9,758 research outputs found
Kepler Observations of Rapid Optical Variability in Active Galactic Nuclei
Over three quarters in 2010-2011, Kepler monitored optical emission from four
active galactic nuclei (AGN) with ~30 min sampling, >90% duty cycle, and <~0.1%
repeatability. These data determined the AGN optical fluctuation power spectral
density functions (PSDs) over a wide range in temporal frequency. Fits to these
PSDs yielded power law slopes of -2.6 to -3.3, much steeper than typically seen
in the X-rays. We find evidence that individual AGN exhibit intrinsically
different PSD slopes. The steep PSD fits are a challenge to recent AGN
variability models but seem consistent with first order MRI theoretical
calculations of accretion disk fluctuations.Comment: Accepted by Astrophysical Journal Letters, 31 Oct 201
The astrophysics of visible-light orbital phase curves in the space age
The field of visible-light continuous time series photometry is now at its
golden age, manifested by the continuum of past (CoRoT, Kepler), present (K2),
and future (TESS, PLATO) space-based surveys delivering high precision data
with a long baseline for a large number of stars. The availability of the high
quality data has enabled astrophysical studies not possible before, including
for example detailed asteroseismic investigations and the study of the
exoplanet census including small planets. This has also allowed to study the
minute photometric variability following the orbital motion in stellar binaries
and star-planet systems which is the subject of this review. We focus on
systems with a main sequence primary and a low-mass secondary, from a small
star to a massive planet. The orbital modulations are induced by a combination
of gravitational and atmospheric processes, including the beaming effect, tidal
ellipsoidal distortion, reflected light, and thermal emission. Therefore, the
phase curve shape contains information about the companion's mass and
atmospheric characteristics, making phase curves a useful astrophysical tool.
For example, phase curves can be used to detect and measure the mass of
short-period low-mass companions orbiting hot fast-rotating stars, out of reach
of other detection methods. Another interesting application of phase curves is
using the orbital phase modulations to look for non-transiting systems, which
comprise the majority of stellar binary and star-planet systems. We discuss the
science done with phase curves, the first results obtained so far, and the
current difficulties and open questions related to this young and evolving
subfield.Comment: Invited Review accepted to PAS
Index for asset value measure obtained from condition monitoring digitalized data interpretation. A railway asset management application
Hosted by the Johannes Kepler University, Linz, Austria. May 23-24, 2019The objective of any asset is to provide value to the organization, being the corner
stone to get a highest possible economic benefit in a sustainable way. An effective
asset value management demands method that allow measuring and comparing the
expected value with the real value realized at any time during its life cycle for value
informed decision-making. Digitalization is providing new data about events and
states related to asset condition and risk, information that can be reinterpreted to
generate value measure strategies. This paper presents a proposal of TVO (Total
Value of Ownership) model where it is possible to quantify and measure the value, including its monitoring throughout the life cycle of the asset and/or system.
Proposed TVO model is focused on Safety, one of the most relevant value factors for
Industry and Infrastructure sectors. Asset events and states are intrinsically linked to
the defined failure modes. Consequently, it is necessary to structure the system
information around the failure modes that have been defined, in order to obtain a
value measurement index. A railway use case is presented
- …