Estimating the final spin of a binary black hole coalescence

We present a straightforward approach for estimating the final black hole spin of a binary black hole coalescence with arbitrary initial masses and spins. Making some simple assumptions, we estimate the final angular momentum to be the sum of the individual spins plus the orbital angular momentum of a test particle orbiting at the last stable orbit around a Kerr black hole with a spin parameter of the final black hole. The formula we obtain is able to reproduce with reasonable accuracy the results from available numerical simulations, but, more importantly, it can be used to investigate what configurations might give rise to interesting dynamics. In particular, we discuss scenarios which might give rise to a ``flip'' in the direction of the total angular momentum of the system. By studying the dependence of the final spin upon the mass ratio and initial spins we find that our simple approach suggests that it is not possible to spin-up a black hole to extremal values through merger scenarios irrespective of the mass ratio of the objects involved.Comment: 9 pages, 8 figure

Orbital Dynamics of Binary Boson Star Systems

We extend our previous studies of head-on collisions of boson stars by considering orbiting binary boson stars. We concentrate on equal mass binaries and study the dynamical behavior of boson/boson and boson/antiboson pairs. We examine the gravitational wave output of these binaries and compare with other compact binaries. Such a comparison lets us probe the apparent simplicity observed in gravitational waves produced by black hole binary systems. In our system of interest however, there is an additional internal freedom which plays a significant role in the system's dynamics, namely the phase of each star. Our evolutions show rather simple behavior at early times, but large differences occur at late times for the various initial configurations.Comment: 10 pages, 14 figure

On characteristic initial data for a star orbiting a black hole

We take further steps in the development of the characteristic approach to enable handling the physical problem of a compact self-gravitating object, such as a neutron star, in close orbit around a black hole. We examine different options for setting the initial data for this problem and, in order to shed light on their physical relevance, we carry out short time evolution of this data. To this end we express the matter part of the characteristic gravity code so that the hydrodynamics are in conservation form. The resulting gravity plus matter relativity code provides a starting point for more refined future efforts at longer term evolution. In the present work we find that, independently of the details of the initial gravitational data, the system quickly flushes out spurious gravitational radiation and relaxes to a quasi-equilibrium state with an approximate helical symmetry corresponding to the circular orbit of the star.Comment: 20 pages, 10 figure

Towards the Final Fate of an Unstable Black String

Black strings, one class of higher dimensional analogues of black holes, were shown to be unstable to long wavelength perturbations by Gregory and Laflamme in 1992, via a linear analysis. We revisit the problem through numerical solution of the full equations of motion, and focus on trying to determine the end-state of a perturbed, unstable black string. Our preliminary results show that such a spacetime tends towards a solution resembling a sequence of spherical black holes connected by thin black strings, at least at intermediate times. However, our code fails then, primarily due to large gradients that develop in metric functions, as the coordinate system we use is not well adapted to the nature of the unfolding solution. We are thus unable to determine how close the solution we see is to the final end-state, though we do observe rich dynamical behavior of the system in the intermediate stages.Comment: 17 pages, 7 figure

Robustness of the Blandford-Znajek mechanism

The Blandford-Znajek mechanism has long been regarded as a key ingredient in models attempting to explain powerful jets in AGNs, quasars, blazzars etc. In such mechanism, energy is extracted from a rotating black hole and dissipated at a load at far distances. In the current work we examine the behaviour of the BZ mechanism with respect to different boundary conditions, revealing the mechanism robustness upon variation of these conditions. Consequently, this work closes a gap in our understanding of this important scenario.Comment: 7 pages, accepted in CQ

Resistance profiles and genetic diversity of Escherichia coli strains isolated from acute bovine mastitis

Between March 2011 and February 2012 83 E. coli strains were isolated from mastitis milk samples from 83 different animals (67 farms) and tested for their sensitivity to various antibiotics by means of disk diffusion method and genotyped by determination of the phylogenetic groups as well as by pulsed field gel electrophoresis (PFGE). The antibiotics were chosen on the basis of their licenses for intramammary application in Switzerland. As many as 16.9 % of the isolates were resistant to one or more antimicrobial agents. Amoxicillin-clavulanic acid, gentamicin and third generation cephalosporins proved effective against the majority of these strains. Nevertheless, one blaCTX-M-14 harbouring extended-spectrum-beta-lactamase producing strain was found. Genetic analysis grouped most of the strains (87 %) into phylogenetic groups A and B1. PFGE genotyping demonstrated that E. coli from cows with mastitis even from the same farm were genotypically very diverse. = Entre mars 2011 et février 2012, 83 souches d'E. coli issues de 83 vaches différentes provenant de 67 exploitations ont été collectées et testées quant à leur sensibilité vis-à-vis de divers antibiotiques. Ces antibiotiques ont été choisis sur la base de leurs autorisations pour l'application intra mammaire en Suisse et le test a été effectué par diffusion sur gel d'agar. En outre toutes les souches ont été typisées quant à leur appartenance aux groupes phylogénétiques. 16.9 % des souches présentaient une résistance à un ou plusieurs antibiotiques. L'amoxicilline-acide clavulanique, la gentamicine et les céphalosporines de troisièmes générations se montraient efficaces contre la majorité des souches d'E. coli. On a toutefois trouvé une souche fabriquant un extended-spectrum-beta-lactamase qui portait le gène blaCTX-M-14. L'analyse génétique groupait la majorité des souches (87 %) dans les groupes phylogénétiques A et B1. La génotypisation par PFGE montrait une grande diversité entre les souches, même si elles provenaient de la même exploitation. = Zwischen März 2011 und Februar 2012 wurden 83 E. coli Stämme von 83 verschiedenen Kühen aus 67 Betrieben gesammelt und auf ihre Empfindlichkeit gegenüber verschiedenen Antibiotika getestet. Die Antibiotika wurden aufgrund der Zulassung für eine intramammäre Applikation in der Schweiz ausgesucht und die Empfindlichkeitstestung mittels Agardiffusions-Methode durchgeführt. Zudem wurden alle Stämme hinsichtlich ihrer Zugehörigkeit zu den phylogenetischen Gruppen wie auch mittels Pulsfeldgelelektrophorese (PFGE) genotypisiert. 16.9 % aller Stämme zeigten Resistenzen gegenüber einem oder mehreren Antibiotika. Amoxicillin-Clavulansäure, Gentamicin und Cephalosporine der dritten Generation erwiesen sind als wirksam gegen die Mehrheit der E. coli Stämme. Jedoch wurde ein extended-Spectrum-beta-Lactamase Bildner, welcher das blaCTX-M-14-Gen trägt, gefunden. Die genetische Analyse gruppierte das Gros der Stämme (87 %) in die phylogentischen Guppen A und B1. Die weitere Genotypisierung mittels PFGE zeigte eine grosse Diversität unter den E. coli Stämmen, auch wenn diese vom selben Betrieb stammten

Relativistic MHD and black hole excision: Formulation and initial tests

A new algorithm for solving the general relativistic MHD equations is described in this paper. We design our scheme to incorporate black hole excision with smooth boundaries, and to simplify solving the combined Einstein and MHD equations with AMR. The fluid equations are solved using a finite difference Convex ENO method. Excision is implemented using overlapping grids. Elliptic and hyperbolic divergence cleaning techniques allow for maximum flexibility in choosing coordinate systems, and we compare both methods for a standard problem. Numerical results of standard test problems are presented in two-dimensional flat space using excision, overlapping grids, and elliptic and hyperbolic divergence cleaning.Comment: 22 pages, 8 figure

Understanding possible electromagnetic counterparts to loud gravitational wave events: Binary black hole effects on electromagnetic fields

In addition to producing loud gravitational waves (GW), the dynamics of a binary black hole system could induce emission of electromagnetic (EM) radiation by affecting the behavior of plasmas and electromagnetic fields in their vicinity. We here study how the electromagnetic fields are affected by a pair of orbiting black holes through the merger. In particular, we show how the binary's dynamics induce a variability in possible electromagnetically induced emissions as well as an enhancement of electromagnetic fields during the late-merge and merger epochs. These time dependent features will likely leave their imprint in processes generating detectable emissions and can be exploited in the detection of electromagnetic counterparts of gravitational waves.Comment: 12 page