COSMIC GRAVITATIONAL-WAVE BACKGROUND - LIMITS FROM MILLISECOND PULSAR TIMING

We use seven years of millisecond pulsar-timing data to derive rigorous upper bounds on a cosmic gravitational-wave background. The energy density per logarithmic frequency interval near f=0.14 yr-1 must be <9×10-8 (68% confidence), or <4×10-7 (95% confidence) of the density required to close the Universe. We detect a red-noise signal in the residuals for one pulsar. Its spectrum appears flatter than the f-5 power law expected for a gravity-wave background; likely noise sources include changes in the interstellar medium and pulsar rotational instabilities. © 1990 The American Physical Society.