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D.W. Fox (Caltech), M.P. Muno, W.H.G. Lewin, E.H. Morgan (M.I.T.), L. Bildsten (I.T.P., U.C. Santa Barbara)
We perform a phase connection of the highly coherent 550 Hz pulse train detected in the 1 March 1997 type I X-ray burst from Aql X-1 by Zhang et al. (1998) using the Proportional Counter Array of the Rossi X-ray Timing Explorer satellite. We find that the pulsations last for 10 s, or roughly 5500 cycles, and exhibit a frequency evolution that is consistent with an exponential-relaxation model. Examining the phase of the pulsations by energy band and time throughout the burst, we discover an approximately one radian hard lag during the early portion of the burst, which reverses its sense during the latter portion of the burst to become the one radian soft lag reported by Ford (1999). Ford interpreted the soft lag as the signature of relativistic Doppler effects. However, since hard lags of the pulsations in particular, and reversing, time-variable phase lags in general, cannot be produced by Doppler effects, this interpretation may have been premature. We examine alternative models, and suggest that significant photon scattering (Comptonization) effects may be present. The time-variable, reversing phase lags we observe may help explain why other sensitive searches for phase lags, which integrated data from all of one or several bursts, have failed to make any detections.
The authors gratefully acknowledge NASA and NSF support of this research.
The author(s) of this abstract have provided an email address for comments about the abstract: derekfox@astro.caltech.edu