The steady spin-down rate of 4U 1907+09

Using X-ray data from the Rossi X-ray Timing Explorer, we report the pulse timing results of the accretion-powered, high-mass X-ray binary pulsar 4U 1 907 + 09, covering a time-span of almost two years. We measured three new p ulse periods in addition to the previously measured four pulse periods. We are able to connect pulse arrival times in phase for more than a year. The source has been spinning down almost at a constant rate, with a spin-down r ate of (nu) over dot = (- 3.54 +/- 0.02) x 10(-14) Hz s(-1) for more than 1 5 yr. Residuals of pulse arrival times yield a very low level of random-wal k noise, with a strength of similar to2 x 10(-20) rad(2) s(-3) on a time-sc ale of 383 d, which is 40 times lower than that of the high-mass X-ray bina ry pulsar Vela X-1. The noise strength is only a factor of 5 greater than t hat of the low-mass X-ray binary pulsar 4U 1626-67. The low level of the ti ming noise and the very stable spin-down rate of 4U 1907+09 make this sourc e unique among the high-mass X-ray binary pulsars, providing another exampl e, in addition to 4U 1626-67, of long-term quiet spin down from an accretin g source. These examples show that the extended quiet spin-down episodes ob served in the anomalous X-ray pulsars 1RXS J170849.0-400910 and IE 2259+586 do not necessarily imply that these sources are not accreting pulsars.