Dwarf Novae: Mining Kepler Data for New Discoveries

Cataclysmic variable stars (CVs) are binary phenomena, usually consisting of a white dwarf that is accreting material out of a nearby companion star. As you would imagine, a wide range of CVs in various stages of accretion and subsequent outburst can be detected. When the accretion disk around the white dwarf becomes unstable, we get what is known as a dwarf nova (DN), and in systems with orbital periods less than two hours, there can be much more violent outbursts, feeding off orbital resonances in the orbit of the two stars.

Now we have a newly discovered cataclysmic variable (KSN:BS-C11a) in an interesting configuration, a white dwarf apparently feeding off a brown dwarf companion that is about 10 times less massive. The ‘super-outburst’ from the dwarf nova turned up in data from the decommissioned Kepler Space Telescope. Grad student Ryan Ridden-Harper (Australian National Observatory), lead author of the paper on this work, likes to refer to this cataclysmic variable as ‘a vampire star system.’ Whatever we call it, the level of activity here is noteworthy:

“The incredible data from Kepler reveals a 30-day period during which the dwarf nova rapidly became 1,600 times brighter before dimming quickly and gradually returning to its normal brightness. The spike in brightness was caused by material stripped from the brown dwarf that’s being coiled around the white dwarf in a disk. That disk reached up to 11,700 degrees Celsius at the peak of the super-outburst.”

Image: An artist’s impression of a star ‘feeding’ off a nearby brown dwarf. Credit: NASA and L. Hustak (STScI)

Still unexplained is the slow rise in brightness that preceded the outburst. Ridden-Harper has been working with colleagues at ANU as well as the Space Telescope Science Institute (STScI) and the University of Notre Dame. What catches my eye about the discovery of this bright transient is the data mining that turned it up. The team had been searching for new transients in the K2 and Kepler campaigns in a project known as K2: Background Survey. The idea here is that each science target in the data is accompanied by background pixels that have been observed at high cadence, meaning a short time period before re-observing the same target.

These background pixels, the authors note, can contain transient signals that have thus far gone undetected. The paper describes the K2: Background Survey as:

…a systematic search for transients in K2/Kepler background pixels. K2:BS independently analyses each pixel to detect abnormal behaviour. This is done by searching for pixels that rise above a brightness threshold set from the median brightness and standard deviation through a campaign. Telescope motion presents a challenge in candidate detection as science targets may drift into background pixel, triggering false events. False triggers are screened by vetting of events that last < 1 day, chosen for candidates with the 6 hourly telescope resets. Coincident pixels that pass the vetting procedure are grouped into an event mask. All candidate events are checked against the NASA/IPAC Extragalactic Database (NED) 1 and the SIMBAD database (Wenger et al. 2000) to identify potential hosts.

As to the parameters of the system, the brown dwarf orbits the white dwarf every 83 minutes at a distance of approximately 400,000 kilometers, close enough to allow the rapid growth of the accretion disk as material spirals from the brown dwarf inward toward the host star. The Kepler cadence of 30 minutes turns out to have been the key to making these observations, which are particularly useful because only about 100 such dwarf novae systems have been catalogued.

Image: This is Figure 3 from the paper. Caption: The K2/Kepler light curve of the transient KSN:BS-C11a shown with the 30-minute cadence. The time axis is shown in barycentric Julian days and the flux has been converted to Kepler magnitudes. Credit: Ridden-Harper et al.

Thus Kepler/K2 keeps on producing good science, in this case finding a transient whose rarity stems partially from the years or decades such a system can spend between outbursts. We shouldn’t be too surprised that Kepler data can produce such results — after all, the telescope was designed to study the transits of planets across the face of a star, making it ideal for spotting any objects that brighten or dim over time — but now we can see the potential for detecting other rare transient events both in archival data and data from ongoing missions like TESS.

The paper is Harper et al., “Discovery of a new WZ Sagittae-type cataclysmic variable in the Kepler/K2 data,” Monthly Notices of the Royal Astronomical Society,” Vol. 490, Issue 4 pp. 5551-5559 (abstract).