RX J1716.6+6708: A young cluster at z=0.81

Clusters of galaxies at redshifts nearing 1 are of special importance since they may be caught at the epoch of formation. At these high redshifts ther e are very few known clusters. We present follow-up ASCA, ROSAT High Resolu tion Imager, and Keck LRIS observations of the cluster RX J1716.6 + 6708, w hich was discovered during the optical identification of X-ray sources in t he north ecliptic pole region of the ROSAT All-Sky Survey. At z = 0.809, RX J1716.6+6708 is the second most distant X-ray-selected cluster so far publ ished and the only one with a large number of spectroscopically determined cluster member velocities. The optical morphology of RX J1716.6+6708 resemb les an inverted S-shape filament with the X-rays coming from the midpoint o f the filament. The X-ray contours have an elongated shape that roughly coi ncides with the weak lensing contours. The cluster has a low temperature, k T = 5.661(-0.58)(+1.37) keV, and a very high velocity dispersion sigma(los) = 1522(-150)(+215) km s(-1). While the temperature is commensurate with it s X-ray luminosity of (8.19 +/- 0.43) x 10(44) h(50)(-2) ergs s(-1) (2-10 k eV rest frame), its velocity dispersion is much higher than expected from t he sigma-T-X relationship of present-day clusters with comparable X-ray lum inosity. RX J1716.6 + 6708 could be an example of a protocluster, where mat ter is flowing along filaments and the X-ray flux is maximum at the impact point of the colliding streams of matter.