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

The different scales of accretion

In this first post I will try and describe the project I will be mainly undertaking during my time in Leuven. It has to do with accretion, and specifically the similarities observed within different accreting systems. Accretion is a ubiquitous phenomenon in the Universe, and is responsible for the growth of objects of all masses, from stellar-like objects to super-massive black holes at the centre of galaxies.

My project will specifically focus on a subset of accreting close interacting binary stars called cataclysmic variables. These systems usually consist of two stellar mass objects orbiting each other, but one of them is not actually a star but a white dwarf. The white dwarf is more massive that the star (although not by much on astronomical scales) however it's radius is much smaller. To make the point, one can imagine a white dwarf as having the same mass of our Sun, but all squished in a ball whose radius is that of planet Earth! The interesting and fascinating thing about cataclysmic variables (and indeed all close interacting binaries) is that the central compact object (a white dwarf in the case of cataclysmic variables) and the companion star are so close to each other that the gravitational potential of the compact object strips matter from the envelope of the secondary star.

Schematic view of a compact interacting binary. Credit: R. Hynes

This stripped matter is then on it's way to the white dwarf, and forms a disk in order to conserve angular momentum. This disk is very dynamic, and for material to get closer to the central white dwarf, angular momentum must be lost through interactions with surrounding material. For example, viscous interactions is a mechanism by which angular momentum is lost (but defiantly not the only one!), and matter can gradually get closer and closer to the white dwarf, until it can settle on it's surface and increase it's mass by a bit. The issue however is that the laws that govern the physics within accretion disks are very hard to grasp, since within these disks many effects come to play, like viscosity, magnetic fields, radiation, and in some cases also relativity. Models which seek to reproduce the observed behaviour within these disks sometimes get it right, but most times are wrong! If we were to understand fully how these disks behave and how they relate to the systems which produce them we would be able to also understand better how the systems evolve in the first place, and what their fate will be. This in turn can have direct implications to how galaxies evolve, since white dwarfs are the end-state of most stars in the Universe and most stars are also found in binaries. Supernova Ia are also the result of a white dwarf gaining so much mass (presumably by accretion) that it surpasses the Chandrasekhar limit, resulting in a supernova explosion which can be seen throughout the Universe and can be used to measure it's size! 

My project will relate some of the observed phenomena within the disks of cataclysmic variables to their more massive counterparts: X-ray binaries and Active Galactic Nuclei. X-ray binaries are also close interacting binary systems, however the accretor in these cases is not a white dwarf but a more massive black hole or neutron star. On top of being more massive, black holes and neutron stars are also much smaller! In fact, if a white dwarf has the mass of the sun squished into the radius of the Earth, a neutron star is even more massive, and is squished into the size of a city! Black holes take it even a step further and are so dense that we don't even refer to it's surface any more but to the event horizon: the point where nothing, not even light, can escape from it's gravitational pull. For comparison, a stellar-mass black hole will have it's event horizon at just a few kilometres! Active Galactic Nuclei on the other hand are not binary systems, but accreting super-massive black holes (with masses reaching a few million times the mass of the Sun), usually situated at the centre of Galaxies. Although at first all these systems might seem wildly different in both mass and size, they all have one common feature: an accretion disk! It is the similarities between these different sized disks which fuelled a lot of (what I think is) interesting research in the last few years: the fact that although these disks are seemingly different, there exist relations which, if scaled properly, allow us the see that they all behave in a very similar way. Cataclysmic variables offer a nice way to study these similarities since there are many more of these systems in the galaxy than there are X-ray binaries. If we can learn from the numerous cataclysmic variables and find similar behaviour to the X-ray binaries, then we can hope to get a step closer in understanding how all these different systems behave and relate to each other, including Active Galactic Nuclei!

Ciao!
Simo

Back on the blog!

Hello again! It's been a while since I last posted here! Hopefully I will make up for it by posting more often from now on. This is something I actually want to do for various reasons, but never actually get to do it as I loose my self in lines of code trying to analyse some data. Firstly, by posting on this blog I will have the chance to share some of my research and ideas with the wider public, but also with some experts in the field (hopefully!). Consequently, I hope some readers will comment back to ask questions and/or point out flaws in what I am doing. Secondly, I hope to fuel more ideas both for myself and others. Writing about what I do will help me to better focus on my research, whilst readers can find inspiration (or the lack of it!). Lastly, I recently got awarded a fellowship (!!!) to carry out 3 years of research at the Institute for Astronomy at KU Leuven, and in my proposal there was a specific section describing this blog post, and how I envisioned it. Well, now that I got the award I guess I need to stick to my word ;)

What I hope to do is to post every week or two a brief summary of what I am (trying!) to work on, or understand. This can be research I am actively doing myself, other peoples research I find interesting, or as sometimes will happen, observing runs at telescopes. The idea is to make this very informal, and try to keep the content simple enough so that people without a dedicated training in astrophysics can follow, but not too much as to loose the essence of some interesting results. I really hope I can manage this with this blog, and additionally I hope some readers bumping into this blog will comment back to let me know: 1) what they think of it 2) how I can improve it and 3) help me understand why some of those twinkly objects in the sky behave the way they do! :)

As I have not posted here for a while, I will post a few things soon to try and catch up! First a brief introduction on the project I am meant to carry out here in Leuven, and then a brief summary of some of articles of mine which recently got published (or are in the process of). Hopefully this will then set the scene! Hope you enjoy the reading!

Ciao
Simo