0940023

We propose a Target of Opportunity Observation whenever one of the transient accreting pulsars from the provided list of sources enters an outburst. The main scientific goal of the proposal is to identify and study different accretion regimes that might be at work in the observed source depending on the luminosity level - a new very promising approach to understand the physics inside the emitting accretion column/mound on the neutron star. Different accretion regimes lead to different luminosity-dependencies of the broad-band spectral continuum and the cyclotron feature. We demonstrate that the observed slopes of the dependencies at different luminosities can quantitatively be compared with theoretical predictions. We will also search for transitions from one accretion regime to another that so far have not been seen in any source but if observed would provide an ultimate test for the theory of the X-ray emitting structure on accreting pulsars. The proposal is focused on those X-ray pulsars the spectra of which contain an evidence or an indication of a cyclotron scattering feature but for which the evolution of the broad-band X-ray spectrum with luminosity (i.e. throughout an outburst) has not been so far studied in details. We request a total of 800 ksec of exposure time divided into several pointings to follow the spectral evolution with flux of the source entered in outburst. [truncated! Please see actual data for full text]