Archive for SWIFT

The Edge of Darkness

Posted in The Universe and Stuff with tags , , , on October 29, 2009 by telescoper

I just picked up an item from the BBC Website that refers to news announced in this week’s edition of Nature of the discovery of a gamma-ray burst detected by NASA’s Swift satellite.  The burst itself was detected in April this year and I had a sneak preview that something exciting was going to be announced earlier this month at the Royal Astronomical Society meeting on October 9th. However, today’s press releases still managed to catch me on the hop owing to the fact that a rather different story had distracted my attention…

In fact, detections of gamma-ray bursts are not all that rare. Swift observes one every few days on average. Once such a source is found through its gamma-ray emission, a signal is sent to astronomers around the world who then work like crazy to detect an optical counterpart. If and when they find one, they try to measure the spectrum of light emitted in order to determine the source’s redshift. This is very difficult for the distant ones, and is not  always successful.

However, what happened in this case – called GRB 090423 – was that a spectrum was that not one but two independent teams obtained optical spectra of the  object in which the gamma-ray burst must have happened. What each time found was that their spectrum showed a sharp cut-off at wavelengths shorter than a given limiting value.

Hydrogen is very effective at absorbing radiation with wavelengths shorter than 91.2 nm (the so-called Lyman limit, which is in the ultraviolet part of the spectrum), and all galaxies contain large amounts of hydrogen; hence galaxies are virtually dark at wavelengths shorter than 91.2 nm in their rest-frame. The position of the break in an observed frame will be at a different wavelength owing to the effect of the cosmological redshift.

The Lyman break for the host of  GRB 090423 appears not in the ultraviolet but in the infrared, indicating a very large redshift. In fact, it’s a truly spectacular  8.2.

Together with the direct observations of galaxies at high redshifts I blogged about a month or so ago, this discovery helps push back the frontiers of our knowledge of the Universe not just in space but also in time. A quick calculation reveals that in the standard cosmological model, light from a source at redshift 8.2 has taken about 13.1 billion light years to reach us. The gamma-ray burst therefore exploded about 600 million years after the Big Bang.

Another interesting thing about this source is its duration. The optical afterglow of a gamma-ray burst  decays with time. Gamma-ray bursts are usually classified as either short or long, depending on the decay time with the dividing line between the two classes being around 2 seconds. The optical afterglow of GRB 090423 lasted about ten seconds. But that doesn’t make it a long burst. We actually see the afterglow stretched out in time by the same redshift factor as an individual photon’s wavelength. So in the rest frame of the source the optical glow was only a bit over a second in duration, i.e. it was a short burst.

Long gamma-ray bursts are thought to be associated with core-collapse supernovae which arise from the self-destruction of very massive stars with very short lifetimes. The fact that such things die young means that they are only found where star formation has happened very recently. One might expect the earliest gamma-ray bursts to therefore be of this type.

I don’t think anyone is really sure what the shorter ones really are, but they  seem to happen in regions without active star formation in which the stellar populations are quite old, such as in elliptical galaxies. The fact that the most distant GRB yet discovered happens to be a short burst is very interesting. How can there be an old stellar population at a time when the  Universe itself was so young?

If the Big Bang theory is correct, astronomers  should eventually be able to reach back so far in time that the Universe was so young that no stars had had time to form. There would be no sources of light to detect so we would have reached the edge of darkness. We’re not there yet, but we’re getting closer.

Darwin and After

Posted in Biographical, Science Politics with tags , , , , , , , , on October 10, 2009 by telescoper

Another sign that the academic year is back into full swing is that the monthly meetings of the Royal Astronomical Society have started up again after the usual summer hiatus. Since I’ve got a very heavy week coming up, I thought I’d take the advantage of a bit of breathing space in my timetable to attend yesterday’s meeting and catch up with the gossip at the Club afterwards.

The highlight of the day’s events was the annual George Darwin Lecture which was given this year by Neil Gehrels from the NASA Goddard Space Flight Center on the subject Gamma Ray Bursts and the Birth of Black Holes: Discoveries by SWIFT. This is a very hot topic (of course) and the lecture did full justice to it. The RAS has two other “prize” lectures – the Gerald Whitrow Lecture and the Harold Jeffreys Lecture – which are used to invite eminent speakers from around the world. They’re not always successful as lectures because the speakers sometimes try to make them too specialised and too detailed, but this one was exceptionally clear and well delivered. I enjoyed it, as well as learning a lot; that’s the essence of a good lecture I think.

The main task for visiting speakers when it comes to the George Darwin Lecture is to give their talk without revealing the fact that they hadn’t realised that Charles Darwin had a famous astronomical son!

Then to the Athenaeum, for drinks and dinner, where the current financial crisis at STFC was in the background of a lot of the conversation. Rumours abounded but I didn’t pick up any hard information about what is likely to happen to our funding next year. I suspect that’s because even STFC doesn’t know. After a bit of wine, though, conversation moved onto other,  less depressing, things including football, cheese and the Welsh landscape.

The colleague sitting next to me (an old friend from Queen Mary days, now at Imperial College) reminded me that in January last year Joao Magueijo invited me to give the vote of thanks at his inaugural lecture (as long as I promised to try to make my speech as short and as funny as possible). It turns out his lecture was only twenty minutes long, which didn’t give as much time as I’d hoped to think of something to say so I resorted to a couple of off-colour jokes and a facetious remark about how the brevity of Imperial’s lectures explained why their students never seemed to know anything. I got a very good laugh from the packed lecture theatre, but was told off afterwards by a senior physicist from the Imperial physics department. That particular episode is something I often think about, the pomposity of some of the staff reminding me that I’m not unhappy at not getting a job there I applied for a few years ago.

Actually, I just remembered that they took pictures at the party afterwards so here’s one of me and Joao having a chuckle afterwards. Notice I had put a tie on for the occasion, but Joao’s wardrobe is strictly T-shirts only.

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After Friday’s dinner (roast partridge, if you want to know) I got the last train back to Cardiff from Paddington, snoozing comfortably for a large part of the journey. On time until just outside Cardiff Central, the train then sat motionless on the track almost within sight of the platform owing to the presence of a broken down goods train in front of us. We finally got into the station 50 (FIFTY) minutes late, and I didn’t get home until well after 2am.