The X-ray spectrum of A0620—00

A REMARKABLE change in the X-ray spectrum of the transient source A0620—00 (ref. 1) was observed by the Ariel V Sky Survey Experiment (SSE) during the rise to maximum brightness, The SSE detector being used has an effective area (behind its field collimator) of 290 cm2 and covers the energy band 2–18 keV in 8 channels. The channel boundaries are marked in Fig. 1, which shows sample pulse height spectra taken as the source intensified. These raw data have not been corrected for the detector efficiency and at this stage include no input spectral assumptions. Two points emerge clearly: when first detected the X-ray spectrum of A0620—00 was hard and remained so as the overall intensity rose to the precursor peak and, secondly, the subsequent rise to maximum intensity was entirely due to a strong enhancement of X rays below 10 keV, giving a marked softening in the overall spectrum. In fact, the observed flux above 10 keV actually fell during this period.