ASCA OBSERVATIONS OF THE DISTANT CLUSTER A1204

We report the results of ASCA and Ryle Telescope observations of the d istant cluster of galaxies A1204. The X-ray image shows that this sour ce is very compact and that the intracluster gas is strongly centrally concentrated. The brightness temperature decrement of the cosmic micr owave background radiation toward this cluster (the Sunyaev-Zeldovich effect) was not detected by the Ryle Telescope, with a 1 sigma upper l imit of \Delta T\ < 360 mu K. The gas temperature is 3.6 keV, which is considerably lower than that expected from the bright X-ray luminosit y in the ROSAT band and explains the lack of detection. The hot gas ma ss within 1 Mpc is estimated to be less than or similar to 7 x 10(13) M(.). In the wide window of the ASCA Gas Imaging Spectrometer, a very faint X-ray source was found at R.A. = 11(h)12(m)16(s), decl. = +17 de grees 56'40'' (J2000). The observed count rate was too small to identi fy the X-ray emission mechanism in the new source.