10,432 research outputs found
Unbiased Monte Carlo continuum radiative transfer in optically thick regions
Radiative transfer describes the propagation of electromagnetic radiation
through an interacting medium. This process is often simulated by the use of
the Monte Carlo method, which involves the probabilistic determination and
tracking of simulated photon packages. In the regime of high optical depths,
this approach encounters difficulties since a proper representation of the
various physical processes can only be achieved by considering high numbers of
simulated photon packages. As a consequence, the demand for computation time
rises accordingly and thus practically puts a limit on the optical depth of
models that can be simulated. Here we present a method that aims to solve the
problem of high optical depths in dusty media, which relies solely on the use
of unbiased Monte Carlo radiative transfer. For that end, we identified and
precalculated repeatedly occuring and simulated processes, stored their outcome
in a multidimensional cumulative distribution function, and immediately
replaced the basic Monte Carlo transfer during a simulation by that outcome.
During the precalculation, we generated emission spectra as well as deposited
energy distributions of photon packages traveling from the center of a sphere
to its rim. We carried out a performance test of the method to confirm its
validity and gain a boost in computation speed by up to three orders of
magnitude. We then applied the method to a simple model of a viscously heated
circumstellar disk, and we discuss the necessity of finding a solution for the
optical depth problem with regard to a proper temperature calculation. We find
that the impact of an incorrect treatment of photon packages in highly
optically thick regions extents even to optically thin regions, thus, changing
the overall observational appearance of the disk.Comment: 17 pages, 11 figures, 1 tabl
The design and evaluation of grazing incidence relay optics
X-ray astronomy, both solar and celestial, has many needs for high spatial resolution observations which have to be performed with electronic detectors. If the resolution is not to be detector limited, plate scales in excess of 25 microns arc/sec, corresponding to focal lengths greater than 5 m, are required. In situations where the physical size is restricted, the problem can be solved by the use of grazing incidence relay optics. A system was developed which employs externally polished hyperboloid-hyperboloid surfaces to be used in conjunction with a Wolter-Schwarzschild primary. The secondary is located in front of the primary focus and provides a magnification of 4, while the system has a plate scale of 28 microns arc/sec and a length of 1.9 m. The design, tolerance specification, fabrication and performance at visible and X-ray wavelengths of this optical system are described
Continuing data analysis of the AS/E grazing incidence X-ray telescope experiment on the OSO-4 satellite
The work to correct and extend the calculation of the theoretical solar X-ray spectrum produced during earlier OSO-4 data analysis is reported along with the work to formulate models of active regions, and compare these models with the experimental values. An atlas of solar X-ray photographs is included, and solar X-ray observations are correlated with the solar wind
Sofic-Dyck shifts
We define the class of sofic-Dyck shifts which extends the class of
Markov-Dyck shifts introduced by Inoue, Krieger and Matsumoto. Sofic-Dyck
shifts are shifts of sequences whose finite factors form unambiguous
context-free languages. We show that they correspond exactly to the class of
shifts of sequences whose sets of factors are visibly pushdown languages. We
give an expression of the zeta function of a sofic-Dyck shift
Color television study Final report, Nov. 1965 - Mar. 1966
Color television camera for transmission from lunar and earth orbits and lunar surfac
Do topology and ferromagnetism cooperate at the EuS/BiSe interface?
We probe the local magnetic properties of interfaces between the insulating
ferromagnet EuS and the topological insulator BiSe using low energy
muon spin rotation (LE-SR). We compare these to the interface between EuS
and the topologically trivial metal, titanium. Below the magnetic transition of
EuS, we detect strong local magnetic fields which extend several nm into the
adjacent layer and cause a complete depolarization of the muons. However, in
both BiSe and titanium we measure similar local magnetic fields,
implying that their origin is mostly independent of the topological properties
of the interface electronic states. In addition, we use resonant soft X-ray
angle resolved photoemission spectroscopy (SX-ARPES) to probe the electronic
band structure at the interface between EuS and BiSe. By tuning the
photon energy to the Eu anti-resonance at the Eu pre-edge we are able to
detect the BiSe conduction band, through a protective AlO
capping layer and the EuS layer. Moreover, we observe a signature of an
interface-induced modification of the buried BiSe wave functions and/or
the presence of interface states
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