The Astrophysics Spectator
The Astrophysics Spectator
The thrust today in projects to observe gamma-ray bursts is to observe them at as many wavelengths as possible. The trigger for each event is the prompt gamma-ray emission that reaches a spacecraft. Gamma-ray detectors produce poor locations, so other methods must be used to derive a precise location. This can be done using the time of arrival of a burst to many widely-separated spacecraft, or this can be done by observing the burst with an x-ray telescope. The first method has been implemented using interplanetary spacecraft. The second method is the method employed by the HETE-2 and the Swift spacecraft.
Once a gamma-ray burst is observed, the burst community is notified of its occurrence through the Global Coordination Center (GCN). This is a computer system that sends automated messages about bursts to optical and radio observatories on the ground for immediate and long-term follow up.
If there are gamma-ray detectors carried on several spacecraft separated by large distances, then a gamma-ray burst position can be derived by comparing the time of arrival of the burst at each spacecraft. With four or more spacecraft in orbits inclined to one another, a very precise position can be derived. With three spacecraft, a precise position and a false position can be derived; the false position is a mirror image of the precise position located on the opposite side of the plane defined by the spacecraft. Two spacecraft give an annulus on the sky for the location.
A network of this type composed of interplanetary spacecraft has existed several time. The most recent network, the Third InterPlanetary Network (IPN), became operational in 1990 with the launch of the Ulysses spacecraft. The last configuration of the network employed the Ulysses, Konus-Wind, Mars Odyssey, HETE-2, RHESSE, and INTEGRAL spacecraft. Ulysses is a solar observatory that is in an orbit at 5AU that takes the spacecraft over the Sun's poles. Mars Odyssey is a survey satellite orbiting Mars. Konus-Wind which is studying the solar wind, is in a very complex orbit around Earth that takes it far from Earth. RHESSE, which is a solar spectroscopy spacecraft, and HETE-2 and INTEGRAL, which are gamma-ray observatories, are all in orbit around Earth. Currently, the gamma-ray detector on Ulysses is shut off, and the network is dormant; being out of the ecliptic and far from Earth and Mars, Ulysses is vital for deriving a precise position, and without it, the positions derived with the remaining spacecraft are too inaccurate to be useful.
The first spacecraft designed specifically to study gamma-ray bursts was the High Energy Transient Experiment (HETE). It is designed to watch half of the sky for bursts at gamma-ray, x-ray, and ultraviolet energies. After this spacecraft was lost in an unsuccessful launch, a replacement was built that followed almost the same design; the one change was that the HETE ultraviolet instruments were replaced with soft-x-ray cameras on HETE-2. HETE-2 was successfully placed into orbit on October 9, 2000, and is now observing the sky for gamma-ray bursts.
HETE-2 is in a nearly equatorial orbit around Earth. It is triggered by an upsurge in the gamma-ray background. With its soft x-ray camera, it normally locates the position of a burst on the sky to an accuracy of 2 to 2.5 arc minutes. The four instruments are the gamma-ray detector, which detects gamma-rays of energies between 6 and 400keV, the wide-field x-ray monitor, which detects x-rays of energies between 2 and 25keV, and the soft x-ray camera, which detects x-rays of energies between 0.5 and 10keV. The wide-field x-ray monitor can localize a burst to 10 arc minutes.,and the soft x-ray cameras can localize a burst to 30 arc seconds. The instruments have a field of view that is less than 15% of the sky. HETE-2 observes about 25 bursts per year.
The interesting feature of HETE-2 is that it's designed to derive and send a burst location to ground observatories immediately. With a special radio receiver, anyone can receive burst alerts directly from the spacecraft.
The latest generation of gamma-ray burst experiments is Swift, a satellite that is to be launched on November 11 Like HETE, this spacecraft is designed to observe gamma-ray bursts over a broad photon energy range. Swift will observe at the gamma-ray, x-ray, and optical frequencies. The gamma-ray detector, which serves to alert the spacecraft that a burst is occurring, sees gamma-rays between 15 and 150keV over 15% of the sky; it can localize a burst to between 1 and 4 arc minutes. The X-ray telescope has an energy range of 0.2 to 10 keV. It has a field of view of 23.6 by 23.6 arc minutes., and can localize a burst to 3 to 5 arc seconds. The ultraviolet-optical telescope observed over the wavelengths of 170 to 650 nm; it has a 17 by 17 arc second field of view, and a visual magnitude limit of 7. This satellite will only be able to see prompt optical emission, if it exists, because the magnitude of the supernovae associated with other gamma-ray bursts is around magnitude 24.
When a gamma-ray bursts in the gamma-ray detector's field of view arrives at the spacecraft, the spacecraft turns to bring it into the field of view of the x-ray and optical telescopes. This takes about 50 seconds. Once the burst is acquired, a very accurate x-ray position can be derived and transmitted to Earth. This is expected to take 70 seconds from when a burst begins. For comparison, normal bursts last a couple of hundred seconds in duration, so roughly one-third of the bursts that trigger Swift will still be in progress when notification is sent to the ground. Swift is expected to trigger on more than 100 gamma-ray bursts a year.
