NGC 6624中低质量X射线双星4U 1820-30令人困惑的负轨道周期导数
The Puzzling Negative Orbit-Period Derivative of the Low-Mass X-Ray Binary 4U 1820-30 in NGC 6624
摘要：
4 U 1820-30是一颗低质量的X射线联星，位于球状星团NGC 6624的中心附近，至少由一颗中子星和一颗氦白矮星组成。通过分析罗西X射线定时探测器（RXTE）16年的数据，我们可以测量出它的轨道周期和时间导数，其精确度前所未有，分别为P = 685.01197 +- 0.00003 s和dP/dt /P = -5.3 +- 0.3x10^-8 yr^-1。因此，我们确认，周期导数是显着的负在>17西格玛的水平，相反的理论预期的一个孤立的X射线二进制。我们讨论了可以解释这种差异的可能情况，并得出结论，NGC 6624的中心很可能包含大量的非发光物质，如黑暗遗迹。我们还讨论了NGC 6624内部IMBH的可能性，或者接近4U 1820-30的暗残留物导致观察到的位移。
英文摘要：
4U 1820-30 is a low-mass X-ray binary near the center of the globular cluster NGC 6624 consisting of, at least, one neutron star and one helium white dwarf. Analyzing 16 years of data from the Rossi X-ray Timing Explorer (RXTE) allows us to measure its orbital period and its time derivative with unprecedented accuracy to be P = 685.01197 +- 0.00003 s and dP/dt /P = -5.3 +- 0.3x10^-8 yr^-1. Hence, we confirm that the period derivative is significantly negative at the >17 sigma level, contrary to theoretical expectations for an isolated X-ray binary. We discuss possible scenarios that could explain this discrepancy, and conclude that the center of NGC 6624 most likely contains large amounts of non-luminous matter such as dark remnants. We also discuss the possibility of an IMBH inside NGC 6624, or that a dark remnant close to 4U 1820-30 causes the observed shift.
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