OJ287 – a case of squizophrenia?

Like many quasars, OJ287 has been observed many times by accident over the years and these measures are a quite invaluable record of its behaviour. In the case of OJ287 the first images that were found corresponded to the year 1891 and there were nearly 200 measures in total until it was officially discovered in 1968 and became subject to regular and detailed monitoring. 

When the historical light curve was first compiled, the suggestion was made that it showed evidence of regular fades in brightness that might be eclipses. However, Mauri Valtonen at Tuorla Observatory (Turku, Finland) looked at the evidence and replied sagely that he could not see eclipses, but he could see what seemed to be a series of regular brightenings or outbursts in the light curve. Looking into the data in more detail it seemed that about every 11.6 years over a period of some 90 years OJ287 had suffered a major crisis and become much brighter for a few weeks or months.


Figure 1 – The light curve of the blazar OJ287 between 1893 and 1996. We can see how there seem to be regularly spaced, sharp maxima throughout the light curve. These are the outbursts that occur every 11.5-12 years when the two black holes are at their closest approach in their orbit. A new outburst duly occurred in 1994-95, as predicted. Image prepared by the author.


What could cause a quasar to become as much as 3 magnitudes brighter for a short interval every 12 years and do so in a seemingly regular fashion?

Mauri Valtonen and his collaborators had an extremely interesting idea (Sillanpaa, A.; Haarala, S.; Valtonen, M. J.; Sundelius, B.; Byrd, G. G.: 1988, The Astrophysical Journal, “OJ 287 - Binary pair of supermassive black holes”, 325, 628-634). What if, unlike a normal quasar, OJ287 had two black holes in its heart? They suggested that one was 5 billion solar masses and that the “small” black hole of 20 million times the Sun’s mass orbited around it in a highly eccentric orbit every 9 years which we would see from Earth as a period of 11.6 years thanks to the stretching of time caused by relativity. Every 9 years, when the small back hole passed close to the large one the encounter would cause a massive perturbation of the black hole. The force of gravity of the small black hole would cause an enormous surge of material to fall onto the larger one from its surrounding accretion disk and, with it, a huge increase in the brightness of the blazar.

Initially the idea took some time to capture the imagination of the astronomical community, but it made one extremely interesting prediction. There was a big outburst in the light curve in 1972. Another had followed in 1983. That meant that a further outburst should follow in 1994; what is more, the group made the prediction that it would be a double one, with two outbursts about a year apart. Making predictions is the other way to test a theory, but it is a double-edged sword: get it right and people will be impressed (but even then, you may find that there are alternative explanations suggested as to why your prediction has worked!), get it wrong and you feel very silly.