Discovery of 413.9-second X-ray pulsation from X 1722-36.

X-ray emission above the background level was detected from a 2 x 4 deg field of view centered at alpha = 17h22m, delta = -35.9 deg during a Ginga pointing observation. This error box includes only one catalogued X-ray source, GPS 1722-363. The X-ray flux from this source was also observed during the Galactic plane scan observations in April and August, 1987 with an intensity of 1.0 mCrab and 4.5 mCrab, respectively. In the pointing observation, the X-ray flux was found to be variable on a time scale of hours. At the beginning of the observation on October 10, the X-ray intensity was about 2 mCrab and then gradually decreased below the detectable limit of 0.2-0.3 mCrab after 8 hr. The X-ray spectrum was hard with significant low energy absorption. Clear X-ray pulsation was observed with a period of 413.9 s + or - 0.2 s during the first 8 hr of observation beginning at 10h25m UT, in October 10, 1987. The pulse fraction was more than 80 percent of the total flux.