0320049

Renewing our accepted TOO proposal of AO1 and AO2 we propose to observe an outburst in the Be/X-ray binary pulsar A 0535+26 in two stages each with a 200 ks observation. The first stage is triggered by a normal outburst (200 mCrab 2-10 keV) the second stage is to be done if the outburst develops turns out to be a giant outburst (>=1 Crab).The first observation will measure the broad-band phase averaged spectrum with high significance up to ~200 keV testing the presence of a 55 keV cyclotron line at half the energy of the known 110 keV line. With the high energy resolution of SPI and ISGRI we will study in detail the line shapes. Previous instruments mostly scintillators lacked the energy resolution for these studies.The possible second observation when A 0535+26 is significantly brighter than the Crab below 200 keVwill allow us to perform detailed phase-resolved spectral studies. By comparing the evolution of the spectrum with pulse phase to Monte-Carlo simulations of cyclotron line formation we hope to infer details of the emission region such as the overall geometry the optical depth and the viewing angle of the observer to the magnetic dipole axis.Finally we will search for redshifted gamma-ray lines particularly 2.2 MeV photons from deuterium formation from nuclear processes at the neutron star polar cap. This has the exciting possibility of determining the redshift at the neutron star surface thereby constraining the neutron star equation of state.Because of the typically long (few year) intervals between major outbursts it is possible that only one such event will occur during the Integral mission lifetime. It is therefore important that it be a preassigned TOO in order that any outburst be optimally observed. [truncated! Please see actual data for full text]