M. J. Ricketts

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.

Observations of variable and transient X-ray sources with the Ariel V sky survey experiment

Results obtained during the first six months in orbit of Ariel V with the Leicester Sky Survey are reviewed. Among 80 sources found by a scan of the Milky Way, 16 are new, and 11 UHURU sources in the scanned region are not detected. Some of these sources may be transient. The light curve of Cen X-3 in a binary cycle shows a dip between phase 0.5, and 0.75, and a secondary maximum at the centre of the dip. The dip and the maximum get progressively weaker in the succeeding cycles. These features are interpreted in terms of the stellar wind accretion model. Cyg X-1 observation for 14 days gives a broad minimum around superior conjuction. Four bright transient sources of nova-like light curves have been observed. The light curves and the spectra are given for TrA X-1 (A1524-62) and Tau X-T (A0535+26).

Ariel-6 Medium Energy Spectral Observations of Active Galaxies

X-ray spectra of the BL Lac type object Mkn 421 and several Seyfert type 1 galaxies; IIIZw2, MCG8-11-11 and NGC 4151, have been obtained using the Leicester University instrument on board the Ariel-6 satellite. The Mkn 421 spectrum is best represented by two powerlaw components, the soft component having α ~ 3.4 whilst the hard flux has α ~ 1.0. In MCG8-11-11 there is clear evidence for spectral variability between our observation in late 1979 and that of HEAO-1/A2 in 1977. The Ariel-6 spectrum of MCG8-11-11 can be fitted by a powerlaw of index α ~ 2.1 together with an iron line at ~ 6.2 keV with an equivalent width of ~ 1.6 keV. The first X-ray spectrum of IIIZw2 is also presented, fitting with a powerlaw we find an index of α ~ 1.7. With the exception of NGC 4151 there is no evidence for a significant column of cool material along the line of sight.

Ariel 6 observations of Cyg X-1 in the high state

Whereas nearly all previous observations of Cyg X-1 with good time resolution have been made when the source was in its low emission state, Ariel-6 has been fortunate to observe it during 1980 June–July mainly in its high state, i.e. with enhanced low energy emission. The fluctuations were studied on all time-scales down to 2 ms and appeared similar to those previously reported for the low state. The cross-correlation of rapid fluctuations between two bands above and below 3.5 keV however showed a small but significant asymmetry with the high energy emission delayed.