  Supernova SN 1998bw exploded in the same direction and at the same time as
the gamma-ray burst GRB 980425. Here we report radio observations of this type
Ic supernova, beginning 4 days after the gamma-ray burst. At its peak the radio
source is the most luminous ever seen from a supernova, nu L_nu = 4 x 10^38
erg/s at 5 GHz. More remarkably, the traditional synchrotron interpretation of
the radio emission requires that the radio source be expanding at an apparent
velocity of at least twice the speed of light, indicating that this supernova
was accompanied by a shock wave moving at relativistic speed. The energy U_e
associated with the radio-emitting relativistic electrons must lie between
10^49 erg <U_e< 10^52 erg, and thus represents a significant fraction of the
total kinetic energy ~10^51 erg associated with supernova explosions. The
presence of a relativistic shock in SN 1998bw suggests a physical connection
with the gamma-ray burst GRB 980425. We argue that this represents a second
class of gamma-ray burst, with much lower emitted energy ~10^48 erg in
gamma-rays than the two powerful ~10^53 erg high-redshift gamma-ray bursts
previously identified.
