Eavsdropping on accretion disks: The Kepler view

Following from the previous post, the specific goal of my project is to determine whether the accretion disks in accreting black holes and neutron stars show similar features to the less massive accreting white dwarfs by "listening to their sound". Obviously this is not possible you might think: listening to sound? in space?!?! Technically that's right, you can't "listen" in space, but what you can do is look at tiny variations in the emitted light of these objects, and apply similar techniques as those used to analyse sound waves to understand the seemingly random light variations. These techniques transform lightcurves (time plotted against flux/magnitude, see Fig.1) to Fourier spectra, where one looks at the power (or amplitudes) generated at different frequencies (see Fig.2).

Fig.1: Light curve of the cataclysmic variable (accreting white dwarf) MV Lyrae obtained with the Kepler satellite. The observation consists of over 600 days of quasi-continuos monitoring every ~58 seconds and shows typical flickering observed in accreting white dwarfs.

To this end I am interested in the timing properties of accretion disks, and specifically at the non-periodic variability properties, also referred to as flickering in cataclysmic variables. I am trying to find (or not!) phenomenological similarities between the flickering properties of accreting white dwarfs to those observed in accreting black holes an neutron stars.

Fig.2: Power spectrum showing Frequency vs. Power x Frequency for the MV lyr lightcurve shown in Fig.1. The main features to note are the fact that over a wide range of timescales (thus frequencies) long variations contribute more power than shorter ones, and that at about 10^-3 Hz (tens of minutes) power drops very steeply.

In this respect a lot of work on the different techniques has already been developed to study variability in X-rays for X-ray binaries and Active Galactic Nuclei (accreting black holes or neutron stars). Until recently however these techniques could not have been applied to studies of accreting white dwarfs, since these systems are not X-ray bright sources, but instead emit most of their light in the optical/ultraviolet wavebands. This difference is mainly attributed to the fact that black holes and neutron stars have deep gravitational potential wells when compared to white dwarfs, resulting in the fact that most emission is emitted in X-rays close to the central compact object. Conversely the potential well of a white dwarf is not so deep, and although most of the emission is also originating from close to the central compact object, the emitting region is much further out (see comparison in Fig.3), resulting in the emission being optical/ultraviolet.

Fig.3: Schematic view of the two potential wells.

The major obstacle in applying the techniques developed for X-ray binaries is that optical observations of accreting white dwarfs from Earth are hindered by the fact that observations are not continuous (day/night interruptions) and suffer from atmospheric effects resulting in uncertainties in the data. However this has all changed since the advent of the Kepler satellite. Kepler is a space observatory (an awesome one too!) which observes one, and only one, patch of sky continuously since it's launch. It does this to simultaneously obtain lightcurves for over 150,000 objects in it's field-of-view of about 100 square degrees. It's main science drive is that of finding as many extrasolar planets as possible through continuous monitoring of many stars. The idea of detecting extrasolar planets is simple: when the planet passes in front of it's parent star during it's orbit we perceive a small change in brightness, and this change occurs periodically with the period being that of the planet around it's star. These changes in brightness are minuscule, and because of that Kepler is very sensitive to these small brightness changes. All of these specifications (the accuracy of the obtained lightcurves together with the continuous monitoring of objects) make Kepler a perfect tool to study variability in cataclysmic variables too! In fact thanks to Kepler we can, for the first time, apply the techniques developed to study X-ray binaries and Active Galactic Nuclei to cataclysmic variables. This is where my project lies: comparing the variability properties of cataclysmic variables to those of X-ray binaries with the aim to find similarities and differences between the two types of systems, and consequently learn something about how the accretion disks behave and what are the main physical process governing their dynamics. :)

The Kepler space observatory. Credit: NASA.

Ciao!
Simo