What are ultra-luminous X-ray sources?
In a galaxy far, far away, mysterious objects have been blasting out super bright x-ray bursts towards the Earth. The only galactic objects that are brighter than these are super massive black holes that exist in the centres of galaxies. And so far, they've not been able to work out what these really bright objects known as ultraluminous x-ray sources actually are. But now, Durham University's Dr. Tim Roberts and his colleagues have finally shed some light on what these things could be.
Tim - There's a long standing mystery in x-ray astronomy which is the nature of these objects called ultraluminous x-ray sources. They're the brightest x-ray sources we see in other galaxies outside of the central x-ray source. We've seen some galaxies which are big, what we call a super massive black hole.
We've thought for a while that they are other black holes, smaller black holes, but the real mystery is how big they are, whether they are similar to some black holes we see in our own galaxy, thet are roughly the size of our sun or whether they're something in between, something we call an intermediate mass black hole. And so, what this work, what we do when we look at ultraluminous x-ray sources is we're trying to discover the size of black holes.
Chris - How does looking at the x-ray sources actually tell you how big the black holes are?
Tim - This is very useful physical property of black holes that we think gives an indication of how big they might be, and that's simply how bright they are. It turns out that there's this thing called the Eddington limit which really is if you like, a maximum brightness that they can reach for a given mass for a given size of black hole.
Chris - Why do they pump out these x-rays?
Tim - You get x-rays from black holes is basically actually the best way of finding a black hole, is to look for this characteristic x-ray emission. You get that when a black hole feeds, when a black hole, what we call accretes materials from something else. Now in some black holes, that's just the accretion of gas. Just free gas that floats into space. Generally, that's how you see these super massive black holes at the centre of galaxies. But in certain cases, for the smaller black holes, occasionally, you find them orbiting another star. So, it's like a binary star system, where one of the stars is a black hole, and in those systems occasionally, they're close enough that the black hole can actually pull gas off the outer surface of the star, and that's when you see them. The gas falls into the intense gravity field of the black hole and gains energy. It gets spun around and round quicker and quicker, and gets hot. It gets so hot that it literally glows in x-rays as it approaches the edge of a black hole and that's really how we see it.
Chris - So, this one that you're documenting in Nature, how did you spot it?
Tim - Well, with the satellites we have at the moment with the big Xray observatory missions, there are some objects we tend to look at quite frequently. One of those objects is the Andromeda galaxy. That's because a lot of the x-ray sources you see in neighbouring galaxies are what we call transient then we turn on for short periods of time. They turn on and then they very quickly turn off again. So we have these programmes where we're monitoring the Andromeda galaxy fairly regularly to look for sources that go on and then fade and go off again. This is one of those sources.
Chris - Andromeda is about what, 3 million light years away?
Tim - 2 to 3 million, yeah.
Chris - And when you made these observations, what did you actually see?
Tim - Well, we saw a source that we haven't seen before that had turned on. We looked again a couple of weeks later and it got brighter and over the next few days, it brightened further. Until it got up to luminosity and the units we use are about 1039 ergs per second. Now that's interesting because that is the Eggington limit for a stellar mass black hole - one that's roughly the size of our sun.
Chris - And what did this tell you?
Tim - We saw this incredibly luminesque outburst of radio emission. Even more extraordinary, the radio emission seemed to be changing very rapidly. It was fading away pretty rapidly as we were watching it. That was in combination with a very slow decay of the x-ray emission. Now, that's interesting too, us because we've seen very similar behaviour in sources in our own galaxy when they're at their brightest. And the great thing about the sources in our own galaxy is we know how big the black holes are. The black holes again, are these stellar mass black holes, these ones that are of the same order. They're slightly bigger. They're all of the same order of our own sun. What that means is that ultraluminous x-ray source is coming from stellar mass black hole and that's really interesting to us because it tells us that by extension, a lot of these other mysterious ultraluminous x-ray sources we're seeing in other galaxies potentially are stellar mass black holes and that's the mystery we've been trying to solve for a long time.