31,550 research outputs found
OGLE-2008-BLG-510: first automated real-time detection of a weak microlensing anomaly - brown dwarf or stellar binary?
The microlensing event OGLE-2008-BLG-510 is characterised by an evident
asymmetric shape of the peak, promptly detected by the ARTEMiS system in real
time. The skewness of the light curve appears to be compatible both with
binary-lens and binary-source models, including the possibility that the lens
system consists of an M dwarf orbited by a brown dwarf. The detection of this
microlensing anomaly and our analysis demonstrates that: 1) automated real-time
detection of weak microlensing anomalies with immediate feedback is feasible,
efficient, and sensitive, 2) rather common weak features intrinsically come
with ambiguities that are not easily resolved from photometric light curves, 3)
a modelling approach that finds all features of parameter space rather than
just the `favourite model' is required, and 4) the data quality is most
crucial, where systematics can be confused with real features, in particular
small higher-order effects such as orbital motion signatures. It moreover
becomes apparent that events with weak signatures are a silver mine for
statistical studies, although not easy to exploit. Clues about the apparent
paucity of both brown-dwarf companions and binary-source microlensing events
might hide here.Comment: 17 pages with 8 figures, MNRAS submitte
Large deviations of stochastic systems and applications
This dissertation focuses on large deviations of stochastic systems with applications to optimal control and system identification. It encompasses analysis of two-time-scale Markov processes and system identification with regular and quantized data. First, we develops large deviations principles for systems driven by continuous-time Markov chains with twotime scales and related optimal control problems. A distinct feature of our setup is that the Markov chain under consideration is time dependent or inhomogeneous. The use of two time-scale formulation stems from the effort of reducing computational complexity in a wide variety of applications in control, optimization, and systems theory. Starting with a rapidly fluctuating Markovian system, under irreducibility conditions, both large deviations upper and lower bounds are established first for a fixed terminal time and then for time-varying dynamic systems. Then the results are applied to certain dynamic systems and LQ control problems.
Second, we study large deviations for identifications systems. Traditional system identification concentrates on convergence and convergence rates of estimates in mean squares, in distribution, or in a strong sense. For system diagnosis and complexity analysis, however, it is essential to understand the probabilities of identification errors over a finite data window. This paper investigates identification errors in a large deviations framework. By considering both space complexity in terms of quantization levels and time complexity with respect to data window sizes, this study provides a new perspective to understand the fundamental relationship between probabilistic errors and resources that represent data sizes in computer algorithms, sample sizes in statistical analysis, channel bandwidths in communications, etc.
This relationship is derived by establishing the large deviations principle for quantized identification that links binary-valued data at one end and regular sensors at the other. Under some mild conditions, we obtain large deviations upper and lower bounds. Our results accommodate
independent and identically distributed noise sequences, as well as more general classes of mixing-type noise sequences. Numerical examples are provided to illustrate the theoretical results
ROTSE All Sky Surveys for Variable Stars I: Test Fields
The ROTSE-I experiment has generated CCD photometry for the entire Northern
sky in two epochs nightly since March 1998. These sky patrol data are a
powerful resource for studies of astrophysical transients. As a demonstration
project, we present first results of a search for periodic variable stars
derived from ROTSE-I observations. Variable identification, period
determination, and type classification are conducted via automatic algorithms.
In a set of nine ROTSE-I sky patrol fields covering about 2000 square degrees
we identify 1781 periodic variable stars with mean magnitudes between m_v=10.0
and m_v=15.5. About 90% of these objects are newly identified as variable.
Examples of many familiar types are presented. All classifications for this
study have been manually confirmed. The selection criteria for this analysis
have been conservatively defined, and are known to be biased against some
variable classes. This preliminary study includes only 5.6% of the total
ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will
include more than 32,000 periodic variable stars.Comment: Accepted for publication in AJ 4/00. LaTeX manuscript. (28 pages, 11
postscript figures and 1 gif
Polarized QPOs from the INTEGRAL polar IGRJ14536-5522 (=Swift J1453.4-5524)
We report optical spectroscopy and high speed photometry and polarimetry of
the INTEGRAL source IGRJ14536-5522 (=Swift J1453.4-5524). The photometry,
polarimetry and spectroscopy are modulated on an orbital period of 3.1564(1)
hours. Orbital circularly polarized modulations are seen from 0 to -18 per
cent, unambiguously identifying IGRJ14536-5522 as a polar.
Some of the high speed photometric data show modulations that are consistent
with quasi-periodic oscillations (QPOs) on the order of 5-6 minutes.
Furthermore, for the first time, we detect the (5-6) minute QPOs in the
circular polarimetry. We discuss the possible origins of these QPOs. We also
include details of HIPPO, a new high-speed photo-polarimeter used for some of
our observations.Comment: Accepted for publication by MNRAS. The paper contains 7 figures and 1
tabl
Spectroscopic binaries among Hipparcos M giants II. Binary frequency
This paper is the second one in a series devoted to the study of properties
of binaries involving M giants. The binary frequency of field M giants is
derived and compared with the binary fraction of K giants. Diagrams of the
CORAVEL spectroscopic parameter Sb (measuring the average line-width) vs.
radial-velocity standard deviation for our samples are used to define
appropriate binarity criteria. These then serve to extract the binarity
fraction among the M giants. Comparison is made to earlier data on K giants
binarity frequency. The Sb parameter is discussed in relation to global stellar
parameters and the Sb vs. stellar radius relation is used to identify fast
rotators. We find that the spectroscopic binary detection rate among field M
giants, in a sample with a low number of velocity measurements (~2), unbiased
toward earlier known binaries, is 6.3%. This is less than half of the analogous
rate for field K giants, likely resulting from a real difference. This
difference originates in the greater difficulty of finding binaries among M
giants because of their smaller orbital velocity amplitudes and larger
intrinsic jitter and in the different distributions of K and M giants in the
eccentricity-period diagram. A larger detection rate was obtained in a smaller
M giant sample with more radial velocity measurements per object: 11.1%
confirmed plus 2.7% possible binaries. The CORAVEL spectroscopic parameter Sb
was found to correlate better with the stellar radius than with either
luminosity or effective temperature separately. Two outliers of the Sb vs.
stellar radius relation, HD 190658 and HD 219654, have been recognized as fast
rotators. The rotation is companion-induced, as both objects turn out to be
spectroscopic binaries.Comment: 12 pages, 7 figures, accepted for publication in A&A, language
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