What makes a blazar like OJ287 special?


Even if astronomers have a hard job explaining the singular behaviour of blazars, they have a good idea what makes them so peculiar. OJ287 is unusual for two reasons, one of which is the presumption that it has two black holes in its heart rather than the single one in most quasars and blazars. The second though is a product of the black hole.


Black holes, as we have seen, do not have an insatiable appetite. If more material tries to fall into one than it can swallow the excess will be expelled at almost the speed of light in two jets that will blow out from the poles of the black hole’s accretion disk. Normally we do not see this jet, but what happens if it points towards us? Just at random, a few quasars will happen to line up such that the jet points almost exactly towards the Earth. This is what leads to us seeing a quasar as a blazar; we are looking more or less along the jet. Inside it material is moving towards us at more than 99.9% of the speed of light, giving rise to some odd effects from relativity: firstly, the light is concentrated like a light house beam – the faster the material in the jet is moving, the more concentrated the beam. This means that we see the blazar as much more luminous than it really is – yes, a searchlight is bright, but what makes it seem so intense is that reflector behind it that concentrates the light into a tight beam, that is what makes a blazar seem brilliant. It also means that everything that happens in the blazar appears accelerated: it seems to move much faster and happen much quicker than it does really; blazars appear to vary quickly and violently because the jet magnifies everything that happens inside them.


OJ287 is special because its jet is particularly well lined-up with the Earth – it is off by only about 13º. This makes it one of the best-aligned jets of any known blazar. OJ287 is though also relatively nearby, which means that by being so well aligned with the Earth and being so close, we actually look some way inside the throat of the jet. That is why this object is a particularly extreme blazar that behaves in such an unusual and bizarre manner. Other blazars are more gentle in their behaviour than OJ287 because their jet is less well-aligned with the Earth and so we are only in the fringes of the intense beam of light.