BeppoSAX captured the X-ray afterglow of GRB 970228 just 8 hours after
the burst triggered the satellite’s gamma-ray detector (left). The glow
had faded considerably just 3 days later (right). ASI
Last week marked the 20th anniversary of supernova 1987A's appearance in the Large Magellanic Cloud. Cakes were baked, corks were popped, and scientists rejoiced last week at a workshop in Aspen, Colorado, dedicated to 1987A and exploding stars in general. (You can read about the conference in my previous blogs here and here.)
In a science as rich as astronomy, anniversaries of significant events come around fairly often. But who would have thought we'd mark another cosmic-explosion milestone so soon after 1987A's anniversary? That's precisely what happens this week, as we look back at an event that helped solve the biggest mystery surrounding gamma-ray bursts.
On February 28, 1997, a burst of gamma rays reached Earth from the constellation Orion. Nothing unusual about that — such bursts rain down on us about once a day, on average. What set GRB 970228 apart from all its predecessors was the presence in Earth orbit of a new Italian-Dutch satellite named BeppoSAX.
Before BeppoSAX, other spacecraft detected gamma-ray bursts, but these orbiting and interplanetary spacecraft determined positions only approximately. When astronomers searched these poorly defined areas for a counterpart at some other wavelength, they found a confusing mess littered with dozens, or even hundreds, of potential host objects. Without being able to follow up on the burst, astronomers had few clues where in the universe gamma-ray bursts come from. Some believed the bursts originated in our own galaxy, in a vast halo that extends well beyond the Milky Way's visible borders. Others argued the bursts came from deep in the cosmos, in distant galaxies spread throughout the universe.
Enter BeppoSAX. Not only did the satellite contain a gamma-ray-burst detector, it also possessed a number of X-ray cameras with much better resolution. Just 8 hours after the 80-second-long burst, the satellite had pinpointed an X-ray source in the same vicinity that had not been there before.
The X-ray source faded quickly, but by then, astronomers around the world had aimed their telescopes in the burst's direction, where they glimpsed a faintly glowing object in visible light. Further observations confirmed there was an extended faint glow in the vicinity — the burst's host galaxy.
The mystery was solved — gamma-ray bursts occur in distant galaxies (in GRB 970228's case, several billion light-years from Earth). That makes them the brightest objects in the universe, radiating more energy in a few seconds than the Sun will in its 10-billion-year lifetime.
I'd say that makes this an important anniversary. Please pass me another piece of cake.