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Knowledgebase: IRS
I didn't get a spectrum of my science target because there was a false positive. Was that exceptionally bad luck?
Posted by z-Vandana Desai on 01 December 2008 03:07 PM
The integrated number counts for sources with flux density of 1 mJy at 24 microns at high latitudes (see Marleau et al. 2004, ApJS, 154, 66) give 335 sources (both stars and galaxies) per square degree in a high galactic latitude field. That is one source per 10 peak-up fields. At a 24-micron flux density of 5 mJy, the number counts are only 25 per square degree or one source per ~150 peak-up fields. So if you have a peak-up source with an estimated flux of 1 mJy, it is not that unusual to have a second source in the peak-up array field-of-view that is as bright or brighter. The source density can be much higher in many regions of the sky.

The SOM recommends that your peak-up source flux density be larger than 5-10 mJy for both Blue and Red peak-up. If your source is fainter than this, you should check the peak-up data carefully to ensure your peak-up was performed as intended and, therefore, that your IRS spectrum was obtained for the correct target.