Swift's most distant burst yet
The Swift satellite CREDIT: NASA
In the 1960s, satellites designed to monitor compliance with the nuclear test ban treaty discovered peculiar bursts of radiation coming from outside the Earth. Nuclear explosions produce bursts of gamma rays (photons of light with very short wavelengths) so these satellites watched the surface of the Earth so that if someone broke the treaty and exploded a nuclear bomb in secret it would be detected. What they discovered instead were bursts of gamma rays coming from the sky. The bursts were distributed randomly which immediately ruled out some ideas about where they might come from. If they came from nuclear explosions on Earth, you would only detect them while pointing at the Earth. If they came from our Galaxy then you would see them in a thin strip across the sky corresponding to the disk of the Milky Way. As the distribution was random either the cause of the bursts had to be very local (within the solar system) or very distant (distributed throughout the whole universe).
Astronomers used to favour the nearby hypothesis because of their enegy output. The energy from a source decreases according to the inverse square law, light spreads out as it moves away from a source, so a distant object must be much brighter to appear the same as a nearby object (think about car headlights). Astronomers now think that these objects are very distant. One reason for this is that often these GRBs leave optical afterglows which fade much slower than the initial gamma ray burst and can be detected by ground-based observatories. By using optical telescopes astronomers have managed to take spectra of some of these optical afterglows and determine from their redshifts that they are distant objects. So, although they are not caused by nuclear weapons, they are caused by massive explosions. Models of GRBs suggest that they are far more energetic than supernovae which are pretty big explosions themselves.
The initial bursts are still hard to detect though. Gamma rays do not pass through the Earth's atmosphere very easily so detectors are still sent into space on satellites. One of these, the Swift satellite, was launched in to low Earth orbit aboard a Delta rocket in November 2004. Since then it has detected many bursts. Each time one is found an alert is sent which allows optical observatories to rapidly make follow-up observations to look for the optical afterglows and hence find distances to these objects. As well as the alerts, the team also provide a map of recent bursts which is pretty cool.
The burst profile of GRB 050904A from Swift CREDIT: Swift / NASA
On September 4th 2005, the Swift satellite detected another burst of gamma rays from the distant universe: 090504A (first reported in GCN 3910). This burst is noteworthy because it might be the most distant GRB ever detected. It was also one of the longest, lasting 120 seconds (I told you they were short!). The image on the right shows the light curve, how the brightness of this burst varied with time. Although optical telescopes looked for an afterglow, 050904A was not detected optically, but it was detected at slightly longer wavelengths in the infra-red part of the spectrum. This difference could be due to obscuring dust in the host galaxy which would absorb optical but let through infra-red light, but other parameters determined from the observations would appear to rule this out. The other possibility is that this could be a very distant explosion, the observations would imply a redshift, z, of between 6 and 8 which corresponds to a distance of between 12.7 and 13 billion light years.
On Monday 12th September 2005 NASA will host a press conference on this burst at 2 pm EDT which will also be webcast on NASA TV. Participating scientists will include Neil Gehrels, Swift principal investigator at NASA's Goddard Space Flight Center. Should be interesting.