Optimal Point-Source Extraction for Spitzer IRS Spectra

A new optimal-extraction technique has been developed for deriving point-source spectra from data taken by the Infrared Spectrograph (IRS) on-board the Spitzer Space Telescope. The new technique gives improvements of up to a factor of two in the signal-to-noise ratio (S/N) for faint (< 10 mJy) sources, corresponding to an effective quadrupling of the exposure time. Regular extraction consists of an even-weighted summing of pixel values at the same wavelength. Optimal extraction weights each pixel by its S/N , estimated using the spatial profile of a bright calibration star and data uncertainties. Additionally, the optimal-extraction calculations are performed in “rectified” space, and so a natural by-product of the processing is a useful output file containing the rectified image. The optimal-extraction technique is unsuitable for extended sources and best only for point sources.