One Disk to Rule Them All

Figure 1. A typical dwarf nova. Credit NASA via www.skyandtelescope.com

In a study accepted for publication in the Monthly Notices of the Royal Astronomical Society (MNRAS), astronomers Aranzana et al. reported on Fourier time lags in the dwarf nova SS Cyg.

SS Cyg is a well studied variable star system in the constellation Cygnus the swan. Specifically, it is a special type of cataclysmic variable called dwarf novae (DNe). DNe consist of a binary system made up of a primary white dwarf that every so often siphons off material from its low-mass companion that has overfilled its Roche Lobe. This material forms an accretion disk that swirls around the primary star. It is this accretion disk that will occasionally brighten across a variety of wavelengths. These "brightenings" are associated with outbursts and, along with DNe, are observed across a variety of astrophysical phenomena such as X-ray binaries (XRBs) and Active Galactic Nuclei (AGN).  Although these are widely different phenomena, astronomers have evidence to support the idea that they share the same underlying accretion disk that gives rise to the outbursts. Subsequently, these are interesting targets for research since studying any of these objects could shed light on all the others.

There have been several studies of that give credence to the notion of an accretion disk connection between CVs, XRBs, and AGN. For example, all three objects have demonstrated that increases in brightness are correlated with increases in variability. This suggests mass transfer to the accretion disk resulting in the aforementioned increases in both brightness and variability. Furthermore, fluctuations in the outer, cooler parts of the disk are hypothesized to travel inwards towards hotter regions. This leads to emission of soft photons (lower energy) from the cooler areas of the disk before hard photons (higher energy) from the hotter, inner regions of the disk. Subsequently, there have been X-ray studies where soft photons arrive at astronomers' detectors slightly before hard photons, a so-called 'hard/positive' time lag. Because this phenomena presumably requires mass transfer, it is only observed during an outburst.

However, what about periods of no mass transfer? There have been reports of 'soft/negative' time lags in CVs, meaning that the hard photons arrive before the soft ones. There is debate over the cause of this soft lag. One explanation is that emission from the source (white dwarf) lights up the disk, which in turn creates a reflection spectrum. However, this idea suffers from the report that such a spectrum would lie outside the binary orbit. An alternative explanation is that the photons from the white dwarf are thermally reprocessed in the disk, leading to hard photons being emitted from the accretion disk before soft ones (i.e., a soft/negative lag, see Figure 2).

Figure 2. Diagram showing emission of photons from the white dwarf being reprocessed in the disk. Since the higher energy/hard (u) photons are closer to the source, they get reprocessed before the lower energy/soft (r) photons resulting in the 'soft lag'. Credit Aranzana et al. 2018

The purpose of the study was to gather more evidence that could improve upon or rule out either of the models. SS Cyg was chosen as an optical target because it is one of the brightest DNe during quiescence (no activity). Researchers used the 4.2 meter William Herschel Telescope located in the Canary Islands, Spain over the course of two nights to take images using SDSS filters Ultraviolet (u), Green (g), and Red (r). In astronomy, color is typically represented by combining two filters. For example, r & u is the combination of the red and ultraviolet filters. Figure 3 shows the results for the three different color combinations averaged over both nights. At a frequency of 4 x 10^-3 Hz there is about a -6 second time lag in the g & u combination and -4 second time lag in the r & u combination.

Figure 3. SS Cyg during quiescence demonstrating a significant -4 second time lag for r&u and -6 second time lag for g&u at 0.004 Hz. The r&g Credit Aranzana et al. 2018

Due to the associated error bars, the r & g combination is consistent with no lag. The researchers suggest that this could be due to the fact that the source is also emitting primarily in this band. However, the overall results confirm the existence of soft time lags in the dwarf nova SS Cyg. Further lines of inquiry include expanding the number of CVs studied to see if the time lag is repeatable and consistent and also continued monitoring of the relationship with XRB and AGN time lags. Continued study of these time lags in various types of astronomical phenomena with accretion disks could help understand the underlying physics that unite them.