0840017

We propose to observe the Be/X-ray binary system A 0535+26 in the case it presents a normal and a giant outburst monitoring the rise to the peak and the descending part of the outburst. In the case of a giant outburst one main goal will be to study the luminosity dependence of the cyclotron energy. For the first time high accuracy measurements at high luminosities of the cyclotron energy in A0535+26 will allow to test the hypothesis of a change in the height in the accretion column caused by an increase in the mass accretion rate. If the source enters a normal outburst another important goal will be to study possible magnetospheric instabilities which have recently been discovered for this source. Non-stationary accretion can be probed using INTEGRAL capabilities which will allow to monitor the cyclotron line energy to perform a precise timing analysis and to study the pulse profile evolution with time and luminosity. Recent observations of two normal outbursts with INTEGRAL have revealed new unexpected features making observations of normal outbursts extremely useful tools to study the onset of the accretion onto a neutron star at low accretion rates. [truncated! Please see actual data for full text]