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Three Harmonics of X-ray Quasi-periodic Oscillation seen in Swift/XRT and RXTE/PCA Observations of GX 339-4

ATel #2556; Wenfei Yu (Shanghai Astronomical Observatory
on 14 Apr 2010; 08:13 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

Subjects: Optical, Ultra-Violet, X-ray, Transient

Referred to by ATel #: 2573, 3191

The transient black hole candidate GX 339-4 is currently declining in the hard X-ray as seen with the Swift/BAT, while the soft X-ray flux seen with the RXTE/ASM has increased, giving the BAT/ASM daily flux ratio (in crab unit) reaches below 1.0 (see Yu & Yan 2009), continuing the trend that the source is leaving out of the hard state (see ATel #2545). Swift UVOT/XRT monitoring observations of the source in the past 20 days, aiming at fast optical/UV variability in the hard state and transition, was not able to detect rapid optical/UV variability which was suggested to correlate with the X-rays because of low source count rates (around 1 c/s in UVOT: UVW2 filter in event mode). However, the most recent XRT observation on April 11 showed three narrow QPOs harmonically related at 1.266+0.002 Hz, 2.515+-0.004 Hz, and 4.11+-0.08 Hz, respectively. Two harmonically related QPO showed up in the RXTE observation performed a few hours later and the third harmonic showed up in the RXTE observations 1 day later, suggesting these QPOs are energy dependent. In the most recent RXTE observations (observation 95409-01-14-06, on day 103 of 2010), the three QPO frequencies has increased to 2.431+-0.003 Hz, 4.879+-0.013 Hz, and 7.248+-0.057, with the fractional rms amplitudes being 6.9%, 3.4%, and 2.5%, respectively. Based on the trend of the QPO frequency and amplitude, the harmonically related QPOs are estimated to remain for about 5 days if transition to the soft state finally occurs. Since correlated optical-X-ray rapid variability up to 10 Hz has been reported for GX 339-4 in the hard state (Gandhi et al. 2008, MNRAS, 390, L29; Gandhi 2009, ApJ, 697, L167), fast optical photometric observations targeting at these narrow, strong X-ray QPOs are strongly encouraged. The Swift and RXTE results are located at