  The 1998 September outburst of the black-hole X-ray binary XTE J1550-564 was
monitored at X-ray, optical and radio wavelengths. We divide the outburst
sequence into five phases and discuss their multiwavelength properties. The
outburst starts with a hard X-ray spike, while the soft X-ray flux rises with a
longer timescale. We suggest that the onset of the outburst is determined by an
increased mass transfer rate from the companion star, but the outburst
morphology is determined by the distribution of specific angular momentum in
the accreting matter. The companion in XTE J1550-564 is likely to be an active
magnetic star, with a surface field strong enough to influence the dynamics of
mass transfer. We suggest that its magnetic field can create a magnetic bag
capable of confining gas inside the Roche lobe of the primary. The impulsive
rise in the hard X-rays is explained by the inflow of material with low angular
momentum onto the black hole, on a free-fall timescale, when the magnetic
confinement breaks down. At the same time, high angular momentum matter,
transferred via ordinary Roche-lobe overflow, is responsible for the formation
of a disk.
