The Universal sound of accretion

I'll begin this post by noting that some observational properties of accreting black holes/neutron stars (with mass ranges from a few solar masses to a few million) are very similar to those observed in lower-mass accreting white dwarfs. Some of these similarities have been noted when observing how the spectrum of these objects changes as a function of time during their daily, monthly or yearly evolution. For example most accreting systems become brighter from time to time due to matter in the accretion disk falling onto the compact object relatively quickly. This increase in brightness is soon followed by a burst at radio wavelengths due to the presence of a high velocity jet being ejected from the system (see figure 1 of "The different scales of accretion").

These similarities, however, have been observed on long(-ish) time-scales, and do not probe the fast behaviour of the accretion disks which tells us something about how matter is transported within the disk. Does the disks in accreting white dwarfs, which seemingly go through similar changes to accreting black holes and neutron stars, also show other properties which resemble their more massive cousins? Lightcurves, as opposed to spectra, allows us to probe the fast variations generated within the accretion disk, however at the cost of loosing most spectral information.

It turns out that when we observe X-ray lightcurves of accreting black holes and neutron stars (with mass ranges from a few solar masses to a few million) we find a tight linear correlation between the mean count rate (or simply flux/brightness) and the lightcurve rms (the root-mean-squared or simply the spread of variability): the so-called rms-flux relation. Although this might sound very technical (what is this rms?!?) the implications of the rms-flux relation are quite important, and tell us a lot about what is actually happening in the accretion disk very close to the compact object (which we see directly through X-rays) as well as further out in it's outskirts (which we do not see directly). The rms-flux relation in X-ray lightcurves imply that the different variability mechanisms which produce the observed variations must originate in different places in the disk (maybe different radii?), and must interact in a very specific way to produce the X-ray lightcurves we observe.

One example of this would be if we imagine mass-transfer variations (how much mass goes through per unit time) through the outer-most edges of the accretion disk (see "The different scales of accretion"), and allow these variations to propagate inwards through the disk. As the variations propagate inwards they would couple to the variations produced further in (which will be different from the ones further out), until they reach the inner-most regions of the disk and emit X-rays. What we would then observe in X-ray lightcurves of accreting systems would contain information regarding the mass-transfer variations throughout the whole disk, and not just it's inner-edges.

Figure 1: The sound from the different musicians in the valley will reach the listener at different times, making the song sound like a big mess. However all the information is there to recreate the original song! Excuse my poor art skills! :p

One analogy to the above example which comes to mind is the following: imagine a person standing at the end of a valley and listing to an orchestra play live in front of him/her, but where the musicians of the orchestra are all at different places relative to the listener. Some are at a few tens of meters, whilst others at a few hundred (see Figure 1). Although the whole orchestra will play in tune and keeping the correct rhythm, the listener will not perceive a nice, in-rythm. This because the sound from the different musicians will reach the listener at different times (due to constant sound speed in the valley), making the overall sound a big mess. Within this mess is however a lot of information, which in theory could be used to recreate the original song, taking into account the delays between the different musicians and pitch changes caused by the valley's shape. Although the above example is actually very crude compared to the accretion disks, I think it kind of gets the basic message across when we compare the sounds
of the different instruments travelling through the valley to the variations which travel through the accretion disk.

The inner-edges of the accretion disks in accreting white dwarfs emit mostly optical/UV radiation, as opposed to X-rays emitted by accreting black holes or neutron stars. To determine whether the "sound" of accretion around white dwarfs is similar to that of black holes/neutron stars I recently used optical lightcurves obtained with the Kepler satellite, and showed that one accreting white dwarf system called MV Lyrae also displays the so-called rms-flux relation. MV Lyrae, is in some ways similar to many other accreting white dwarfs which display a lot of variability (also referred to as flickering in the scientific community), and it might be that in fact all accreting white dwarfs might show this same feature, just like in X-rays for accreting neutron stars/black holes.

The above discovery has only been made possible thanks to the high-quality lightcurves obtained with the Kepler satellite, which contrary to observations taken from telescopes on Earth, can observe objects continuously at high precision without being hindered by day/night interruptions and atmospheric uncertainties. The implications of the result is that the accretion disks around white dwarfs seem to behave in a very similar way to the accretion disks surrounding black holes/neutron stars, although the sizes of these disks are over a billion times different(!!!)... but we have to look at the right wavelengths to find out!

Ciao
Simo