Richard Saxton, a Telespazio VEGA senior engineer, looks back over the past two decades of the European Space Agency’s XMM-Newton mission and is confident the satellite will deliver even more critical data in the coming years.
On 10 December 1999, XMM-Newton – the largest ever scientific satellite built in Europe – was launched with the objective of helping solve many cosmic mysteries, ranging from enigmatic black holes to the formation of galaxies. Its name is derived from its x-ray multi-mirror design and honours Sir Isaac Newton.
During the intervening 20 years, the unique x-ray space observatory has simultaneously collected X-rays, visible and ultraviolet light, and consistently demonstrated its role as one of the most important astronomical observatories of all time. This includes measuring the influence of the gravitational field of a neutron star on the light it emits, for the first time ever.
Throughout this period, Telespazio VEGA has played a significant role from the XMM-Newton’s mission’s operational base at ESAC in Spain, working on instrument calibration, observation planning, providing user support, and conducting software and data analysis, higher level processing and scientific studies.
Reaching new frontiers
The combination of its telescope mirrors being amongst the most powerful ever developed and sensitive cameras, means XMM-Newton can see so much more than any previous x-ray satellite.
This immense power and reach enables us to see a galaxy, named GSN 069, about 250 million light years away from Earth. We have been following it since we picked it up in 2010 in a satellite slew, and have made a couple of fairly standard observations of it since then. However, on Christmas Eve 2018, we had another look and found funny bursts in the light curve which were not there previously.
With a signal so regular and clear, it is always tempting to wonder whether there is an intelligence behind it. There is a lot of power here though, with each bursts liberating ~10^45 ergs of energy. Bearing in mind that it takes our sun 10,000 years to generate this much energy, a civilization would have to be very patient to accumulate enough energy even for a single burst.
Bursting with energy
So if they are natural, how are these bursts produced? Like almost all galaxies, GSN 069 contains a black hole at its centre – in this case we have measured it to have a mass of 400,000 times the mass of the sun. Big of course, but actually quite small compared to the black holes in other galaxies, such as the Milky Way which hosts a black hole of four million solar masses. The black hole in GSN 069 is eating matter at an equivalent rate of 12 moons per day.
Using the fact that the peaks go up and down in around half an hour, together with the temperature of the gas being eaten, we can calculate that the flares come from a region which is no more than about 10 times larger than the event horizon of the black hole. This is just a few million km or about five times the radius of the sun.
A property of the flares is that they start off hot (1.2 million degrees) and then drop back to the temperature of the non-flare emission (800,000 K), so whatever is causing the bursts is heating the gas very quickly. We can use an analogy from the behaviour of other accreting black holes to make a first guess at the mechanism. The gas that is constantly getting eaten by the black hole is thought to form a flattish disk. If you imagine a lump of hot matter coming up out of the disk, much like a blob of goo moving up a lava lamp, then it will get hit by many photons. If the matter is hotter than the photons, then it increases their energy and reflects a proportion of them towards the Earth. During this process, the matter cools down and then maybe falls back into the disk.
What causes the ‘eruption’ of matter from the flat disk?
One possibility is that there is a second body – another black hole or a star – which crashes through the disk every 7-9 hours, extracting a stream of material each time it goes through. Alternatively, there may be a physical process, an instability, in the disk which causes it to change shape on a semi-regular basis. A third possibility is that the matter outside the plane of the disk is always present and moves up and down in a kind of bobbing motion.
Whatever is causing it, we’ll continuing studying and evaluating this Quasi-Periodic Eruption (QPE) phenomenon thanks to the power and reach of XMM-Newton.