5,256 research outputs found
Time Variability of Quasars: the Structure Function Variance
Significant progress in the description of quasar variability has been
recently made by employing SDSS and POSS data. Common to most studies is a
fundamental assumption that photometric observations at two epochs for a large
number of quasars will reveal the same statistical properties as well-sampled
light curves for individual objects. We critically test this assumption using
light curves for a sample of 2,600 spectroscopically confirmed quasars
observed about 50 times on average over 8 years by the SDSS stripe 82 survey.
We find that the dependence of the mean structure function computed for
individual quasars on luminosity, rest-frame wavelength and time is
qualitatively and quantitatively similar to the behavior of the structure
function derived from two-epoch observations of a much larger sample. We also
reproduce the result that the variability properties of radio and X-ray
selected subsamples are different. However, the scatter of the variability
structure function for fixed values of luminosity, rest-frame wavelength and
time is similar to the scatter induced by the variance of these quantities in
the analyzed sample. Hence, our results suggest that, although the statistical
properties of quasar variability inferred using two-epoch data capture some
underlying physics, there is significant additional information that can be
extracted from well-sampled light curves for individual objects.Comment: Presented at the "Classification and Discovery in Large Astronomical
Surveys" meeting, Ringberg Castle, 14-17 October, 200
Improved methods for detecting gravitational waves associated with short gamma-ray bursts
In the era of second generation ground-based gravitational wave detectors,
short gamma-ray bursts (GRBs) will be among the most promising astrophysical
events for joint electromagnetic and gravitational wave observation. A targeted
search for gravitational wave compact binary merger signals in coincidence with
short GRBs was developed and used to analyze data from the first generation
LIGO and Virgo instruments. In this paper, we present improvements to this
search that enhance our ability to detect gravitational wave counterparts to
short GRBs. Specifically, we introduce an improved method for estimating the
gravitational wave background to obtain the event significance required to make
detections; implement a method of tiling extended sky regions, as required when
searching for signals associated to poorly localized GRBs from Fermi Gamma-ray
Burst Monitor or the InterPlanetary Network; and incorporate astrophysical
knowledge about the beaming of GRB emission to restrict the search parameter
space. We describe the implementation of these enhancements and demonstrate how
they improve the ability to observe binary merger gravitational wave signals
associated with short GRBs.Comment: 13 pages, 6 figure
Artificial biochemical networks.
Connectionist approaches to Artificial Intelligence are almost always based on Artificial Neural Networks. However, there is another route towards Parallel Distributed Processing, taking as its inspiration the intelligence displayed by single celled creatures called Protoctists (Protists). This is based on networks of interacting proteins. Such networks may be used in Pattern Recognition and Control tasks and are more flexible than most neuron models. In this paper they are demonstrated in Image Recognition applications and in Legged Robot control. They are trained using a Genetic Algorithm and Back Propagation
Merger of white dwarf-neutron star binaries: Prelude to hydrodynamic simulations in general relativity
White dwarf-neutron star binaries generate detectable gravitational
radiation. We construct Newtonian equilibrium models of corotational white
dwarf-neutron star (WDNS) binaries in circular orbit and find that these models
terminate at the Roche limit. At this point the binary will undergo either
stable mass transfer (SMT) and evolve on a secular time scale, or unstable mass
transfer (UMT), which results in the tidal disruption of the WD. The path a
given binary will follow depends primarily on its mass ratio. We analyze the
fate of known WDNS binaries and use population synthesis results to estimate
the number of LISA-resolved galactic binaries that will undergo either SMT or
UMT. We model the quasistationary SMT epoch by solving a set of simple ordinary
differential equations and compute the corresponding gravitational waveforms.
Finally, we discuss in general terms the possible fate of binaries that undergo
UMT and construct approximate Newtonian equilibrium configurations of merged
WDNS remnants. We use these configurations to assess plausible outcomes of our
future, fully relativistic simulations of these systems. If sufficient WD
debris lands on the NS, the remnant may collapse, whereby the gravitational
waves from the inspiral, merger, and collapse phases will sweep from LISA
through LIGO frequency bands. If the debris forms a disk about the NS, it may
fragment and form planets.Comment: 28 pages, 25 figures, 6 table
Adaptive light-sheet fluorescence microscopy with a deformable mirror for video-rate volumetric imaging
Light-sheet fluorescence microscopy (LSFM) achieves optically sectioned imaging with the relatively low photobleaching and phototoxic effect. To achieve high-speed volumetric LSFM imaging without perturbing the sample, it is necessary to use some form of remote refocusing in the detection beam path. Previous work used electrically tunable lenses, tunable acoustic gradient index of refraction lenses, or the remote-refocusing approach of Botcherby et al. [Opt. Lett. 32(14), 2007 (2007)] to achieve remote refocusing. However, these approaches generally only provide low-order defocus correction, which is not compatible with higher-NA objectives that require higher order defocus corrections or reduce the optical throughput. In order to simultaneously achieve high-speed remote refocusing and correct system aberrations, we employ a deformable mirror in the detection path that is capable of providing higher orders of defocus and aberration correction in an optical system with an NA of 0.72–0.75. We demonstrate high-speed volumetric imaging at 26.3 volumes per second and 35 frames per volume for a defocus range of −50 to 50 μm
Quasar Selection Based on Photometric Variability
We develop a method for separating quasars from other variable point sources
using SDSS Stripe 82 light curve data for ~10,000 variable objects. To
statistically describe quasar variability, we use a damped random walk model
parametrized by a damping time scale, tau, and an asymptotic amplitude
(structure function), SF_inf. With the aid of an SDSS spectroscopically
confirmed quasar sample, we demonstrate that variability selection in typical
extragalactic fields with low stellar density can deliver complete samples with
reasonable purity (or efficiency, E). Compared to a selection method based
solely on the slope of the structure function, the inclusion of the tau
information boosts E from 60% to 75% while maintaining a highly complete sample
(98%) even in the absence of color information. For a completeness of C=90%, E
is boosted from 80% to 85%. Conversely, C improves from 90% to 97% while
maintaining E=80% when imposing a lower limit on tau. With the aid of color
selection, the purity can be further boosted to 96%, with C= 93%. Hence,
selection methods based on variability will play an important role in the
selection of quasars with data provided by upcoming large sky surveys, such as
Pan-STARRS and the Large Synoptic Survey Telescope (LSST). For a typical
(simulated) LSST cadence over 10 years and a photometric accuracy of 0.03 mag
(achieved at i~22), C is expected to be 88% for a simple sample selection
criterion of tau>100 days. In summary, given an adequate survey cadence,
photometric variability provides an even better method than color selection for
separating quasars from stars.Comment: (v2) 50 pages, accepted to Ap
Methanol Masers as Tracers of Circumstellar Disks
We show that in many methanol maser sources the masers are located in lines,
with a velocity gradient along them which suggests that the masers are situated
in edge-on circumstellar, or protoplanetary, disks. We present VLBI
observations of the methanol maser source G309.92+0.48, in the 12.2 GHz
transition, which confirm previous observations that the masers in this source
lie along a line. We show that such sources are not only linear in space but,
in many cases, also have a linear velocity gradient. We then model these and
other data in both the 6.7 GHz and the 12.2 GHz transition from a number of
star formation regions, and show that the observed spatial and velocity
distribution of methanol masers, and the derived Keplerian masses, are
consistent with a circumstellar disk rotating around an OB star. We consider
this and other hypotheses, and conclude that about half of these methanol
masers are probably located in edge-on circumstellar disks around young stars.
This is of particular significance for studies of circumstellar disks because
of the detailed velocity information available from the masers.Comment: 38 pages, 13 figures accepted by Ap
Viewpoint: The Human Capital Approach to Inference
The purpose of this essay is to discuss the “human capital” approach to inference. Observed decisions by experts can be used to organize data on their decisions using simple machine learning techniques. The fact that the human capital of these experts is heterogeneous implies that errors in decision making are inevitable, which in turn allows us to identify the conditional average treatment effect for a wider class of situations than would be possible with randomized control trials. This point is illustrated with some data from medical decision making in the context of treating depression, heart disease, and adverse childbirth events
An ultrashort pulse ultra-violet radiation undulator source driven by a laser plasma wakefield accelerator
Narrow band undulator radiation tuneable over the wavelength range of 150–260 nm has been produced by short electron bunches from a 2 mm long laser plasma wakefield accelerator based on a 20 TW femtosecond laser system. The number of photons measured is up to 9 × 106 per shot for a 100 period undulator, with a mean peak brilliance of 1 × 1018 photons/s/mrad2/mm2/0.1% bandwidth. Simulations estimate that the driving electron bunch r.m.s. duration is as short as 3 fs when the electron beam has energy of 120–130 MeV with the radiation pulse duration in the range of 50–100 fs
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