New class of low frequency QPOs: signature of nuclear burning or accretion disk instabilities?

We report the discovery of a new class of low frequency quasi–periodic variations of the X–ray flux in the X-ray bursters 4U1608-52 and 4U1636-536. We report also the occasional detection of similar QPO in Aql X-1. The QPOs, associated with the flux variations at the level of percents, are observed at the frequency of 7–9 ·10−3 Hz. While usually relative amplitude of flux variations is increasing with energy, the newly discovered QPOs are limited to the softest energies (1–5 keV). The observations of 4U1608-52 suggest that these QPOs are present only when the source luminosity/mass accretion rate is within a rather narrow range and they disappear after Xray bursts. Approximately at the same level of the source luminosity the type I X–ray bursts cease to exist. Judging from these complex of properties we speculate that a special mode of a nuclear burning at the neutron star surface is behind the observed flux variations. Alternatively some instabilities in the accretion disk may be responsible for these QPOs.