Millisecond Pulsar PSR 1937+21: A Highly Stable Clock
L. A. RAWLEY 1,
J. H. TAYLOR 1,
M. M. DAVIS 2, and
D. W. ALLAN 3
1 Physics Department, Princeton University, Princeton, NJ 08544.
2 Arecibo Observatory, Arecibo, PR 00613.
3 Time and Frequency Division, National Bureau of Standards, Boulder, CO 80303.
The stable rotation and sharp radio pulses of PSR 1937+21 make this pulsar a clock whose long-term frequency stability approaches and may exceed that of the best atomic clocks. Improvements in measurement techniques now permit pulse arrival times to be determined in 1 hour at the Arecibo radio telescope with uncertainties of about 300 nanoseconds relative to atomic time. Measurements taken approximately every 2 weeks since November 1982 yield estimates of fractional frequency stability that continue to improve with increasing averaging time. The pulsar's frequency stability is at least as good as 6 x 10-14 for averaging times longer than 4 months, and over the longest intervals the measurements appear to be limited by the stability of the reference atomic docks. The data yield a firm upper limit of 7 x 10-36 gram per cubic centimeter for the energy density of a cosmic background of gravitational radiation at frequencies of about 0.23 cycle per year. This limit corresponds to approximately 4 x 10-7 of the density required to close the universe.
