Strong Aperiodic X-ray Variability and Quasi-Periodic Oscillation in   X-ray Nova XTE J1550-564
  We report the discovery of strong aperiodic X-ray variability and quasi-periodic oscillation (QPO) in the X-ray light curves of a new X-ray Nova, XTE J1550-564, and the evolution of the observed temporal properties during the rise of the recent X-ray outburst. The power spectral analysis of the first observation reveals strong aperiodic X-ray variability of the source (~28%), as well as the presence of a QPO at ~82 mHz with fractional rms amplitude ~14% over the 2-60 keV energy range. Also apparent is the first harmonic of the QPO with the amplitude ~9%. As the X-ray flux increases, the source tends to become less variable, and the QPO frequency increases rapidly, from 82 mHz to 4 Hz, over the flux (2-50 keV) range of 1.73-5.75 x 10^{-8} ergs cm^{-2} s^{-1}. The amplitude of the fundamental component of the QPO varies little, while that of the harmonic follows a decreasing trend. The fundamental component strengthens toward high energies, while its harmonic weakens. Initially, the power spectrum is roughly flat at low frequencies and turns into a power law at high frequencies, with the QPO harmonic sitting roughly at the break. In later observations, however, the high-frequency portion of the continuum can actually be better described by a broken power law (as opposed to a simple power law). This effect becomes more apparent at higher energies. The overall amplitude of the continuum shows a similar energy dependence to that of the fundamental component of the QPO. Strong rapid X-ray variability, as well as hard energy spectrum, makes XTE J1550-564 a good black hole candidate. We compare its temporal properties with those of other black hole candidates. 
Wei Cui (MIT), S. N. Zhang (UAH), Wan Chen (UMd/GSFC), and E. H.   Morgan (MIT)