激变变星中傅立叶相关时滞的发现
Discovery of Fourier-dependent time lags in cataclysmic variables
摘要：
我们报告的第一项研究的傅立叶频率相关的相干性和相位/时间滞后在光学波长的激变变量（MV Lyr和LU Cam）显示典型的闪烁变化的白光。使用ULTRACAM在威廉·赫歇尔望远镜上进行了观测。用SDSS滤波器$u'$、$g'$和$r'$同时获得了两个系统的光变曲线，其节奏在$\approx0.5-2$秒之间，并允许我们探测~10^{-3} Hz和~1 Hz之间的时间频率。我们发现在u '，g'和r'光变曲线之间的高相干性至少达到~10^{-2} Hz。此外，我们检测到红色/负滞后，其中红色波段滞后于蓝色波段，处于最低观测频率。对于MV Lyr，观察到的时间滞后长达约3秒，而LU Cam显示的时间滞后更大，约为10秒。试图解释在X射线双星和活动星系核中观察到的红色/负滞后现象的机制涉及到在致密天体附近产生的光子反射到吸积盘的表层上，其中滞后延迟仅仅是光从发射源到反射吸积盘区域的旅行时间。虽然这可能是一个可行的解释MV Lyr中观察到的滞后，在LU Cam中观察到的滞后太大，无法用来自磁盘和/或施主星的反射来解释。我们建议再加工的热时间尺度上的边界层光子到吸积盘作为一种可能的机制，以解释在吸积白矮星，或反向（由内而外）的冲击通过较冷的磁盘区域，因为它们向外移动的磁盘内观察到的滞后。
英文摘要：
We report the first study of Fourier-frequency-dependent coherence and phase/time lags at optical wavelengths of cataclysmic variables (MV Lyr and LU Cam) displaying typical flickering variability in white light. Observations were performed on the William Herschel Telescope using ULTRACAM. Lightcurves for both systems have been obtained with the SDSS filters $u'$, $g'$ and $r'$ simultaneously with cadences between $\approx0.5-2$ seconds, and allow us to probe temporal frequencies between ~10^{-3} Hz and ~1 Hz. We find high levels of coherence between the u', g' and r' lightcurves up to at least ~10^{-2} Hz. Furthermore we detect red/negative lags where the redder bands lag the bluer ones at the lowest observed frequencies. For MV Lyr time lags up to ~3 seconds are observed, whilst LU Cam displays larger time lags of ~10 seconds. Mechanisms which seek to explain red/negative lags observed in X-ray binaries and Active Galactic Nuclei involve reflection of photons generated close to the compact object onto the surface layers of the accretion disk, where the lag delay is simply the light-travel time from the emitting source to the reflecting accretion disk area. Although this could be a viable explanation for the lags observed in MV Lyr, the lags observed in LU Cam are too large to be explained by reflection from the disk and/or the donor star. We suggest reprocessing on the thermal timescale of boundary layer photons onto the accretion disk as a possible mechanism to explain the lags observed in accreting white dwarfs, or reverse (inside-out) shocks within the disk travelling through cooler disk regions as they move outwards.
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