New class of low frequency QPOs: Signature of nuclear burning or accretion disk instabilities?
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 x 10-3 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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- June 2001
- DOI:
- 10.1051/0004-6361:20010434
- arXiv:
- arXiv:astro-ph/0011110
- Bibcode:
- 2001A&A...372..138R
- Keywords:
-
- ACCRETION;
- ACCRETION DISKS;
- INSTABILITIES;
- STARS: BINARIES: GENERAL;
- STARS: NEUTRON X-RAYS: GENERAL;
- X-RAYS: STARS;
- Astrophysics
- E-Print:
- 8 pages, 9 figures, Accepted for publication in A&