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Determining magnitudes and fluxes

uvotsource is the tool used to perform aperture photometry on a single source in a UVOT sky image.

Start up DS9, load the image and determine your extraction and background regions. This must be done with WCS coordinates set (i.e. fk5, not physical) and you may wish to use the source position returned by uvotdetect. The calibration of counts to magnitudes to fluxes has been done with source apertures of radius 5 arcseconds for all filters; hence, this is the recommended aperture to use for photometry. Save these apertures into region files (*.reg). Note that it is preferable to define a background region which is several times larger than the source extraction region; alternatively, a combination of small regions can be used.

We caution that the image binning varies (check the keywords BINX and BINY with fkeyprint or fv), which will change the aperture pixel values you need; i.e. for 1x1 binning, the standard aperture is 10 pixels radius, while it is 5 pixels for 2x2 binning. Both of these radii are equivalent to 5 arcseconds.

The image below demonstrates typical extraction regions - a source region on the left and a background region on the right.

Input image file and extension[]:sw00374210000uuu_sk.img.gz+1
Source region file[]:Usource.reg
Background region file[]:Uback.reg
Background threshold (1:100) []:3
FITS table of values[]:photometryU.fits

uvotsource: running uvotinteg
uvotsource: Source
            Position: RA = 04h 00m 42.59s, Dec = -55d 57m 20.0s (J2000)
            Position: RA = 60.17746, Dec = -55.95556 (J2000)
            Exposure: 245.85 s
              Filter: U
        Significance: 11.5 sigma (stat)
    Background-limit: 3.0 sigma (stat)
uvotsource: Magnitude [mag]
              Source: 18.50 +/- 0.10 (stat) +/- 0.02 (sys)
          Background: 23.83 arcsec^-2
    Background-limit: 20.36
   Coincidence-limit: 11.91
uvotsource: Flux density [erg/s/cm^2/A at 3501 A]
              Source: 1.41 +/- 0.13 (stat) +/- 0.02 (sys) x 10^-16
          Background: 1.04 +/- 0.02 (stat) +/- 0.02 (sys) x 10^-18 arcsec^-2
    Background-limit: 2.55 x 10^-17
   Coincidence-limit: 6.07 x 10^-14
uvotsource: Corrected rate [count/s]
              Source: 0.866 +/- 0.078 (stat)
          Background: 0.0064 arcsec^-2
    Background-limit: 0.156
   Coincidence-limit: 372.234
uvotsource: Raw rate [count/s]
              Source: 0.856 +/- 0.075 (stat)
          Background: 0.0064 arcsec^-2
    Background-limit: 0.155
   Coincidence-limit: 90.644
uvotsource: Flux density [mJy at 8.563 x 10^14 Hz]
              Source: 5.77 +/- 0.52 (stat) +/- 0.09 (sys) x 10^-2
          Background: 4.26 +/- 0.10 (stat) +/- 0.07 (sys) x 10^-4 arcsec^-2
    Background-limit: 1.04 x 10^-2
   Coincidence-limit: 2.48 x 10^1
uvotsource: running ftcreate
uvotsource: running fthedit
uvotsource: running fthedit

The resulting FITS file provides the same information as is written to the screen: detection significance and coincidence-corrected count rate, magnitude and flux density in erg cm-2 s-1 A-1 and in mJy with 1-sigma error (or "Background threshold"-sigma upper limits if the source is undetected - 3-sigma in the above example).

If you require a different size of source extraction region from 5 arcsec, then aperture corrections can be applied using the option apercorr: see fhelp uvotsource for more details.

There are small areas on the UVOT detector where the throughput is lower than in other areas, leading to incorrectly low (up to 34% lower) count rates (see the GSFC website). If you think your data may be affected, you should run uvotsource (or uvotmaghist) with the additional command lssfile=sssfile5.fits (sssfile5.fits can be downloaded). If the source of interest is located in a bad area, its magnitude will come out as 99 (and the Large Scale Sensitivity - LSS - factor as -99.9) and these data should be disgarded. IMPORTANT: This check disables the usual LSS correction, so remember to reset this by including lssfile=CALDB for further analysis of sources not affected by the problem.