Afterglow
In 1993, Bohdan Paczyński and James E. Rhoads published an article arguing that, regardless of the type of explosion that causes GRBs, the extreme energetics of GRBs meant that matter from the host body must be ejected at relativistic speeds during the explosion. They predicted that the interaction between the ejecta and interstellar matter would create a shock front. Should this shock front occur in a magnetic field, accelerated electrons in it would emit long-lasting synchrotron radiation in the radio frequencies, a phenomenon that would later be referred to as a radio afterglow. Jonathan Katz later concluded that this lower-energy emission would not be limited to radio waves, but should range in frequency from radio waves to x-rays, including visible light.
The Narrow Field Instruments on board BeppoSAX began making observations of the GRB 970228's position within eight hours of its detection. A transient x-ray source was detected which faded with a power-law slope in the days following the burst. This x-ray afterglow was the first GRB afterglow ever detected. Power-law decays have since been recognized as a common feature in GRB afterglows, although most afterglows decay at differing rates during different phases of their lifetimes.
Optical images were taken of GRB 970228's position on 1 and 8 March using the William Herschel Telescope and the Isaac Newton Telescope. Comparison of the images revealed an object which had decreased in luminosity in both visible light and infrared light. This was the burst's optical afterglow. The predicted radio afterglow was never observed for this burst. At the time of this burst's discovery, GRBs were believed to emit radiation isotropically. The afterglows from this burst and several others—such as GRB 970508 and GRB 971214—provided early evidence that GRBs emit radiation in collimated jets, a characteristic which lowers the total energy output of a burst by several orders of magnitude.
Read more about this topic: GRB 970228