**Authors**: Adam Pound

**Date**: 24 Jan 2012

**Abstract**: Using a rigorous method of matched asymptotic expansions, I derive the equation of motion of a small, compact body in an external vacuum spacetime through second order in the body's mass (neglecting effects of internal structure). The motion is found to be geodesic in a certain locally defined regular geometry satisfying Einstein's equation at second order. I provide a practical scheme for numerically obtaining both the metric of that regular geometry and the complete second-order metric perturbation produced by the body.

1201.5089
(/preprints)

2012-01-25, 10:34
**[edit]**

**Authors**: Evghenii Gaburov, Anders Johansen, Yuri Levin

**Date**: 23 Jan 2012

**Abstract**: In this paper we report on the formation of magnetically-levitating accretion disks around supermassive black holes. The structure of these disks is calculated by numerically modelling tidal disruption of magnetized interstellar gas clouds. We find that the resulting disks are entirely supported by the pressure of the magnetic fields against the component of gravitational force directed perpendicular to the disks. The magnetic field shows ordered large-scale geometry that remains stable for the duration of our numerical experiments extending over 10% of the disk lifetime. Strong magnetic pressure allows high accretion and inhibits disk fragmentation. This in combination with the repeated feeding of manetized molecular clouds to a supermassive black hole yields a possible solution to the long-standing puzzle of black hole growth in the centres of galaxies.

1201.4873
(/preprints)

2012-01-25, 10:34
**[edit]**

**Authors**: A. Hees, B. Lamine, S. Reynaud, M.-T. Jaekel, C. Le Poncin-Lafitte, V. Lainey, A. Füzfa, J.-M. Courty, V. Dehant, P. Wolf

**Date**: 24 Jan 2012

**Abstract**: In this paper, we focus on the possibility to test General Relativity in the Solar System with radioscience measurements. To this aim, we present a new software that simulates Range and Doppler signals directly from the space-time metric. This flexible approach allows one to perform simulations in General Relativity and in alternative metric theories of gravity. In a second step, a least-squares fit of the different initial conditions involved in the situation is performed in order to compare anomalous signals produced by a given alternative theory with the ones obtained in General Relativity. This software provides orders of magnitude and signatures stemming from hypothetical alternative theories of gravity on radioscience signals. As an application, we present some simulations done for the Cassini mission in Post-Einsteinian Gravity and in the context of MOND External Field Effect. We deduce constraints on the Post-Einsteinian parameters but find that the considered arc of the Cassini mission is not useful to constrain the MOND External Field Effect.

1201.5041
(/preprints)

2012-01-25, 10:33
**[edit]**

**Authors**: Miroslav Shaltev

**Date**: 23 Jan 2012

**Abstract**: We derive two different methods to compute the minimal required integration time of a fully coherent follow-up of candidates produced in wide parameter space semi-coherent searches, such as global correlation StackSlide searches using Einstein@Home. We numerically compare these methods in terms of integration duration and computing cost. In a Monte Carlo study we confirm that we can achieve the required detection probability.

1201.4656
(/preprints)

2012-01-24, 17:13
**[edit]**

**Authors**: The LIGO Scientific Collaboration: J. Abadie, B. P. Abbott, T. D. Abbott, R. Abbott, M. Abernathy, C. Adams, R. Adhikari, C. Affeldt, P. Ajith, B. Allen, G. S. Allen, E. Amador Ceron, D. Amariutei, R. S. Amin, S. B. Anderson, W. G. Anderson, K. Arai, M. A. Arain, M. C. Araya, S. M. Aston, D. Atkinson, P. Aufmuth, C. Aulbert, B. E. Aylott, S. Babak, P. Baker, S. Ballmer, D. Barker, S. Barnum, B. Barr, P. Barriga, L. Barsotti, M. A. Barton, I. Bartos, R. Bassiri, M. Bastarrika, J. Bauchrowitz, B. Behnke, A. S. Bell, I. Belopolski, M. Benacquista, A. Bertolini, J. Betzwieser, N. Beveridge, P. T. Beyersdorf, I. A. Bilenko, G. Billingsley, J. Birch, R. Biswas, E. Black, J. K. Blackburn, L. Blackburn, D. Blair, B. Bland, O. Bock, T. P. Bodiya, C. Bogan, R. Bondarescu, R. Bork, M. Born, S. Bose, M. Boyle, P. R. Brady, V. B. Braginsky, J. E. Brau, J. Breyer, D. O. Bridges, M. Brinkmann, M. Britzger, A. F. Brooks, D. A. Brown, A. Brummitt, A. Buonanno, J. Burguet-Castell, O. Burmeister, R. L. Byer, L. Cadonati, J. B. Camp, P. Campsie, J. Cannizzo, K. Cannon, J. Cao, C. Capano, S. Caride, S. Caudill, M. Cavaglia, C. Cepeda, T. Chalermsongsak, E. Chalkley, P. Charlton, S. Chelkowski, Y. Chen, N. Christensen, S. S. Y. Chua, S. Chung, C. T. Y. Chung, F. Clara, D. Clark, J. Clark, J. H. Clayton, R. Conte, D. Cook, T. R. C. Corbitt, N. Cornish, C. A. Costa, M. Coughlin, D. M. Coward, D. C. Coyne, J. D. E. Creighton, T. D. Creighton, A. M. Cruise, A. Cumming, L. Cunningham, R. M. Culter, K. Dahl, S. L. Danilishin, R. Dannenberg, K. Danzmann, K. Das, B. Daudert, H. Daveloza, G. Davies, E. J. Daw, T. Dayanga, D. DeBra, J. Degallaix, T. Dent, V. Dergachev, R. DeRosa, R. DeSalvo, S. Dhurandhar, I. Di Palma, M. Diaz, F. Donovan, K. L. Dooley, S. Dorsher, E. S. D. Douglas, R. W. P. Drever, J. C. Driggers, J. -C. Dumas, S. Dwyer, T. Eberle, M. Edgar, M. Edwards, A. Effler, P. Ehrens, R. Engel, T. Etzel, M. Evans, T. Evans, M. Factourovich, S. Fairhurst, Y. Fan, B. F. Farr, D. Fazi, H. Fehrmann, D. Feldbaum, L. S. Finn, M. Flanigan, S. Foley, E. Forsi, N. Fotopoulos, M. Frede, M. Frei, Z. Frei, A. Freise, R. Frey, T. T. Fricke, D. Friedrich, P. Fritschel, V. V. Frolov, P. Fulda, M. Fyffe, J. Garcia, J. A. Garofoli, I. Gholami, S. Ghosh, J. A. Giaime, S. Giampanis, K. D. Giardina, C. Gill, E. Goetz, L. M. Goggin, G. Gonzalez, M. L. Gorodetsky, S. Gossler, C. Graef, A. Grant, S. Gras, C. Gray, R. J. S. Greenhalgh, A. M. Gretarsson, R. Grosso, H. Grote, S. Grunewald, C. Guido, R. Gupta, E. K. Gustafson, R. Gustafson, B. Hage, J. M. Hallam, D. Hammer, G. Hammond, J. Hanks, C. Hanna, J. Hanson, J. Harms, G. M. Harry, I. W. Harry, E. D. Harstad, M. T. Hartman, K. Haughian, K. Hayama, J. Heefner, M. A. Hendry, I. S. Heng, A. W. Heptonstall, V. Herrera, M. Hewitson, S. Hild, D. Hoak, K. A. Hodge, K. Holt, T. Hong, S. Hooper, D. J. Hosken, J. Hough, E. J. Howell, B. Hughey, S. Husa, S. H. Huttner, D. R. Ingram, R. Inta, T. Isogai, A. Ivanov, W. W. Johnson, D. I. Jones, G. Jones, R. Jones, L. Ju, P. Kalmus, V. Kalogera, S. Kandhasamy, J. B. Kanner, E. Katsavounidis, W. Katzman, K. Kawabe, S. Kawamura, F. Kawazoe, W. Kells, M. Kelner, D. G. Keppel, A. Khalaidovski, F. Y. Khalili, E. A. Khazanov, N. Kim, H. Kim, P. J. King, D. L. Kinzel, J. S. Kissel, S. Klimenko, V. Kondrashov, R. Kopparapu, S. Koranda, W. Z. Korth, D. Kozak, V. Kringel, S. Krishnamurthy, B. Krishnan, G. Kuehn, R. Kumar, P. Kwee, M. Landry, B. Lantz, N. Lastzka, A. Lazzarini, P. Leaci, J. Leong, I. Leonor, J. Li, P. E. Lindquist, N. A. Lockerbie, D. Lodhia, M. Lormand, P. Lu, J. Luan, M. Lubinski, H. Luck, A. P. Lundgren, E. Macdonald, B. Machenschalk, M. MacInnis, M. Mageswaran, K. Mailand, I. Mandel, V. Mandic, A. Marandi, S. Marka, Z. Marka, E. Maros, I. W. Martin, R. M. Martin, J. N. Marx, K. Mason, F. Matichard, L. Matone, R. A. Matzner, N. Mavalvala, R. McCarthy, D. E. McClelland, S. C. McGuire, G. McIntyre, J. McIver, D. J. A. McKechan, G. Meadors, M. Mehmet, T. Meier, A. Melatos, A. C. Melissinos, G. Mendell, R. A. Mercer, S. Meshkov, C. Messenger, M. S. Meyer, H. Miao, J. Miller, Y. Mino, V. P. Mitrofanov, G. Mitselmakher, R. Mittleman, O. Miyakawa, B. Moe, P. Moesta, S. D. Mohanty, D. Moraru, G. Moreno, K. Mossavi, C. M. Mow-Lowry, G. Mueller, S. Mukherjee, A. Mullavey, H. Muller-Ebhardt, J. Munch, D. Murphy, P. G. Murray, T. Nash, R. Nawrodt, J. Nelson, G. Newton, A. Nishizawa, D. Nolting, L. Nuttall, B. O'Reilly, R. O'Shaughnessy, E. Ochsner, J. O'Dell, G. H. Ogin, R. G. Oldenburg, C. Osthelder, C. D. Ott, D. J. Ottaway, R. S. Ottens, H. Overmier, B. J. Owen, A. Page, Y. Pan, C. Pankow, M. A. Papa, P. Patel, M. Pedraza, L. Pekowsky, S. Penn, C. Peralta, A. Perreca, M. Phelps, M. Pickenpack, I. M. Pinto, M. Pitkin, H. J. Pletsch, M. V. Plissi, J. Podkaminer, J. Pold, F. Postiglione, V. Predoi, L. R. Price, M. Prijatelj, M. Principe, S. Privitera, R. Prix, L. Prokhorov, O. Puncken, V. Quetschke, F. J. Raab, H. Radkins, P. Raffai, M. Rakhmanov, C. R. Ramet, B. Rankins, S. R. P. Mohapatra, V. Raymond, K. Redwine, C. M. Reed, T. Reed, S. Reid, D. H. Reitze, R. Riesen, K. Riles, P. Roberts, N. A. Robertson, C. Robinson, E. L. Robinson, S. Roddy, J. Rollins, J. D. Romano, J. H. Romie, C. Rover, S. Rowan, A. Rudiger, K. Ryan, S. Sakata, M. Sakosky, F. Salemi, M. Salit, L. Sammut, L. Sancho de la Jordana, V. Sandberg, V. Sannibale, L. SantamarÌa, I. Santiago-Prieto, G. Santostasi, S. Saraf, B. S. Sathyaprakash, S. Sato, P. R. Saulson, R. Savage, R. Schilling, S. Schlamminger, R. Schnabel, R. M. S. Schofield, B. Schulz, B. F. Schutz, P. Schwinberg, J. Scott, S. M. Scott, A. C. Searle, F. Seifert, D. Sellers, A. S. Sengupta, A. Sergeev, D. A. Shaddock, M. Shaltev, B. Shapiro, P. Shawhan, T. Shihan Weerathunga, D. H. Shoemaker, A. Sibley, X. Siemens, D. Sigg, A. Singer, L. Singer, A. M. Sintes, G. Skelton, B. J. J. Slagmolen, J. Slutsky, R. Smith, J. R. Smith, M. R. Smith, N. D. Smith, K. Somiya, B. Sorazu, J. Soto, F. C. Speirits, A. J. Stein, J. Steinlechner, S. Steinlechner, S. Steplewski, M. Stefszky, A. Stochino, R. Stone, K. A. Strain, S. Strigin, A. S. Stroeer, A. L. Stuver, T. Z. Summerscales, M. Sung, S. Susmithan, P. J. Sutton, G. P. Szokoly, D. Talukder, D. B. Tanner, S. P. Tarabrin, J. R. Taylor, R. Taylor, P. Thomas, K. A. Thorne, K. S. Thorne, E. Thrane, A. Thuring, K. V. Tokmakov, C. Torres, C. I. Torrie, G. Traylor, M. Trias, K. Tseng, D. Ugolini, K. Urbanek, H. Vahlbruch, B. Vaishnav, M. Vallisneri, C. Van Den Broeck, M. V. van der Sluys, A. A. van Veggel, S. Vass, R. Vaulin, A. Vecchio, J. Veitch, P. J. Veitch, C. Veltkamp, A. E. Villar, C. Vorvick, S. P. Vyachanin, S. J. Waldman, L. Wallace, A. Wanner, R. L. Ward, P. Wei, M. Weinert, A. J. Weinstein, R. Weiss, L. Wen, S. Wen, P. Wessels, M. West, T. Westphal, K. Wette, J. T. Whelan, S. E. Whitcomb, D. White, B. F. Whiting, C. Wilkinson, P. A. Willems, H. R. Williams, L. Williams, B. Willke, L. Winkelmann, W. Winkler, C. C. Wipf, A. G. Wiseman, G. Woan, R. Wooley, J. Worden, J. Yablon, I. Yakushin, K. Yamamoto, H. Yamamoto, H. Yang, D. Yeaton-Massey, S. Yoshida, P. Yu, M. Zanolin, L. Zhang, Z. Zhang, C. Zhao, N. Zotov, M. E. Zucker, J. Zweizig, M. A. Bizouard, A. Dietz, G. M. Guidi, M. Was

**Date**: 21 Jan 2012

**Abstract**: We present the results of a LIGO search for gravitational waves (GWs) associated with GRB 051103, a short-duration hard-spectrum gamma-ray burst (GRB) whose electromagnetically determined sky position is coincident with the spiral galaxy M81, which is 3.6 Mpc from Earth. Possible progenitors for short-hard GRBs include compact object mergers and soft gamma repeater (SGR) giant flares. A merger progenitor would produce a characteristic GW signal that should be detectable at the distance of M81, while GW emission from an SGR is not expected to be detectable at that distance. We found no evidence of a GW signal associated with GRB 051103. Assuming weakly beamed gamma-ray emission with a jet semi-angle of 30 deg we exclude a binary neutron star merger in M81 as the progenitor with a confidence of 98%. Neutron star-black hole mergers are excluded with > 99% confidence. If the event occurred in M81 our findings support the the hypothesis that GRB 051103 was due to an SGR giant flare, making it the most distant extragalactic magnetar observed to date.

1201.4413
(/preprints)

2012-01-24, 17:00
**[edit]**

**Authors**: Roland Haas, Roman V. Shcherbakov, Tanja Bode, Pablo Laguna

**Date**: 20 Jan 2012

**Abstract**: We present numerical relativity results of tidal disruptions of white dwarfs from ultra-close encounters with a spinning, intermediate mass black hole. These encounters require a full general relativistic treatment of gravity. We show that the disruption process and prompt accretion of the debris strongly depend on the magnitude and orientation of the black hole spin. However, the late-time accretion onto the black hole follows the same decay, $\dot{M}$ ~ tˆ{-5/3}, estimated from Newtonian gravity disruption studies. We compute the spectrum of the disk formed from the fallback material using a slim disk model. The disk spectrum peaks in the soft X-rays and sustains Eddington luminosity for 1-3 yrs after the disruption. For arbitrary black hole spin orientations, the disrupted material is scattered away from the orbital plane by relativistic frame dragging, which often leads to obscuration of the inner fallback disk by the outflowing debris. The disruption events also yield bursts of gravitational radiation with characteristic frequencies of ~3.2 Hz and strain amplitudes of ~10ˆ{-18} for galactic intermediate mass black holes. The optimistic rate of considered ultra-close disruptions is consistent with no sources found in ROSAT all-sky survey. The future missions like Wide-Field X-ray Telescope (WFXT) could observe dozens of events.

1201.4389
(/preprints)

2012-01-24, 16:59
**[edit]**

**Authors**: Reinhard Prix, Miroslav Shaltev

**Date**: 20 Jan 2012

**Abstract**: Coherent wide parameter-space searches for continuous gravitational waves are typically limited in sensitivity by their prohibitive computing cost. Therefore semi-coherent methods (such as StackSlide) can often achieve a better sensitivity. We develop an analytical method for finding optimal StackSlide parameters at fixed computing cost under ideal conditions of gapless data with Gaussian stationary noise. This solution separates two regimes: an unbounded regime, where it is always optimal to use all the data, and a bounded regime with a finite optimal observation time. Our analysis of the sensitivity scaling reveals that both the fine- and coarse-grid mismatches contribute equally to the average StackSlide mismatch, an effect that had been overlooked in previous studies. We discuss various practical examples for the application of this optimization framework, illustrating the potential gains in sensitivity compared to previous searches.

1201.4321
(/preprints)

2012-01-22, 22:47
**[edit]**

**Authors**: Matthew Pitkin

**Date**: 17 Jan 2012

**Abstract**: Pulsar timing arrays (PTAs) are being used to search for very low frequency gravitational waves. Gravitational waves imprint their signal in the observed pulse time of arrivals from when they passed the pulsar and as they pass the Earth. In searches for gravitational wave bursts with PTAs (e.g. Finn & Lommen, 2010) the pulsar term is generally ignored as only the Earth term will be coherent between all pulsars in the array, whereas signals in the pulsar terms may be separated by delays on the order of the pulsar distance. However, we show that for a set of pulsars (made up from those in the International Pulsar Timing Array) there are areas of the sky where the alignment between pairs, or more, of pulsars and a source are serendipitously placed to give pulsar terms that are separated by feasible (10-20 year) observing times. The data from these pulsars can therefore be coherently combined, with the appropriate sky position delay, to search for gravitational wave bursts. This increases the time-span over which bursts could be observed to be many times that covered by the PTA observation span. Assuming perfectly known pulsar distances we show that sources over approximately 70 per cent of the sky produce pulsar term signals separated by less than 10 years within at least one pair of pulsars. We study the effect of pulsar distance uncertainties on the sky coverage. We also assess a simplified method for detecting burst sources from these sky positions with a toy two-pulsar array.

1201.3573
(/preprints)

2012-01-19, 13:10
**[edit]**

**Authors**: Tania Regimbau, Thomas Dent, Walter Del Pozzo, Stefanos Giampanis, Tjonnie G. F. Li, Craig Robinson, Chris Van Den Broeck, Duncan Meacher, Carl Rodriguez, Bangalore S. Sathyaprakash, Katarzyna Wójcik

**Date**: 17 Jan 2012

**Abstract**: Einstein Telescope (ET) is conceived to be a third generation gravitational-wave observatory. Its amplitude sensitivity would be a factor ten better than advanced LIGO and Virgo and it could also extend the low-frequency sensitivity down to 1--3\,Hz, compared to the 10--20\,Hz of advanced detectors. Such an observatory will have the potential to observe a variety of different GW sources, including compact binary systems at cosmological distances. ET's expected reach for binary neutron star (BNS) coalescences is out to redshift $z\simeq 2$ and the rate of detectable BNS coalescences could be as high as one every few tens or hundreds of seconds, each lasting up to several days. %in the sensitive frequency band of ET. With such a signal-rich environment, a key question in data analysis is whether overlapping signals can be discriminated. In this paper we simulate the GW signals from a cosmological population of BNS and ask the following questions: Does this population create a confusion background that limits ET's ability to detect foreground sources? How efficient are current algorithms in discriminating overlapping BNS signals? Is it possible to discern the presence of a population of signals in the data by cross-correlating data from different detectors in the ET observatory? We find that algorithms currently used to analyze LIGO and Virgo data are already powerful enough to detect the sources expected in ET, but new algorithms are required to fully exploit ET data.

1201.3563
(/preprints)

2012-01-19, 13:10
**[edit]**

**Authors**: Shuo Li, F. K. Liu, Peter Berczik, Xian Chen, Rainer Spurzem

**Date**: 17 Jan 2012

**Abstract**: Supermassive black hole binaries (SMBHBs) are the products of frequent galaxy mergers. The coalescence of the SMBHBs is a distinct source of gravitational wave (GW) radiation. The detections of the strong GW radiation and their possible electromagnetic counterparts are essential. Numerical relativity suggests that the post-merger supermassive black hole (SMBH) gets a kick velocity up to 4000 km/s due to the anisotropic GW radiations. Here we investigate the dynamical co-evolution and interaction of the recoiling SMBHs and their galactic stellar environments with one million direct N-body simulations including the stellar tidal disruption by the recoiling SMBHs. Our results show that the accretion of disrupted stars does not significantly affect the SMBH dynamical evolution. We investigate the stellar tidal disruption rates as a function of the dynamical evolution of oscillating SMBHs in the galactic nuclei. Our simulations show that most of stellar tidal disruptions are contributed by the unbound stars and occur when the oscillating SMBHs pass through the galactic center. The averaged disruption rate is ~10ˆ{-6} M_\odot yrˆ{-1}, which is about an order of magnitude lower than that by a stationary SMBH at similar galactic nuclei. Our results also show that a bound star cluster is around the oscillating SMBH of about ~ 0.7% the black hole mass. In addition, we discover a massive cloud of unbound stars following the oscillating SMBH. We also investigate the dependence of the results on the SMBH masses and density slopes of the galactic nuclei.

1201.3407
(/preprints)

2012-01-19, 13:07
**[edit]**

**Authors**: Pau Amaro-Seoane, Sofiane Aoudia, Stanislav Babak, Pierre Binétruy, Emanuele Berti, Alejandro Bohé, Chiara Caprini, Monica Colpi, Neil J. Cornish, Karsten Danzmann, Jean-François Dufaux, Jonathan Gair, Oliver Jennrich, Philippe Jetzer, Antoine Klein, Ryan N. Lang, Alberto Lobo, Tyson Littenberg, Sean T. McWilliams, Gijs Nelemans, Antoine Petiteau, Edward K. Porter, Bernard F. Schutz, Alberto Sesana, Robin Stebbins, Tim Sumner, Michele Vallisneri, Stefano Vitale, Marta Volonteri, Henry Ward

**Date**: 17 Jan 2012

**Abstract**: This document introduces the exciting and fundamentally new science and astronomy that the European New Gravitational Wave Observatory (NGO) mission (derived from the previous LISA proposal) will deliver. The mission (which we will refer to by its informal name "eLISA") will survey for the first time the low-frequency gravitational wave band (about 0.1 mHz to 1 Hz), with sufficient sensitivity to detect interesting individual astrophysical sources out to z = 15. The eLISA mission will discover and study a variety of cosmic events and systems with high sensitivity: coalescences of massive black holes binaries, brought together by galaxy mergers; mergers of earlier, less-massive black holes during the epoch of hierarchical galaxy and black-hole growth; stellar-mass black holes and compact stars in orbits just skimming the horizons of massive black holes in galactic nuclei of the present era; extremely compact white dwarf binaries in our Galaxy, a rich source of information about binary evolution and about future Type Ia supernovae; and possibly most interesting of all, the uncertain and unpredicted sources, for example relics of inflation and of the symmetry-breaking epoch directly after the Big Bang. eLISA's measurements will allow detailed studies of these signals with high signal-to-noise ratio, addressing most of the key scientific questions raised by ESA's Cosmic Vision programme in the areas of astrophysics and cosmology. They will also provide stringent tests of general relativity in the strong-field dynamical regime, which cannot be probed in any other way. This document not only describes the science but also gives an overview on the mission design and orbits.

1201.3621
(/preprints)

2012-01-19, 13:04
**[edit]**

**Authors**: Idan Ginsburg, Abraham Loeb, Gary A. Wegner

**Date**: 6 Jan 2012

**Abstract**: The disruption of a binary star system by the massive black hole at the Galactic Centre, SgrA*, can lead to the capture of one star around SgrA* and the ejection of its companion as a hypervelocity star (HVS). We consider the possibility that these stars may have planets and study the dynamics of these planets. Using a direct $N$-body integration code, we simulated a large number of different binary orbits around SgrA*. For some orbital parameters, a planet is ejected at a high speed. In other instances, a HVS is ejected with one or more planets orbiting around it. In these cases, it may be possible to observe the planet as it transits the face of the star. A planet may also collide with its host star. In such cases the atmosphere of the star will be enriched with metals. In other cases, a planet is tidally disrupted by SgrA*, leading to a bright flare.

1201.1446
(/preprints)

2012-01-17, 14:55
**[edit]**

**Authors**: Andreas Burkert, Mark Schartmann, Christian Alig, Stefan Gillessen, Reinhard Genzel, Tobias Fritz, Frank Eisenhauer

**Date**: 6 Jan 2012

**Abstract**: The origin, structure and evolution of the small gas cloud, G2, is investigated, that is on an orbit almost straight into the Galactic central supermassive black hole (SMBH). G2 is a sensitive probe of the hot accretion zone of Sgr A*, requiring gas temperatures and densities that agree well with models of captured shock-heated stellar winds. Its mass is equal to the critical mass below which cold clumps would be destroyed quickly by evaporation. Its mass is also constrained by the fact that at apocenter its sound crossing timescale was equal to its orbital timescale. Our numerical simulations show that the observed structure and evolution of G2 can be well reproduced if it formed in pressure equilibrium with the surrounding in 1995 at a distance from the SMBH of 7.6e16 cm. If the cloud would have formed at apocenter in the 'clockwise' stellar disk as expected from its orbit, it would be torn into a very elongated spaghetti-like filament by 2011 which is not observed. This problem can be solved if G2 is the head of a larger, shell-like structure that formed at apocenter. Our numerical simulations show that this scenario explains not only G2's observed kinematical and geometrical properties but also the Br_gamma observations of a low surface brightness gas tail that trails the cloud. In 2013, while passing the SMBH G2 will break up into a string of droplets that within the next 30 years mix with the surrounding hot gas and trigger cycles of AGN activity.

1201.1414
(/preprints)

2012-01-17, 14:54
**[edit]**

**Authors**: Richard H. Price, Joseph D. Romano

**Date**: 12 Aug 2005

**Abstract**: The expansion of the universe is often viewed as a uniform stretching of space that would affect compact objects, atoms and stars, as well as the separation of galaxies. One usually hears that bound systems do not take part in the general expansion, but a much more subtle question is whether bound systems expand partially. In this paper, a very definitive answer is given for a very simple system: a classical "atom" bound by electrical attraction. With a mathemical description appropriate for undergraduate physics majors, we show that this bound system either completely follows the cosmological expansion, or -- after initial transients -- completely ignores it. This "all or nothing" behavior can be understood with techniques of junior-level mechanics. Lastly, the simple description is shown to be a justifiable approximation of the relativistically correct formulation of the problem.

0508052
(/preprints/gr-qc)

2012-01-17, 14:53
**[edit]**

**Authors**: J. H. J. de Bruijne

**Date**: 16 Jan 2012

**Abstract**: Gaia is the next astrometry mission of the European Space Agency (ESA), following up on the success of the Hipparcos mission. With a focal plane containing 106 CCD detectors, Gaia will survey the entire sky and repeatedly observe the brightest 1,000 million objects, down to 20th magnitude, during its 5-year lifetime. Gaia's science data comprises absolute astrometry, broad-band photometry, and low-resolution spectro-photometry. Spectroscopic data with a resolving power of 11,500 will be obtained for the brightest 150 million sources, down to 17th magnitude. The thermo-mechanical stability of the spacecraft, combined with the selection of the L2 Lissajous point of the Sun-Earth/Moon system for operations, allows stellar parallaxes to be measured with standard errors less than 10 micro-arcsecond (muas) for stars brighter than 12th magnitude, 25 muas for stars at 15th magnitude, and 300 muas at magnitude 20. Photometric standard errors are in the milli-magnitude regime. The spectroscopic data allows the measurement of radial velocities with errors of 15 km/s at magnitude 17. Gaia's primary science goal is to unravel the kinematical, dynamical, and chemical structure and evolution of the Milky Way. In addition, Gaia's data will touch many other areas of science, e.g., stellar physics, solar-system bodies, fundamental physics, and exo-planets. The Gaia spacecraft is currently in the qualification and production phase. With a launch in 2013, the final catalogue is expected in 2021. The science community in Europe, organised in the Data Processing and Analysis Consortium (DPAC), is responsible for the processing of the data.

1201.3238
(/preprints)

2012-01-17, 14:49
**[edit]**

**Authors**: C. Palomba, for the LIGO Scientific Collaboration, for the Virgo Collaboration

**Date**: 16 Jan 2012

**Abstract**: We present results from searches of recent LIGO and Virgo data for continuous gravitational wave signals (CW) from spinning neutron stars and for a stochastic gravitational wave background (SGWB). The first part of the talk is devoted to CW analysis with a focus on two types of searches. In the targeted search of known neutron stars a precise knowledge of the star parameters is used to apply optimal filtering methods. In the absence of a signal detection, in a few cases, an upper limit on strain amplitude can be set that beats the spindown limit derived from attributing spin-down energy loss to the emission of gravitational waves. In contrast, blind all-sky searches are not directed at specific sources, but rather explore as large a portion of the parameter space as possible. Fully coherent methods cannot be used for these kind of searches which pose a non trivial computational challenge. The second part of the talk is focused on SGWB searches. A stochastic background of gravitational waves is expected to be produced by the superposition of many incoherent sources of cosmological or astrophysical origin. Given the random nature of this kind of signal, it is not possible to distinguish it from noise using a single detector. A typical data analysis strategy relies on cross-correlating the data from a pair or several pairs of detectors, which allows discriminating the searched signal from instrumental noise. Expected sensitivities and prospects for detection from the next generation of interferometers are also discussed for both kind of sources.

1201.3176
(/preprints)

2012-01-17, 14:49
**[edit]**

**Authors**: Rahul Biswas, Patrick R. Brady, Jordi Burguet-Castell, Kipp Cannon, Jessica Clayton, Alexander Dietz, Nickolas Fotopoulos, Lisa M. Goggin, Drew Keppel, Chris Pankow, Larry R. Price, Ruslan Vaulin

**Date**: 13 Jan 2012

**Abstract**: There is a broad class of astrophysical sources that produce detectable, transient, gravitational waves. Some searches for transient gravitational waves are tailored to known features of these sources. Other searches make few assumptions about the sources. Typically events are observable with multiple search techniques. This work describes how to combine the results of searches that are not independent, treating each search as a classifier for a given event. This will be shown to improve the overall sensitivity to gravitational-wave events while directly addressing the problem of consistent interpretation of multiple trials.

1201.2959
(/preprints)

2012-01-17, 14:49
**[edit]**

**Authors**: Samir D. Mathur

**Date**: 10 Jan 2012

**Abstract**: Many relativists have been long convinced that black hole evaporation leads to information loss or remnants. String theorists have however not been too worried about the issue, largely due to a belief that the Hawking argument for information loss is flawed in its details. A recently derived inequality shows that the Hawking argument for black holes with horizon can in fact be made rigorous. What happens instead is that in string theory black hole microstates have no horizons. Thus the evolution of radiation quanta with E ~ kT is modified by order unity at the horizon, and we resolve the information paradox. We discuss how it is still possible for E >> kT objects to see an approximate black hole like geometry. We also note some possible implications of this physics for the early Universe.

1201.2079
(/preprints)

2012-01-17, 14:49
**[edit]**

**Authors**: Kimitake Hayasaki, Kent Yagi, Takahiro Tanaka, Shin Mineshige

**Date**: 13 Jan 2012

**Abstract**: When binary black holes are embedded in a gaseous environment, a rotating disk surrounding them, the so-called circumbinary disk, will be formed. The binary exerts a gravitational torque on the circumbinary disk and thereby the orbital angular momentum is transferred to it, while the angular momentum of the circumbinary disk is transferred to the binary through the mass accretion. The binary undergoes an orbital decay due to both the gravitational wave emission and the binary-disk interaction. This causes the phase evolution of the gravitational wave signal. The precise measurement of the gravitational wave phase thus may provide information regarding the circumbinary disk. In this paper, we assess the detectability of the signature of the binary-disk interaction using the future space-borne gravitational wave detectors such as DECIGO and BBO by the standard matched filtering analysis. We find that the effect of the circumbinary disk around binary black holes in the mass range $6M_sun\le{M}\lesssim3\times10ˆ3M_sun$ is detectable at a statistically significant level in five year observation, provided that gas accretes onto the binary at a rate greater than $\dot{M}\sim1.4\times10ˆ{17} [gsˆ{-1}] jˆ{-1}(M/10M_sun)ˆ{33/23}$ with 10% mass-to-energy conversion efficiency, where j represents the efficiency of the angular momentum transfer from the binary to the circumbinary disk. We show that $O(0.1)$ coalescence events are expected to occur in sufficiently dense molecular clouds in five year observation. We also point out that the circumbinary disk is detectable, even if its mass at around the inner edge is by over 10 orders of magnitude less than the binary mass.

1201.2858
(/preprints)

2012-01-17, 14:49
**[edit]**

**Authors**: Jacob D. Bekenstein

**Date**: 13 Jan 2012

**Abstract**: The impressive success of the standard cosmological model has suggested to many that its ingredients are all one needs to explain galaxies and their systems. I summarize a number of known problems with this program. They might signal the failure of standard gravity theory on galaxy scales. The requisite hints as to the alternative gravity theory may lie with the MOND paradigm which has proved an effective summary of galaxy phenomenology. A simple nonlinear modified gravity theory does justice to MOND at the nonrelativistic level, but cannot be consistently promoted to relativistic status. The obstacles were first sidestepped with the formulation of TeVeS, a covariant modified gravity theory. I review its structure, its MOND and Newtonian limits, and its performance in face of galaxy phenomenology. I also summarize features of TeVeS cosmology and describe the confrontation with data from strong and weak gravitational lensing

1201.2759
(/preprints)

2012-01-17, 14:49
**[edit]**

**Authors**: Rahul Biswas, Patrick R. Brady, Jordi Burguet-Castell, Kipp Cannon, Jessica Clayton, Alexander Dietz, Nickolas Fotopoulos, Lisa M. Goggin, Drew Keppel, Chris Pankow, Larry R. Price, Ruslan Vaulin

**Date**: 13 Jan 2012

**Abstract**: There is a broad class of astrophysical sources that produce detectable, transient, gravitational waves. Some searches for transient gravitational waves are tailored to known features of these sources. Other searches make few assumptions about the sources. Typically events are observable with multiple search techniques. This work describes how to combine the results of searches that are not independent, treating each search as a classifier for a given event. This will be shown to improve the overall sensitivity to gravitational-wave events while directly addressing the problem of consistent interpretation of multiple trials.

1201.2964
(/preprints)

2012-01-17, 14:49
**[edit]**

**Authors**: Carlos O. Lousto, Yosef Zlochower, Massimo Dotti, Marta Volonteri

**Date**: 9 Jan 2012

**Abstract**: We explore the newly discovered "hangup-kick" effect, which greatly amplifies the recoil for configuration with partial spin- orbital-angular momentum alignment, by studying a set of 48 new simulations of equal-mass, spinning black-hole binaries. We propose a phenomenological model for the recoil that takes this new effect into account and then use this model, in conjunction with statistical distributions for the spin magnitude and orientations, based on accretion simulations, to find the probabilities for observing recoils of several thousand km/s. In addition, we provide initial parameters, eccentricities, radiated linear and angular momentum, precession rates and remnant mass, spin, and recoils for all 48 configurations. Our results indicate that surveys exploring peculiar (redshifted or blueshifted) differential line-of-sight velocities should observe at least one case above 2000 km/s out of four thousand merged galaxies. The probability that a remnant BH receives a total recoil exceeding the ~2000 km/s escape velocity of large elliptical galaxies is ten times larger. Probabilities of recoils exceeding the escape velocity quickly rise to 5% for galaxies with escape velocities of 1000 km/s and nearly 20% for galaxies with escape velocities of 500 km/s. In addition the direction of these large recoils is strongly peaked toward the angular momentum axis, with very low probabilities of recoils exceeding 350 km/s for angles larger than 45 deg. with respect to the orbital angular momentum axis.

1201.1923
(/preprints)

2012-01-17, 14:49
**[edit]**

**Authors**: Brynmor Haskell, Nathalie Degenaar, Wynn C.G. Ho

**Date**: 10 Jan 2012

**Abstract**: Rapidly rotating Neutron Stars in Low Mass X-ray Binaries (LMXBs) may be an interesting source of Gravitational Waves (GWs). In particular, several modes of stellar oscillation may be driven unstable by GW emission, and this can lead to a detectable signal. Here we illustrate how current X-ray and ultra-violet (UV) observations can constrain the physics of the r-mode instability. We show that the core temperatures inferred from the data would place many systems well inside the unstable region predicted by standard physical models. However, this is at odds with theoretical expectations. We discuss different mechanisms that could be at work in the stellar interior, and we show how they can modify the instability window and make it consistent with the inferred temperatures.

1201.2101
(/preprints)

2012-01-17, 14:49
**[edit]**

**Authors**: R. O'Shaughnessy (1), J. Healy (2), L. London (2), Z. Meeks (2), D. Shoemaker (2) ((1) Center for Gravitation and Cosmology, University of Wisconsin-Milwaukee, (2) Center for Relativistic Astrophysics, Georgia Tech)

**Date**: 10 Jan 2012

**Abstract**: The gravitational wave signature emitted from a merging binary depends on the orientation of an observer relative to the binary. Previous studies suggest that emission along the total initial or total final angular momenta leads to both the strongest and simplest signal from a precessing compact binary. In this paper we describe a concrete counterexample: a binary with $m_1/m_2=4$, $a_1=0.6 \hat{x} = -a_2$, placed in orbit in the x,y plane. We extract the gravitational wave emission along several proposed emission directions, including the initial (Newtonian) orbital angular momentum; the final (~ initial) total angular momentum; and the dominant principal axis of $<L_a L_b>_M$. Using several diagnostics, we show that the suggested preferred directions are not representative. For example, only for a handful of other directions (< 15%) will the gravitational wave signal have comparable shape to the one extracted along each of these fiducial directions, as measured by a generalized overlap (>0.95). We conclude that the information available in just one direction (or mode) does not adequately encode the complexity of orientation-dependent emission for even short signals from merging black hole binaries. Future investigations of precessing, unequal-mass binaries should carefully explore and model their orientation-dependent emission.

1201.2113
(/preprints)

2012-01-17, 14:48
**[edit]**

**Authors**: Drew Keppel

**Date**: 9 Jan 2012

**Abstract**: Singular-value decomposition is a powerful technique that has been used in the analysis of matrices in many fields. In this paper, we summarize how it has been applied to the analysis of gravitational-wave data. These include producing basis waveforms for matched filtering, decreasing the computational cost of searching for many waveforms, improving parameter estimation, and providing a method of waveform interpolation.

1201.1739
(/preprints)

2012-01-17, 14:48
**[edit]**

**Authors**: Glenn D. Starkman

**Date**: 9 Jan 2012

**Abstract**: The combination of GR and the Standard Model disagrees with numerous observations on scales from our Solar System up. In the concordance model of cosmology, these contradictions are removed or alleviated by the introduction of three completely independent new components of stress-energy -- the inflaton, dark matter, and dark energy. Each of these in its turn is meant to have (or to currently) dominate the dynamics of the universe. There is still no non-gravitational evidence for any of these dark sectors; nor for the required extensions of the standard model. An alternative is to imagine that GR itself must be modified. Certain coincidences of scale even suggest that one might expect not to have to make three independent. Because they must address the most different types of data, attempts to replace dark matter with modified gravity are the most controversial. A phenomenological model (or family of models), Modified Newtonian Dynamics, has, over the last few years seen several covariant realizations. We discuss a number of challenges that any model that seeks to replace dark matter with modified gravity must face: the loss of Birkhoff's Theorem, and the calculational simplifications it implies; the failure to explain clusters, whether static or interacting, and the consequent need to introduce dark matter of some form, whether hot dark matter neutrinos, or dark fields that arise in new sectors of the modified gravity theory; the intrusion of cosmological expansion into the modified force law, that arises precisely because of the coincidence in scale between the centripetal acceleration at which Newtonian gravity fails in galaxies, and the cosmic acceleration. We conclude with the observation that, although modified gravity may indeed manage to replace dark matter, it is likely to do so by becoming or incorporating, a dark matter theory itself.

1201.1697
(/preprints)

2012-01-17, 14:48
**[edit]**

**Authors**: Feryal Ozel (Arizona), Dimitrios Psaltis (Arizona), Ramesh Narayan (Harvard), Antonio Santos Villarreal (Arizona)

**Date**: 4 Jan 2012

**Abstract**: We investigate the distribution of neutron star masses in different populations of binaries, employing Bayesian statistical techniques. In particular, we explore the differences in neutron star masses between sources that have experienced distinct evolutionary paths and accretion episodes. We find that the distribution of neutron star masses in non-recycled eclipsing high-mass binaries as well as of slow pulsars, which are all believed to be near their birth masses, has a mean of 1.28 M_solar and a dispersion of 0.24 M_solar. These values are consistent with expectations for neutron star formation in core-collapse supernovae. On the other hand, double neutron stars, which are also believed to be near their birth masses, have a much narrower mass distribution, peaking at 1.33 M_solar but with a dispersion of only 0.06 M_solar. Such a small dispersion cannot easily be understood and perhaps points to a particular and rare formation channel. The mass distribution of neutron stars that have been recycled has a mean of 1.48 M_solar and a dispersion of 0.2 M_solar, consistent with the expectation that they have experienced extended mass accretion episodes. The fact that only a very small fraction of recycled neutron stars in the inferred distribution have masses that exceed ~2 M_solar suggest that only a few of these neutron stars cross the mass threshold to form low mass black holes.

1201.1006
(/preprints)

2012-01-06, 10:27
**[edit]**

**Authors**: J. Veitch, I. Mandel, B. Aylott, B. Farr, V. Raymond, C. Rodriguez, M. van der Sluys, V. Kalogera, A. Vecchio

**Date**: 5 Jan 2012

**Abstract**: The advanced versions of the LIGO and Virgo ground-based gravitational-wave detectors are expected to operate from three sites: Hanford, Livingston, and Cascina. Recent proposals have been made to place a fourth site in Australia or India; and there is the possibility of using the Large Cryogenic Gravitational Wave Telescope in Japan to further extend the network. Using Bayesian parameter-estimation analyses of simulated gravitational-wave signals from a range of coalescing-binary locations and orientations at fixed distance or signal-to-noise ratio, we study the improvement in parameter estimation for the proposed networks. We find that a fourth detector site can break degeneracies in several parameters; in particular, the localization of the source on the sky is improved by a factor of ~ 3--4 for an Australian site, or ~ 2.5--3.5 for an Indian site, with more modest improvements in distance and binary inclination estimates. This enhanced ability to localize sources on the sky will be crucial in any search for electromagnetic counterparts to detected gravitational-wave signals.

1201.1195
(/preprints)

2012-01-06, 10:27
**[edit]**

**Authors**: Pierre Binétruy, Alejandro Bohé, Chiara Caprini, Jean-François Dufaux

**Date**: 4 Jan 2012

**Abstract**: We review the main cosmological backgrounds of gravitational waves accessible to detectors in space sensitive to the range $10ˆ{-4}$ to $10ˆ{-1}$ Hz, with a special emphasis on those backgrounds due to phase transitions or networks of cosmic strings. We apply this to identify the scientific potential of the NGO/eLISA mission of ESA, regarding the detectability of such cosmological backgrounds.

1201.0983
(/preprints)

2012-01-05, 09:41
**[edit]**

**Authors**: Justin Alsing, Emanuele Berti, Clifford Will, Helmut Zaglauer

**Date**: 21 Dec 2011

**Abstract**: We derive the equations of motion, the periastron shift, and the gravitational radiation damping for quasicircular compact binaries in a massive variant of the Brans-Dicke theory of gravity. We also study the Shapiro time delay and the Nordtvedt effect in this theory. By comparing with recent observational data, we put bounds on the two parameters of the theory: the Brans-Dicke coupling parameter \omega_{BD} and the scalar mass m_s. We find that the most stringent bounds come from Cassini measurements of the Shapiro time delay in the Solar System, that yield a lower bound \omega_{BD}>40000 for scalar masses m_s<2.5x10ˆ{-20} eV, to 95% confidence. In comparison, observations of the Nordtvedt effect using Lunar Laser Ranging (LLR) experiments yield \omega_{BD}>1000 for m_s<2.5x10ˆ{-20} eV. Observations of the orbital period derivative of the quasicircular white dwarf-neutron star binary PSR J1012+5307 yield \omega_{BD}>1250 for m_s<10ˆ{-20} eV. A first estimate suggests that bounds comparable to the Shapiro time delay may come from observations of radiation damping in the eccentric white dwarf-neutron star binary PSR J1141-6545, but a quantitative prediction requires the extension of our work to eccentric orbits.

1112.4903
(/preprints)

2012-01-03, 08:44
**[edit]**

**Authors**: Barry Wardell, Ian Vega, Jonathan Thornburg, Peter Diener

**Date**: 29 Dec 2011

**Abstract**: A leading approach to the modelling of extreme mass ratio inspirals involves the treatment of the smaller mass as a point particle and the computation of a regularized self-force acting on that particle. In turn, this computation requires knowledge of the regularized retarded field generated by the particle. A direct calculation of this regularized field may be achieved by replacing the point particle with an effective source and solving directly a wave equation for the regularized field. This has the advantage that all quantities are finite and require no further regularization. In this work, we present a method for computing an effective source which is finite and continuous everywhere, and which is valid for a scalar point particle in arbitrary geodesic motion in an arbitrary background spacetime. We explain in detail various technical and practical considerations that underlie its use in several numerical self-force calculations. We consider as examples the cases of a particle in a circular orbit about Schwarzschild and Kerr black holes, and also the case of a particle following a generic time-like geodesic about a highly spinning Kerr black hole. We provide numerical C code for computing an effective source for various orbital configurations about Schwarzschild and Kerr black holes.

1112.6355
(/preprints)

2012-01-03, 08:42
**[edit]**

**Authors**: The LIGO Scientific Collaboration, The Virgo Collaboration

**Date**: 27 Dec 2011

**Abstract**: Aims. The detection and measurement of gravitational-waves from coalescing neutron-star binary systems is an important science goal for ground-based gravitational-wave detectors. In addition to emitting gravitational-waves at frequencies that span the most sensitive bands of the LIGO and Virgo detectors, these sources are also amongst the most likely to produce an electromagnetic counterpart to the gravitational-wave emission. A joint detection of the gravitational-wave and electromagnetic signals would provide a powerful new probe for astronomy.

Methods. During the period between September 19 and October 20, 2010, the first low-latency search for gravitational-waves from binary inspirals in LIGO and Virgo data was conducted. The resulting triggers were sent to electromagnetic observatories for followup. We describe the generation and processing of the low-latency gravitational-wave triggers. The results of the electromagnetic image analysis will be described elsewhere.

Results. Over the course of the science run, three gravitational-wave triggers passed all of the low-latency selection cuts. Of these, one was followed up by several of our observational partners. Analysis of the gravitational-wave data leads to an estimated false alarm rate of once every 6.4 days, falling far short of the requirement for a detection based solely on gravitational-wave data.

1112.6005
(/preprints)

2012-01-03, 08:42
**[edit]**

© M. Vallisneri 2012 — last modified on 2010/01/29

*Tantum in modicis, quantum in maximis*