8th HEAD Meeting, 8-11 September, 2004
Session 17 Neutron Stars and X-ray Binaries
Poster, Thursday, September 9, 2004, 9:00am-10:00pm

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[17.13] Photon-Energy Dependence of the Phase of Quasi-Periodic Oscillations in Black Holes and Neutron Stars and the Implications for Models of Their X-Ray Emission

W. Yu (Center for Theoretical Astrophysics, University of Illinois at Urbana-Champaign), F. K. Lamb (Center for Theoretical Astrophsycis, University of Illinois at Urbana-Champaign)

Using a new method to determine the relative phases of quasi-periodic oscillations (QPOs) at low and high photon energies, we find that the ~6 Hz oscillations of the QPOs in the black hole X-ray binary XTE J1550-564 and in the ~6 Hz normal branch oscillation (NBO) in the neutron star LMXB Sco X-1 exhibit similar trends. In both oscillations, the waveforms shift to later times with increasing photon energy up to about 6--8 keV. Above this energy the trend reverses and the waveforms shift to earlier times with increasing photon energy, eventually leading the waveforms observed at low photon energies. This behavior rules out the possibility that Compton up/down scattering mechanism is the only mechanism responsible for generating the phase shifts. We discuss the implications for models of QPO X-ray spectra and the origin of spectral variability in these X-ray sources.

This research was supported in part by NASA grant NAG~5-12030, NSF grant AST~0098399, and the funds of the Fortner Endowed Chair at Illinois. WY thanks M. van der Klis for stimulating comments and the support of NWO grant 614.051.002 at the University of Amsterdam where this study was initiated.

The author(s) of this abstract have provided an email address for comments about the abstract: wenfei@uiuc.edu

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© 2004. The American Astronomical Soceity.