ABSTRACT
The first clear detection of X-rays from a celestial body was of the Sun, from an Aerobee rocket in 1949, by Friedman using photographic emulsion. The Sun is by far the strongest X-ray source in the sky. By the end of the twentieth century, the most sensitive X-ray telescope, the Chandra X-ray Observatory, could detect sources that are 1016 times fainter, a rapid rate of progress for any field. During solar flares, the intensity may increase rapidly by a factor of 103 for several minutes to several hours, and the spectral hardness also increases temporarily. Intense, rapidly varying levels of hard X-rays are often emitted during the rise phase of a flare, and gamma rays are seen from the largest events.
