Dr Mark Kidger, (European Space Agency, European Space Astronomy Centre, PLATO Instrument Calibration Scientist & Herschel Science Centre), 6th Dec 2019
This speaker gave an interesting story of how the strange periodicity of changes in brightness of a quasar was explained and how amateur astronomers contributed to the discoveries.
A Quasar means a “Quasi Stellar Object” and these are objects that look like feint blue stars but are actually incredibly distant galaxies. Quasars are powered by supermassive black holes that are feeding on the mass of their (usually invisible) host galaxy. The absorption of huge amounts of mass into these enormous black holes generates the emission of light and other radiation from the black hole causing quasars to be extremely bright ; some quasars are 1000 times more luminous than the entire Milky Way galaxy!
Quasars were discovered by accident. In the 1950’s only 8 radio sources had been discovered in the sky (one of which was the Andromeda galaxy). In the 1960s astronomers were investigating radio sources in more detail and Tom Matthews identified a radio source, 3C 48, with what he thought was a feint star. Astronomers used interferometry by combining signals from two radio telescopes to calculate the size of the object and concluded the source of the radio emissions must be extremely small. They also looked at the spectra of light emitted from the source and discovered hydrogen lines in its spectrum that had been very red shifted so concluded that the object must be moving away from us at huge speeds and must be extremely distant. All quasars have these characteristics of a powerful radio source, extreme distance and moving away from us at extreme speeds.
OJ287 is a quasar in the constellation of Cancer and many amateur astronomers had noticed large changes in its brightness (between magnitude 8 and magnitude 14) which can happen rapidly over a period of minutes. Unusually the changes in brightness didn’t seem to have any regular periodicity. It was also unusual in that we didn’t see the typical spectral lines that were expected and astronomers concluded the lack of lines was because the beam of radiation from this quasar was pointed almost directly at Earth so was drowning out the rest of the light from the quasar.
A huge outburst was observed from OJ287 in 1972 which caused heightened brightness for 4 years but since then the brightness has varied rapidly and seemingly randomly. This wasn’t what was expected if the quasar was pulsating or orbiting something in a regular manner. Luckily more information was found on the changes in brightness of OJ287 because old photographic plates were found of the region of sky containing OJ287 and they allowed measurement of changes in brightness of the quasar back to the year 1900. Using these data astronomers were able to find periodicity in the light with regular outbursts in which it brightened every 11 years. They concluded that the best explanation of these outbursts is that there are two black holes orbiting each other, one larger and one smaller. There are discs of material around both black holes and when the smaller one passes through the disc around the larger one material from the disc is transferred to the larger black hole and this transfer of material causes the outburst. Further calculations showed the smaller black hole to have the mass of 100 million solar masses and the larger one to have the mass of 17 billion solar equivalents and a diameter 20 times bigger than the orbit of Pluto!
However computer modelling of the outbursts expected for two orbiting black holes could not perfectly explain the frequency of the outbursts seen. The explanation for these differences was found to be intense gravitational waves emitted by both black holes. It was concluded that the gravity fields are so intense that the gravity waves emitted interfere with each other. This interference sometimes slows and sometimes speeds up the black holes in their orbit, an effect which had been predicted by Einstein but never seen before. When computer models of the expected outbursts were updated with this new understanding the forecast of the outbursts was expected to be much more accurate and another large outburst was forecast for 31st July 2019. Unfortunately on this date OJ287 was very close to the Sun in the sky, making observations of an outburst very difficult. The team approached NASA to see if they would agree to observe OJ287 using the Spitzer Space Observatory which at that time was 260 million miles from Earth and able to see OJ287 further from the Sun in the sky. Such was the importance of verifying the hypothesis of gravitational wave interference that NASA agreed and Spitzer was able to observe that the outburst occurred exactly when predicted. Calculations show the next major outbursts should be on 19th July 2022 and 30th May 2031 so astronomers will be observing OJ287 on those dates to see if their model holds up and their explanation for the outbursts is correct.
Interestingly the speaker said the two black holes are actually getting closer to each other after each orbit (the orbits precess at a rate of 39 degrees each orbit) and in another 10,000 years will crash into each other and form one giant black hole. This is a very short period of time in astronomical terms so we are lucky to be living at a time when we can make these observations and calculations.
So observing the changes in brightness of quasars, including those by amateur astronomers, has allowed development of a new theory of how they work and allowed us to check and verify some of the most detailed predictions of Einstein’s general theory of relativity, a cornerstone of modern astronomy.
Notes and summary by Tony Hersh.