The effect of magnetic fields on gamma-ray bursts inferred from multi-wavelength observations of the burst of 23 January 1999

Gamma-ray bursts (GRBs) are thought to arise when an extremely relativistic outflow of particles from a massive explosion (the nature of which is stil l unclear) Interacts with material surrounding the site of the explosion. o bservations of the evolving changes in emission at many wavelengths allow u s to Investigate the origin of the photons, and so potentially determine th e nature of the explosion. Here we report the results of gamma-ray, optical , Infrared, submillimetre, millimetre and radio observations of the burst G RB990123 and Its afterglow. Our Interpretation of the data Indicates that t he initial and afterglow emissions are associated with three distinct regio ns In the fireball. The peak flux of the afterglow, one day after the burst , has a lower frequency than observed for other bursts; this explains tbe s hort-lived radio emission. We suggest that the differences between bursts r eflect variations In the magnetic-field strength in the afterglow-emitting regions.