1,018 research outputs found
Disentangling jet and disc emission from the 2005 outburst of XTE J1118+480
The black hole X-ray transient, XTE J1118+480, has now twice been observed in
outburst - 2000 and 2005 - and on both occasions remained in the low/hard X-ray
spectral state. Here we present radio, infrared, optical, soft X-ray and hard
X-ray observations of the more recent outburst. We find that the lightcurves
have very different morphologies compared with the 2000 event and the optical
decay is delayed relative to the X-ray/radio. We attribute this lesser degree
of correlation to contributions of emission from multiple components, in
particular the jet and accretion disc. Whereas the jet seemed to dominate the
broadband spectrum in 2000, in 2005 the accretion disc seems to be more
prominent and we use an analysis of the lightcurves and spectra to distinguish
between the jet and disc emission. There also appears to be an optically thin
component to the radio emission in the 2005 data, possibly associated with
multiple ejection events and decaying as the outburst proceeds. These results
add to the discussion that the term "low/hard state'" covers a wider range of
properties than previously thought, if it is to account for XTE J1118+480
during these two outbursts.Comment: Accepted for publication in MNRA
Minimum-Fuel Trajectory Design in Multiple Dynamical Environments Utilizing Direct Transcription Methods and Particle Swarm Optimization
Particle swarm optimization is used to generate an initial guess for designing fuel-optimal trajectories in multiple dynamical environments. Trajectories designed in the vicinity of Earth use continuous or finite low-thrust burning and transfer from an inclined or equatorial circular low-Earth-orbit to a geostationary orbit. In addition, a trajectory from near-Earth to a periodic orbit about the cislunar Lagrange point with minimized impulsive burn costs is designed within a multi-body dynamical environment. Direct transcription is used in conjunction with a nonlinear optimizer to find locally-optimal trajectories given the initial guess. The near-Earth transfers are propagated at low-level thrust where neither the very-low-thrust spiral solution nor the impulsive transfer is an acceptable starting point. The very-high-altitude transfer is designed in a multi-body dynamical environment lacking a closed-form analytical solution. Swarming algorithms excel given a small number of design parameters.When continuous control time histories are needed, employing a polynomial parameterization facilitates the generation of feasible solutions. For design in a circular restricted three-body system, particle swarm optimization gains utility due to a more global search for the solution, but may be more sensitive to boundary constraints. Computation time and constraint weighting are areas where a swarming algorithm is weaker than other approaches
Evidence for a black-hole in the X-ray transient XTE J1859+226
We present the results of time-resolved optical photometry and spectroscopy
of the X-ray transient XTE J1859+226 (V406 Vul). Photometric observations taken
during 2000 and 2008 reveals the presence of the secondary star's ellipsoidal
modulation. Further photometry obtained in 2010 shows the system ~1 mag
brighter than its quiescence level and the ellipsoidal modulation diluted by
strong flaring activity. Spectroscopic data obtained with the 10.4-m GTC in
2010 reveals radial velocity variations of ~500 km/s over 3 h. A simultaneous
fit to the photometry and spectroscopy using sinusoids to represent the
secondary star's ellipsoidal and radial velocity variations, yields an orbital
period of 6.58+-0.05 h and a secondary star's radial velocity semi-amplitude of
K_2= 541+-70 km/s. The implied mass function is f(M)=4.5+-0.6 Msun,
significantly lower than previously reported but consistent with the presence
of a black hole in XTE J1859+226. The lack of eclipses sets an upper limit to
the inclination of 70 degrees which yields a lower limit to the black hole mass
of 5.42 Msun.Comment: Accepted for publication in MNRAS. Contains 5 pages and 4 figure
How usefulness shapes neural representations during goal-directed behavior
Value is often associated with reward, emphasizing its hedonic aspects. However, when circumstances change, value must also change (a compass outvalues gold, if you are lost). How are value representations in the brain reshaped under different behavioral goals? To answer this question, we devised a new task that decouples usefulness from its hedonic attributes, allowing us to study flexible goal-dependent mapping. Here, we show that, unlike sensory cortices, regions in the prefrontal cortex (PFC)—usually associated with value computation—remap their representation of perceptually identical items according to how useful the item has been to achieve a specific goal. Furthermore, we identify a coding scheme in the PFC that represents value regardless of the goal, thus supporting generalization across contexts. Our work questions the dominant view that equates value with reward, showing how a change in goals triggers a reorganization of the neural representation of value, enabling flexible behavior
Topology and Interactions in the photonic Creutz and Creutz-Hubbard ladders
The latest advances in the field of photonics have enabled the simulation of an increasing number of quantum models in photonic systems, turning them into an important tool for realizing exotic quantum phenomena. In this paper, different ways in which these systems can be used to study the interplay between flat band dynamics, topology, and interactions in a well-known quasi-1 D topological insulator-the Creutz ladder-are suggested. First, a simple experimental protocol is proposed to observe the Aharonov-Bohm localization in the noninteracting system, and the different experimental setups that might be used for this are reviewed. The inclusion of a repulsive Hubbard-type interaction term, which can give rise to repulsively bound pairs termed doublons, is then considered. The dynamics of these quasiparticles are studied for different points of the phase diagram, including a regime in which pairs are localized and particles are free to move. Finally, a scheme for the photonic implementation of a two-particle bosonic Creutz-Hubbard model is presented
Structure and reactivity of titania-supported oxides. Part 1: vanadium oxide on titania in the sub- and super-monolayer regions
Vanadium oxide has been deposited on TiO2 (washed anatase, 10 m2g−1; Degussa P-25, 55 ±3 m2g−1; Eurotitania, 46 m2g−1) by aqueous impregnation of (NH4)2[VO(C2O4)2] and by reaction with VOCl3, VO(OR)3 (R=iBu) and VO(acac)2 in organic solvents. Single applications of the last tree reagents form not more than a monolayer of vanadium oxide VOx, a monolayer being defined as 0.10 wt.% V2O5 per m2 of surface. When less than about four monolayers of VOx are present, there is in most cases only a single TPR peak: Tmax values, which increase with V2O5 content, are almost independent of the method used but vary slightly with the support (P-25 < Eurotitania < washed anatase). The 995 cm−1 band, characteristic of V&z.dbnd;O in V2O5, only appears when more than a monolayer of VOx is present.\ud
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In the sub-monolayer region, VOx is best formulated as an oxohydroxy species bonded to two surface oxygens. As the V2O5 content is increased, layers of disordered V2O5 are formed on limited areas of the surface, but crystalline V2O5 only occurs, probably on top of the disordered V2O5, when the V2O5 content exceeds about four monolayers, and takes the form of acicular crystals exposing only planes perpendicular to the a and b axes
BSM physics at the LHeC and the FCC-he
Electron-proton (e-p) colliders are an ideal laboratory to study common features of electron and quarks with production via electroweak bosons, leptoquarks, multi-jet final states and very forward physics, due to their impressive pseudorapidity coverage. In addition to these physics cases, there exist a broad Beyond the Standard Model (BSM) program aimed at exploring the capabilities of the LHeC [1] and FCC-he [2] for several New Physics scenarios. Although their centre-of-mass energy is down with respect to a pp collider by a factor of √Ep/Ee ∼ 10 (30) for the LHeC (FCC-he), they can be an invaluable tool to characterize BSM physics hints at ee and pp machines. The aim of this talk is to provide, on behalf of the BSM e-p Working Group, an overview of the aforementioned BSM program, by briefly summarizing the existing studies and reporting on the most recent progress. We expect that the ample scope in terms of NP models to be tested would enhance the synergies between the BSM and e-p communities
Seven parton amplitudes from recursion relations
We present the first calculation of two-quark and five-gluon tree amplitudes
using on-shell recursion relations. These amplitudes are needed for tree level
5-jet cross-section and an essential ingredient for next-to-leading order 4-jet
and next-to-next-to-leading order 3-jet production at hadronic colliders. Very
compact expressions for all possible helicity configurations are provided,
allowing for direct implementation in Monte-Carlo codes.Comment: 11 page
Time-resolved optical photometry of the ultra-compact binary 4U0614+091
We present a detailed optical study of the ultra-compact X-ray binary
4U0614+091. We have used 63 hrs of time-resolved optical photometry taken with
three different telescopes (IAC80, NOT and SPM) to search for optical
modulations. The power spectra of each dataset reveals sinusoidal modulations
with different periods, which are not always present. The strongest modulation
has a period of 51.3 mins, a semi-amplitude of 4.6 mmags, and is present in the
IAC80 data. The SPM and NOT data show periods of 42 mins and 64 mins
respectively, but with much weaker amplitudes, 2.6 mags and 1.3 mmags
respectively. These modulations arise from either X-ray irradiation of the
inner face of the secondary star and/or a superhump modulation from the
accretion disc, or quasi-periodic modulations in the accretion disc. It is
unclear whether these periods/quasi-periodic modulations are related to the
orbital period, however, the strongest period of 51.3 mins is close to earlier
tentative orbital periods. Further observations taken over a long base-line are
encouraged.Comment: Accepted for publication in PAS
Underground urbanism in Africa: splintered subterranean space in Lagos, Nigeria
Africa is rapidly urbanising and is likely to home to some of the most populous cities within the next decade. Such rapid growth has made the prevention of urban sprawl a Sisyphean Quest in many African cities, as rural fringes are rapidly being transformed into urbanised space. A strategy proposed around the world to address some of the urban challenges is the increasing adoption of a volumetric lens to planning the city. Specifically, to use the urban underground as a strategic site to place infrastructure and free-up superficial urban surface space, in turn potentially helping to create more sustainable, liveable, equitable and just urban environments. Yet, so far, little attention has been paid to the urban underground in Africa cities. In this paper, mobilising Lagos, Nigeria as a case study, we start addressing this lacuna. We provide a critical long-term analysis – spanning the colonial and since independence eras – of how the urban underground has been used in Lagos, focussing on utility (energy, telecommunications, water) and transport infrastructure. We follow this with an analysis of how political economies have shaped underground use and access, with a particular consideration on informal interactions, and how they shape underground use and access. We conclude by offering an assessment of the possibilities and challenges that the urban underground presents for the future of Lagos and other African cities, with a critical consideration of the dynamism of localised volumes and the practices around them
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