Chakrabarty Deepto, Morgan Edward H, Muno Michael P, Galloway Duncan K, Wijnands Rudy, Van Der Klis Michiel, Markwardt Craig B
Department of Physics and Center for Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Nature. 2003 Jul 3;424(6944):42-4. doi: 10.1038/nature01732.
Millisecond pulsars are neutron stars that are thought to have been spun-up by mass accretion from a stellar companion. It is not known whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many neutron stars that are accreting mass from a companion star exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces. Millisecond-period brightness oscillations during bursts from ten neutron stars (as distinct from other rapid X-ray variability that is also observed) are thought to measure the stellar spin, but direct proof of a rotational origin has been lacking. Here we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting neutron stars, corroborating earlier evidence. The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most neutron-star models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars.
毫秒脉冲星是中子星，被认为是通过从恒星伴星吸积物质而加速自转的。目前尚不清楚这个过程是否存在自然刹车机制，或者它是否会一直持续下去，直到在亚毫秒周期达到离心瓦解极限。许多从伴星吸积物质的中子星会出现持续数十秒的热核X射线爆发，这是由其表面不稳定的核燃烧引起的。十个中子星爆发期间的毫秒周期亮度振荡（与观测到的其他快速X射线变异性不同）被认为可以测量恒星的自转，但一直缺乏旋转起源的直接证据。在这里，我们报告了在一个吸积毫秒脉冲星已知自转频率下对爆发振荡的探测，并且我们表明这些振荡总是具有相同的旋转相位。这有力地确立了爆发振荡是追踪吸积中子星自转的核动力脉动，证实了早期的证据。对于大多数中子星模型，11个核动力脉冲星的自转频率分布在远低于瓦解频率处截止，支持了引力辐射损失可以限制在加速毫秒脉冲星过程中吸积扭矩的理论预测。
