No QPO time lags from Sco X-1 as seen with EXOSAT: a comparison with Cyg X-2

Citation
Sw. Dieters et al., No QPO time lags from Sco X-1 as seen with EXOSAT: a comparison with Cyg X-2, ASTRON ASTR, 353(1), 2000, pp. 203-210
Citations number
39
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ASTRONOMY AND ASTROPHYSICS
ISSN journal
0004-6361 → ACNP
Volume
353
Issue
1
Year of publication
2000
Pages
203 - 210
Database
ISI
SICI code
0004-6361(200001)353:1<203:NQTLFS>2.0.ZU;2-9
Abstract
We have measured the phase-delay and rms amplitude spectra of Cyg X-2 and S co X-l. Using EXOSAT data from the normal branch of Cyg X-2 we confirm earl ier (Ginga) results, showing that at an energy near 6 keV there is both a m inimum in the quasi-periodic oscillations QPO rms amplitude spectrum and a 150 degrees phase jump in the QPO phase-delay spectrum. Surprisingly, using EXOSAT and Ginga data, we find no evidence for a phase jump of this kind in the phase-delay spectrum of Sco X-l on either the norm al or flaring branch. Upper limits (90% confidence) of 42 degrees can be se t on any phase jump in the energy range 2-10 keV on the normal branch, and 88 degrees on the flaring branch. The QPO rms amplitude spectrum of Sco X-l increases steeply with energy on both the normal and flaring branches. The se results suggest that the X-ray spectrum pivots about an energy of less t han or similar to 2keV or greater than or similar to 10 keV or that normal branch QPO of Sco X-l does not have a pivoting spectrum. We discuss the imp lications of these results in terms of the radiation-hydrodynamic model for normal branch QPO.