## Friday, 7 October 2011

### My Dissertation, Decisions Decisions! - X-ray Binaries

Recently I was given the suggested list of topics for my dissertation. However none of them took my fancy, so started my search for another topic. I thought about Active Galactic Nuclei(AGN) as it is the topic for which I would like to study for my Ph.D. Alas it had been done last year and they strongly recommend against something that was done in the previous couple of years.

So I was trying to think of a reasonably exciting topic and hopefully something with some sort of relation to AGN. In my lab work this week we where talking about Binary systems (I've got to update some predictions of when there gonna eclipse) this got me thinking about X-ray Binaries. So I've had a little bit of look around for some paper and books -still in the process of reading them. They're pretty interesting and accretion powered just like AGN which is a win win.

Heres a bit of background on binary star systems their when 2 stars orbit a common centre of mass(around each other) in an X-ray binary system one of the stars is a compact object(neutron star or a black hole). As the compact object has a strong gravitational field is can sometimes take some of the matter from is 'companion' star this produces X-rays hence there name.

So here's what I know so far. X-ray binaries come in two types - when companion star is high or low mass. In the high mass case the companion is massive enough to resist the compact objects gravitational pull however due to its high mass it also burns much hotter and this drives off some matter(this is known as stellar wind), which is then collected by the compact object. These systems tend to undergo pulsations in X-ray observations and there companion star tends to dominate the optical emissions.

The second type is when the low mass companion star can not resist the pull of the compact object. In this situation the matter of the companion closest to the compact object pulled towards it more than the matter on the other side. This creates a distortion and allows some of the matter to pass point at which the companion can pull it back -this is called a Lagrangian point. Once passed the Lagrangian point the matter heads straight for the compact object- This is called Roche-lobe overflow. It can't fall directly in as angular momentum must be conserved so the matter forms a disk around the compact object-this is also true for the former case.

Once in the accretion disk the material loses angular momentum as it does this it gets closer to the compact object before in the inner regions it falls into it. It is from the inner regions of the which the X-rays are emitted.

The energy gained in accretion is gravitational potential energy. Its really just a fancy way of saying the energy given to matter when its pulled on by other matter and as gravity gets stronger the closer the masses. The more massive the object pulling the matter in and the closer the matter gets to the object the more energy is released. So as the compact objects are very very massive the matter falling into them gains a huge amount of energy.

Thats no all I've learned but its getting pretty late and this post has gotten quite long so I'll leave it here for now.