New millisecond pulsar in a binary system

Abstract

Recent observations^1,2 at the Arecibo Observatory have resulted in the discovery of PSR1855+09, a pulsar with period P=5.362 ms, moving in a nearly circular orbit of period 12.3 days. The pulsar is only the third one known with P < 10 ms, and the sixth known radio pulsar in a binary system. (Discovery of a seventh binary pulsar is announced in an accompanying paper³.) Three of the seven binaries are among the fastest five of more than 400 pulsars—a fact that provides strong support for the conclusion that fast pulsars are ‘recycled’ neutron stars, spun up during a phase of mass accretion from an evolving companion star. The pulsar has a small dispersion measure (13.3 cm⁻³ pc), suggesting a distance of only ∼350 pc. The proximity of this pulsar and its location within 25 pc of the galactic plane argue that millisecond pulsars form a significant fraction (∼10%) of the pulsar population, leaving many detectable ones undiscovered^4,5. Its signal is strong enough to permit pulse-arrival-time measurements with single-day uncertainties of <3µs. Timing observations already suggest that PSR1855+09, like the 1.5-ms pulsar PS R1937+21, will prove to be a natural clock of extremely high stability⁶. The existence of a second pulsar with extremely small timing uncertainties will greatly aid the search for background gravitational waves using millisecond pulsars as detectors.