ON August 3 a faint new X-ray source was discovered (at 15 Ariel counts s−1) near the boundary of the Monoceros and Orion constellations and 6° from the galactic plane, during routine monitoring of the Milky Way with the Ariel V Sky Survey Experiment (SSE). During the following week, the source—designated A0620—00—increased rapidly in intensity to become the brightest in the sky (at 2–18 keV). The X-ray light curve from continued observation with the SSE to August 16 is shown in Fig. 1. The ordinate is in Ariel counts s−1 and for comparison, the corresponding count rates for the Crab Nebula and Sco X-1 are 400 s−1 and 3,200 s−1, respectively. The precursor peak on August 6 is real and is qualitatively similar to the feature seen1 in the rise to maximum intensity of the earlier Ariel transient A1524—62. Subsequent observations of A0620—00 with the Ariel V experiments pointing along the spin axis of the spacecraft have shown the intensity to have remained very high at least until August 27.