9,900 research outputs found
Stochastic model of optical variability of BL Lacertae
We use optical photometric and polarimetric data of BL Lacertae that cover a
period of 22 years to study the variability of the source. The long-term
observations are employed for establishing parameters of a stochastic model
consisting of the radiation from a steady polarized source and a number of
variable components with different polarization parameters, proposed by
Hagen-Thorn et al. earlier. We infer parameters of the model from the
observations using numerical simulations based on a Monte Carlo method, with
values of each model parameter selected from a Gaussian distribution. We
determine the best set of model parameters by comparing model distributions to
the observational ones using the chi-square criterion. We show that the
observed photometric and polarimetric variability can be explained within a
model with a steady source of high polarization, ~40%, and with direction of
polarization parallel to the parsec scale jet, along with 10+-5 sources of
variable polarization.Comment: 4 pages, 10 figures, published by Astronomy and Astrophysics; v2:
typos correcte
The quantitative measure and statistical distribution of fame
Fame and celebrity play an ever-increasing role in our culture. However,
despite the cultural and economic importance of fame and its gradations, there
exists no consensus method for quantifying the fame of an individual, or of
comparing that of two individuals. We argue that, even if fame is difficult to
measure with precision, one may develop useful metrics for fame that correlate
well with intuition and that remain reasonably stable over time. Using datasets
of recently deceased individuals who were highly renowned, we have evaluated
several internet-based methods for quantifying fame. We find that some
widely-used internet-derived metrics, such as search engine results, correlate
poorly with human subject judgments of fame. However other metrics exist that
agree well with human judgments and appear to offer workable, easily accessible
measures of fame. Using such a metric we perform a preliminary investigation of
the statistical distribution of fame, which has some of the power law character
seen in other natural and social phenomena such as landslides and market
crashes. In order to demonstrate how such findings can generate quantitative
insight into celebrity culture, we assess some folk ideas regarding the
frequency distribution and apparent clustering of celebrity deaths.Comment: 17 pages, 6 figure
Perturbative Expansion in the Galilean Invariant Spin One-Half Chern-Simons Field Theory
A Galilean Chern-Simons field theory is formulated for the case of two
interacting spin-1/2 fields of distinct masses M and M'. A method for the
construction of states containing N particles of mass M and N' particles of
mass M' is given which is subsequently used to display equivalence to the
spin-1/2 Aharonov-Bohm effect in the N = N' =1 sector of the model. The latter
is then studied in perturbation theory to determine whether there are
divergences in the fourth order (one loop) diagram. It is found that the
contribution of that order is finite (and vanishing) for the case of parallel
spin projections while the antiparallel case displays divergences which are
known to characterize the spin zero case in field theory as well as in quantum
mechanics.Comment: 14 pages LaTeX, including 2 figures using eps
The effect of time-dependent coupling on non-equilibrium steady states
Consider (for simplicity) two one-dimensional semi-infinite leads coupled to
a quantum well via time dependent point interactions. In the remote past the
system is decoupled, and each of its components is at thermal equilibrium. In
the remote future the system is fully coupled. We define and compute the non
equilibrium steady state (NESS) generated by this evolution. We show that when
restricted to the subspace of absolute continuity of the fully coupled system,
the state does not depend at all on the switching. Moreover, we show that the
stationary charge current has the same invariant property, and derive the
Landau-Lifschitz and Landauer-Buttiker formulas.Comment: 30 pages, submitte
Fluctuations of the intergalactic ionization field at redshift z ~ 2
(Abridged) Aims. To probe the spectral energy distribution (SED) of the
ionizing background radiation at z ~ 2 and to specify the sources contributing
to the intergalactic radiation field. Methods. The spectrum of a bright quasar
HS1103+6416 (zem = 2.19) contains five successive metal-line absorption systems
at zabs = 1.1923, 1.7193, 1.8873, 1.8916, and 1.9410. The systems are optically
thin and reveal multiple lines of different metal ions with the ionization
potentials lying in the extreme ultraviolet (EUV) range (1 Ryd to 0.2 keV). For
each system, the EUV SED of the underlying ionization field is reconstructed by
means of a special technique developed for solving the inverse problem in
spectroscopy. For the zabs = 1.8916 system, the analysis also involves the HeI
resonance lines of the Lyman series and the HeI 504 A continuum, which are seen
for the first time in any cosmic object except the Sun. Results. From one
system to another, the SED of the ionizing continuum changes significantly,
indicating that the intergalactic ionization field at z ~ 2 fluctuates at the
scale of at least Delta_z ~ 0.004. This is consistent with Delta_z ~ 0.01
estimated from HeII and HI Lyman-alpha forest measurements between the
redshifts 2 and 3.Comment: 29 pages, 18 figures, 3 tables, accepted for publication in A\&
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