  X-ray bursts are thermonuclear explosions on the surface of accreting neutron
stars in low mass X-ray binaries. As most of the known X-ray bursters are
frequently observed by INTEGRAL, an international collaboration have been
taking advantage of its instrumentation to specifically monitor the occurrence
of exceptional burst events lasting more than ~10 minutes. Half of the
so-called intermediate long bursts registered so far have been observed by
INTEGRAL. The goal is to derive a comprehensive picture of the relationship
between the nuclear ignition processes and the accretion states of the system
leading up to such long bursts. Depending on the composition of the accreted
material, these bursts may be explained by either the unstable burning of a
large pile of mixed hydrogen and helium, or the ignition of a thick pure helium
layer. Intermediate long bursts are particularly expected to occur at very low
accretion rates and make possible to study the transition from a hydrogen-rich
bursting regime to a pure helium regime.
