On the importance of background subtraction in the analysis of coronal loops observed with TRACE

Aims. Using TRACE coronal observations, we compare the analysis and diagnostics of coronal loop after subtracting the background with two different and independent methods. Methods. We analyze sequences of images in the 171 Å and 195 Å filter bands of TRACE. One background subtraction method consists of considering background values obtained by interpolating between concentric strips around the analyzed loop. Another involves a pixel-to-pixel subtraction of the final image after the loop has completely faded out, used by Reale and Ciaravella. Results. We compare the emission distributions along the loop obtained with the two methods and find that they differ considerably. We also find differences in the related filter ratio and temperature profiles. In particular, the pixel-to-pixel subtraction leads to coherent diagnostics of a cooling loop. After applying the other type of subtraction, the diagnostics are much less clear. Conclusions. The background subtraction should be treated with care when analyzing a loop. The pixel-to-pixel subtraction appears to be more reliable, but its application is not always possible. Subtraction by means of interpolation between surrounding regions can produce higher systematic errors, because of intersecting structures and the large amount of subtracted emission in TRACE observations