Michele Vallisneri's BlogSpace
http://www.vallis.org/blogspace
My Experimental Blosxom Weblog.enEmacs trick: adding Python mode to Emacs
http://www.vallis.org/blogspace/2016/04/11#emacs-python-mode
<p class="story_para">
Drop <a href="https://launchpad.net/python-mode">python-mode.el</a> into site-lisp (in <a href="http://www.porkrind.org/emacs/">Carbon Emacs</a>, it is in /Applications/Emacs.app/Contents/Resources/share/emacs/site-lisp/); byte-compile it,
</p>
<pre>
C-x C-f /path/to/python-mode.el RET
M-x byte-compile-file RET
</pre>
<p class="story_para">
then modify .emacs to autoload,
</p>
<pre>
(autoload `python-mode "python-mode" "Python editing mode." t)
</pre>
<p class="story_para">
to associate files,
</p>
<pre>
(setq auto-mode-alist
(cons `("\\.py$" . python-mode) auto-mode-alist))
(setq interpreter-mode-alist
(cons `("python" . python-mode)
interpreter-mode-alist))
</pre>
<p class="story_para">
and to have syntax highlighting by default,
</p>
<pre>
(global-font-lock-mode t)
(setq font-lock-maximum-decoration t)
</pre>
<p class="story_para">
done.</p>
[1302.6049] Studies of waveform requirements for intermediate mass-ratio coalescence searches with advanced detectors
http://www.vallis.org/blogspace/2014/01/07#1302.6049
<p class="story_para">
<b>Authors</b>: R. J. E. Smith, I. Mandel, A. Vecchio
</p>
<p class="story_para">
<b>Date</b>: 25 Feb 2013
</p>
<p class="story_para">
<b>Abstract</b>: The coalescence of a stellar-mass compact object into an intermediate-mass black hole (intermediate mass-ratio coalescence; IMRAC) is an important astrophysical source for ground-based gravitational-wave interferometers in the so-called advanced configuration. However, the ability to carry out effective matched-filter based searches for these systems is limited by the lack of reliable waveforms. Here we consider binaries in which the intermediate-mass black hole has mass in the range 24 - 200 solar masses with a stellar-mass companion having masses in the range 1.4 - 18.5 solar masses. In addition, we constrain the mass ratios, q, of the binaries to be in the range 1/140 < q < 1/10 and we restrict our study to the case of circular binaries with non-spinning components. We investigate the relative contribution to the signal-to-noise ratio (SNR) of the three different phases of the coalescence: inspiral, merger and ringdown. We show that merger and ringdown contribute to a substantial fraction of the total SNR over a large portion of the mass parameter space, although in a limited portion the SNR is dominated by the inspiral phase. We further identify three regions in the IMRAC mass-space in which: (i) inspiral-only searches could be performed with losses in detection rates L in the range 10% < L < 27%, (ii) searches based on inspiral-only templates lead to a loss in detection rates in the range 27% < L < 50%$, and (iii) templates that include merger and ringdown are essential to prevent losses in detection rates greater than 50%. We investigate the effectiveness with which the inspiral-only portion of the IMRAC waveform space is covered by comparing several existing waveform families in this regime. Our results reinforce the importance of extensive numerical relativity simulations of IMRACs and the need for further studies of suitable approximation schemes in this mass range.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1302.6049">abs</a>
<a href="http://www.arxiv.org/pdf/1302.6049">pdf</a></p>
[1312.1289] On incorporating post-Newtonian effects in N-body dynamics
http://www.vallis.org/blogspace/2013/12/05#1312.1289
<p class="story_para">
<b>Authors</b>: Clifford M. Will
</p>
<p class="story_para">
<b>Date</b>: 4 Dec 2013
</p>
<p class="story_para">
<b>Abstract</b>: The increasing role of general relativity in the dynamics of stellar systems with central massive black holes and in the evolution of hierarchical triple systems inspires a close examination of how post-Newtonian effects are incorporated into N-body dynamics. The majority of approaches incorporate relativity by adding to the Newtonian N-body equations the standard two-body post-Newtonian terms for a given star around the black hole or for the close binary in a triple system. We argue that, for calculating the evolution of such systems over timescales comparable to the relativistic pericenter advance timescale, it is essential to include ‘cross terms’ in the equations of motion. These are post-Newtonian terms that represent a coupling between the potential of the central black hole and the potential due to other stars in the system. For hierarchical triple systems, these are couplings between the potential of the inner binary and that of the distant third body. Over pericenter precession timescales, the effects of such terms can actually be ‘boosted’ to amplitudes of Newtonian order. We write down the post-Newtonian N-body equations of motion including a central black hole in a truncated form that includes all the relevant cross terms, in a format ready to use for numerical implementation. We do the same for hierarchical triple systems, and illustrate explicitly the effects of cross terms on the orbit-averaged equations of evolution for the orbit elements of the inner binary for the special case where the third body is on a circular orbit. We also describe the inspiration for this investigation: the motion of a test body about a central body with a Newtonian quadrupole moment, including the relativistic pericenter advance, whose correct solution for the conserved total Newtonian energy requires including PN cross terms between the mass monopole and quadrupole potentials.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1312.1289">abs</a>
<a href="http://www.arxiv.org/pdf/1312.1289">pdf</a></p>
[1312.0775] Gravitational Self-Torque and Spin Precession in Compact Binaries
http://www.vallis.org/blogspace/2013/12/03#1312.0775
<p class="story_para">
<b>Authors</b>: Sam R. Dolan, Niels Warburton, Abraham I. Harte, Alexandre Le Tiec, Barry Wardell, Leor Barack
</p>
<p class="story_para">
<b>Date</b>: 3 Dec 2013
</p>
<p class="story_para">
<b>Abstract</b>: We calculate the effect of self-interaction on the "geodetic" spin precession of a compact body in a strong-field orbit around a black hole. Specifically, we consider the spin precession angle $\psi$ per radian of orbital revolution for a particle carrying mass $\mu$ and spin $s \ll (G/c) \muˆ2$ in a circular orbit around a Schwarzschild black hole of mass $M \gg \mu$. We compute $\psi$ through $O(\mu/M)$ in perturbation theory, i.e, including the correction $\delta\psi$ (obtained numerically) due to the torque exerted by the conservative piece of the gravitational self-field. Comparison with a post-Newtonian (PN) expression for $\delta\psi$, derived here through 3PN order, shows good agreement but also reveals strong-field features which are not captured by the latter approximation. Our results can inform semi-analytical models of the strong-field dynamics in astrophysical binaries, important for ongoing and future gravitational-wave searches.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1312.0775">abs</a>
<a href="http://www.arxiv.org/pdf/1312.0775">pdf</a></p>
[1311.7174] Optimizing gravitational-wave searches for a population of coalescing binaries: Intrinsic parameters
http://www.vallis.org/blogspace/2013/12/03#1311.7174
<p class="story_para">
<b>Authors</b>: Thomas Dent, John Veitch
</p>
<p class="story_para">
<b>Date</b>: 27 Nov 2013
</p>
<p class="story_para">
<b>Abstract</b>: We revisit the problem of searching for gravitational waves from inspiralling compact binaries in Gaussian coloured noise. For binaries with quasicircular orbits and non-precessing component spins, considering dominant mode emission only, if the intrinsic parameters of the binary are known then the optimal statistic for a single detector is the well-known two-phase matched filter. However, the matched filter signal-to-noise ratio is /not/ in general an optimal statistic for an astrophysical population of signals, since their distribution over the intrinsic parameters will almost certainly not mirror that of noise events, which is determined by the (Fisher) information metric. Instead, the optimal statistic for a given astrophysical distribution will be the Bayes factor, which we approximate using the output of a standard template matched filter search. We then quantify the possible improvement in number of signals detected for various populations of non-spinning binaries. The method may easily be generalized to binaries with non-precessing spins.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1311.7174">abs</a>
<a href="http://www.arxiv.org/pdf/1311.7174">pdf</a></p>
[1311.7231] Pulsar timing residuals due to individual non-evolving gravitational wave sources
http://www.vallis.org/blogspace/2013/12/03#1311.7231
<p class="story_para">
<b>Authors</b>: Ming-Lei Tong, Cheng-Shi Zhao, Bao-Rong Yan, Ting-Gao Yang, Yu-Ping Gao
</p>
<p class="story_para">
<b>Date</b>: 28 Nov 2013
</p>
<p class="story_para">
<b>Abstract</b>: The pulsar timing residuals induced by gravitational waves from non-evolving single binary sources are affected by many parameters related to the relative positions of the pulsar and the gravitational wave sources. We will fully analyze the effects due to different parameters one by one. The standard deviations of the timing residuals will be calculated with a variable parameter fixing a set of other parameters. The orbits of the binary sources will be generally assumed to be elliptical. The influences of different eccentricities on the pulsar timing residuals will also studied in detail. We find that effects of the related parameters are quite different, and some of them present certain regularities.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1311.7231">abs</a>
<a href="http://www.arxiv.org/pdf/1311.7231">pdf</a></p>
[1311.7539] A Hamiltonian Monte Carlo method for Bayesian Inference of Supermassive Black Hole Binaries
http://www.vallis.org/blogspace/2013/12/03#1311.7539
<p class="story_para">
<b>Authors</b>: Edward K. Porter, J&#xe9;r&#xf4;me Carr&#xe9;
</p>
<p class="story_para">
<b>Date</b>: 29 Nov 2013
</p>
<p class="story_para">
<b>Abstract</b>: We investigate the use of a Hamiltonian Monte Carlo to map out the posterior density function for supermassive black hole binaries. While previous Markov Chain Monte Carlo (MCMC) methods, such as Metropolis-Hastings MCMC, have been successfully employed for a number of different gravitational wave sources, these methods are essentially random walk algorithms. The Hamiltonian Monte Carlo treats the inverse likelihood surface as a "gravitational potential" and by introducing canonical positions and momenta, dynamically evolves the Markov chain by solving Hamilton's equations of motion. We present an implementation of the Hamiltonian Markov Chain that is faster, and more efficient by a factor of approximately the dimension of the parameter space, than the standard MCMC.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1311.7539">abs</a>
<a href="http://www.arxiv.org/pdf/1311.7539">pdf</a></p>
[1310.3050] Validity of the Effective Fisher matrix for parameter estimation analysis: Comparing to the analytic Fisher matrix
http://www.vallis.org/blogspace/2013/10/13#1310.3050
<p class="story_para">
<b>Authors</b>: Hee-Suk Cho, Chang-Hwan Lee
</p>
<p class="story_para">
<b>Date</b>: 11 Oct 2013
</p>
<p class="story_para">
<b>Abstract</b>: The effective Fisher matrix method recently introduced by Cho et al. is a semi-analytic approach to the Fisher matrix, in which a local overlap surface is fitted by using a quadratic fitting function. Mathematically, the effective Fisher matrix should be consistent with the analytic one at the infinitesimal fitting scale. In this work, using the frequency-domain waveform (TaylorF2), we give brief comparison results between the effective and analytic Fisher matrices for several non-spinning binaries consisting of binary neutron stars with masses of (1.4, 1.4)M_sun, black hole-neutron star of (1.4, 10)M_sun, and binary black holes of (5, 5) and (10, 10)M_sun for a fixed signal to noise ratio (SNR=20) and show a good consistency between two methods. We also give a comparison result for an aligned-spin black hole-neutron star binary with a black hole spin of \chi=1, where we define new mass parameters (Mc, \etaˆ-1, \chiˆ7/2) to find good fitting functions to the overlap surface. The effective Fisher matrix can also be computed by using the time-domain waveforms which are generally more accurate than frequency-domain waveform. We show comparison results between the frequency-domain and time-domain waveforms (TaylorT4) for both the non-spinning aligned-spin binaries.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1310.3050">abs</a>
<a href="http://www.arxiv.org/pdf/1310.3050">pdf</a></p>
[1301.2556] Information field theory
http://www.vallis.org/blogspace/2013/08/19#1301.2556
<p class="story_para">
<b>Authors</b>: Torsten En&#xdf;lin
</p>
<p class="story_para">
<b>Date</b>: 11 Jan 2013
</p>
<p class="story_para">
<b>Abstract</b>: Non-linear image reconstruction and signal analysis deal with complex inverse problems. To tackle such problems in a systematic way, I present information field theory (IFT) as a means of Bayesian, data based inference on spatially distributed signal fields. IFT is a statistical field theory, which permits the construction of optimal signal recovery algorithms even for non-linear and non-Gaussian signal inference problems. IFT algorithms exploit spatial correlations of the signal fields and benefit from techniques developed to investigate quantum and statistical field theories, such as Feynman diagrams, re-normalisation calculations, and thermodynamic potentials. The theory can be used in many areas, and applications in cosmology and numerics are presented.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1301.2556">abs</a>
<a href="http://www.arxiv.org/pdf/1301.2556">pdf</a></p>
[1304.2052] Equation-of-state-independent relations in neutron stars
http://www.vallis.org/blogspace/2013/08/09#1304.2052
<p class="story_para">
<b>Authors</b>: Andrea Maselli, Vitor Cardoso, Valeria Ferrari, Leonardo Gualtieri, Paolo Pani
</p>
<p class="story_para">
<b>Date</b>: 7 Apr 2013
</p>
<p class="story_para">
<b>Abstract</b>: Neutron stars are extremely relativistic objects which abound in our universe and yet are poorly understood, due to the high uncertainty on how matter behaves in the extreme conditions which prevail in the stellar core. It has recently been pointed out that the moment of inertia I, the Love number lambda and the spin-induced quadrupole moment Q of an isolated neutron star, are related through functions which are practically independent of the equation of state. These surprising universal I-lambda-Q relations pave the way for a better understanding of neutron stars, most notably via gravitational-wave emission. Gravitational-wave observations will probe highly-dynamical binaries and it is important to understand whether the universality of the I-lambda-Q relations survives strong-field and finite-size effects. We apply a Post-Newtonian-Affine approach to model tidal deformations in compact binaries and show that the I-lambda relation depends on the inspiral frequency, but is insensitive to the equation of state. We provide a fit for the universal relation, which is valid up to a gravitational wave frequency of ~900 Hz and accurate to within a few percent. Our results strengthen the universality of I-lambda-Q relations, and are relevant for gravitational-wave observations with advanced ground-based interferometers. We also discuss the possibility of using the Love-compactness relation to measure the neutron-star radius with an uncertainty of about 10% or smaller from gravitational-wave observations.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1304.2052">abs</a>
<a href="http://www.arxiv.org/pdf/1304.2052">pdf</a></p>
[1307.4676] Measurement of Quantum Geometry Using Laser Interferometry
http://www.vallis.org/blogspace/2013/07/25#1307.4676
<p class="story_para">
<b>Authors</b>: Craig Hogan
</p>
<p class="story_para">
<b>Date</b>: 9 Jul 2013
</p>
<p class="story_para">
<b>Abstract</b>: New quantum degrees of freedom of space-time, originating at the Planck scale, could create a coherent indeterminacy and noise in the transverse position of massive bodies on macroscopic scales. An experiment is under development at Fermilab designed to detect or rule out a transverse position noise with Planck spectral density, using correlated signals from an adjacent pair of Michelson interferometers. A detection would open an experimental window on quantum space-time.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1307.4676">abs</a>
<a href="http://www.arxiv.org/pdf/1307.4676">pdf</a></p>
[1307.4418] Gravitational Waveforms for Precessing, Quasicircular Compact Binaries with Multiple Scale Analysis: Small Spin Expansion
http://www.vallis.org/blogspace/2013/07/25#1307.4418
<p class="story_para">
<b>Authors</b>: Katerina Chatziioannou, Antoine Klein, Nicolas Yunes, Neil Cornish
</p>
<p class="story_para">
<b>Date</b>: 16 Jul 2013
</p>
<p class="story_para">
<b>Abstract</b>: We obtain analytical gravitational waveforms in the frequency-domain for precessing, quasi-circular compact binaries with small spins, applicable, for example, to binary neutron star inspirals. We begin by calculating an analytic solution to the precession equations, obtained by expanding in the dimensionless spin parameters and using multiple-scale analysis to separate timescales. We proceed by analytically computing the Fourier transform of time-domain waveform through the stationary phase approximation. We show that the latter is valid for systems with small spins. Finally, we show that these waveforms have a high overlap with numerical waveforms obtained through direct integration of the precession equations and discrete Fourier transformations. The resulting, analytic waveform family is ideal for detection and parameter estimation of gravitational waves emitted by inspiraling binary neutron stars with ground-based detectors.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1307.4418">abs</a>
<a href="http://www.arxiv.org/pdf/1307.4418">pdf</a></p>
[1307.4523] Extended Gravity: State of the Art and Perspectives
http://www.vallis.org/blogspace/2013/07/25#1307.4523
<p class="story_para">
<b>Authors</b>: Salvatore Capozziello, Mariafelicia De Laurentis
</p>
<p class="story_para">
<b>Date</b>: 17 Jul 2013
</p>
<p class="story_para">
<b>Abstract</b>: We shortly review the state of art and perspectives of Extended Theories of Gravity.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1307.4523">abs</a>
<a href="http://www.arxiv.org/pdf/1307.4523">pdf</a></p>
[1307.4812] Astrophysical signatures of boson stars: quasinormal modes and inspiral resonances
http://www.vallis.org/blogspace/2013/07/25#1307.4812
<p class="story_para">
<b>Authors</b>: Caio F. B. Macedo, Paolo Pani, Vitor Cardoso, Lu&#xed;s C. B. Crispino
</p>
<p class="story_para">
<b>Date</b>: 18 Jul 2013
</p>
<p class="story_para">
<b>Abstract</b>: Compact bosonic field configurations, or boson stars, are promising dark matter candidates which have been invoked as an alternative description for the supermassive compact objects in active galactic nuclei. Boson stars can be comparable in size and mass to supermassive black holes and they are hard to distinguish by electromagnetic observations. However, boson stars do not possess an event horizon and their global spacetime structure is different from that of a black hole. This leaves a characteristic imprint in the gravitational-wave emission, which can be used as a discriminant between black holes and other horizonless compact objects. Here we perform a detailed study of boson stars and their gravitational-wave signatures in a fully relativistic setting, a study which was lacking in the existing literature in many respects. We construct several fully relativistic boson star configurations, and we analyze their geodesic structure and free oscillation spectra, or quasinormal modes. We explore the gravitational and scalar response of boson star spacetimes to an inspiralling stellar-mass object and compare it to its black hole counterpart. We find that a generic signature of compact boson stars is the resonant-mode excitation by a small compact object on stable quasi-circular geodesic motion.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1307.4812">abs</a>
<a href="http://www.arxiv.org/pdf/1307.4812">pdf</a></p>
[1307.6232] Inspiral-merger-ringdown waveforms of spinning, precessing black-hole binaries in the effective-one-body formalism
http://www.vallis.org/blogspace/2013/07/25#1307.6232
<p class="story_para">
<b>Authors</b>: Yi Pan, Alessandra Buonanno, Andrea Taracchini, Lawrence E. Kidder, Abdul H. Mroue, Harald P. Pfeiffer, Mark A. Scheel, Bela Szilagyi
</p>
<p class="story_para">
<b>Date</b>: 23 Jul 2013
</p>
<p class="story_para">
<b>Abstract</b>: We describe a general procedure to generate spinning, precessing waveforms that include inspiral, merger and ringdown stages in the effective-one-body (EOB) approach. The procedure uses a precessing frame in which precession-induced amplitude and phase modulations are minimized, and an inertial frame, aligned with the spin of the final black hole, in which we carry out the matching of the inspiral-plunge to merger-ringdown waveforms. As a first application, we build spinning, precessing EOB waveforms for the gravitational modes l=2 such that in the nonprecessing limit those waveforms agree with the EOB waveforms recently calibrated to numerical-relativity waveforms. Without recalibrating the EOB model, we then compare EOB and post-Newtonian precessing waveforms to two numerical-relativity waveforms produced by the Caltech-Cornell-CITA collaboration. The numerical waveforms are strongly precessing and have 35 and 65 gravitational-wave cycles. We find a remarkable agreement between EOB and numerical-relativity precessing waveforms and spins' evolutions. The phase difference is ~ 0.2 rad at merger, while the mismatches, computed using the advanced-LIGO noise spectral density, are below 2% when maximizing only on the time and phase at coalescence and on the polarization angle.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1307.6232">abs</a>
<a href="http://www.arxiv.org/pdf/1307.6232">pdf</a></p>
[1307.6219] Strong Binary Pulsar Constraints on Lorentz Violation in Gravity
http://www.vallis.org/blogspace/2013/07/25#1307.6219
<p class="story_para">
<b>Authors</b>: Kent Yagi, Diego Blas, Nicolas Yunes, Enrico Barausse
</p>
<p class="story_para">
<b>Date</b>: 23 Jul 2013
</p>
<p class="story_para">
<b>Abstract</b>: Binary pulsars are excellent laboratories to test the building blocks of Einstein's theory of General Relativity. One of these is Lorentz symmetry which states that physical phenomena appear the same for all inertially moving observers. We study the effect of violations of Lorentz symmetry in the orbital evolution of binary pulsars and find that it induces a much more rapid decay of the binary's orbital period due to the emission of dipolar radiation. The absence of such behavior in recent observations allows us to place the most stringent constraints on Lorentz violation in gravity, thus verifying one of the cornerstones of Einstein's theory much more accurately than any previous gravitational observation.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1307.6219">abs</a>
<a href="http://www.arxiv.org/pdf/1307.6219">pdf</a></p>
[1307.6237] Where angular momentum goes in a precessing black hole binary
http://www.vallis.org/blogspace/2013/07/25#1307.6237
<p class="story_para">
<b>Authors</b>: Carlos O. Lousto, Yosef Zlochower
</p>
<p class="story_para">
<b>Date</b>: 23 Jul 2013
</p>
<p class="story_para">
<b>Abstract</b>: We evolve a set of 32 equal-mass black-hole binaries with collinear spins (with intrinsic spin magnitudes 0.8 of the maximum allowed) to study the effects of precession in the highly nonlinear plunge and merger regimes. We compare the direction of the instantaneous radiated angular momentum, to the directions of the total angular momentum and the orbital angular momentum$. We find that the radiated angular momentum approximately follows the orbital angular momentum throughout the evolution. During the orbital evolution and merger, we observe that the angle between the orbital angular momentum and total spin is approximately conserved, which allows us to propose and test models for the merger remnant black hole's mass and spin. For instance, we verify that the hangup effect is the dominant effect and largely explains the observed total energy and angular momentum radiated by these precessing systems. We also find that the total angular momentum, while significantly decreasing in magnitude during the simulations, varies from its initial direction by < 5 deg. The maximum variation in the direction of the total angular momentum occurs when the spins are nearly antialigned with the orbital angular momentum. Based on our results, we conjecture that transitional precession, which would lead to large variations in the direction of the total angular momentum, is not possible for similar-mass binaries.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1307.6237">abs</a>
<a href="http://www.arxiv.org/pdf/1307.6237">pdf</a></p>
[1307.6283] Migration of massive black hole binaries in self--gravitating accretion discs: Retrograde versus prograde
http://www.vallis.org/blogspace/2013/07/25#1307.6283
<p class="story_para">
<b>Authors</b>: Constanze Roedig, Alberto Sesana
</p>
<p class="story_para">
<b>Date</b>: 24 Jul 2013
</p>
<p class="story_para">
<b>Abstract</b>: We study the interplay between mass transfer, accretion and gravitational torques onto a black hole binary migrating in a self-gravitating, retrograde circumbinary disc. A direct comparison with an identical prograde disc shows that: (i) because of the absence of resonances, the cavity size is a factor a(1+e) smaller for retrograde discs; (ii) nonetheless the shrinkage of a circular binary semi--major axis, a, is identical in both cases; (iii) a circular binary in a retrograde disc remains circular while eccentric binaries grow more eccentric. For non-circular binaries, we measure the orbital decay rates and the eccentricity growth rates to be exponential as long as the binary orbits in the plane of its disc. Additionally, for these co-planar systems, we find that interaction (~ non--zero torque) stems only from the cavity edge plus a(1+e) in the disc, i.e. for dynamical purposes, the disc can be treated as a annulus of small radial extent. We find that simple 'dust' models in which the binary- disc interaction is purely gravitational can account for all main numerical results, both for prograde and retrograde discs. Furthermore, we discuss the possibility of an instability occurring for highly eccentric binaries causing it to leave the disc plane, secularly tilt and converge to a prograde system. Our results suggest that there are two stable configurations for binaries in self-gravitating discs: the special circular retrograde case and an eccentric (e~ 0.6) prograde configuration as a stable attractor.
</p>
<p class="story_para">
<a href="http://www.arxiv.org/abs/1307.6283">abs</a>
<a href="http://www.arxiv.org/pdf/1307.6283">pdf</a></p>
[1307.6331] A strongly magnetized pulsar within grasp of the Milky Way's supermassive black hole
http://www.vallis.org/blogspace/2013/07/25#1307.6331
<p class="story_para">
<b>Authors</b>: N. Rea, P. Esposito, J. A. Pons, R. Turolla, D. F. Torres, G. L. Israel, A. Possenti, M. Burgay, D. Vigano&#x27;, R. Perna, L. Stella, G. Ponti, F. Baganoff, D. Haggard, A. Papitto, A. Camero-Arranz, S. Zane, A. Minter, S. Mereghetti, A. Tiengo, R. Schoedel, M. Feroci, R. Mignani, D. Gotz
</p>
<p class="story_para">
<b>Date</b>: 24 Jul 2013
</p>
<p class="story_para">
<b>Abstract</b>: The center of our Galaxy hosts a supermassive black hole, Sagittarius (Sgr) A*. Young, massive stars within 0.5 pc of SgrA* are evidence of an episode of intense star formation near the black hole a few Myr ago. Some of them might have left behind a young neutron star traveling deep into SgrA*'s gravitational potential. However, no neutron star closer than ~200 parsec from the supermassive black hole has been detected so far. On 2013 April 25, a short X-ray burst was observed from the direction of the Galactic center. Thanks to the superb spatial resolution of the Chandra X-ray Observatory, we could pinpoint the associated, persistent pulsating X-ray source at an angular distance of 2.4+/-0.3 arcsec from SgrA*. Using a series of Chandra and Swift observations we have refined the spin period and its derivative (P=3.7635443(3) and \dot{P}=7.6(2)x10ˆ{-12} s/s), confirmed by radio observations performed with the Green Bank (GBT) and Parkes single dish antennas, that also measured a Dispersion Measure of DM=1750 +/-50 pc cmˆ{-3}, the highest ever detected for a radio pulsar. We have found that this X-ray source is a young magnetar at only ~0.07-2 pc away from SgrA*, and that with high probability (~90%), it is in a bound orbit around the black hole. The passage of radiation fronts produced by the past activity from the magnetar through the molecular clouds surrounding the Galactic center region might also partially explain the light echoes observed in the Fe fluorescence features.
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<p class="story_para">
<a href="http://www.arxiv.org/abs/1307.6331">abs</a>
<a href="http://www.arxiv.org/pdf/1307.6331">pdf</a></p>
[1301.7074] Gravitational and electromagnetic outputs from binary neutron star mergers
http://www.vallis.org/blogspace/2013/07/25#1301.7074
<p class="story_para">
<b>Authors</b>: Carlos Palenzuela, Luis Lehner, Marcelo Ponce, Steven L. Liebling, Matthew Anderson, David Neilsen, Patrick Motl
</p>
<p class="story_para">
<b>Date</b>: 29 Jan 2013
</p>
<p class="story_para">
<b>Abstract</b>: The late stage of an inspiraling neutron star binary gives rise to strong gravitational wave emission due to its highly dynamic, strong gravity. Moreover, interactions between the stellar magnetospheres can produce considerable electromagnetic radiation. We study this scenario using fully general relativistic, resistive magneto-hydrodynamics simulations. We show that these interactions extract kinetic energy from the system, dissipate heat, and power radiative Poynting flux, as well as develop current sheets. Our results indicate that this power can: (i) outshine pulsars in binaries, (ii) display a distinctive angular- and time-dependent pattern, and (iii) radiate within large opening angles. These properties suggest that some binary neutron star mergers are ideal candidates for multimessenger astronomy.
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<p class="story_para">
<a href="http://www.arxiv.org/abs/1301.7074">abs</a>
<a href="http://www.arxiv.org/pdf/1301.7074">pdf</a></p>