Late last month, space and astronomy blogs and news sources were abuzz over the discovery of what may be the most distant event ever detected in the Universe.
The event was something referred to as a Gamma Ray Burst officially designated as GRB 090429B, and it was detected by the ‘Burst Alert Telescope’ which is part of NASA’s ‘Swift’ space satellite. The satellite, launched in 2004, orbits at an altitude of 600 km above the Earth. Here, I take a closer look at what a “gamma ray burst” really is and why this observation is interesting.
A whole lot of “bursting” going on?
First of all, what is all this “bursting” about and why is it important to astronomers? Gamma ray bursts (or GRBs) are really huge energetic explosions, which as the name suggests produce gamma rays. Gamma rays have the smallest wavelengths and highest energies of any other wave in the electromagnetic spectrum. On Earth, they are produced in nuclear explosions as well as by radioactive atoms. In fact, we use gamma rays on Earth because their high energy means that they can kill living cells. In cancer treatment, radiotherapy involves using targeted gamma rays to destroy cancerous cells.
The discovery of Gamma Ray Bursts coming from space was actually more to do with chance than anything else. During the 1960s, the United States Air Force launched a series of satellites, (known as the Vela satellites) to monitor compliance with the Nuclear Test Ban Treaty. These satellites were sensitive to gamma radiation, since a nuclear blast in the Earth’s atmosphere would produce a large amount of gamma-rays. However a strange thing happened: the detectors onboard the satellites began to pick up intense bursts of gamma rays which were very different to those expected– rather like a flashlight shining brightly for a short time, coming from random directions in the sky. By the early 1970’s GRB’s had been discovered. It wasn’t until the early 1990’s though that scientists were able to conclude that these burst events were coming from deep space – with cosmological distances i.e distances ranging from hundreds of millions to billions of light-years away. Despite how very luminous these explosions are, don’t expect to witness one in the night sky. Most of the energy reaches us as gamma rays and our human eyes simply aren’t sensitive to this form of radiation: in short – we can’t see them. In addition, the Earth’s atmosphere does not allow gamma rays to penetrate so we have to rely on space-borne satellites to make observations.
What do we know about Gamma Ray Bursts now?
We now know a lot more about these bursts of light. They can last anywhere from a few milliseconds to a few minutes with most lasting in the 20-40 second range. The initial burst is usually followed by a longer-lived “afterglow” emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared ,etc.) The light itself is extremely bright – hundreds of times brighter than an average supernova and about a million trillion times as bright as the Sun. In fact, GRBs can release more energy in 10 seconds than the Sun will emit in its entire 10 billion-year lifetime.
Because they are the most energetic form of light in the universe, they are only produced in the very hottest regions. Most GRBs occur when a massive star at least 30 times the mass of our Sun, dies (runs out of fuel). These stars collapse dramatically to form black holes or neutron stars and when they do, a huge burst of high energy gamma rays and particles gets ejected in an explosion. Most of this energy is focussed into two distinct ‘beams’ which race out through space heating everything in their path, producing the aforementioned “afterglow effect”. The fact that GRBs (unlike supernovae) shoot out radiation in these narrow beams like a laser make them more difficult to observe – depending on which direction the beams are pointed. These gamma-ray bursts, appear like cosmic flashbulbs from any direction, flicker, and then fade after briefly dominating the gamma-ray sky:
To put it in perspective, GRB’s are the brightest events in our Universe since the Big Bang. Currently orbiting satellites detect an average of about one gamma-ray burst per day. Because gamma-ray bursts are visible to distances encompassing most of the observable universe, which is home to many billions of galaxies, this suggests that gamma-ray bursts must be very rare events per galaxy. Perhaps occurring at a rate of a few, per galaxy per million years. So far, astronomers have found most GRBS detected originate in distant galaxies. And this is precisely why this new detection of a GRB is important. When something is so bright – we can observe it even when we may not even be able to observe light from the galaxy containing the GRB. So GRB’s allow us to measure distances in space greater than we have ever been able to before. Because light travels at a constant, finite speed – this also means that the farther away we see something in the Universe, the further back in time we are looking. If the Universe is assumed to be 13.7 billion years old then the light from the very first stars will take approximately 13.7 billion years to reach us. So the very farthest object we could ever see would be 13.7 billion light years away.
Bring in GRB 090429B
And so we get to last month’s announcement at the American Astronomical Society meeting that astronomers may have discovered the most distant object in the Universe: GRB 090429B. Okay, so perhaps not a name that encompasses how important this may be. Perhaps a picture will help:
Again, perhaps not what we expect as the image of a massive exploding star in the throes of death. However, this is the actual image that NASA’s Swift satellite snapped of the Gamma Ray Burst. Given that this GRB lasted a mere 10 seconds, it is clear how important it is to have a satellite like Swift which slews quickly to point at the source of a GRB within seconds of detecting the burst. Unlike many NASA satellites, “Swift” is not an acronym but instead refers to this ability to quickly change angle and position. Swift is also equipped with X-ray, UV and optical telescopes which measure the “afterglow” of the GRB. What we actually see in the image above is this “afterglow”.
How far away is this thing?
Calculations have estimated the distance to GRB 090429B at 13.14 billion light years. Yes, billion. I mentioned earlier that the age of the Universe is an estimated 13.7 billion years, and so the furthest object we could see would be 13.7 billion light years away. This also means the universe had only been around for a mere 600 million years when this massive explosion took place. The galaxy which contained the GRB was possibly one of the very first formed after the Big Bang- in fact it may still have been in the process of forming when this burst occurred! And on April 29, 2009, courtesy of the Swift satellite – we saw it! (The numerical designation 090429B actually tells us when the GRB was observed.) You can tell by the lapse in time between observation and the announcement last month that painstaking analysis was required to give scientists enough confidence in the data that they believe that this GRB event did in fact happen 13.14 billion years ago, at a distance 96% of the way to the furthest we could ever hope to observe.
How did scientists calculate this distance?
How on earth are we able to say that this could be the most distant object or event in the Universe to date? Astronomers commonly use the term “redshift” to give an idea of how far away an object in space is from us. This is based on the more familiar Doppler effect where wavelengths are stretched out as an object moves away from us. When light travels from the vast distance we are talking about here, we refer to a different type of redshift – the cosmological redshift. This redshift is actually caused by the expansion of space. The wavelength of light increases as it traverses the expanding universe between its point of emission and its point of detection by the same amount that space has expanded during the crossing time. At huge distances, far ultraviolet light for example gets stretched or redshifted all the way into the visible part of the spectrum. We can observe this light that reaches us with ground based telescopes, and by looking at how the wavelength of the light has changed we can calculate how far it has travelled!
In this case, because the light travelled so far much of the UV radiation is absorbed by gas in the universe which the visible light from the burst passed through. The visible light gets redshifted all the way to the infrared. So if the light really had travelled this enormous distance we’d expect to see no visible light (formerly UV light) from the GRB at the earth, but we would see the infrared light that started off its journey as visble light. It turns out this is exactly what we saw, which suggests we are looking at something very distant that happened very early on in the life of our universe.
Ideally, astronomers would measure a complete spectrum of the afterglow of the GRB using several ground based telescopes but thanks to bad weather – this wasn’t possible in this case, making a precise distance measurement difficult. Because GRB’s are so short lived, time is of the essence. However the research team (led by former Penn State University graduate student Antonino Cucchiara) did gather data from the Gemini telescope in Hawai’i and were able to combine this with data from other telescopes which they feel has given them a good level of confidence in estimating the distance to GRB090429B.
Another key piece of information is the fact that astronomers were never able to detect anything in the spot where they saw the afterglow . “We looked with Gemini, the Hubble Space Telescope and also with the Very Large Telescope in Chile and never saw anything once the afterglow faded.” said Cucchiara. “This means that this GRB’s host galaxy is so distant that it couldn’t be seen with any existing telescopes. Because of this, and the information provided by the Swift satellite, our confidence is extremely high that this event happened very, very early in the history of our universe.” Astronomers tend to quantify large distances in terms of this redshift, ‘z’, where higher values of z indicate greater distance and greater lookback time into the early universe. The previous GRB record holder has an estimated z value of around 8.2, with GRB 090429B estimated at 9.4.
So is this REALLY, DEFINITELY, ABSOLUTELY the most distant object ever observed?
It is definitely a convincing candidate but ask some of the Hubble Space telescope astronomers and cosmologists and they may have a different answer. As with all things astronomy, there is a lot of error and uncertainty. Swift’s principal Investigator, Antonino Cucchiara puts it like this: “Like any finding of this sort there are uncertainties. However, if I were in Vegas, I would never bet against the odds that this is the most distant GRB ever seen and we estimate that there is even a 23% chance that it is the most distant object ever observed in the universe.”
Whether or not GRB 090429B is the most distant object we’ve observed to date, the observation itself is a testament to the sensitivity and advancement of both space satellite and ground based telescope technology. Who knows how much further back in time and out in space we’ll be looking in the coming years?
I, for one, am excited at the prospect.