Michele's wiki/blog

Authors: M. De Laurentis, S. Capozziello

Date: 2 Feb 2012

Abstract: We review black hole solutions and self-gravitating structures in f(R)-gravity.

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Authors: P. Ajith, Michael Boyle, Duncan A. Brown, Bernd Brügmann, Luisa T. Buchman, Laura Cadonati, Manuela Campanelli, Tony Chu, Zachariah B. Etienne, Stephen Fairhurst, Mark Hannam, James Healy, Ian Hinder, Sascha Husa, Lawrence E. Kidder, Badri Krishnan, Pablo Laguna, Yuk Tung Liu, Lionel London, Carlos O. Lousto, Geoffrey Lovelace, Ilana MacDonald, Pedro Marronetti, Satya Mohapatra, Philipp Mösta, Doreen Müller, Bruno C. Mundim, Hiroyuki Nakano, Frank Ohme, Vasileios Paschalidis, Larne Pekowsky, Denis Pollney, Harald P. Pfeiffer, Marcelo Ponce, Michael Pürrer, George Reifenberger, Christian Reisswig, Lucía Santamaría, Mark A. Scheel, Stuart L. Shapiro, Deirdre Shoemaker, Carlos F. Sopuerta, Ulrich Sperhake, Béla Szilágyi, Nicholas W. Taylor, Wolfgang Tichy, Petr Tsatsin, Yosef Zlochower

Date: 25 Jan 2012

Abstract: The Numerical INJection Analysis (NINJA) project is a collaborative effort between members of the numerical relativity and gravitational wave data analysis communities. The purpose of NINJA is to study the sensitivity of existing gravitational-wave search and parameter-estimation algorithms using numerically generated waveforms, and to foster closer collaboration between the numerical relativity and data analysis communities. The first NINJA project used only a small number of injections of short numerical-relativity waveforms, which limited its ability to draw quantitative conclusions. The goal of the NINJA-2 project is to overcome these limitations with long post-Newtonian - numerical relativity hybrid waveforms, large numbers of injections, and the use of real detector data. We report on the submission requirements for the NINJA-2 project and the construction of the waveform catalog. Eight numerical relativity groups have contributed 63 hybrid waveforms consisting of a numerical portion modelling the late inspiral, merger, and ringdown stitched to a post-Newtonian portion modelling the early inspiral. We summarize the techniques used by each group in constructing their submissions. We also report on the procedures used to validate these submissions, including examination in the time and frequency domains and comparisons of waveforms from different groups against each other. These procedures have so far considered only the $(\ell,m)=(2,2)$ mode. Based on these studies we judge that the hybrid waveforms are suitable for NINJA-2 studies. We note some of the plans for these investigations.

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Authors: David Keitel, Reinhard Prix, Maria Alessandra Papa, Maham Siddiqi

Date: 25 Jan 2012

Abstract: Continuous gravitational waves (CW) are expected from spinning neutron stars with non-axisymmetric deformations. A network of interferometric detectors (LIGO, Virgo and GEO600) is looking for these signals. They are predicted to be very weak and retrievable only by integration over long observation times. One of the standard methods of CW data analysis is the multi-detector F-statistic. In a typical search, the F-statistic is computed over a range in frequency, spin-down and sky position, and the candidates with highest F values are kept for further analysis. However, this detection statistic is susceptible to a class of noise artifacts, strong monochromatic lines in a single detector. By assuming an extended noise model - standard Gaussian noise plus single-detector lines - we can use a Bayesian odds ratio to derive a generalized detection statistic, the line veto (LV-) statistic. In the absence of lines, it behaves similarly to the F-statistic, but it is more robust against line artifacts. In the past, ad-hoc post-processing vetoes have been implemented in searches to remove these artifacts. Here we provide a systematic framework to develop and benchmark this class of vetoes. We present our results from testing this LV-statistic on simulated data.

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Authors: Michal Was, Patrick J. Sutton, Gareth Jones, Isabel Leonor

Date: 26 Jan 2012

Abstract: We present the performance of searches for gravitational wave bursts associated with external astrophysical triggers as a function of the search sky region. We discuss both the case of Gaussian noise and real noise of gravitational wave detectors for arbitrary detector networks. We demonstrate the ability to reach Gaussian limited sensitivity in real non-Gaussian data, and show the conditions required to attain it. We find that a single sky position search is ~20% more sensitive than an all-sky search of the same data.

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Authors: Enrico Barausse

Date: 27 Jan 2012

Abstract: [Abridged] […] In this paper, we study the mass and spin evolution of massive black holes within a semianalytical galaxy-formation model that follows the evolution of dark-matter halos along merger trees, as well as that of the baryonic components (hot gas, stellar and gaseous bulges, and stellar and gaseous galactic disks). This allows us to study the mass and spin evolution of massive black holes in a self-consistent way, by taking into account the effect of the gas present in galactic nuclei both during the accretion phases and during mergers. Also, we present predictions, as a function of redshift, for the fraction of gas-rich black-hole mergers -- in which the spins prior to the merger are aligned due to the gravito-magnetic torques exerted by the circumbinary disk -- as opposed to gas-poor mergers, in which the orientation of the spins before the merger is roughly isotropic. These predictions may be tested by LISA or similar spaced-based gravitational-wave detectors such as eLISA/NGO or SGO.

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Authors: Carlos F. Sopuerta, Nicolas Yunes

Date: 27 Jan 2012

Abstract: We describe a new kludge scheme to model the dynamics of generic extreme-mass-ratio inspirals (EMRIs; stellar compact objects spiraling into a spinning supermassive black hole) and their gravitational-wave emission. The Chimera scheme is a hybrid method that combines tools from different approximation techniques in General Relativity: (i) A multipolar, post-Minkowskian expansion for the far-zone metric perturbation (the gravitational waveforms) and for the local prescription of the self-force; (ii) a post-Newtonian expansion for the computation of the multipole moments in terms of the trajectories; and (iii) a BH perturbation theory expansion when treating the trajectories as a sequence of self-adjusting Kerr geodesics. The EMRI trajectory is made out of Kerr geodesic fragments joined via the method of osculating elements as dictated by the multipolar post-Minkowskian radiation-reaction prescription. We implemented the proper coordinate mapping between Boyer-Lindquist coordinates, associated with the Kerr geodesics, and harmonic coordinates, associated with the multipolar post-Minkowskian decomposition. The Chimera scheme is thus a combination of approximations that can be used to model generic inspirals of systems with extreme to intermediate mass ratios, and hence, it can provide valuable information for future space-based gravitational-wave observatories, like LISA, and even for advanced ground detectors. The local character in time of our multipolar post-Minkowskian self-force makes this scheme amenable to study the possible appearance of transient resonances in generic inspirals.

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Authors: John G. Baker, James Ira Thorpe

Date: 26 Jan 2012

Abstract: We consider a class of proposed gravitational wave detectors based on multiple atomic interferometers separated by large baselines and referenced by common laser systems. We compute the sensitivity limits of these detectors due to intrinsic phase noise of the light sources, non-inertial motion of the light sources, and atomic shot noise and compare them to sensitivity limits for traditional light interferometers. We find that atom interferometers and light interferometers are limited in a nearly identical way by intrinsic phase noise and that both require similar mitigation strategies (e.g. multiple arm instruments) to reach interesting sensitivities. The sensitivity limit from motion of the light sources is slightly different and favors the atom interferometers in the low-frequency limit, although the limit in both cases is severe.

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Authors: J. Abadie et al.*

Date: 28 Jan 2012

Abstract: We present the results of a weakly modeled burst search for gravitational waves from mergers of non-spinning intermediate mass black holes (IMBH) in the total mass range 100--450 solar masses and with the component mass ratios between 1:1 and 4:1. The search was conducted on data collected by the LIGO and Virgo detectors between November of 2005 and October of 2007. No plausible signals were observed by the search which constrains the astrophysical rates of the IMBH mergers as a function of the component masses. In the most efficiently detected bin centered on 88+88 solar masses, for non-spinning sources, the rate density upper limit is 0.13 per Mpcˆ3 per Myr at the 90% confidence level.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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