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XMM­Newton CCF Release Note
XMM­CCF­REL­23
OM Photometry
R. Much
October 3, 2000
1 CCF components
Name of CCF VALDATE List of Blocks
changed
CAL VERSION XSCS flag
OM PHOTTONAT 0002 2000­01­01T00:00:00 PHOTTONAT Yes
OM COLORTRANS 0004 2000­01­01T00:00:00 COLORMAG No
2 Changes
First release
The om phottonat file contains the detector charcteristica, such as CCD reset times, shift in
horizontal and vertical direction, flush time and position of readout area within CCD. The numbers
were derived from laboratory measurements and are supposed not to change inorbit.
The om colortrans file includes the defintion of the zero points of the various broadband filters and
the parameters of the colour transformation of the OM­U, OM­B and OM­V­filter into the standard
systems. The zero point was derived in the following way:
ffl Matching the OM repsonse using the measurements of BPM16274 and LBB227.
ffl the zeropoints in U, B, V are set in a way that the simulations with the Vega spectrum (from
ESO ftp server) matches the average brightness found in the literature which is (0.025, 0.03,
0.03) mag respectively.
ffl In the UV filters the ESO Vega brightness is arbitrarily set to the U value, i.e. 0.025 mag
ffl Magnifier and White brightness of Vega are arbitrarily set to 0.03 mag
1

XMM­Newton CCF Release XMM­CCF­REL­23 Page: 2
The colour transformation analysis was performed by comparing simulated count rates of model
spectra for different stellar types with values from literature and using the results of the LBB227
and BPM16274 data analysis. Input model spectra were taken from Pickles (1998, PASP, 110, 863).
The Pickles spectra are normalized to 1. at the wavelength of 555.6 nm. Therefore the spectra had
to be renormalized to allow absolute calibration. The analysis comprised of the following steps:
1. The UBV response was taken from the literature (Allen, Astrophysical Quantities). Stepsize
is 20 nm.
2. The flux of Vega was calculated for the three filters.
3. The Vega flux was corrected to mag 0, as the average literature flux of Vega is non­zero
(v=b=0.03, u=uvw1=uvm2=uvw2=0.025). In this way the flux of a zero magnitude star in
the V­filter was calculated.
4. The spectrum of each spectral type in the Pickles library was normalized to yield the flux
corresponding to a 0 mag star in the V­filter.
5. The brightness of each library spectrum was calculated for the B and V filter as well. The
derived colours agree with the values published by Pickles within 0.05 mag, whereby the largest
differences were found for the U­B colour of early types.
6. The expected OM count rates for the filters V, B, U, UVW1, UVM2 and UVW2 were calcu­
lated.
7. applying the methode of Harris, the coefficients of the colour transformation were calculated.
All spectra were considered, i.e. for this version the solution were not calculated for subranges
of the colour index.
The current analysis is limited by a number of factors, these include:
ffl analysis is mainly based on simulations with model spectra.
ffl the throughput calibration is based only on data of the two calibrators LBB227 and BPM16274.
ffl colour transformation for different colour intervals is not yet done. The quality of the colour
transformation gets worse as the spectral type gets later. This problem can be overcome by
defining colour several transformation each only valid in certain colour intervals.
ffl the quality of the transformation still has to be assessed in detail, i.e. the inspection of outliners
is still TBD.
Both files describe the redundant OM detector chain.
We expect the file om colortrans to be updated once colour transformations are calculated for
different intervals and once the results of the dedicated ground based photometric observation pro­
gramme become available. Early results of the ground observations are expected in October 2000.

XMM­Newton CCF Release XMM­CCF­REL­23 Page: 3
3 Scientific Impact of this Update
First release
4 Estimated Scientific Quality
An inorbit update of the parameters in the PHOTONAT file is very unlikely and the ground based
data probably contain the best achievable values.
The colour transformation of model spectra was found to be generally good at the 0.1 mag level or
even better. However there are outliners, which need inspection. The colour transformation gets
worse as one goes towards late type spectra (B­V ? 1). Recommendation: assume accuracy of 0.5
if redder than B­V ?1.0.
The accuracy of U­B transformation is generally better than 0.1 mag, however there are again some
outliners, which need inspection. The U­V transformation is generally better than 0.1 mag, but
there are some outliners for U­V ? 1. BV transformation is generally better than 0.05 mag for the
range \Gamma0:5 ! B \Gamma V ! 1. Towards the red, i.e. with increasing colour index the transformation gets
worse with B­V residuals up to ­0.3 mag. The V magnitude is good within 0.05 mag, with some
outliners with a residual up to 0.2 mag.
5 Acknowledgements
Thanks to OM team members, especially Dirk Pandel (UCSB) and Hajime Kawakami (MSSL) for
providing inputs.