6,832 research outputs found
A systematic search for massive black hole binaries in SDSS spectroscopic sample
We present the results of a systematic search for massive black hole binaries
in the Sloan Digital Sky Survey spectroscopic database. We focus on bound
binaries, under the assumption that one of the black holes is active. In this
framework, the broad lines associated to the accreting black hole are expected
to show systematic velocity shifts with respect to the narrow lines, which
trace the rest-frame of the galaxy. For a sample of 54586 quasars and 3929
galaxies at redshifts 0.1<z<1.5 we brute-force model each spectrum as a mixture
of two quasars at two different redshifts. The spectral model is a data-driven
dimensionality reduction of the SDSS quasar spectra based on a matrix
factorization. We identified 32 objects with peculiar spectra. Nine of them can
be interpreted as black hole binaries. This doubles the number of known black
hole binary candidates. We also report on the discovery of a new class of
extreme double-peaked emitters with exceptionally broad and faint Balmer lines.
For all the interesting sources, we present detailed analysis of the spectra,
and discuss possible interpretations.Comment: 10 pages, 2 figures, accepted for publication in Ap
The faint-galaxy hosts of gamma-ray bursts
The observed redshifts and magnitudes of the host galaxies of gamma-ray
bursts (GRBs) are compared with the predictions of three basic GRB models, in
which the comoving rate density of GRBs is (1) proportional to the cosmic star
formation rate density, (2) proportional to the total integrated stellar
density and (3) constant. All three models make the assumption that at every
epoch the probability of a GRB occuring in a galaxy is proportional to that
galaxy's broad-band luminosity. No assumption is made that GRBs are standard
candles or even that their luminosity function is narrow. All three rate
density models are consistent with the observed GRB host galaxies to date,
although model (2) is slightly disfavored relative to the others. Models (1)
and (3) make very similar predictions for host galaxy magnitude and redshift
distributions; these models will be probably not be distinguished without
measurements of host-galaxy star-formation rates. The fraction of host galaxies
fainter than 28 mag may constrain the faint end of the galaxy luminosity
function at high redshift, or, if the fraction is observed to be low, may
suggest that the bursters are expelled from low-luminosity hosts. In all
models, the probability of finding a z<0.008 GRB among a sample of 11 GRBs is
less than 10^(-4), strongly suggesting that GRB 980425, if associated with
supernova 1998bw, represents a distinct class of GRBs.Comment: 7 pages, ApJ in press, revised to incorporate yet more new and
revised observational result
Private Database Queries Using Quantum States with Limited Coherence Times
We describe a method for private database queries using exchange of quantum
states with bits encoded in mutually incompatible bases. For technology with
limited coherence time, the database vendor can announce the encoding after a
suitable delay to allow the user to privately learn one of two items in the
database without the ability to also definitely infer the second item. This
quantum approach also allows the user to choose to learn other functions of the
items, such as the exclusive-or of their bits, but not to gain more information
than equivalent to learning one item, on average. This method is especially
useful for items consisting of a few bits by avoiding the substantial overhead
of conventional cryptographic approaches.Comment: extended to generalized (POVM) measurement
Steady-state simulations using weighted ensemble path sampling
We extend the weighted ensemble (WE) path sampling method to perform rigorous
statistical sampling for systems at steady state. The straightforward
steady-state implementation of WE is directly practical for simple landscapes,
but not when significant metastable intermediates states are present. We
therefore develop an enhanced WE scheme, building on existing ideas, which
accelerates attainment of steady state in complex systems. We apply both WE
approaches to several model systems confirming their correctness and efficiency
by comparison with brute-force results. The enhanced version is significantly
faster than the brute force and straightforward WE for systems with WE bins
that accurately reflect the reaction coordinate(s). The new WE methods can also
be applied to equilibrium sampling, since equilibrium is a steady state
Single-shot single-gate RF spin readout in silicon
For solid-state spin qubits, single-gate RF readout can help minimise the
number of gates required for scale-up to many qubits since the readout sensor
can integrate into the existing gates required to manipulate the qubits
(Veldhorst 2017, Pakkiam 2018). However, a key requirement for a scalable
quantum computer is that we must be capable of resolving the qubit state within
single-shot, that is, a single measurement (DiVincenzo 2000). Here we
demonstrate single-gate, single-shot readout of a singlet-triplet spin state in
silicon, with an average readout fidelity of at a
measurement bandwidth. We use this technique to measure a triplet to
singlet relaxation time of in precision donor quantum
dots in silicon. We also show that the use of RF readout does not impact the
maximum readout time at zero detuning limited by the to decay,
which remained at approximately . This establishes single-gate
sensing as a viable readout method for spin qubits
Catalytic hollow fiber membranes prepared using layer-by-layer adsorption of polyelectrolytes and metal nanoparticles
Immobilization of metalnanoparticles in hollowfibermembranes via alternating adsorption of polyelectrolytes and negatively charged Au nanoparticles yields catalytic reactors with high surface areas. SEM images show that this technique deposits a high density of unaggregated metalnanoparticles both on the surfaces and in the pores of the hollowfibers. Catalytic reduction of 4-nitrophenol with NaBH4, which can be easily monitored by UV–vis spectrophotometry, demonstrates that the nanoparticles in the hollowfibermembrane are highly catalytically active. In a single pass through the membrane, >99% of the 4-nitrophenol is reduced to 4-aminophenol, but this conversion decreases over time. The conversion decline may stem from catalyst fouling caused by by-products of 4-aminophenol oxidation
Socially intelligent reasoning for autonomous agents
Socially intelligent agents are autonomous problem solvers that have to achieve their objectives by interacting with other similarly autonomous entities. A major concern, therefore, is with the design of the decision-making mechanism that such agents employ in order to determine which actions to take to achieve their goals. An attractive and much sought after property of this mechanism is that it produces decisions that are rational from the perspective of the individual agent. However, some agents are also inherently social. Moreover, individual and social concerns often conflict, leading to the possibility of inefficient performance of the individual and the system. To address these problems we propose a framework for making socially acceptable decisions, based on social welfare functions, that combines social and individual perspectives in a unified and flexible manner. The framework is realized in an exemplar computational setting and an empirical analysis is made of the relative performance of varying sociable decision-making functions in a range of environments. This analysis is then used to design an agent that adapts its decision-making to reflect the resource constraints that it faces at any given time. A further round of empirical evaluation shows how adding such a metalevel mechanism enhances the performance of the agent by directing reasoning to adopt different strategies in different contexts. Finally, the possibility and efficacy of making the metalevel mechanism adaptive, so that experience of past encounters can be factored into the decision-making, is demonstrated
Probing Dark Matter Substructure in Lens Galaxies
We investigate the effects of numerous dark matter subhalos in a galaxy-sized
halo on the events of strong lensing, to assess their presence as expected from
the cold dark matter scenario. Lens galaxies are represented by a smooth
ellipsoid in an external shear field and additional cold dark matter subhalos
taken from Monte Carlo realizations which accord with recent N-body results. We
also consider other possible perturbers, globular clusters and luminous dwarf
satellites, for comparison. We then apply the models to the particular lens
systems with four images, B1422+231 and PG1115+080, for which smooth lens
models are unable to reproduce both the positions of the images and their radio
flux ratios or dust-free optical flux ratios simultaneously. We show that the
perturbations by both globular clusters and dwarf satellites are too small to
change the flux ratios, whereas cold dark matter subhalos are most likely
perturbers to reproduce the observed flux ratios in a statistically significant
manner. This result suggests us the presence of numerous subhalos in lens
galaxies, which is consistent with the results of cosmological N-body
simulations.Comment: 19 pages, including 5 figures, ApJ in pres
Quantum Portfolios
Quantum computation holds promise for the solution of many intractable
problems. However, since many quantum algorithms are stochastic in nature they
can only find the solution of hard problems probabilistically. Thus the
efficiency of the algorithms has to be characterized both by the expected time
to completion {\it and} the associated variance. In order to minimize both the
running time and its uncertainty, we show that portfolios of quantum algorithms
analogous to those of finance can outperform single algorithms when applied to
the NP-complete problems such as 3-SAT.Comment: revision includes additional data and corrects minor typo
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