AAS 199th meeting, Washington, DC, January 2002
Session 24. Cosmology
Oral, Monday, January 7, 2002, 10:00-11:30am, International Ballroom Center

## [24.03] New Limits on the Gravitational Wave Background and Other Results from the Pulsar Timing Array

A. N. Lommen, D. C. Backer (U.C. Berkeley)

Other Gravitational Wave detectors have been built, or are being built (e.g., see Phinney 2000), but no human-made detector is sensitive to the wavelength range to which pulsar timing is sensitive, namely wavelengths near 1 year. Measuring the Gravitational Wave Background (GWB) would contribute significantly to our knowledge of both the epoch of inflation and the early merging of galaxies. We present new results from the Pulsar Timing Array" (PTA) which is an array of millisecond pulsars (MSPs) being used to detect the GWB. Our goal is to make the PTA as sensitive as possible. We have connected pre-upgrade Arecibo telescope data, with Green Bank 140-ft telescope data, with post-upgrade Arecibo data to make 17-yr data sets on both PSRs B1855+09 and B1937+21, and a 9-yr data set on PSR J1713+0747. These long baselines allow us to refine model parameters, and to suggest that a planet orbits PSR B1937+21. We update the curve of Timing Noise vs Period Derivative and show that we need not be as pessimistic as previously thought about timing noise limiting the sensitivity of the PTA. We show that MSP timing will soon have a lower fractional stability than terrestrial time standards. Finally, we place a new limit on the energy density in the stochastic background of gravitational radiation at nHz frequency of \Omegag h2 < 2 \times 10-9. This is more than an order of magnitude lower than the previous result by Kaspi, Taylor, & Ryba (1994).