X ? Ray Photon Spectroscopy with the Yohkoh Soft X ? Ray Telescope

Individual X ? ray photons in the keV energy range produce hundreds of photoelectrons in a single pixel of a CCD array detector. The number of photoelectrons produced is a linear function of the photon energy, allowing the measurement of spectral information with an imaging detector system. The Yohkoh Soft X ? ray Telescope uses a CCD in an integrating mode and makes temperature estimates from multiband lter photometry. We show how the SXT can be used in a mew way to perform a limited type of photon spectroscopy. By measuring the variance in intensity through a single lter of an x ? ray source on repeated SXT images, the mean energy per detected photon can be determined. This value is related to the underlying coronal spectrum, and hence can be used to deduce the plasma temperature. We compare the results of the temperatures derived using this new technique on a series of SXT images of a post-are loop system with the temperatures derived using the standard ux-ratio method. We demonstrate that the bright posttare loops really are cooler than the surrounding material, as shown by Tsuneta et al. (1992). Given the large dynamic range of the soft x ? ray ux observed from the Sun, we describe the requirements for a future instrument that would take advantage of photon spectroscopy.