Multiwavelength Observations of GX 339-4 in 1996.
II. Rapid X-ray Variability
Smith, I. A., and Liang, E. P.
As part of our multiwavelength campaign of GX 339-4 observations in 1996 we
present the rapid X-ray variability observed July 26 using the RXTE when the
source was in a hard state (= soft X-ray low state). We found that the source
was extremely variable, with many bright flares. The flares have relatively
symmetric time profiles. There are a few time intervals where the flux rises
steadily and then drops suddenly, sometimes to a level lower than the average
before the increase. Hardness ratios showed that the source was slightly
softer when the flux was brighter. The power density spectra (PDS) were also
complicated and we found that broken power laws do not provide adequate fits
to any of them. Instead a pair of zero-centered Lorentzians gives a good
general description of the shape of the PDS. We found several quasi-periodic
oscillations (QPO), including some that are harmonically spaced with the most
stable frequency at 0.35 Hz. While the overall rms variability of the source
was close to being constant throughout the observation (29% integrating between
0.01 and 50 Hz), there is a small but significant change in the PDS shape with
time. More importantly, we show that the soft 2-5 keV band is more variable
than the harder 5-10 and 10-40 keV bands, which is unusual for this source and
for other black hole candidates. Cross correlation functions (CCF) between
these bands show that the light curve for the 10-40 keV band lags that of the
2-5 keV band by 5 msec.
Status:
1999 July 10, ApJ, 519, 771.
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