## Friday, 28 December 2012

### Almost Time

I will soon be embarking on my 3(hopefully) year journey that is my PhD.I can't say I'm not nervous but I must say I'm more excited. I've had quite a long break(almost 7 months) from doing anything really, (bar demonstrating in labs and a bit of reading) so I will have to see how easy it is to get back into the groove of work.

I start Jan 7th quite a strange time but my supervisor has been over in america so I had to wait for him to come back. At least I'm not alone in this I have a fellow PhD student starting with me.

### AGN

I suppose tell you about the subject of my PhD. I'm going to be observing AGN (Active Galactic Nuclei) using x-rays - what about them has not yet been confirmed still talking to my supervisor about that one.
AGN are thought to be made up of 5-6 regions the first of which is A super massive (billions of times the mass of the sun) black holes which are accreting matter. This liberates the matter of its gravitational potential, as the viral theorem states that it must radiate half of it's energy during it's in fall. Before the matter can accrete on to the central black hole it must lose it's angular momentum. This is done in the second region an accretion disk which forms from in falling matter which is flattened into a disk by it's rotation around the central black hole. The accretion disk emits as a black body with the innermost regions being the hottest and emitting in the extreme ultra-violet. Annoyingly the energy of a photon from the extreme ultra-violet part of the EM spectrum has enough energy to ionise hydrogen, and due to the hydrogen rich nature of our galaxy is completely absorbed.
The third and fifth regions are observed in the spectra of AGN they are the two line emitting regions. The BLR (Broad Line Region) so called as the lines in the spectra are broadened (who said physics is complicated ) the reason for the broadening is thought to be due to it's closer proximity to the black hole. The NLR (Narrow Line Region) is therefore further out than the BLR but it is also less dense as it contains forbidden lines which are emission lines that are normally collisionally suppressed at higher densities (atoms are not allowed to emit there lines normal as they take too long allowing other atoms bump into them) from this knowledge we can limits to the density of the NLR. Both regions are thought to be clumpy with the matter arranged in clouds.
The awake ones of you will have noticed that I missed out the forth region it is known as the dusty torus(ring doughnut shape) which absorbs radiation from the accretion disk and BLR, which heats it causing it to emit in the IR (infra-red). It has the effect of screening the accretion disk and BLR from the observer if it falls in the line of site. This is the basis behind the unified model of AGN, that the difference in AGN types are due to orientation which the AGN is observed from as illustrated in this image.

There is also another distinction between different types of AGN radio loud and radio quite. Radio loud AGN emit much more more in the radio band than their quite counterparts hence the name. This thought to be due to the presence(/or lack of) a relativistic jet.

Hope you have enjoyed my very brief overview the structure of AGN.

-Micky

P.S. This blog is going to become a place for me to keep track of my day to day work hope you guy's enjoy it, although it's really more for my benefit.