Searching from the LOTIS Position
Searching from the LOTIS Position
September 18, 1997
more news on gamma-ray bursts
While most of the data on gamma ray bursts have come from the Burst and Transient Source Experiment aboard the Compton Gamma Ray Observatory, it is important to remember that many of the important discoveries are coming from instruments on other spacecraft.
The future of the gamma-ray burst field is based on instruments being developed now. Those instruments fall generally into two major groups, those that look for optical and other counterparts as the burst flares up, and those that look for the afterglow of counterparts as the bursts fade away. Of the two tasks, the first is more demanding because bursts appear and reach their peak within a few seconds, while the afterglow may last for minutes, hours, or - in the case of the GRB 970228 counterpart observed by the Hubble Space Telescope - at least six months.
Lawrence Livermore Laboratories in Berkeley, California, built a neat little apparatus called LOTIS, the Livermore Optical Transient Imaging Sensor, to find optical counterparts. It comprises four electronic cameras, each with a 200 mm telephoto lens. The cameras have 2,048 x 2,048 pixel charge-coupled device (CCD) detectors so the array covers a field of view about 35 times wider than the Moon. LOTIS is mounted on a special pointing system and connected to a computer so the cameras can take pictures with 10 seconds of a burst being detected by BATSE or a different experiment in space.
So far, it has not found any optical counterparts, partly because of weather and the position of the Earth, and partly because nothing appeared in the pictures. In science, this is not a failure but an upper limit on the brightness of the bursts. The team now is working on Super LOTIS, a larger telescope with a 50 cm aperture that will capture fainter objects. The Los Alamos National Laboratory is building a similar array in Robotic Optical Transient Search Experiment.
France's National Center for Science Research (CNRS) is building its own version, the Rapid Action Telescope for Transient Objects (the acronym of the French name is TAROT. Using a converted telescope, it should start operating presently.
The Astronomical Institute Ondrejov in the Czech Republic is getting into the act by adapting its portion of the European Fireball Network which was set up to photograph and track meteors as they enter the atmosphere.The Lowell Observatory outside Flagstaff, Arizona, has adapted its Near-Earth Object (LONEOS) telescope to help, too. LONEOS is a 58 cm aperture Schmidt telescope looking for asteroids and comets that may imperil the Earth. Its film plates are checked for signs of bursts if it was taking pictures in the vicinity of a burst location.
Still, the best place to hunt for bursts is space. The weather does not interfere with the view, and the atmosphere does not absorb the gamma rays.
The next major burst instrument to be launched is the High Energy Transient Experiment (HETE 2; HETE 1 was lost during launch last year). HETE 2, to be launched in 1999, will not have the all-sky capability of BATSE, but it will be able to collect more detailed locations and spectra on about 25 bursts a year.
The European Space Agency plans to launch the Integral spacecraft in 2001. Its instrument suite includes an imager and a spectrometer that also should help locate bursts with greater accuracy and see their spectra in greater detail.
A Burst and All Sky Imaging Survey (BASIS) Explorer has been proposed for the Small Explorer program which is part of NASA's "faster, smaller, cheaper" initiative. BASIS will have an array of detectors behind a mask peppered with pinholes. BASIS will map the sky in hard X-rays and, because the mask acts like a sophisticated pinhole camera, will provide more precise locations of bursts that happen to be in its 40 degree field of view.
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