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
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.
>uvotsource 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.