X-ray Spectral Variability in Initial Rise of Xte J1550-564

X-ray variability of the microquasar XTE J1550-564 is studied with time domain techniques for the data from the RXTE/PCA observation in September 8, 1998. The 2–60 keV averaged shot is obtained from superposing shots with one millisecond time bin through aligning their peaks. The spectral behavior during the averaged shot exhibits prominent differences from that observed in Cyg X1. The hardness ratio of (13–60 keV)/(2–13 keV) or (16–60 keV)/(13–16 keV) during a shot is lower or higher than that of steady emission respectively. The correlation coefficient between intensity and hardness ratio of (13–60 keV)/(2– 13 keV) or (16–60 keV)/(13–16 keV) is negative or positive respectively. These results may indicate that physical processes in the low state of XTE J1550-564 are different from those in Cyg X-1. Subject headings: accretion, accretion disks — black hole physics — stars: individual (XTE J1550-564) — X-rays: stars