The Neil Gehrels Swift Observatory

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Small Scale Sensitivity check for UVOT

In the UVOT we have discovered some small areas where the throughput is lower than for the rest of the detector. The UV filters show the effect more strongly than the optical. In the UV the bad areas cover just 0.6% of the whole field of view, however this is 7% of the central 5'x5' square. For the optical filters, 0.7% of the central area is affected. See uvotcaldb_sss_01.pdf for more information.

The calibration team are working on a correction to the photometry for sources imaged within these regions, but in the meantime you can check whether your data is likely to be affected.

Firstly you will need to ensure you have HEASOFT version 6.18 or later installed.

Secondly you will need this caldb-style file: sssfile5.fits.gz. In this SSS file, the SSS factor images are almost entirely filled with 1.0s but wherever there is a bad patch, the SSS factor is set to -99.9. You can use fv or ds9 to look at the images in the file.

The check can be performed by running any of the UVOT routines:


e.g. > uvotmaghist lssfile=sssfile5.fits infile=sw00055761030uw1_sk.img.gz outfile=maghist.fits srcreg=55761.reg bkgreg=55761-bk.reg plotfile=plotout.gif

as normal but defining the parameter LSSFILE=sssfile5.fits. This replaces the normal CALDB LSS file with this special SSS file. In the event that your source is located on a bad patch, the LSS factor in the output file will be -99.9 and the magnitude of the source will come out as 99. The photometry for these sources has to be discarded.

NOTE: When using this SSS file in this way, you are disabling the normal LSS correction. Therefore it is important to re-run the analysis using the normal LSSFILE (LSSFILE=CALDB, or just use the default setting) to get the best photometry for those sources that are not being discarded.

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