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