60
GB of real-world Chinese news data, although our ideas are not
language-dependent and can easily be extended to other languages, through
detailed pilot user experience studies. The results demonstrate the superior
capability of Story Forest to accurately identify events and organize news text
into a logical structure that is appealing to human readers, compared to
multiple existing algorithm frameworks.Comment: Accepted by CIKM 2017, 9 page
The Discovery of Quasisoft and Supersoft Sources in External Galaxies
We apply a uniform procedure to select very soft sources from point sources
observed by Chandra in 4 galaxies. This sample includes one elliptical galaxy
(NGC 4967), 2 face-on spirals (M101 and M83), and an interacting galaxy (M51).
We have found very soft X-ray sources (VSSs) in every galaxy. Some of these fit
the criteria for canonical supersoft sources (SSSs), while others are somewhat
harder. These latter have characteristic values of kT < 300 eV; we refer to
them as quasisoft sources (QSSs). We found a combined total of 149 VSSs in the
4 galaxies we considered; 77 were SSSs and 72 were QSSs. (See the paper for the
original long abstract)Comment: 20 pages, 6 figures. Accepted for publication in Ap
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