X射线双星SAX J1808 .4-3658中光学周期调制的精确测定
An Accurate Determination of the Optical Periodic Modulation in the X-Ray Binary SAX J1808.4-3658
摘要:
我们报告的光学成像的X射线二进制SAX J1808.4-3658与8米双子座南望远镜。双星,包含一个吸积动力毫秒脉冲星,似乎有一个大的周期性调制在其静态光发射。为了阐明这种调制的起源,我们在五天内获得了三个时间分辨的$r'$波段光变曲线(LC)的源。LC可以用正弦曲线来描述,它们之间的长时间跨度使我们能够确定MJD 54599.0处的光学周期P=7251.9 s和相位0.671(TDB;相位0.0对应于脉冲星轨道的升交点),不确定度分别为2.8 s和0.008(90%置信度)。这种周期性与X射线轨道星历高度一致。通过考虑这种一致性和LC的正弦形状,我们排除了由吸积盘引起的调制的可能性。我们的研究支持先前的建议,X射线脉冲星成为旋转供电在静止,其能量输出照射伴星,造成光学调制。虽然也有人认为吸积盘会被脉冲星蒸发,但我们认为吸积盘是存在的,并产生持续的光辐射。盘的存在可以通过对静止源的长期多波长光学监测来验证,因为根据标准盘不稳定性模型,预计来自源的通量和光谱变化会增加。
英文摘要:
We report on optical imaging of the X-ray binary SAX J1808.4-3658 with the 8-m Gemini South Telescope. The binary, containing an accretion-powered millisecond pulsar, appears to have a large periodic modulation in its quiescent optical emission. In order to clarify the origin of this modulation, we obtained three time-resolved $r'$-band light curves (LCs) of the source in five days. The LCs can be described by a sinusoid, and the long time-span between them allows us to determine optical period P=7251.9 s and phase 0.671 at MJD 54599.0 (TDB; phase 0.0 corresponds to the ascending node of the pulsar orbit), with uncertainties of 2.8 s and 0.008 (90 % confidence), respectively. This periodicity is highly consistent with the X-ray orbital ephemeris. By considering this consistency and the sinusoidal shape of the LCs, we rule out the possibility of the modulation arising from the accretion disk. Our study supports the previous suggestion that the X-ray pulsar becomes rotationally powered in quiescence, with its energy output irradiating the companion star, causing the optical modulation. While it has also been suggested that the accretion disk would be evaporated by the pulsar, we argue that the disk exists and gives rise to the persistent optical emission. The existence of the disk can be verified by long-term, multi-wavelength optical monitoring of the source in quiescence, as an increasing flux and spectral changes from the source would be expected based on the standard disk instability model.
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