2,304 research outputs found

### Marginalizing the likelihood function for modeled gravitational wave searches

Matched filtering is a commonly used technique in gravitational wave searches
for signals from compact binary systems and from rapidly rotating neutron
stars. A common issue in these searches is dealing with four extrinsic
parameters which do not affect the phase evolution of the system: the overall
amplitude, initial phase, and two angles determining the overall orientation of
the system. The F-statistic maximizes the likelihood function analytically over
these parameters, while the B-statistic marginalizes over them. The
B-statistic, while potentially more powerful and capable of incorporating
astrophysical priors, is not as widely used because of the computational
difficulty of performing the marginalization. In this paper we address this
difficulty and show how the marginalization can be done analytically by
combining the four parameters into a set of complex amplitudes. The results of
this paper are applicable to both transient non-precessing binary coalescence
events, and to long lived signals from rapidly rotating neutron stars.Comment: 26 page

### New Coordinates for the Amplitude Parameter Space of Continuous Gravitational Waves

The parameter space for continuous gravitational waves (GWs) can be divided
into amplitude parameters (signal amplitude, inclination and polarization
angles describing the orientation of the source, and an initial phase) and
phase-evolution parameters. The division is useful in part because the
Jaranowski-Krolak-Schutz (JKS) coordinates on the four-dimensional amplitude
parameter space allow the GW signal to be written as a linear combination of
four template waveforms with the JKS coordinates as coefficients. We define a
new set of coordinates on the amplitude parameter space, with the same
properties, which is more closely connected to the physical amplitude
parameters. These naturally divide into two pairs of Cartesian-like coordinates
on two-dimensional subspaces, one corresponding to left- and the other to
right-circular polarization. We thus refer to these as CPF (circular
polarization factored) coordinates. The corresponding two sets of polar
coordinates (known as CPF-polar) can be related in a simple way to the physical
parameters. We illustrate some simplifying applications for these various
coordinate systems, such as: a calculation of Jacobians between various
coordinate systems; an illustration of the signal coordinate singularities
associated with left- and right-circular polarization, which correspond to the
origins of the two two-dimensional subspaces; and an elucidation of the form of
the log-likelihood ratio between hypotheses of Gaussian noise with and without
a continuous GW signal. These are used to illustrate some of the prospects for
approximate evaluation of a Bayesian detection statistic defined by
marginalization over the physical parameter space. Additionally, in the
presence of simplifying assumptions about the observing geometry, we are able
to explicitly evaluate the integral for the Bayesian detection statistic, and
compare it to the approximate results.Comment: REVTeX, 18 pages, 8 image files included in 7 figure

### Towards the graviton from spinfoams: higher order corrections in the 3d toy model

We consider the recent calculation gr-qc/0508124 of the graviton propagator
in the spinfoam formalism. Within the 3d toy model introduced in gr-qc/0512102,
we test how the spinfoam formalism can be used to construct the perturbative
expansion of graviton amplitudes. Although the 3d graviton is a pure gauge, one
can choose to work in a gauge where it is not zero and thus reproduce the
structure of the 4d perturbative calculations. We compute explicitly the next
to leading and next to next to leading orders, corresponding to one-loop and
two-loop corrections. We show that while the first arises entirely from the
expansion of the Regge action around the flat background, the latter receives
contributions from the microscopic, non Regge-like, quantum geometry.
Surprisingly, this new contribution reduces the magnitude of the next to next
to leading order. It thus appears that the spinfoam formalism is likely to
substantially modify the conventional perturbative expansion at higher orders.
This result supports the interest in this approach. We then address a number
of open issues in the rest of the paper. First, we discuss the boundary state
ansatz, which is a key ingredient in the whole construction. We propose a way
to enhance the ansatz in order to make the edge lengths and dihedral angles
conjugate variables in a mathematically well-defined way. Second, we show that
the leading order is stable against different choices of the face weights of
the spinfoam model; the next to leading order, on the other hand, is changed in
a simple way, and we show that the topological face weight minimizes it.
Finally, we extend the leading order result to the case of a regular, but not
equilateral, tetrahedron.Comment: 24 pages, many figure

### The PyCBC search for gravitational waves from compact binary coalescence

We describe the PyCBC search for gravitational waves from compact-object
binary coalescences in advanced gravitational-wave detector data. The search
was used in the first Advanced LIGO observing run and unambiguously identified
two black hole binary mergers, GW150914 and GW151226. At its core, the PyCBC
search performs a matched-filter search for binary merger signals using a bank
of gravitational-wave template waveforms. We provide a complete description of
the search pipeline including the steps used to mitigate the effects of noise
transients in the data, identify candidate events and measure their statistical
significance. The analysis is able to measure false-alarm rates as low as one
per million years, required for confident detection of signals. Using data from
initial LIGO's sixth science run, we show that the new analysis reduces the
background noise in the search, giving a 30% increase in sensitive volume for
binary neutron star systems over previous searches.Comment: 29 pages, 7 figures, accepted by Classical and Quantum Gravit

### The TcEG1 beetle (Tribolium castaneum) cellulase produced in transgenic switchgrass is active at alkaline pH and auto-hydrolyzes biomass for increased cellobiose release

Background
Genetically engineered biofuel crops, such as switchgrass (Panicum virgatum L.), that produce their own cell wall-digesting cellulase enzymes would reduce costs of cellulosic biofuel production. To date, non-bioenergy plant models have been used in nearly all studies assessing the synthesis and activity of plant-produced fungal and bacterial cellulases. One potential source for cellulolytic enzyme genes is herbivorous insects adapted to digest plant cell walls. Here we examine the potential of transgenic switchgrass-produced TcEG1 cellulase from Tribolium castaneum (red flour beetle). This enzyme, when overproduced in Escherichia coliand Saccharomyces cerevisiae, efficiently digests cellulose at optima of 50 Â°C and pH 12.0. Results TcEG1 that was produced in green transgenic switchgrass tissue had a range of endoglucanase activity of 0.16â€“0.05 units (ÂµM glucose release/min/mg) at 50 Â°C and pH 12.0. TcEG1 activity from air-dried leaves was unchanged from that from green tissue, but when tissue was dried in a desiccant oven (46 Â°C), specific enzyme activity decreased by 60%. When transgenic biomass was â€śdropped-inâ€ť into an alkaline buffer (pH 12.0) and allowed to incubate at 50 Â°C, cellobiose release was increased up to 77% over non-transgenic biomass. Saccharification was increased in one transgenic event by 28%, which had a concurrent decrease in lignin content of 9%. Histological analysis revealed an increase in cell wall thickness with no change to cell area or perimeter. Transgenic plants produced more, albeit narrower, tillers with equivalent dry biomass as the control. Conclusions This work describes the first study in which an insect cellulase has been produced in transgenic plants; in this case, the dedicated bioenergy crop switchgrass. Switchgrass overexpressing the TcEG1 gene appeared to be morphologically similar to its non-transgenic control and produced equivalent dry biomass. Therefore, we propose TcEG1 transgenics could be bred with other transgenic germplasm (e.g., low-lignin lines) to yield new switchgrass with synergistically reduced recalcitrance to biofuel production. In addition, transgenes for other cell wall degrading enzymes may be stacked with TcEG1 in switchgrass to yield complementary cell wall digestion features and complete auto-hydrolysis

### Acceleration of generalized hypergeometric functions through precise remainder asymptotics

We express the asymptotics of the remainders of the partial sums {s_n} of the
generalized hypergeometric function q+1_F_q through an inverse power series z^n
n^l \sum_k c_k/n^k, where the exponent l and the asymptotic coefficients {c_k}
may be recursively computed to any desired order from the hypergeometric
parameters and argument. From this we derive a new series acceleration
technique that can be applied to any such function, even with complex
parameters and at the branch point z=1. For moderate parameters (up to
approximately ten) a C implementation at fixed precision is very effective at
computing these functions; for larger parameters an implementation in higher
than machine precision would be needed. Even for larger parameters, however,
our C implementation is able to correctly determine whether or not it has
converged; and when it converges, its estimate of its error is accurate.Comment: 36 pages, 6 figures, LaTeX2e. Fixed sign error in Eq. (2.28), added
several references, added comparison to other methods, and added discussion
of recursion stabilit

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