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.