XMM-Newton Science Analysis System: User Guide

6.7.2 Counts conversion to magnitudes and fluxes

The task ommag converts the count rates to magnitudes in the appropriate instrumental bandpasses. The rates are taken from a source-list produced by omdetect or from a time series produced by evselect. The output file will be a FITS file identical to the input source-list or time-series except that the count-rates have been converted into instrumental magnitudes (in the specified filter bandpass: U, B, V, UVW1, UVM2, UVW2) and then written as an extra column in the original FITS file. The programme also estimates the limiting magnitude and writes the value in the FITS header.

Since SAS 5.4, flux conversion factors for U, B, V, UVW1, UVM2 and UVW2 filters are added in the keyword in the COLORMAG extension of the CCF file, OM_COLORTRANS. These flux conversion factors help users to get a rough direct estimation of flux (expressed in erg/cm$^2$/s/Å) from count rates.

For each filter, the flux (in erg/cm$^2$/s/Å) can be obtained multiplying the count rates (counts/s) from SAS by the following values:

Table 13: OM filter effective wavelengths and the white dwarf (WD) based zero points and conversion factors from count rates to magnitudes and fluxes

	V	B	U	UVW1	UVM2	UVW2
effective wavelength (Å)	5430	4500	3440	2910	2310	2120
flux conversion factors	2.49E-16	1.29E-16	1.94E-16	4.76E-16	2.20E-15	5.71E-15
WD magnitiude zero points	17.96	19.27	18.26	17.20	15.77	14.87

These flux conversion values have been obtained from observations of white dwarfs standard stars. They reflect the current status of the OM in-flight calibration, and are therefore constantly verified and updated. The users should make sure that the most updated calibration is always employed.

It should also be pointed out that these flux conversion numbers can only provide an approximated measurement of the flux without an a priori knowledge of the spectral type. For a more accurate determination of the flux, the users will find in our SAS watchout page updated values of the flux conversion factors for a given spectral type.