Best-fit and 1σ limits (solid line and shaded region, respectively) on additional Gaussian flux in the LW X-ray spectrum at different energies. The dashed line and shaded region show the results for a simulated spectrum without any additional line. The 3σ uncertainties are plotted as dotted lines. The black crosses show the expected flux from selected previous detections of the line with 1σ uncertainties in flux and the typical energy resolution of the instruments (∼100 eV). The second y axis shows the corresponding mixing angle.