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

¹ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117810 Moscow, Russia
² Max-Planck-Institute für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany

Corresponding author: M. Revnivtsev, revnivtsev@hea.iki.rssi.ru

Received: 6 November 2000
Accepted: 28 February 2001

Abstract

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 also report an occasional detection of a similar QPO in Aql X-1. The QPOs, associated with flux variations at the level of percents, are observed at a frequency of 7-9 Hz. While usually the relative amplitude of flux variations increases 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 X-ray luminosity is within a rather narrow range and they disappear after X-ray bursts. Approximately at the same level of the source luminosity, type I X-ray bursts cease to exist. Judging from this complex of properties, we speculate that a special mode of nuclear burning at the neutron star surface is responsible for the observed flux variations. Alternatively, some instabilities in the accretion disk may be responsible for these QPOs.