The stochastic gravitational-wave background from massive black hole binary systems: implications for observations with Pulsar Timing Arrays

Massive black hole binary systems, with masses in the range ~10^4-10^10 \msun, are among the primary sources of gravitational waves in the frequency window ~10^-9 Hz - 0.1 Hz. Pulsar Timing Arrays (PTAs) and the Laser Interferometer Space Antenna (LISA) are the observational means by which we will be able to observe gravitational radiation from these systems. We carry out a systematic study of the generation of the stochastic gravitational-wave background from the cosmic population of massive black hole binaries. We consider a wide variety of assembly scenarios and we estimate the range of signal strength in the frequency band accessible to PTAs. We show that, taking into account the uncertainties surrounding the actual key model parameters, the amplitude lies in the interval h_c(f = 10^-8 Hz)~5x10^-16 - 8x10^-15. The most optimistic predictions place the signal level at a factor of ~3 below the current sensitivity of Pulsar Timing Arrays, but within the detection range of the complete Parkes PTA for a wide variety of models, and of the future Square-Kilometer-Array PTA for all the models considered here. We also show that at frequencies >10^-8 Hz the frequency dependency of the generated background follows a power-law significantly steeper than f^-2/3, that has been considered so far. Finally we show that LISA observations of individual resolvable massive black hole binaries are complementary and orthogonal to PTA observations of a stochastic background from the whole population in the Universe. In fact, the detection of gravitational radiation in both frequency windows will enable us to fully characterise the cosmic history of massive black holes.Comment: 21 pages, 14 figures, minor revisions, accepted for publication in MNRA

Prospects for Stochastic Background Searches Using Virgo and LSC Interferometers

We consider the question of cross-correlation measurements using Virgo and the LSC Interferometers (LIGO Livingston, LIGO Hanford, and GEO600) to search for a stochastic gravitational-wave background. We find that inclusion of Virgo into the network will substantially improve the sensitivity to correlations above 200 Hz if all detectors are operating at their design sensitivity. This is illustrated using a simulated isotropic stochastic background signal, generated with an astrophysically-motivated spectrum, injected into 24 hours of simulated noise for the LIGO and Virgo interferometers.Comment: 11 pages, uses IOP style files, submitted to CQG for GWDAW11 proceedings; revised in response to referee comment

Konzorcium, fő p.: Gravitáció és asztro-részecske fizika = Consortional main: Gravitation and astro-particle physics

Elsősorban az elliptikus pályán keringő kompakt kettősök által keltett gravitációs sugárzást tanulmányoztuk, de foglalkoztunk hiperbolikus pályákon mozgó kettősök által kisugárzott gravitációs hullámok leírásával is. Egy újszerű számítási eljárást fejlesztettünk ki, amivel hatékonyan számolható az, hogy a LISA gravitációs hullámdetektor mennyire pontosan fogja tudni meghatározni a forrás égi pozícióját. Összeolvadó szupermasszív feketelyuk kettősökre kiszámítottuk a mérhető jel/zaj viszony értékeit. A Fisher-analízis segítségével megbecsültük a kettősrendszer paramétereinek relatív hibáit. Kimutattuk, hogy a pálya kezdő excentricitásától lényegesen függenek a paraméterek relatív hibái. Rámutattunk egy újfajta mechanizmusra, ami a földi interferometrikus detektorokkal észlelhető gravitációshullám jelet produkál. Amennyiben a galaxismagokban levő naptömegű fekete lyukak és neutroncsillagok elegendően közel haladnak el egymás mellett, ezek rövid gravitációshullám felvillanásokat generálnak, s megmutattuk, hogy ezen jelek gyakorisága összemérhető a többi forráséval. Lassan forgó csillagok anyagát leíró lehetséges fizikai állapotegyenletek egy nagy osztályára megmutattuk, hogy a kvadrupólus momentum mindig nagyobb, mint amilyen a megfelelő Kerr megoldás kvadrupólus momentuma. Magnetárok környezetében gyorsuló protonok pion sugárzását vizsgáltuk, figyelembevéve a téridő görbületét, s megmutattuk, hogy az a magnetár felszínéről kirepülő protonok energiáját lényegesen befolyásolja. | We have most intensively studied gravitational radiation emitted by compact binary systems, but have also considered gravitational wave emission by binaries on hyperbolic orbits. A new computational method has been developed making the precise determination of the source’s celestial position very effective by LISA. For coalescing supermassive black hole binaries the measurable signal to noise ratio has been computed. By a Fisher-type analysis we have estimated the relative errors of the parameters of the binaries and have shown that these relative errors strongly depend on the initial eccentricity. We have found a new mechanism producing gravitational wave signals originating from close encounters by the black holes and neutron stars present in galactic cores measurable by earth-based interferometric detectors. These close encounters generate gravitational wave bursts, and the expected signal is comparable to all others hitherto known. It has been demonstrated that for a very large class of physical equations of state, the quadrupole moment of slowly rotating stars is always greater than that of the corresponding Kerr black hole. The pion radiation by protons accelerating near magnetars has been studied taking into account the curvature of space-time. It has been found that space-time geometry influences the energy of the protons significantly

Upper limits on the strength of periodic gravitational waves from PSR J1939+2134

The first science run of the LIGO and GEO gravitational wave detectors presented the opportunity to test methods of searching for gravitational waves from known pulsars. Here we present new direct upper limits on the strength of waves from the pulsar PSR J1939+2134 using two independent analysis methods, one in the frequency domain using frequentist statistics and one in the time domain using Bayesian inference. Both methods show that the strain amplitude at Earth from this pulsar is less than a few times $10^{-22}$.Comment: 7 pages, 1 figure, to appear in the Proceedings of the 5th Edoardo Amaldi Conference on Gravitational Waves, Tirrenia, Pisa, Italy, 6-11 July 200

Search for gravitational wave bursts in LIGO's third science run

We report on a search for gravitational wave bursts in data from the three LIGO interferometric detectors during their third science run. The search targets subsecond bursts in the frequency range 100-1100 Hz for which no waveform model is assumed, and has a sensitivity in terms of the root-sum-square (rss) strain amplitude of hrss ~ 10^{-20} / sqrt(Hz). No gravitational wave signals were detected in the 8 days of analyzed data.Comment: 12 pages, 6 figures. Amaldi-6 conference proceedings to be published in Classical and Quantum Gravit

Improving the sensitivity to gravitational-wave sources by modifying the input-output optics of advanced interferometers

We study frequency dependent (FD) input-output schemes for signal-recycling interferometers, the baseline design of Advanced LIGO and the current configuration of GEO 600. Complementary to a recent proposal by Harms et al. to use FD input squeezing and ordinary homodyne detection, we explore a scheme which uses ordinary squeezed vacuum, but FD readout. Both schemes, which are sub-optimal among all possible input-output schemes, provide a global noise suppression by the power squeeze factor, while being realizable by using detuned Fabry-Perot cavities as input/output filters. At high frequencies, the two schemes are shown to be equivalent, while at low frequencies our scheme gives better performance than that of Harms et al., and is nearly fully optimal. We then study the sensitivity improvement achievable by these schemes in Advanced LIGO era (with 30-m filter cavities and current estimates of filter-mirror losses and thermal noise), for neutron star binary inspirals, and for narrowband GW sources such as low-mass X-ray binaries and known radio pulsars. Optical losses are shown to be a major obstacle for the actual implementation of these techniques in Advanced LIGO. On time scales of third-generation interferometers, like EURO/LIGO-III (~2012), with kilometer-scale filter cavities, a signal-recycling interferometer with the FD readout scheme explored in this paper can have performances comparable to existing proposals. [abridged]Comment: Figs. 9 and 12 corrected; Appendix added for narrowband data analysi

Searching for a Stochastic Background of Gravitational Waves with LIGO

The Laser Interferometer Gravitational-wave Observatory (LIGO) has performed the fourth science run, S4, with significantly improved interferometer sensitivities with respect to previous runs. Using data acquired during this science run, we place a limit on the amplitude of a stochastic background of gravitational waves. For a frequency independent spectrum, the new limit is $\Omega_{\rm GW} < 6.5 \times 10^{-5}$. This is currently the most sensitive result in the frequency range 51-150 Hz, with a factor of 13 improvement over the previous LIGO result. We discuss complementarity of the new result with other constraints on a stochastic background of gravitational waves, and we investigate implications of the new result for different models of this background.Comment: 37 pages, 16 figure

Stochastic background from extra dimensions

The existence of extra dimensions is a common feature in almost all quantum theories of gravity. These extra dimensions, whose size and number vary from theory to theory, have a signature on the gravitational stochastic background. Here we review the predictions of the individual theories and the hope of revealing such signals with earthbound detectors