Spectrum of 4U 0614+091
The high-resolution spectrum of 4U 0614+091 collected by two Reflection Grating Spectrometers (RGS) aboard XMM-Newton. On top of the data is overlaid the best-fit model, excluding the contribution of the oxygen line, with the abundances of oxygen, neon and iron as derived from local fits.
The bottom panel displays the residuals between the data and the best fit model, clearly showing a broad emission feature at 19 Å, corresponding to 0.7 keV. This feature is interpreted as an oxygen line - more precisely, the O VIII Lyman-alpha line, produced by the transition to a lower energy state of the only electron remaining in an oxygen ion; the line profile is broadened both by relativistic effects and by Compton scattering.