One of a class of celestial x-ray sources which produce bursts of x-rays in the 1-20-kiloelectronvolt range and which are characterized by rise times of less than a few seconds and decay times of a few seconds to a few minutes; the peak luminosity is of the order of 10³⁸ ergs per second (10³¹ watts) and the sources have an average equivalent temperature of 10⁸ K.