Solar Hard X‐Ray Images Observed by Astro‐A

The Solar X‐ray Telescope (SXT) on board the Astro‐A has observed many events since its launch on February 21, 1981. Several of the largest events with counting rates <104 c/s, have been analyzed to reveal very compact sources for the large hard X‐ray bursts. Although a few limb events show some extended features up to about one arcmin, most events have linear dimensions less than the FWHM of the SXT triangular response, which is about 30 arcseconds. This compactness of the largest events may conflict with traditional models of hard X‐ray sources, including thin and thick target models. In this paper, two typical large events are presented. A disk event on April 2, 1981, shows a single source with a very small diameter, while a April 27, 1981, limb event shows a double source structure with unbalanced intensities.