The Sky Survey Instrument (SSI) on Ariel V scanned the sky for 5.5 yr (∼ 30 000 orbits) in the energy band 2–18 keV, with a time resolution of one orbit (≈ 100 min). We have systematically searched the SSI database for fast-transient outbursts lasting one, or sometimes a few orbits. This is, to our knowledge, the first systematic all-sky survey of such outbursts. We have discovered 27 fast-transient sources above our sensitivity limit (5.5 σ) of 8 SSI count s−1, only two of which exhibited more than one outburst. We estimate that an isotropic detector otherwise similar to the SSI would detect the start of one outburst every ∼ 3 days. The sources are isotropically distributed on the sky thereby favouring a nearby or extragalactic origin. 11 of the sources are identified, including six with nearby (≲ 100 pc) RS CVn-like systems for which our observations favour a ‘large loop’ (few |R_|) model. It is probable that many of the presently unidentified fast-transients also arise from nearby RS CVn (or related) systems although our observations do allow the possibility of outbursts from extragalactic objects out to a redshift ∼ 0.06. However, one of our sources is associated with a gamma-ray burst source and the possibility remains that this, or other as yet unknown classes ot object, could responsible for some of the remaining unidentified sources.
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Pye et al. (1983) studied this question.