PSR J1909-3744: A Binary Millisecond Pulsar with a Very Small Duty Cycle

We report the discovery of PSR J1909-3744, a 2.95 millisecond pulsar in anearly circular 1.53 day orbit. Its narrow pulse width of 43 microsecondsallows pulse arrival times to be determined with great accuracy. We havespectroscopically identified the companion as a moderately hot (T = 8500 K)white dwarf with strong absorption lines. Radial velocity measurements of thecompanion will yield the mass ratio of the system. Our timing data suggest thepresence of Shapiro delay; we expect that further timing observations, combinedwith the mass ratio, will allow the first accurate determination of amillisecond pulsar mass. We have measured the timing parallax and proper motionfor this pulsar which indicate a transverse velocity of 140 (+80/-40) km/s.This pulsar's stunningly narrow pulse profile makes it an excellent candidatefor precision timing experiments that attempt to detect low frequencygravitational waves from coalescing supermassive black hole binaries.Comment: 12 pages, 4 figures. Accepted for publication in ApJ