was a gamma-ray burst
(GRB) detected on May 8, 1997, at 21:42 UTC
. A gamma-ray burst is a highly luminous
flash associated with an explosion in a distant galaxy and producing gamma rays
, the most energetic form of electromagnetic radiation
, and often followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray
, and radio
GRB 970508 was detected by the Gamma Ray Burst Monitor on the Italian–Dutch X-ray astronomy satellite BeppoSAX. Astronomer Mark Metzger determined that GRB 970508 occurred at least 6 billion light years from Earth; this was the first measurement of the distance to a gamma-ray burst.
Until this burst, astronomers had not reached a consensus regarding how far away GRBs occur from Earth. Some supported the idea that GRBs occur within the Milky Way, but are visibly faint because they are not highly energetic. Others concluded that GRBs occur in other galaxies at cosmological distances and are extremely energetic. Although the possibility of multiple types of GRBs meant that the two theories were not mutually exclusive, the distance measurement unequivocally placed the source of the GRB outside the Milky Way, effectively ending the debate.
GRB 970508 was also the first burst with an observed radio frequency afterglow. By analyzing the fluctuating strength of the radio signals, astronomer Dale Frail calculated that the source of the radio waves had expanded almost at the speed of light. This provided strong evidence that GRBs are relativistically expanding explosions.