The 1998 outburst of XTE J1550 - 564 started with a hard X-ray spike , rising in less than a day and declining after 3-4 days ; at the same time , the soft X-ray flux was rising with a longer timescale ( \sim 10 days ) .
We suggest that the soft and the initial hard X-ray emission are produced by two different components of the accretion flow : a higher angular momentum flow , which forms the disk , and a lower angular momentum flow feeding the hot inner region .
Thus , we argue that the onset of the outburst is determined by an increased mass transfer rate from the companion star , but the outburst morphology is also determined by the distribution of specific angular momentum in the accreting matter .
In XTE J1550 - 564 , we attribute the initial , impulsive low angular momentum accretion to the breaking down of magnetic confinement by the magnetically active secondary star .
We show that a hard X-ray spike is seen at the onset of an outburst in other BHCs .