2,422 research outputs found
A New Model for the Hard Time Lags in Black Hole X-Ray Binaries
The time-dependent Comptonized output of a cool soft X-ray source drifting
inward through an inhomogeneous hot inner disk or corona is numerically
simulated. We propose that this scenario can explain from first principles the
observed trends in the hard time lags and power spectra of the rapid aperiodic
variability of the X-ray emission of Galactic black-hole candidates.Comment: 10 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted
for ApJ Letter
Feller Processes: The Next Generation in Modeling. Brownian Motion, L\'evy Processes and Beyond
We present a simple construction method for Feller processes and a framework
for the generation of sample paths of Feller processes. The construction is
based on state space dependent mixing of L\'evy processes.
Brownian Motion is one of the most frequently used continuous time Markov
processes in applications. In recent years also L\'evy processes, of which
Brownian Motion is a special case, have become increasingly popular.
L\'evy processes are spatially homogeneous, but empirical data often suggest
the use of spatially inhomogeneous processes. Thus it seems necessary to go to
the next level of generalization: Feller processes. These include L\'evy
processes and in particular Brownian motion as special cases but allow spatial
inhomogeneities.
Many properties of Feller processes are known, but proving the very existence
is, in general, very technical. Moreover, an applicable framework for the
generation of sample paths of a Feller process was missing. We explain, with
practitioners in mind, how to overcome both of these obstacles. In particular
our simulation technique allows to apply Monte Carlo methods to Feller
processes.Comment: 22 pages, including 4 figures and 8 pages of source code for the
generation of sample paths of Feller processe
H.E.S.S. discovery of very-high-energy gamma-ray emission of PKS 1440-389
Blazars are the most abundant class of known extragalactic very-high-energy
(VHE, E>100 GeV) gamma-ray sources. However, one of the biggest difficulties in
investigating their VHE emission resides in their limited number, since less
than 60 of them are known by now. In this contribution we report on H.E.S.S.
observations of the BL Lac object PKS 1440-389. This source has been selected
as target for H.E.S.S. based on its high-energy gamma-ray properties measured
by Fermi-LAT. The extrapolation of this bright, hard-spectrum gamma-ray blazar
into the VHE regime made a detection on a relatively short time scale very
likely, despite its uncertain redshift. H.E.S.S. observations were carried out
with the 4-telescope array from February to May 2012 and resulted in a clear
detection of the source. Contemporaneous multi-wavelength data are used to
construct the spectral energy distribution of PKS 1440-389 which can be
described by a simple one-zone synchrotron-self Compton model.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherland
Metastable precursors during the oxidation of the Ru(0001) surface
Using density-functional theory, we predict that the oxidation of the
Ru(0001) surface proceeds via the accumulation of sub-surface oxygen in
two-dimensional islands between the first and second substrate layer. This
leads locally to a decoupling of an O-Ru-O trilayer from the underlying metal.
Continued oxidation results in the formation and stacking of more of these
trilayers, which unfold into the RuO_2(110) rutile structure once a critical
film thickness is exceeded. Along this oxidation pathway, we identify various
metastable configurations. These are found to be rather close in energy,
indicating a likely lively dynamics between them at elevated temperatures,
which will affect the surface chemical and mechanical properties of the
material.Comment: 11 pages including 9 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Absolute velocity measurements in sunspot umbrae
In sunspot umbrae, convection is largely suppressed by the strong magnetic
field. Previous measurements reported on negligible convective flows in umbral
cores. Based on this, numerous studies have taken the umbra as zero reference
to calculate Doppler velocities of the ambient active region. To clarify the
amount of convective motion in the darkest part of umbrae, we directly measured
Doppler velocities with an unprecedented accuracy and precision. We performed
spectroscopic observations of sunspot umbrae with the Laser Absolute Reference
Spectrograph (LARS) at the German Vacuum Tower Telescope. A laser frequency
comb enabled the calibration of the high-resolution spectrograph and absolute
wavelength positions. A thorough spectral calibration, including the
measurement of the reference wavelength, yielded Doppler shifts of the spectral
line Ti i 5713.9 {\AA} with an uncertainty of around 5 m s-1. The measured
Doppler shifts are a composition of umbral convection and magneto-acoustic
waves. For the analysis of convective shifts, we temporally average each
sequence to reduce the superimposed wave signal. Compared to convective
blueshifts of up to -350 m s-1 in the quiet Sun, sunspot umbrae yield a
strongly reduced convective blueshifts around -30 m s-1. {W}e find that the
velocity in a sunspot umbra correlates significantly with the magnetic field
strength, but also with the umbral temperature defining the depth of the
titanium line. The vertical upward motion decreases with increasing field
strength. Extrapolating the linear approximation to zero magnetic field
reproduces the measured quiet Sun blueshift. Simply taking the sunspot umbra as
a zero velocity reference for the calculation of photospheric Dopplergrams can
imply a systematic velocity error.Comment: 10 pages, 7 figures, 2 tables, Appendix with 5 figure
Composition, structure and stability of RuO_2(110) as a function of oxygen pressure
Using density-functional theory (DFT) we calculate the Gibbs free energy to
determine the lowest-energy structure of a RuO_2(110) surface in thermodynamic
equilibrium with an oxygen-rich environment. The traditionally assumed
stoichiometric termination is only found to be favorable at low oxygen chemical
potentials, i.e. low pressures and/or high temperatures. At realistic O
pressure, the surface is predicted to contain additional terminal O atoms.
Although this O excess defines a so-called polar surface, we show that the
prevalent ionic model, that dismisses such terminations on electrostatic
grounds, is of little validity for RuO_2(110). Together with analogous results
obtained previously at the (0001) surface of corundum-structured oxides, these
findings on (110) rutile indicate that the stability of non-stoichiometric
terminations is a more general phenomenon on transition metal oxide surfaces.Comment: 12 pages including 5 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Bandwidth, expansion, treewidth, separators, and universality for bounded degree graphs
We establish relations between the bandwidth and the treewidth of bounded
degree graphs G, and relate these parameters to the size of a separator of G as
well as the size of an expanding subgraph of G. Our results imply that if one
of these parameters is sublinear in the number of vertices of G then so are all
the others. This implies for example that graphs of fixed genus have sublinear
bandwidth or, more generally, a corresponding result for graphs with any fixed
forbidden minor. As a consequence we establish a simple criterion for
universality for such classes of graphs and show for example that for each
gamma>0 every n-vertex graph with minimum degree ((3/4)+gamma)n contains a copy
of every bounded-degree planar graph on n vertices if n is sufficiently large
VHE Gamma-Ray Induced Pair Cascades in Blazars and Radio Galaxies: Application to NGC 1275
Recent blazar detections by HESS, MAGIC, and VERITAS suggest that
very-high-energy (VHE, E > 100 GeV) gamma-rays may be produced in most, if not
all, types of blazars, including those that possess intense circumnuclear
radiation fields. In this paper, we investigate the interaction of nuclear VHE
gamma-rays with the circumnuclear radiation fields through gamma-gamma
absorption and pair production, and the subsequent Compton-supported pair
cascades. We have developed a Monte-Carlo code to follow the spatial
development of the cascade in full 3-dimensional geometry, and calculate the
radiative output due to the cascade as a function of viewing angle with respect
to the primary VHE gamma-ray beam (presumably the jet axis of the blazar). We
show that even for relatively weak magnetic fields, the cascades can be
efficiently isotropized, leading to substantial off-axis cascade emission
peaking in the Fermi energy range at detectable levels for nearby radio
galaxies. We demonstrate that this scenario can explain the Fermi flux and
spectrum of the radio galaxy NGC 1275.Comment: Accepted for publication in The Astrophysical Journa
Opacity in compact extragalactic radio sources and its effect on radio-optical reference frame alignment
Accurate alignment of the radio and optical celestial reference frames
requires detailed understanding of physical factors that may cause offsets
between the positions of the same object measured in different spectral bands.
Opacity in compact extragalactic jets (due to synchrotron self-absorption and
external free-free absorption) is one of the key physical phenomena producing
such an offset, and this effect is well-known in radio astronomy ("core
shift"). We have measured the core shifts in a sample of 29 bright compact
extragalactic radio sources observed using very long baseline interferometry
(VLBI) at 2.3 and 8.6 GHz. We report the results of these measurements and
estimate that the average shift between radio and optical positions of distant
quasars would be of the order of 0.1-0.2 mas. This shift exceeds positional
accuracy of GAIA and SIM. We suggest two possible approaches to carefully
investigate and correct for this effect in order to align accurately the radio
and optical positions. Both approaches involve determining a Primary Reference
Sample of objects to be used for tying the radio and optical reference frames
together.Comment: 4 pages, 1 figure; to appear in IAU Symposium 248 Proceedings, "A
Giant Step: from Milli- to Micro-arcsecond Astrometry", eds. W.-J. Jin, I.
Platais, M. Perryma
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