NGTS和HST对GW Librae中长周期调制的见解
NGTS and HST insights into the long period modulation in GW Librae
摘要：
作为下一代凌日调查的一部分，获得了2017年跨越7.5个月的白矮星增积器GW Librae的光变曲线。该数据集包括来自148个晴朗夜晚的787小时测光，允许在比以前实现的更长的观测基线上从夜晚到夜晚跟踪长（小时）和短周期（20分钟）调制信号的行为。在GW Lib的观测中间歇性地检测到的长周期调制被发现是一个持续的特征，在几天的时间尺度上以~ 83分钟和2- 4小时的周期在状态之间演化。发现20 min信号在活动期间具有广泛稳定的振幅和频率，但确认了先前注意到的相位不稳定性。利用哈勃太空望远镜上的宇宙起源摄谱仪获得的紫外线观测结果将4小时调制的紫外线与光通量之比限制为~5，并将20分钟调制的紫外线与光通量之比限制为<=1，并对非同时观测提出了警告。这些结果增加了进一步的观测证据，表明这些神秘的信号必须来自白矮星，突出了我们在理论上理解驱动它们的机制方面的持续差距。
英文摘要：
Light curves of the accreting white dwarf pulsator GW Librae spanning a 7.5 month period in 2017 were obtained as part of the Next Generation Transit Survey. This data set comprises 787 hours of photometry from 148 clear nights, allowing the behaviour of the long (hours) and short period (20min) modulation signals to be tracked from night to night over a much longer observing baseline than has been previously achieved. The long period modulations intermittently detected in previous observations of GW Lib are found to be a persistent feature, evolving between states with periods ~83min and 2-4h on time-scales of several days. The 20min signal is found to have a broadly stable amplitude and frequency for the duration of the campaign, but the previously noted phase instability is confirmed. Ultraviolet observations obtained with the Cosmic Origin Spectrograph onboard the Hubble Space Telescope constrain the ultraviolet-to-optical flux ratio to ~5 for the 4h modulation, and <=1 for the 20min period, with caveats introduced by non-simultaneous observations. These results add further observational evidence that these enigmatic signals must originate from the white dwarf, highlighting our continued gap in theoretical understanding of the mechanisms that drive them.
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