Upper limits on stellar flare X-ray emission from OSO-3.

The OSO-3 X-ray telescope scanned 82 optically-observed flares on UV Cet, EV Lac, YZ CMi, and AD Leo as well as four radio flares on UV Cet. These data are examined individually and by superposed-epoch analysis. Upper limits of about 1000 and about 10 million are determined for the X-ray/optical and X-ray/radio luminosity ratios, respectively. The upper limits conflict with theories of stellar flaring which directly utilize nonthermal particles to excite the main part of the flare optical emission, but agree with observations of solar flares. Moderate improvement of the X-ray observations should make possible a direct test of the solar-flare analogy in the near future. The integrated X-ray luminosity of stellar flares may contribute significantly to the low-energy diffuse X-ray background in the plane of the Galaxy.