[Loops] [1002.2121] On the importance of background subtraction in the analysis of coronal loops observed with TRACE

[Fabio Reale]

Dear friends
    let me inform you about the following preprint on Astro-PH,

http://arxiv.org/abs/1002.2121

regarding:


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


by Sergio Terzo and Fabio Reale, accepted for publication on A&A

Abstract
In the framework of TRACE coronal observations, we compare the analysis 
and diagnostics of a loop after subtracting the background with two 
different and independent methods. The dataset includes sequences of 
images in the 171 A, 195 A filter bands of TRACE. One background 
subtraction method consists in taking as background values those 
obtained from interpolation between concentric strips around the 
analyzed loop. The other method is a pixel-to-pixel subtraction of the 
final image when the loop had completely faded out, already used by 
Reale & Ciaravella 2006. We compare the emission distributions along the 
loop obtained with the two methods and find that they are considerably 
different. We find differences as well in the related derive filter 
ratio and temperature profiles. In particular, the pixel-to-pixel 
subtraction leads to coherent diagnostics of a cooling loop. With the 
other subtraction the diagnostics are much less clear. The background 
subtraction is a delicate issue in the analysis of a loop. The 
pixel-to-pixel subtraction appears to be more reliable, but its 
application is not always possible. Subtraction from interpolation 
between surrounding regions can produce higher systematic errors, 
because of intersecting structures and of the large amount of subtracted 
emission in TRACE observations.



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