Spin-down rate of 1E 2259+586 from RXTE observation

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We present new X-ray observations of the X-ray pulsar 1E 2259+586, obtained during March 1997, with the Rossi X-Ray Timing Explorer (RXTE). We have measured the pulse frequency derivative ν= (-1.08 ± 0.04) × 10⁻¹⁴ Hz s⁻¹ from pulse arrival times obtained in a sequence of 5 observations spread over one month. This ν is consistent with the long term spin-down trend. We also found that the observed X-ray luminosity is consistent with that measured at quiescent X-ray flux levels by previous missions. Our observations imply that 1E 2259+586 was spinning down steadily without exhibiting any stochastic torque noise fluctuations during the month covered by our observations.