by Anders, P. and Baumgardt, H. and Gaburov, E. and Zwart, S. Portegies
14 pages incl. 3 pages with figures and 4 pages of tables (analysis results), MNRAS in press

Most recent progress in understanding the dynamical evolution of star clusters relies on direct N-body simulations. Owing to the computational demands, and the desire to model more complex and more massive star clusters, hardware calculational accelerators, such as GRAPE special-purpose hardware or, more recently, GPUs (i.e. graphics cards), are generally utilised. In addition, simulations can be accelerated by adjusting parameters determining the calculation accuracy (i.e. changing the internal simulation time step used for each star).

We extend our previous thorough comparison (Anders et al. 2009) of basic quantities as derived from simulations performed either with STARLAB/KIRA or NBODY6. Here we focus on differences arising from using different hardware accelerations (including the increasingly popular graphic card accelerations/GPUs) and different calculation accuracy settings.

We use the large number of star cluster models (for a fixed stellar mass function, without stellar/binary evolution, primordial binaries, external tidal fields etc) already used in the previous paper, evolve them with STARLAB/KIRA (and NBODY6, where required), analyse them in a consistent way and compare the averaged results quantitatively. For this quantitative comparison, we apply the bootstrap algorithm for functional dependencies developed in our previous study.

In general we find very high comparability of the simulation results, independent of the used computer hardware (including the hardware accelerators) and the used N-body code. For the tested accuracy settings we find that for reduced accuracy (i.e. time step at least a factor 2.5 larger than the standard setting) most simulation results deviate significantly from the results using standard settings. The remaining deviations are comprehensible and explicable.

by Farr, Will M. and Ames, Jeff and Hut, Piet and Makino, Junichiro and McMillan, Steve and Muranushi, Takayuki and Nakamura, Koichi and Nitadori, Keigo and Zwart, Simon Portegies
5 pages; submitted to New Astronomy

We present a data format for the output of general N-body simulations, allowing the presence of individual time steps. By specifying a standard, different N-body integrators and different visualization and analysis programs can all share the simulation data, independent of the type of programs used to produce the data. Our Particle Stream Data Format, PSDF, is specified in YAML, based on the same approach as XML but with a simpler syntax. Together with a specification of PSDF, we provide background and motivation, as well as specific examples in a variety of computer languages. We also offer a web site from which these examples can be retrieved, in order to make it easy to augment existing codes in order to give them the option to produce PSDF output.

by Giersz, Mirek and Heggie, Douglas C. and Hurley, Jarrod and Hypki, Arkadiusz
15 pages, 24 figures

We describe a major upgrade of a Monte Carlo code which has previously been used for many studies of dense star clusters. We outline the steps needed in order to calibrate the results of the new Monte Carlo code against N-body simulations for large $latex N$ systems, up to N=200000. The new version of the Monte Carlo code (called MOCCA), in addition to the old version, incorporates direct FewBody integrator for three- and four-body interactions, and new treatment of the escape process based on Fokushige and Heggie (2000). Now stars which fulfil the escape criterion are not removed immediately, but can stay in the system for a certain time which depends on the excess of the energy of a star above the critical energy. They are called potential escapers. FewBody integrator allows to follow all interaction channels, which are important for the rate of creation of various types of objects observed in star clusters, and assures that the energy generation by binaries is treated in a meaner similar to the N-body model.

There are at most three parameters which have to be adjusted against N-body simulations for large N. Two (or one, depends on the chosen approach) connected with the escape process and one responsible for determination of the interaction probabilities. The adopted free parameters are independent on N. They allow MOCCA code to reproduce N-body results, in a reasonably precision, not only for the rate of cluster evolution and the cluster mass distribution, but also for the detailed distributions of mass and binding energy of binaries.

The MOCCA code is at present the most advanced code for simulations of real star clusters. It can follow the cluster evolution in details comparable to N-body code, but orders of magnitude faster.

by Taylor, Paul A. and Miller, John C.
14 pages, 9 figures, submitted to MNRAS

A commonly cited drawback of SPH is the introduction of spurious shear viscosity by the artificial viscosity term in situations involving rotation. Existing approaches for understanding its effect include approximative analytic formulae and disc-averaged behaviour in specific ring-spreading simulations, based on the kinematic contribution of the artificial viscosity. In this work, we have developed a simple, general technique for evaluating the local effect of artificial viscosity directly from the entropic function of each SPH particle. This is simple and quick to implement, and it allows a detailed characterization of its effects as a function of position. Several advantages of this local method are discussed, including its ease in evaluation, its greater accuracy and its broad applicability to arbitary flow geometries and equations of state. Here, we apply this approach to various disc flows, including simulations which implement the commonly-used Balsara switch. Comparisons with existing analytic estimates are made, and examples of quantifying explicit dependencies of the effective viscosity in terms of SPH and flow parameters are given. Additionally, a method for the initial placement of SPH particles which reduces numerical fluctuations is discussed and utilised.

by Nakasato, Naohito
Journal of Computational Science, 2011; See our recent update at http://galaxy.u-aizu.ac.jp/trac/note/wiki/Octree_On_GPU

The kd-tree is a fundamental tool in computer science. Among other applications, the application of kd-tree search (by the tree method) to the fast evaluation of particle interactions and neighbor search is highly important, since the computational complexity of these problems is reduced from O(N^2) for a brute force method to O(N log N) for the tree method, where N is the number of particles. In this paper, we present a parallel implementation of the tree method running on a graphics processing unit (GPU). We present a detailed description of how we have implemented the tree method on a Cypress GPU. An optimization that we found important is localized particle ordering to effectively utilize cache memory. We present a number of test results and performance measurements. Our results show that the execution of the tree traversal in a force calculation on a GPU is practical and efficient.

by de Araujo, J. C. N. and Aguiar, O. D. and Alves, M. E. S. and Tinto, M.
21 pages, 9 eps figures

We analyze the sensitivities of a geostationary gravitational wave interferometer mission operating in the sub-Hertz band. Our proposed Earth-orbiting detector is expected to meet some of the Laser Interferometer Space Antenna (LISA) mission science goals in the lower part of its accessible frequency band ($latex 10^{-4} – 2 \times 10^{-2}$ Hz), and to outperform them by a large margin in the higher-part of it ($latex 2 \times 10^{-2} – 10$ Hz). Since our proposed interferometer will be more sensitive than LISA to supermassive black holes (SMBHs) of masses smaller than $latex \sim 10^{6}$ M$latex _{\odot}$, we will be able to more accurately probe scenarios that account for their formation.

by Conklin, J. W. and Buchman, S. and Aguero, V. and Alfauwaz, A. and Aljadaan, A. and Almajed, M. and Altwaijry, H. and Al-Saud, T. and Balakrishnan, K. and Byer, R. L. and Bower, K. and Costello, B. and Cutler, G. D. and DeBra, D. B. and Faied, D. M. and Foster, C. and Genova, A. L. and Hanson, J. and Hooper, K. and Hultgren, E. and Jaroux, B. and Klavins, A. and Lantz, B. and Lipa, J. A. and Palmer, A. and Plante, B. and Sanchez, H. S. and Saraf, S. and Schaechter, D. and Sherrill, T. and Smith, E. and Shu, K. -L. and Tenerelli, D. and Vanbezooijen, R. and Vasudevan, G. and Williams, S. D. and Worden, S. P. and Zhou, J. and Zoellner, A.
Comments: 24 pages, to be submitted to Classical and Quantum Gravity

We describe a new space gravitational wave observatory design called LAGRANGE that maintains all important LISA science at about half the cost and with reduced technical risk. It consists of three drag-free spacecraft in the most stable geocentric formation, the Earth-Moon L3, L4, and L5 Lagrange points. Fixed antennas allow continuous contact with the Earth, solving the problem of communications bandwidth and latency. A 70 mm diameter AuPt sphere with a 35 mm gap to its enclosure serves as a single inertial reference per spacecraft, which is operated in “true” drag-free mode (no test mass forcing). This is the core of the Modular Gravitational Reference Sensor whose other advantages are: a simple caging design based on the DISCOS 1972 drag-free mission, an all optical read-out with pm fine and nm coarse sensors, and the extensive technology heritage from the Honeywell gyroscopes, and the DISCOS and Gravity Probe B drag-free sensors. An Interferometric Measurement System, designed with reflective optics and a highly stabilized frequency standard, performs the inter-test mass ranging and requires a single optical bench with one laser per spacecraft. Two 20 cm diameter telescopes per spacecraft, each with in-field pointing, incorporate novel technology developed for advanced optical systems by Lockheed Martin, who also designed the spacecraft based on a multi-flight proven bus structure. Additional technological advancements include the drag-free propulsion, thermal control, charge management systems, and materials. LAGRANGE sub-systems are designed to be scalable and modular, making them interchangeable with those of LISA or other gravitational science missions. We plan to space qualify critical technologies on small and nano satellite flights, with the first launch (UV-LED Sat) in 2013.

by Garcia-Senz, Domingo and Cabezon, Ruben M. and Escartin, Jose Antonio
15 pages, 12 figures, accepted for publication in Astronomy & Astrophysics

In this paper we develop and check a fully conservative SPH scheme based on a tensor formulation which can be applied to simulate astrophysical systems. In the proposed scheme derivatives are calculated from an integral expression which leads to a tensor, rather than vectorial, estimation of gradients and reduces to the standard formulation in the continuum limit. The new formulation improves the interpolation of physical magnitudes, leading to a set of conservative equations which looks similar to those of standard SPH. The resulting scheme was checked using a variety of well known tests, all of them simulated in two dimensions. An application of the proposed tensor method to astrophysics was also discussed by simulating the stability of a sun-like polytrope calculated in three dimensions.

by Laibe, Guillaume and Price, Daniel J.
Accepted for publication in MNRAS

In a companion paper (Laibe & Price 2011b), we have presented an algorithm for simulating two-fluid gas and dust mixtures in Smoothed Particle Hydrodynamics (SPH). In this paper, we develop an implicit timestepping method that preserves the exact conservation of the both linear and angular momentum in the underlying SPH algorithm, but unlike previous schemes, allows the iterations to converge to arbitrary accuracy and is suited to the treatment of non- linear drag regimes. The algorithm presented in Paper I is also extended to deal with realistic astrophysical drag regimes, including both linear and non-linear Epstein and Stokes drag. The scheme is benchmarked against the test suite presented in Paper I, including i) the analytic solutions of the dustybox problem and ii) solutions of the dustywave, dustyshock, dustysedov and dustydisc obtained with explicit timestepping. We find that the implicit method is 1- 10 times faster than the explicit temporal integration when the ratio r between the the timestep and the drag stopping time is 1 < r < 1000.

by Laibe, Guillaume and Price, Daniel J.
Accepted for publication in MNRAS

We present a new algorithm for simulating two-fluid gas and dust mixtures in Smoothed Particle Hydrodynamics (SPH), systematically addressing a number of key issues including the generalised SPH density estimate in multi-fluid systems, the consistent treatment of variable smoothing length terms, finite particle size, time step stability, thermal coupling terms and the choice of kernel and smoothing length used in the drag operator. We find that using double-hump shaped kernels improves the accuracy of the drag interpolation by a factor of several hundred compared to the use of standard SPH bell-shaped kernels, at no additional computational expense. In order to benchmark our algorithm, we have developed a comprehensive suite of standardised, simple test problems for gas and dust mixtures: dustybox, dustywave, dustyshock, dustysedov and dustydisc, the first three of which have known analytic solutions. We present the validation of our algorithm against all of these tests. In doing so, we show that the spatial resolution criterion \Delta < cs ts is a necessary condition in all gas+dust codes that becomes critical at high drag (i.e. small stopping time ts) in order to correctly predict the dynamics. Implicit timestepping and the implementation of realistic astrophysical drag regimes are addressed in a companion paper.

by Rein, Hanno and Liu, Shang-Fei
10 pages, 9 figures, re-submitted to A&A, source code available at https://github.com/hannorein/rebound

REBOUND is a new multi-purpose N-body code which is freely available under an open-source license. It was designed for collisional dynamics such as planetary rings but can also solve the classical N-body problem. It is highly modular and can be customized easily to work on a wide variety of different problems in astrophysics and beyond.

REBOUND comes with three symplectic integrators: leap-frog, the symplectic epicycle integrator (SEI) and a Wisdom-Holman mapping (WH). It supports open, periodic and shearing-sheet boundary conditions. REBOUND can use a Barnes-Hut tree to calculate both self-gravity and collisions. These modules are fully parallelized with MPI as well as OpenMP. The former makes use of a static domain decomposition and a distributed essential tree. Two new collision detection modules based on a plane-sweep algorithm are also implemented. The performance of the plane-sweep algorithm is superior to a tree code for simulations in which one dimension is much longer than the other two and in simulations which are quasi-two dimensional with less than one million particles.

In this work, we discuss the different algorithms implemented in REBOUND, the philosophy behind the code’s structure as well as implementation specific details of the different modules. We present results of accuracy and scaling tests which show that the code can run efficiently on both desktop machines and large computing clusters.

by Congedo, G. and Ferraioli, L. and Hueller, M. and De Marchi, F. and Vitale, S. and Armano, M. and Hewitson, M. and Nofrarias, M.
16 pages (two columns), 15 figures, 5 tables, submitted to Phys. Rev. D

LISA is the upcoming space-based Gravitational Wave telescope. LISA Pathfinder, to be launched in the coming years, will prove and verify the detection principle of the fundamental Doppler link of LISA on a flight hardware identical in design to that of LISA. LISA Pathfinder will collect a picture of all noise disturbances possibly affecting LISA, achieving the unprecedented pureness of geodesic motion necessary for the detection of gravitational waves. The first steps of both missions will crucially depend on a very precise calibration of the key system parameters. Moreover, robust parameters estimation is of fundamental importance in the correct assessment of the residual force noise, an essential part of the data processing for LISA. In this paper we present a maximum likelihood parameter estimation technique in time domain being devised for this calibration and show its proficiency on simulated data and validation through Monte Carlo realizations of independent noise runs. We discuss its robustness to non-standard scenarios possibly arising during the real-life mission, as well as its independence to the initial guess and non-gaussianities. Furthermore, we apply the same technique to data produced in mission-like fashion during operational exercises with a realistic simulator provided by ESA.

by Bender, Peter L.
Submitted to Proc. 46th Rencontres de Moriond: Gravitational Waves and Experimental Gravity, March 20 – 27, 2011, La Thuile, Italy

One of the atom interferometer gravitational wave missions proposed by Dimopoulos et al.1 in 2008 was called AGIS-Sat. 2. It had a suggested gravitational wave sensitivity set by the atom state detection shot noise level that started at 1 mHz, was comparable to LISA sensitivity from 1 to about 20 mHz, and had better sensitivity from 20 to 500 mHz. The separation between the spacecraft was 1,000 km, with atom interferometers 200 m long and shades from sunlight used at each end. A careful analysis of many error sources was included, but requirements on the time-stability of both the laser wavefront aberrations and the atom temperatures in the atom clouds were not investigated. After including these considerations, the laser wavefront aberration stability requirement to meet the quoted sensitivity level is about 1\times10-8 wavelengths, and is far tighter than for LISA. Also, the temperature fluctuations between atom clouds have to be less than 1 pK. An alternate atom interferometer GW mission in Earth orbit called AGIS-LEO with 30 km satellite separation has been suggested recently. The reduction of wavefront aberration noise by sending the laser beam through a high-finesse mode-scrubbing optical cavity is discussed briefly, but the requirements on such a cavity are not given. Unfortunately, such an Earth-orbiting mission seems to be considerably more difficult to design than a non-geocentric mission and does not appear to have comparably attractive scientific goals.

by Bédorf, Jeroen and Gaburov, Evghenii and Zwart, Simon Portegies
Submitted to Journal of Computational Physics. 34 pages, 13 figures, single column

We present parallel algorithms for constructing and traversing sparse octrees on graphics processing units (GPUs). The algorithms are based on parallel-scan and sort methods. To test the performance and feasibility, we implemented them in CUDA in the form of a gravitational tree-code which completely runs on the GPU.(The code is publicly available at: http://castle.strw.leidenuniv.nl/software.html) The tree construction and traverse algorithms are portable to many-core devices which have support for CUDA or OpenCL programming languages. The gravitational tree-code outperforms tuned CPU code during the tree-construction and shows a performance improvement of more than a factor 20 overall, resulting in a processing rate of more than 2.8 million particles per second.

by Canizares, Priscilla and Sopuerta, Carlos F.
4 pages, 2 figures, JPCS latex style. Submitted to JPCS (special issue for the proceedings of the Spanish Relativity Meeting (ERE2010))

When a stellar-mass compact object is captured by a supermassive black hole located in a galactic centre, the system losses energy and angular momentum by the emission of gravitational waves. Subsequently, the stellar compact object evolves inspiraling until plunging onto the massive black hole. These EMRI systems are expected to be one of the main sources of gravitational waves for the future space-based Laser Interferometer Space Antenna (LISA). However, the detection of EMRI signals will require of very accurate theoretical templates taking into account the gravitational self-force, which is the responsible of the stellar-compact object inspiral. Due to its potential applicability on EMRIs, the obtention of an efficient method to compute the scalar self-force acting on a point-like particle orbiting around a massive black hole is being object of increasing interest. We present here a review of our time-domain numerical technique to compute the self-force acting on a point-like particle and we show its suitability to deal with both circular and eccentric orbits.

by Thorpe, James Ira and Maghami, Peiman and Livas, Jeffrey
Submitted to Phys. Rev. D

Arm locking is a technique that has been proposed for reducing laser frequency fluctuations in the Laser Interferometer Space Antenna (LISA), a gravitational-wave observatory sensitive in the milliHertz frequency band. Arm locking takes advantage of the geometric stability of the triangular constellation of three spacecraft that comprise LISA to provide a frequency reference with a stability in the LISA measurement band that exceeds that available from a standard reference such as an optical cavity or molecular absorption line. We have implemented a time-domain simulation of arm locking including the expected limiting noise sources (shot noise, clock noise, spacecraft jitter noise, and residual laser frequency noise). The effect of imperfect a priori knowledge of the LISA heterodyne frequencies and the associated ‘pulling’ of an arm locked laser is included. We find that our implementation meets requirements both on the noise and dynamic range of the laser frequency.

by Pitkin, Matthew and Reid, Stuart and Rowan, Sheila and Hough, Jim
Submitted as a major update to Living Rev. Relativity 3, (2000), 3

Significant progress has been made in recent years on the development of gravitational wave detectors. Sources such as coalescing compact binary systems, neutron stars in low-mass X-ray binaries, stellar collapses and pulsars are all possible candidates for detection. The most promising design of gravitational wave detector uses test masses a long distance apart and freely suspended as pendulums on Earth or in drag-free craft in space. The main theme of this review is a discussion of the mechanical and optical principles used in the various long baseline systems in operation around the world – LIGO (USA), Virgo (Italy/France), TAMA300 and LCGT (Japan), and GEO600 (Germany/U.K.) – and in LISA, a proposed space-borne interferometer. A review of recent science runs from the current generation of ground-based detectors will be discussed, in addition to highlighting the astrophysical results gained thus far. Looking to the future, the major upgrades to LIGO (Advanced LIGO), Virgo (Advanced Virgo), LCGT and GEO600 (GEO-HF) will be completed over the coming years, which will create a network of detectors with significantly improved sensitivity required to detect gravitational waves. Beyond this, the concept and design of possible future “third generation” gravitational wave detectors, such as the Einstein Telescope (ET), will be discussed.

by Diaz-Aguilo, Marc and Lobo, Alberto and García-Berro, Enrique

LISA Pathfinder is a science and technology demonstrator of the European Space Agency within the framework of its LISA mission, which aims to be the first space-borne gravitational wave observatory. The payload of LISA Pathfinder is the so-called LISA Technology Package, which is designed to measure relative accelerations between two test masses in nominal free fall. Its disturbances are monitored and dealt by the diagnostics subsystem. This subsystem consists of several modules, and one of these is the magnetic diagnostics system, which includes a set of four tri-axial fluxgate magnetometers, intended to measure with high precision the magnetic field at the positions of the test masses. However, since the magnetometers are located far from the positions of the test masses, the magnetic field at their positions must be interpolated. It has been recently shown that because there are not enough magnetic channels, classical interpolation methods fail to derive reliable measurements at the positions of the test masses, while neural network interpolation can provide the required measurements at the desired accuracy. In this paper we expand these studies and we assess the reliability and robustness of the neural network interpolation scheme for variations of the locations and possible offsets of the magnetometers, as well as for changes in environmental conditions. We find that neural networks are robust enough to derive accurate measurements of the magnetic field at the positions of the test masses in most circumstances.

by Antonucci, F and Armano, M and Audley, H and Auger, G and Benedetti, M and Binetruy, P and Boatella, C and Bogenstahl, J and Bortoluzzi, D and Bosetti, P and Brandt, N and Caleno, M and Cavalleri, A and Cesa, M and Chmeissani, M and Ciani, G and Conchillo, A and Congedo, G and Cristofolini, I and Cruise, M and Danzmann, K and De Marchi, F and Diaz-Aguilo, M and Diepholz, I and Dixon, G and Dolesi, R and Dunbar, N and Fauste, J and Ferraioli, L and Fertin, D and Fichter, W and Fitzsimons, E and Freschi, M and Marin, A García and Marirrodriga, C García and Gerndt, R and Gesa, L and Giardini, D and Gibert, F and Grimani, C and Grynagier, A and Guillaume, B and Guzmán, F and Harrison, I and Heinzel, G and Hewitson, M and Hollington, D and Hough, J and Hoyland, D and Hueller, M and Huesler, J and Jeannin, O and Jennrich, O and Jetzer, P and Johlander, B and Killow, C and Llamas, X and Lloro, I and Lobo, A and Maarschalkerweerd, R and Madden, S and Mance, D and Mateos, I and McNamara, P W and Mendestì, J and Mitchell, E and Monsky, A and Nicolini, D and Nicolodi, D and Nofrarias, M and Pedersen, F and Perreur-Lloyd, M and Perreca, A and Plagnol, E and Prat, P and Racca, G D and Rais, B and Ramos-Castro, J and Reiche, J and Perez, J A Romera and Robertson, D and Rozemeijer, H and Sanjuan, J and Schleicher, A and Schulte, M and Shaul, D and Stagnaro, L and Strandmoe, S and Steier, F and Sumner, T J and Taylor, A and Texier, D and Trenkel, C and Tombolato, D and Vitale, S and Wanner, G and Ward, H and Waschke, S and Wass, P and Weber, W J and Zweifel, P
Proceedings of the 8th LISA Symposium. Submitted to Classical and Quantum Gravity

This paper presents a quantitative assessment of the performance of the upcoming LISA Pathfinder geodesic explorer mission. The findings are based on the results of extensive ground testing and simulation campaigns using flight hardware and flight control and operations algorithms. The results show that, for the central experiment of measuring the stray differential acceleration between the LISA test masses, LISA Pathfinder will be able to verify the overall acceleration noise to within a factor two of the LISA requirement at 1 mHz and within a factor 10 at 0.1 mHz. We also discuss the key elements of the physical model of disturbances, coming from LISA Pathfinder and ground measurement, that will guarantee the LISA performance.

by Gürkan, M. Atakan
5 pages, 5 figures. Accepted for publication in MNRAS Letters

In this paper, we propose a method to study the nature of resonant relaxation in near-Keplerian systems. Our technique is based on measuring the fractal dimension of the angular momentum trails and we use it to analyze the outcome of N-body simulations. With our method, we can reliably determine the timescale for resonant relaxation, as well as the rate of change of angular momentum in this regime. We find that growth of angular momentum is more rapid than random walk, but slower than linear growth. We also determine the presence of long term correlations, arising from the bounds on angular momentum growth. We develop a toy model that reproduces all essential properties of angular momentum evolution.

by Canizares, Priscilla and Conchillo, Aleix and Diaz–Aguilo, Marc and Garcia-Berro, Enrique and Gesa, Lluis and Gibert, Ferran and Grimani, Catia and Lloro, Ivan and Lobo, Alberto and Mateos, Ignacio and Nofrarias, Miquel and Ramos-Castro, Juan and Sanjuan, Josep and Sopuerta, Carlos F
11 pages, 7 figures, prepared for the Proceedings of the 8th International LISA Symposium, Classical and Quantum Gravity

The LISA PathFinder DMU (Data Management Unit) flight model was formally accepted by ESA and ASD on 11 February 2010, after all hardware and software tests had been successfully completed. The diagnostics items are scheduled to be delivered by the end of 2010. In this paper we review the requirements and performance of this instrumentation, specially focusing on the Radiation Monitor and the DMU, as well as the status of their programmed use during mission operations, on which work is ongoing at the time of writing.

by Konstantinidis, Simos and Kokkotas, Kostas D.
24 pages, 28 figures

We present a new C++ code for collisional N-body simulations of star clusters. The code uses the Hermite fourth-order scheme with block time steps, for advancing the particles in time, while the forces and neighboring particles are computed using the GRAPE-6 board. Special treatment is used for close encounters, binary and multiple sub-systems that either form dynamically or exist in the initial configuration. The structure of the code is modular and allows the appropriate treatment of more physical phenomena, such as stellar and binary evolution, stellar collisions and evolution of close black-hole binaries. Moreover, it can be easily modified so that the part of the code that uses GRAPE-6, could be replaced by another module that uses other accelerating-hardware like the Graphics Processing Units (GPUs). Appropriate choice of the free parameters give a good accuracy and speed for simulations of star clusters up to and beyond core collapse. Simulations of Plummer models consisting of equal-mass stars reached core collapse at t~17 half-mass relaxation times, which compares very well with existing results, while the cumulative relative error in the energy remained below 0.001. Also, comparisons with published results of other codes for the time of core collapse for different initial conditions, show excellent agreement. Simulations of King models with an initial mass-function, similar to those found in the literature, reached core collapse at t~0.17, which is slightly smaller than the expected result from previous works. Finally, the code accuracy becomes comparable and even better than the accuracy of existing codes, when a number of close binary systems is dynamically created in a simulation. This is due to the high accuracy of the method that is used for close binary and multiple sub-systems.

by de Vine, Glenn and Ware, Brent and McKenzie, Kirk and Spero, Robert E. and Klipstein, William M. and Shaddock, Daniel A.
4 pages, 4 figures, to appear in Physical Review Letters end of May 2010

We report on the first demonstration of time-delay interferometry (TDI) for LISA, the Laser Interferometer Space Antenna. TDI was implemented in a laboratory experiment designed to mimic the noise couplings that will occur in LISA. TDI suppressed laser frequency noise by approximately 10^9 and clock phase noise by 6×10^4, recovering the intrinsic displacement noise floor of our laboratory test bed. This removal of laser frequency noise and clock phase noise in post-processing marks the first experimental validation of the LISA measurement scheme.

by Finn, Lee Samuel and Lommen, Andrea N.
43 pages, 13 figures, submitted to ApJ.

Efforts to detect gravitational waves by timing an array of pulsars have focused traditionally on stationary gravitational waves: e.g., stochastic or periodic signals. Gravitational wave bursts — signals whose duration is much shorter than the observation period — will also arise in the pulsar timing array waveband. Sources that give rise to detectable bursts include the formation or coalescence of supermassive black holes (SMBHs), the periapsis passage of compact objects in highly elliptic or unbound orbits about a SMBH, or cusps on cosmic strings. Here we describe how pulsar timing array data may be analyzed to detect and characterize these bursts. Our analysis addresses, in a mutually consistent manner, a hierarchy of three questions: \emph{i}) What are the odds that a dataset includes the signal from a gravitational wave burst? \emph{ii}) Assuming the presence of a burst, what is the direction to its source? and \emph{iii}) Assuming the burst propagation direction, what is the burst waveform’s time dependence in each of its polarization states? Applying our analysis to synthetic data sets we find that we can \emph{detect} gravitational waves even when the radiation is too weak to either localize the source of infer the waveform, and \emph{detect} and \emph{localize} sources even when the radiation amplitude is too weak to permit the waveform to be determined. While the context of our discussion is gravitational wave detection via pulsar timing arrays, the analysis itself is directly applicable to gravitational wave detection using either ground or space-based detector data.

by Neumayer, Nadine
8 pages, accepted for publication in PASA, contribution for “The Many Faces of Centaurus A” conference in Sydney, 2009

At less than 4 Mpc distance the radio galaxy NGC 5128 (Centaurus A) is the prime example to study the supermassive black hole and its influence on the environment in great detail. To model and understand the feeding and feedback mechanisms one needs an accurate determination of the mass of the supermassive black hole. The aim of this review is to give an overview of the recent studies that have been dedicated to measure the black hole mass in Centaurus A from both gas and stellar kinematics. It shows how the advancement in observing techniques and instrumentation drive the field of black hole mass measurements and concludes that adaptive optics assisted integral field spectroscopy is the key to identify the effects of the AGN on the surrounding ionised gas. Using data from SINFONI at the ESO Very Large Telescope, the best-fit black hole mass is M_BH=4.5 +1.7/-1.0 x 10^7 Msolar (from H_2 kinematics) and M_BH= (5.5 +/- 3.0) x 10^7 Msolar (from stellar kinematics; both with 3 sigma errors). This is one of the cleanest gas vs star comparison of a M_BH determination, and brings Centaurus A into agreement with the M_BH-sigma relation.

by Capuzzo-Dolcetta, R.
Invited talk to the 2009 general Conference of the Societa’ Astronomica Italiana. 6 pages including 1 figure

In this paper I will outline some of the aspects and problems of modern celestial mechanics and stellar dynamics, in the context of the quickly growing computing facilities. I will point the attention on the great advantages in using, for astrophysical simulations, the modern, fast and cheap Graphic Processing Units (GPUs) acting as true supercomputers. Finally, I present and discuss some characteristics and performances of a new double-parallel code exploiting the joint power of multicore CPUs and GPUs.

by Nakasato, N.
Poster paper to be appeared in SC09

The kd-tree is a fundamental tool in computer science. Among others, an application of the kd-tree search (oct-tree method) to fast evaluation of particle interactions and neighbor search is highly important since computational complexity of these problems are reduced from O(N^2) with a brute force method to O(N log N) with the tree method where N is a number of particles. In this paper, we present a parallel implementation of the tree method running on a graphic processor unit (GPU). We successfully run a simulation of structure formation in the universe very efficiently. On our system, which costs roughly $900, the run with N ~ 2.87×10^6 particles took 5.79 hours and executed 1.2×10^13 force evaluations in total. We obtained the sustained computing speed of 21.8 Gflops and the cost per Gflops of 41.6/Gflops that is two and half times better than the previous record in 2006.

by Stroeer, Alexander and Veitch, John
18 pages, 8 figures, 3 tables, submitted to PRD

The Laser Interferometer Space Antenna (LISA) defines new demands on data analysis efforts in its all-sky gravitational wave survey, recording simultaneously thousands of galactic compact object binary foreground sources and tens to hundreds of background sources like binary black hole mergers and extreme mass ratio inspirals. We approach this problem with an adaptive and fully automatic Reversible Jump Markov Chain Monte Carlo sampler, able to sample from the joint posterior density function (as established by Bayes theorem) for a given mixture of signals “out of the box”, handling the total number of signals as an additional unknown parameter beside the unknown parameters of each individual source and the noise floor. We show in examples from the LISA Mock Data Challenge implementing the full response of LISA in its TDI description that this sampler is able to extract monochromatic Double White Dwarf signals out of colored instrumental noise and additional foreground and background noise successfully in a global fitting approach. We introduce 2 examples with fixed number of signals (MCMC sampling), and 1 example with unknown number of signals (RJ-MCMC), the latter further promoting the idea behind an experimental adaptation of the model indicator proposal densities in the main sampling stage. We note that the experienced runtimes and degeneracies in parameter extraction limit the shown examples to the extraction of a low but realistic number of signals.