We develop a novel statistical approach to identify emission features or set upper limits in high-resolution spectra in the presence of high background. The method relies on detecting differences from the background using smooth tests and using classical likelihood ratio tests to characterise known shapes like emission lines. We perform signal detection or place upper limits on line fluxes while accounting for the problem of multiple comparisons. We illustrate the method by applying it to a Chandra LETGS+HRC-S observation of symbiotic star RT Cru, successfully detecting previously known features like the Fe line emission in the 6-7 keV range and the Iridium-edge due to the mirror coating on Chandra. We search for thermal emission lines from Ne X, Fe XVII, O VIII, and O VII, but do not detect them, and place upper limits on their intensities consistent with a $\approx$1 keV plasma. We serendipitously detect a line at 16.93 $\unicode{x212B}$ that we attribute to photoionisation or a reflection component.

翻译：我们提出了一种新颖的统计方法，用于在高背景条件下识别高分辨率光谱中的发射特征或设定上限值。该方法通过平滑检验检测与背景的差异，并利用经典似然比检验表征发射线等已知形状。我们在考虑多重比较问题的同时，进行信号检测或对谱线通量设定上限。通过将该方法应用于钱德拉卫星LETGS+HRC-S对共生星RT Cru的观测数据，我们成功检测到先前已知的特征，如6-7 keV范围内的Fe谱线发射以及由钱德拉镜面涂层产生的铱吸收边。我们搜索了Ne X、Fe XVII、O VIII和O VII的热发射谱线，但未检测到这些谱线，并对其强度设定了与≈1 keV等离子体一致的上限。此外，我们偶然在16.93 Å处检测到一条谱线，将其归因于光电离或反射成分。