Puzzling fluorescent emission from Orion

The authors examine the temporal behavior of the Fe Kα 6.4 keV fluorescent line in about two dozen flaring sources in the COUP survey, many more than previously reported (using Chandra archival data). The line is isolated by computing the spectral energy distribution and measuring the equivalent width. The authors wrote that "Fluorescent emission is found to accompany a flare in some cases, sometimes it is a persistent feature, and in other cases it appears "out of nothing" with no obvious response by the soft X-ray continuum." No unique identification of the exciting mechanism is presented; instead, it compares the energetic and radiative transfer requirements for photoionization and nonthermal electron collisional excitation.