High-Energy Astrophysics -
More Work for Variable Star Observers!

Gordon Spear

Gordon Spear's talk was one of those whose drift was clearly going to appeal to the purely visual observers (such as myself). It was good to hear that not only are amateur observers still in demand by the professional community, but seem to be becoming more so! It needs to be borne in mind, however, that amateur observers these days are increasingly using detection methods other than the eye, the most notable being CCD's. And, for all the expense and initialisation problems that can accompany the CCD set-up, this will become an ever more important addition to the amateur observer's arsenal.

It is of course necessary for amateurs to continue following those stars they have traditionally followed such as the Mira stars and eruptive variables of all kinds. However, this talk outlined some of the hitherto-neglected type of object we could keep our eyes (or CCDs) on. Chief among these are the AGN's (Active Galactic Nuclei). These can be divided into some distinct types, though the difference between their appearance and observed behaviour is solely due to the spatial angle between us and the object concerned, and does not presume any physical diference between the types themselves. It is rather like close double stars - if at some point one star passes between us and its neighbour, we will have an eclipsing binary. The stars are not actually, physically variable. By the same token, the different types of AGN are not physically different. All that changes is our view of the system's various radiating components.

What all Active Galaxies have in common is a Supermassive Black Hole (SMBH) at their centres. Because there is so much material in the central regions of most galaxies, there is no shortage of 'fuel' for these monster black holes. The material swirls around the SMBH in what is known as an Accretion Disk - not to be confused with the accretion disks of Cataclysmic Variables, for those of you familiar with them! By a process not completely understood, but possibly caused by magnetic / gravitational effects, vast amounts of extremely energetic particles are ejected from the 'poles' of the black hole (i.e., at right-angles to the accretion disk) in the form of jets. These jets are often visible over distances of millions of light-years.

Depending on the orientation of the jet to us, we observe either a Quasar (if the Galaxy is so far away that it appears starlike), a Blazar if we are looking directly 'down' a jet, or a 'conventional' Active Galactic Nucleus otherwise.

The Blazar 3C279. We are looking down the jet right into the high-energy gamma radiation. Lower frequency radiation has been 'burnt out' by the highly energetic gamma raysActive Galaxy Centaurus A, with one jet visible. No gamma radiation can be seen; instead lower energy photons such as x-rays and visible light show up, revealing also some diffuse gas in the centre.

Blazars are named after the first such object to be discovered, BL Lacertae. Its name tells you that it was thought to be a variable star, since it appears stellar. Blazars vary on all sots of timescales with no discernible period. BL Lac itself has varied by about half a magnitude in half an hour! There also appears to be some correlation between variation rates and red shift - the greater the red shift, the more variation is observed. So BL Lac must be a very distantobject indeed.

There are already a few AGN's in the AAVSO's observational programme, and one hopes that more will be added. I know that BL Lacertae at least has been well followed in recent years by the Variable Star Section of the British Astronomical Association, since the late Walter Pennell enthused over it in past issues of "The Astronomer". To those observers who like to see some action, with their unpredictable and sometimes drastic light changes, they could prove popular objects to observe.