Question ID = 0151
Question ID = 0151
Assigned Member (担当者)2011-12-27, Y.TeradaName and Institute (質問者/機関)Yang HuanUniversity of Science and Technology of China (Graduated student, first year) Question (質問)The question is about XIS source region selection and uncertainties in the ARF. The Suzaku abc guide says: "For a point source, circular extraction regions centered on the source should be used to extract source spectra." But the peaks of the PSF have an offset from the "Ra Dec" of the source in the image of many observations of point sources, such as AGN. Why this offset exist? Should the source region be centered on the peak of the PSF or on the "Ra Dec" of the source? I extract spectra and responses form both regions, and find differences between the ARF of the two region. (ARF were generated using the XISSIMARFGEN. The regions are circles with radius=220 arcsec.) Does the offset between the peaks of the PSF and "Ra Dec" of the source bring some uncertainties to the ARF?Answer from(回答の文責)Suzaku HelpdeskAnswer (答え)As described in the ABC guide you pointed out, a (RA DEC) position of an object may slightly different from the peak of the image center of the data. This is due to systematic errors in determination of the SKY coordinate using the attitude of the spacecraft and miss-alignment information of detectors, etc. According to Uchiyama et al 2008 PASJ vol 60 page S35, the systematic error of the coordinate in the SKY X and Y of the Suzaku XIS is about 19 arcsec. You can calculate the error budget in uncertainties of the ARF from this value and the spatial distribution of the PSF (i.e., Figure 3.4 in the technical description). When you use the arf region with the radius of 220 arcsec, the uncertainties in the effective area from the error of the coordinate determination is less than a few %.Status （詳細なステータス）2011-12-27 Accept2011-12-27 Discussion under Suzaku Helpdesk 2011-12-28 Prepare the answer 2011-12-31 Closed 
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