Puzzling fluorescent emission from Orion

Abstract

Fluorescent X-ray emission offers a rare possibility for studying cool material surrounding active, young stars in the X-ray regime. In this work, we develop a new method to search for fluorescent emission and analyze its temporal behavior, which we apply to a sample of <i>106<i/> young, active stars in Orion. Our analysis yields a sample of <i>23<i/> X-ray sources with fluorescent emission, including 6 objects already reported on in an earlier study. The fluorescent sources show a wide variety of temporal behavior. While the fluorescent emission is associated with soft X-ray flares in some cases, it sometimes appears as a (quasi) persistent feature, or is seen during truly quiescent periods. We conclude that fluorescent X-ray emission can be observed in a much higher fraction of young, active stars than previously believed. Whether photoionization alone is the excitation mechanism of fluorescent X-ray emission or if electronic collisional excitation also contributes remains debatable. The temporal variability is often hard to reconcile with the photoionization model, which remains plausible if we allow for suitable geometries. Photoionization is preferred to electronic, collisional excitation mainly because the energetics of the latter challenge our current physical understanding.