Chris Lintott’s Universe

An American Astronomical Society tradition is the party held on the Wednesday evening. I’m sorry to let you down by not bringing you all the gossip- what happens at the AAS party stays at the AAS party – but it’s reassuring to see that the lecture hall is busy this morning for the first talk of the day, so let me talk about that instead.

It’s been given by Steve Ritz, one of the team behind Fermi, NASA’s new gamma-ray space telescope which was launched in June last year. Fermi was known, before its launch, as GLAST – the Gamma-Ray Large Area Space Telescope. Gamma-rays mark the most energetic end of the electromagnetic spectrum, and so Fermi sees some of the most dramatic events and most energetic objects anywhere in the Universe.

The gamma-ray sky looks very different from that our eyes can see; the lecturer pointed out that Fermi operates in the only region of the spectrum where the Moon is brighter than the Sun, for example! Satellites have looked at the sky at these short wavelengths before; the most famous was the Compton gamma-ray observatory, and it’s with these previous results that Ritz began his talk. For starters, there’s an obvious background glow – the source of which is unknown – in front of which we see emission from our galaxy (the long stripe across the all-sky map seen below). There are some individual sources, too; nearly 300 of them, but the identity of half of these is unknown.

EGRET all-sky gamma ray map

Fermi is a much more sophisticated spacecraft than Compton was, and even after only six months there are plenty of new results for astronomers to chew over. It carries two instruments; the Large Area Telescope can view more than 20% of the sky at once, while the Gamma-ray burst monitor scans the whole sky for dramatic sudden changes. Instead of pointing the spacecraft at specific objects, Fermi normally scans across the sky repeatedly, and in total each pixel on the sky has already been viewed for 2 million seconds. As this process continues, the team will see fainter and fainter objects or watch the brighter ones change.

Faint means faint, here, too. Ritz mentioned that one of the goals of the mission is to monitor pulsars – the spinning remnants of massive stars which shine out boldly in the radio. By contrast, Fermi might be lucky to receive ten individual photons of light a day, and yet they’ve managed to draw conclusions from their observations. One of the most remarkable first results is a pulsar (CTA1) that only shines in gamma-rays – not in any other part of the spectrum. This strange behaviour is most likely a result of our particular viewing angle, but much more work needs to be done.

Perhaps the most dramatic feature of the gamma-ray sky is the presence of gamma-ray bursts, ‘the biggest bangs since the Big one’. Fermi is on course to detect roughly 250 of these each and every year, and the selection it’s already found include the burst with the highest apparent energy ever seen. These events remain mysterious, despite the sterling work in discovery provided by the Swift satellite over the last few years, and Fermi’s ability to look in many different wavelengths at once will be very important here.

There’s plenty in the data set for those of us that study galaxies, too. Fermi sees the most active black holes in the nearby Universe, lurking in the centre of active galactic nuclei (AGN). These objects can and do change extremely rapidly, flaring and sputtering as (presumably) the fuel drops into the black hole, and there will be much discussion of variations in brightness seen in the data, particularly in the 20 or so bright galaxies that Fermi will be concentrating on.

The talk was clearly an early and whirlwind tool through a huge host of topics. Ritz closed by reminding astronomers that they are invited to apply to use the observatory for their own projects, and I suspect keeping up with Fermi’s progress will be essential for years to come. As the lecture’s final slide said – ‘join the fun’!