Gamma-Ray Burst Locations
It became clear shortly after the announcement of GRBs' existence in 1973 that it would be necessary to indentify a GRB counterpart at optical or radio wavelengths in order to really understand what caused these massive explosions. The study of the high-energy radiation can tell us much about the physical processes that produce the radiation, but they tell us little about what caused those processes to begin, and they are notoriously hard to localize. In order to determine what they are, it would be necessary to determine where they were coming from.
Gamma-rays are the tsunami of the electromagnetic waves. They carry so much energy that they barrel through almost anything trying to stop them. Unlike optical light, they cannot be focussed into a sharp image. This makes it very difficult to determine where they are coming from. For many years, the best method was to determine when the burst arrived at far-flung satellites, and then calculate which locations for the burst source on the sky would allow the burst to be detected by those satellites at those times.
However, this method takes time. At the very least, the data must be retrieved from satellites all over the solar system, which could take hours. Typically by the time a position is available, the show is over; nothing to see here. Move along. Another ingenious approach, used by the BATSE instrument, involved putting a detector on each corner of a cube. These detectors were built such that the more obliquely the radiation hit the detector, the weaker that radiation looked. By comparing the relative weakness of different detectors' recordings of the same burst, one could derive the origin position on the sky to ten degrees or so (about the size of two fists held together at arm's length). This is far, far too large an area to be searched by typical scientific telescopes.
Two methods were developed to attack this problem: First, try to use X-rays to determine more accurate positions (with X-rays, one can derive a location of a GRB to a few arcminutes: about half the width of a pencil held at arm's length), and second, try to develop fast-moving, wide-field optical telescopes that could actually take an image of a ten-degree field all at once. Since 2000, these two approaches have converged, but for a while, they operated independently. The key factor in both approaches was the fast distribution of GRB coordinates from satellite detectors over the internet. This Gamma-Ray Burst Coordinate Network, or GCN, was established by Scott Barthelmy at NASA's Goddard Space Flight Center. Originally this service only sent out targets from BATSE, but Scott expanded operations to allow any experiment to contribute. With the speed of the internet, researchers on the ground could have the coordinates in hand within seconds of the time when the first gamma-rays began hitting the orbiting detectors.
Finding the Afterglow
In 1997, all the pieces were in place for the study of GRBs to make the next leap forward. It had taken 30 years, but on February 28, scientists unambiguously associated optical emission with a GRB for the first time. The Dutch-Italian satellite BeppoSAX was able to distribute GRB coordinates with arcminte accuracy from their X-ray detectors within a few hours of the event, and several groups of astronomers were ready to get telescopes on target within a few hours of that. This work led to the first detection of a fading "afterglow" from a GRB source.
In the years since then, many more afterglows have been detected and studied with instruments all over the world: the VLA radio dishes in New Mexico, the giant Keck optical telescope in Hawaii, the aptly named Very Large Telescope in Chile. From these studies have emerged a widely accepted general picture of what is going on in a gamma-ray burst, and the very first question that was unequivocally settled was that GRBs are indeed coming from other, far distant galaxies, rather than in our own Milky Way. With deep, sensitive followup, many of these galaxies could be identified. Studying variability in the afterglow has led to evidence that (at least some) GRBs occur during supernovae. Studying the evolution of the afterglow spectra has indicated that GRB emission is most likely beamed. All in all, the data have provided strong support for the "relativistic fireball model" as the best explanation for the origin of the GRB radiation.
Chasing the Prompt Emission
However, this still leaves the ultimate question of what caused the fireball unanswered. All the fireball model needs is some mechanism that releases a vast amount of energy within a small volume. According to the model, the GRB itself is produced far away from the site of the explosion, when collisions form in the ejected material: either through it falling over itself like a car pileup in a traffic jam, if the ejected material moves at different speeds, or through interaction with the surrounding inter-stellar gas, like an ocean wave crashing on the shore. To determine what source ejected the material in the first place, we need more information about the burst itself; we need to observe the cataclysm directly, while it is still happening. This is the primary goal of the ROTSE project.
The first ROTSE instrument was optimized for working in conjuction with the BATSE instrument on the Compton Gamma-Ray Observatory. ROTSE-I used four digital cameras with telephoto lenses to cover a 16°×16° field of view on the sky (the moon, by comparison, is only half-a-degree wide), since BATSE provided positions that were only accurate to about that size. ROTSE-I operated fully automatically, and on January 23, 1999, succeeded in observing the first optical counterpart to a GRB still in progress! To date this remains the only GRB to be simultaneously observed in optical, X-, and gamma-rays.
The Future for ROTSE
The CGRO was brought down into the Pacific Ocean by NASA in June of 2000. ROTSE-I was shut off in January of 2002. Newer instruments for detecting GRBs such as HETE-2 and the upcoming Swift satellite promise to provide localizations to better than the size of the moon, so the plans for a new ROTSE instrument were modified to allow a smaller field of view. Further, to increase the number of GRBs observed, the ROTSE-III telescope program was expanded to include four telescopes at sites around the globe. With more prompt observations of GRBs, we hope to address a host of current mysteries about GRB emission. [List of questions]
With four telescopes at our disposal, there are a quite a few other scientific topics we can study. GRB alerts are expected to arrive at best two or three times a week during the Swift era. In the off-time, we plan to have the ROTSE-III telescopes performing automated sky surveys, as well as regular monitoring of interesting targets. This will enable us to provide uniform coverage of the entire sky on a regular basis, generating a database of sky variability that is unique in scope. We will be able to study thousands of previously unknown variable stars, monitor the behavior of many active galactic nuclei (AGN), and look for transient flashes and explosions such as those from flare stars, X-ray Binaries, and Soft Gamma-Ray Repeaters (SGRs).