Tsing Hua Univ, Dept Engn Phys, Beijing 100084, Peoples R China
; Tsing Hua Univ, Ctr Astrophys, Beijing 100084, Peoples R China
; Tsing Hua Univ, Dept Phys, Beijing 100084, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Particle Astrophys Lab, Beijing 100049, Peoples R China
X-ray variability of the microquasar XTE J1550 - 564 is studied with time-domain techniques using data from an RXTE PCA observation on 1998 September 8. The 2-60 keV average shot profile is obtained by superposing shots with 1 ms time binning after aligning their peaks. The spectral behavior during the averaged shot exhibits prominent differences from that previously observed in Cyg X-1. The 13-60 keV/ 2-13 keV and 16-60 keV/ 13-16 keV hardness ratios during a shot are lower and higher, respectively, than those for steady emission. The correlation coefficients between intensity and 13-60 keV/ 2-13 keV and 16-60 keV/ 13-16 keV hardness ratios are also negative and positive, respectively. These results indicate that the physical processes at work in the low state of XTE J1550 - 564 may be different from those in Cyg X-1.