New Limits on Gravitational Radiation using Pulsars

We calculate a new gravitational wave background limit using timing residuals from PSRs J1713+0747, B1855+09, and B1937+21. The new limit is based on 17 years of continuous data pieced together from 3 different observing projects: 2 at the Arecibo Observatory and 1 at the 140ft Green Bank Telescope. This project represents the earliest results from the `Pulsar Timing Array' which will soon be able detect the stochastic background from early massive black hole mergers.Comment: 12 pages, 11 figures, 3 tables. To appear in the Proceedings of the 270. WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants, Jan. 21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J. Truemper. Proceedings are available as MPE-Report 27

Quantum effects in gravitational wave signals from cuspy superstrings

We study the gravitational emission, in Superstring Theory, from fundamental strings exhibiting cusps. The classical computation of the gravitational radiation signal from cuspy strings features strong bursts in the special null directions associated to the cusps. We perform a quantum computation of the gravitational radiation signal from a cuspy string, as measured in a gravitational wave detector using matched filtering and located in the special null direction associated to the cusp. We study the quantum statistics (expectation value and variance) of the measured filtered signal and find that it is very sharply peaked around the classical prediction. Ultimately, this result follows from the fact that the detector is a low-pass filter which is blind to the violent high-frequency quantum fluctuations of both the string worldsheet, and the incoming gravitational field.Comment: 16 pages, no figur

Gravitational Waves Probe the Coalescence Rate of Massive Black Hole Binaries

We calculate the expected nHz--$\mu$Hz gravitational wave (GW) spectrum from coalescing Massive Black Hole (MBH) binaries resulting from mergers of their host galaxies. We consider detection of this spectrum by precision pulsar timing and a future Pulsar Timing Array. The spectrum depends on the merger rate of massive galaxies, the demographics of MBHs at low and high redshift, and the dynamics of MBH binaries. We apply recent theoretical and observational work on all of these fronts. The spectrum has a characteristic strain $h_c(f)~10^{-15} (f/yr^{-1})^{-2/3}$, just below the detection limit from recent analysis of precision pulsar timing measurements. However, the amplitude of the spectrum is still very uncertain owing to approximations in the theoretical formulation of the model, to our lack of knowledge of the merger rate and MBH population at high redshift, and to the dynamical problem of removing enough angular momentum from the MBH binary to reach a GW-dominated regime.Comment: 31 Pages, 8 Figures, small changes to match the published versio

Constraints on cosmic string tension imposed by the limit on the stochastic gravitational wave background from the European Pulsar Timing Array

We investigate the constraints that can be placed on the cosmic string tension by using the current Pulsar Timing Array limits on the stochastic gravitational wave background (SGWB). We have developed a code to compute the spectrum of gravitational waves (GWs) based on the widely accepted one-scale model. In its simplest form the one-scale model allows one to vary: (i) the string tension, G\mu/c^2; (ii) the size of cosmic string loops relative to the horizon at birth, \alpha; (iii) the spectral index of the emission spectrum, q; (iv) the cut-off in the emission spectrum, n_*; and (v) the intercommutation probability, p. The amplitude and slope of the spectrum in the nHz frequency range is very sensitive to these unknown parameters. We have also investigated the impact of more complicated scenarios with multiple initial loop sizes, in particular the 2-\alpha models proposed in the literature and a log-normal distribution for \alpha. We have computed the constraint on G\mu/c^2 due to the limit on a SGWB imposed by data from the European Pulsar Timing Array. Taking into account all the possible uncertainties in the parameters we find a conservative upper limit of G\mu/c^2<5.3x 10^{-7} which typically occurs when the loop production scale is close to the gravitational backreaction scale, \alpha\approx\Gamma G\mu/c^2. Stronger limits are possible for specific values of the parameters which typically correspond to the extremal cases \alpha\ll \Gamma G\mu/c^2 and \alpha\gg \Gamma G\mu/c^2. This limit is less stringent than the previously published limits which are based on cusp emission, an approach which does not necessarily model all the possible uncertainties. We discuss the prospects for lowering this limit by two orders of magnitude, or even a detection of the SGWB, in the very near future in the context of the Large European Array for Pulsars and the Square Kilometre Array.Comment: 24 pages, 14 figures, accepted for publication in Physical Review D. Minor corrections and additional comments - updated to match the published versio

Using Pulsars to Detect Massive Black Hole Binaries via Gravitational Radiation: Sagittarius A* and Nearby Galaxies

Pulsar timing measurements can be used to detect gravitational radiation from massive black hole binaries. The ~106d quasi-periodic flux variations in Sagittarius A* at radio wavelengths reported by Zhao, Bower, & Goss (2001) may be due to binarity of the massive black hole that is presumed to be responsible for the radio emission. A 106d equal-mass binary black hole is unlikely based on its short inspiral lifetime and other arguments. Nevertheless the reported quasi-periodicity has led us to consider whether the long-wavelength gravitational waves from a conjectured binary might be detected in present or future precision timing of millisecond pulsars. While present timing cannot reach the level expected for an equal-mass binary, we estimate that future efforts could. This inquiry has led us to further consider the detection of binarity in the massive black holes now being found in nearby galaxies. For orbital periods of ~2000d where the pulsar timing measurements are most precise, we place upper limits on the mass ratio of binaries as small as 0.06.Comment: 7 pages, 2 eps figures, accepted for publication in Ap

PSR J0609+2130: A disrupted binary pulsar?

We report the discovery and initial timing observations of a 55.7-ms pulsar, J0609+2130, found during a 430-MHz drift-scan survey with the Arecibo radio telescope. With a spin-down rate of $3.1 \times 10^{-19}$ s s$^{-1}$ and an inferred surface dipole magnetic field of only $4.2 \times 10^{9}$ G, J0609+2130 has very similar spin parameters to the isolated pulsar J2235+1506 found by Camilo, Nice & Taylor (1993). While the origin of these weakly magnetized isolated neutron stars is not fully understood, one intriguing possibility is that they are the remains of high-mass X-ray binary systems which were disrupted by the supernova explosion of the secondary star.Comment: 5 pages, 2 figures, accepted for publication in MNRAS (letters