Testing numerical relativity with the shifted gauge wave

Computational methods are essential to provide waveforms from coalescing black holes, which are expected to produce strong signals for the gravitational wave observatories being developed. Although partial simulations of the coalescence have been reported, scientifically useful waveforms have so far not been delivered. The goal of the AppleswithApples (AwA) Alliance is to design, coordinate and document standardized code tests for comparing numerical relativity codes. The first round of AwA tests have now being completed and the results are being analyzed. These initial tests are based upon periodic boundary conditions designed to isolate performance of the main evolution code. Here we describe and carry out an additional test with periodic boundary conditions which deals with an essential feature of the black hole excision problem, namely a non-vanishing shift. The test is a shifted version of the existing AwA gauge wave test. We show how a shift introduces an exponentially growing instability which violates the constraints of a standard harmonic formulation of Einstein's equations. We analyze the Cauchy problem in a harmonic gauge and discuss particular options for suppressing instabilities in the gauge wave tests. We implement these techniques in a finite difference evolution algorithm and present test results. Although our application here is limited to a model problem, the techniques should benefit the simulation of black holes using harmonic evolution codes.Comment: Submitted to special numerical relativity issue of Classical and Quantum Gravit

Nova Sco and coalescing low mass black hole binaries as LIGO sources

Double neutron star binaries, analogous to the well known Hulse--Taylor pulsar PSR 1913+16, are guaranteed-to-exist sources of high frequency gravitational radiation detectable by LIGO. There is considerable uncertainty in the estimated rate of coalescence of such systems, with conservative estimates of ~1 per million years per galaxy, and optimistic theoretical estimates one or more magnitude larger. Formation rates of low-mass black hole-neutron star binaries may be higher than those of NS-NS binaries, and may dominate the detectable LIGO signal rate. We estimate the enhanced coalescence rate for BH-BH binaries due to weak asymmetric kicks during the formation of low mass black holes like Nova Sco, and find they may contribute significantly to the LIGO signal rate, possibly dominating the phase I detectable signals if the range of BH masses for which there is significant kick is broad enough. For a standard Salpeter IMF, assuming mild natal kicks, we project that the R6 merger rate of BH-BH systems is ~0.5, smaller than that of NS-NS systems. However, the higher chirp mass of these systems produces a signal nearly four times greater, on average, with a commensurate increase in search volume. The BH-BH coalescence channel considered here also predicts that a substantial fraction of BH-BH systems should have at least one component with near-maximal spin (a/M ~ 1).The waveforms produced by the coalescence of such a system should produce a clear spin signature, so this hypothesis could be directly tested by LIGO.Comment: 16 pages, LaTeX/AASTeX, 5 figure

A very massive runaway star from Cygnus OB2

Aims: We analyze the available information on the star BD+43 3654 to investigate the possibility that it may have had its origin in the massive OB association Cygnus OB2. Methods: We present new spectroscopic observations allowing a reliable spectral classification of the star, and discuss existing MSX observations of its associated bow shock and astrometric information not previously studied. Results: Our observations reveal that BD+43 3654 is a very early and luminous star of spectral type O4If, with an estimated mass of (70 +/- 15) solar masses and an age of about 1.6 Myr. The high spatial resolution of the MSX observations allows us to determine its direction of motion in the plane of the sky by means of the symmetry axis of the well-defined bow shock, which matches well the orientation expected from the proper motion. Tracing back its path across the sky we find that BD+43 3654 was located near the central, densest region of Cygnus OB2 at a time in the past similar to its estimated age. Conclusions: BD+43 3654 turns out to be one of the three most massive runaway stars known, and it most likely formed in the central region of Cygnus OB2. A runaway formation mechanism by means of dynamical ejection is consistent with our results.Comment: Accepted by Astronomy and Astrophysics (letters); 5 pages, 3 figure

Gallavotti-Cohen-Type symmetry related to cycle decompositions for Markov chains and biochemical applications

We slightly extend the fluctuation theorem obtained in \cite{LS} for sums of generators, considering continuous-time Markov chains on a finite state space whose underlying graph has multiple edges and no loop. This extended frame is suited when analyzing chemical systems. As simple corollary we derive in a different method the fluctuation theorem of D. Andrieux and P. Gaspard for the fluxes along the chords associated to a fundamental set of oriented cycles \cite{AG2}. We associate to each random trajectory an oriented cycle on the graph and we decompose it in terms of a basis of oriented cycles. We prove a fluctuation theorem for the coefficients in this decomposition. The resulting fluctuation theorem involves the cycle affinities, which in many real systems correspond to the macroscopic forces. In addition, the above decomposition is useful when analyzing the large deviations of additive functionals of the Markov chain. As example of application, in a very general context we derive a fluctuation relation for the mechanical and chemical currents of a molecular motor moving along a periodic filament.Comment: 23 pages, 5 figures. Correction

Fourth order indirect integration method for black hole perturbations: even modes

On the basis of a recently proposed strategy of finite element integration in time domain for partial differential equations with a singular source term, we present a fourth order algorithm for non-rotating black hole perturbations in the Regge-Wheeler gauge. Herein, we address even perturbations induced by a particle plunging in. The forward time value at the upper node of the $(r^*,t)$ grid cell is obtained by an algebraic sum of i) the preceding node values of the same cell, ii) analytic expressions, related to the jump conditions on the wave function and its derivatives, iii) the values of the wave function at adjacent cells. In this approach, the numerical integration does not deal with the source and potential terms directly, for cells crossed by the particle world line. This scheme has also been applied to circular and eccentric orbits and it will be object of a forthcoming publication.Comment: This series of papers deals with EMRI for LISA. With the respect to the v1 version, the algorithm has been improved; convergence tests and references have been added; v2 is composed by 23 pages, and 6 figures. Paper accepted by Class. Quantum Gravity for the special issue on Theory Meets Data Analysis at Comparable and Extreme Mass Ratios (Capra and NRDA) at Perimeier Institute in June 201

On the Rarity of X-Ray Binaries with Naked Helium Donors

The paucity of known High-Mass X-Ray Binaries (HMXB) with naked He donor stars (hereafter He star) in the Galaxy has been noted over the years as a surprising fact, given the significant number of Galactic HMXBs containing H-rich donors, which are expected to be their progenitors. This contrast has further sharpened in light of recent observations uncovering a preponderance of HMXBs hosting loosely bound Be donors orbiting neutron stars (NS), which would be expected to naturally evolve into He-HMXBs through dynamical mass transfer onto the NS and a common-envelope (CE) phase. Hence, reconciling the large population of Be-HMXBs with the observation of only one He-HMXB can help constrain the dynamics of CE physics. Here, we use detailed stellar structure and evolution models and show that binary mergers of HMXBs during CE events must be common in order to resolve the tension between these observed populations. We find that, quantitatively, this scenario remains consistent with the typically adopted energy parameterization of CE evolution, yielding expected populations which are not at odds with current observations. However, future observations which better constrain the underlying population of loosely bound O/B-NS binaries are likely to place significant constraints on the efficiency of CE ejection.Comment: 9 pages, 5 figures, In Pres

A fully relativistic radial fall

Radial fall has historically played a momentous role. It is one of the most classical problems, the solutions of which represent the level of understanding of gravitation in a given epoch. A {\it gedankenexperiment} in a modern frame is given by a small body, like a compact star or a solar mass black hole, captured by a supermassive black hole. The mass of the small body itself and the emission of gravitational radiation cause the departure from the geodesic path due to the back-action, that is the self-force. For radial fall, as any other non-adiabatic motion, the instantaneous identity of the radiated energy and the loss of orbital energy cannot be imposed and provide the perturbed trajectory. In the first part of this letter, we present the effects due to the self-force computed on the geodesic trajectory in the background field. Compared to the latter trajectory, in the Regge-Wheeler, harmonic and all others smoothly related gauges, a far observer concludes that the self-force pushes inward (not outward) the falling body, with a strength proportional to the mass of the small body for a given large mass; further, the same observer notes an higher value of the maximal coordinate velocity, this value being reached earlier on during infall. In the second part of this letter, we implement a self-consistent approach for which the trajectory is iteratively corrected by the self-force, this time computed on osculating geodesics. Finally, we compare the motion driven by the self-force without and with self-consistent orbital evolution. Subtle differences are noticeable, even if self-force effects have hardly the time to accumulate in such a short orbit.Comment: To appear in Int. J. Geom. Meth. Mod. Phy

Spin-transfer in an open ferromagnetic layer: from negative damping to effective temperature

Spin-transfer is a typical spintronics effect that allows a ferromagnetic layer to be switched by spin-injection. Most of the experimental results about spin transfer are described on the basis of the Landau-Lifshitz-Gilbert equation of the magnetization, in which additional current-dependent damping factors are added, and can be positive or negative. The origin of the damping can be investigated further by performing stochastic experiments, like one shot relaxation experiments under spin-injection in the activation regime of the magnetization. In this regime, the N\'eel-Brown activation law is observed which leads to the introduction of a current-dependent effective temperature. In order to justify the introduction of these counterintuitive parameters (effective temperature and negative damping), a detailed thermokinetic analysis of the different sub-systems involved is performed. We propose a thermokinetic description of the different forms of energy exchanged between the electric and the ferromagnetic sub-systems at a Normal/Ferromagnetic junction. The corresponding Fokker Planck equations, including relaxations, are derived. The damping coefficients are studied in terms of Onsager-Casimir transport coefficients, with the help of the reciprocity relations. The effective temperature is deduced in the activation regime.Comment: 65 pages, 10 figure

Fluctuation theorem for currents and Schnakenberg network theory

A fluctuation theorem is proved for the macroscopic currents of a system in a nonequilibrium steady state, by using Schnakenberg network theory. The theorem can be applied, in particular, in reaction systems where the affinities or thermodynamic forces are defined globally in terms of the cycles of the graph associated with the stochastic process describing the time evolution.Comment: new version : 16 pages, 1 figure, to be published in Journal of Statistical Physic

Early warning system of natural hazards and decrease of climat impact from aviation; ALARM funded project

Aviation safety can be jeopardised by multiple hazards arising from natural phenomena, e.g., severe weather, aerosols/gases from natural hazard, and space weather. Furthermore, there are the anthropogenic emissions and climate impact of aviation that could be reduced. To mitigate such risk and/or to decrease climate impact, tactical decision-making processes could be enhanced through the development of multihazard monitoring and Early Warning System (EWS). With this objective in mind, ALARM consortium has implemented alert products (i.e., observations, detection and data access in near realtime) and tailored product (notifications, flight level — FL contamination, risk area, and visualization of emission/risk level) related to Natural Airborne Hazard (NAH, i.e., volcanic, dust and smoke clouds) and environmental hotspots. New selective detection, nowcasting and forecasts of such risks for aviation have been implemented as part of ALARM prototype EWS. This system has two functionalities. One is to provide alerts on a global coverage using remote sensing from satellites and models (focus on NAH, space weather activity and environmental hotspots). A second focuses on detecting severe weather and exceptional SO2 conditions around a selection of few airports, on providing nowcasts and forecasts of risk conditions