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OM count rates can be obtained with SAS. The chain omichain allows any user to process OM image data. Among other products, it produces source list files where the count rates for all objects detected in an image are given. These rates, as from SAS 6.5, are corrected from all known instrumental effects, mainly coincidence losses, dead time and time sensitivity degradation. The count rates can also be obtained from the PPS pipeline products delivered by the SSC at Leicester.

In case a user wants to measure the count rates by him/herself, we provide a recipe at the end of this document.

**
**

- multiply the count rate obtained with SAS (files
*SWSRLI*, corrected count rate in CORR_RATE column) by the
corresponding average conversion
factors listed in the following Table.
**Filter****Effective**

wavelength(nm)**Conversion factor**

(erg/cm/cm/A/cnt)V 543 2.49E-16

B 450 1.29E-16

U 344 1.94E-16

UVW1 291 4.76E-16

UVM2 231 2.20E-15

UVW2 212 5.71E-15

These conversion factors have been
derived from observations of white dwarf standard stars. They give an
average transformation which can be used in all cases where no
information is available on the spectral characteristics of the
source. The error of the fluxes obtained by using these factors is less
than 10%.

In case of stellar objects for which the spectral type is known, the user may prefer to use Method 2.

In case of stellar objects for which the spectral type is known, the user may prefer to use Method 2.

- choose the most appropriate spectral type (in case of white dwarf, refer to Method 1)
- convert from count rate, obtained with SAS (files
*SWSRLI*, corrected count rate in CORR_RATE column), to flux, selecting
the most
appropriate factor for the chosen spectral type and filter
from the following Tables:
**Filter****A0V****B0V****F0V****G0V****K0V****M0V****Vega**V 2.50E-16 2.48E-16 2.52E-16 2.54E-16 2.56E-16 2.65E-16 2.50E-16 B 1.36E-16 1.16E-16 1.41E-16 1.53E-16 1.60E-16 1.81E-16 1.34E-16 U 1.71E-16 1.94E-16 1.80E-16 1.83E-16 1.88E-16 2.01E-16 1.70E-16 UVW1 4.96E-16 4.72E-16 4.96E-16 4.51E-16 3.88E-16 1.09E-16 4.86E-16 UVM2 2.20E-15 2.14E-15 2.10E-15 1.84E-15 1.66E-15 n.a.

2.19E-15 UVW2 6.06E-15 5.56E-15 7.15E-15 6.05E-15 5.76E-16 n.a.

5.88E-15

These numbers have been obtained by folding the spectral library of Pickles with the in-flight response curves of the OM. For Vega we have used the alpha_lyr_stis_002 calibrated spectrum provided by the HST Calspec database.

If we compare these factors with the ones derived from white dwarfs
(Method 1) we see that they are very similar except for the UV filters
in cool stars. These is mostly due to the fact that the library spectra
for these stars have very low or zero flux at ultraviolet wavelengths.

Similar results are obtained with a different library: the
Bruzual-Persson-Gunn-Stryker Spectra:

Filter |
A0V |
B0Ib |
F0IV |
G0V |
K0V |
M0V |

V | 2.48E-16 | 2.50E-16 | 2.50E-16 | 2.55E-16 | 2.56E-16 | 2.61E-16 |

B | 1.29E-16 | 1.17E-16 | 1.38E-16 | 1.44E-16 | 1.55E-16 | 1.80E-16 |

U | 1.66E-16 | 1.97E-16 | 1.77E-16 | 1.88E-16 | 1.85E-16 | 1.94E-16 |

UVW1 | 4.79E-16 | 4.76E-16 | 4.84E-16 | 5.02E-16 | 5.15E-16 | 3.14E-16 |

UVM2 | 2.15E-15 | 2.17E-15 | 2.18E-15 | 2.27E-15 | 2.02E-15 | 1.42E-15 |

UVW2 | 5.56E-15 | 5.25E-15 | 6.14E-15 | 6.50E-15 | 6.34E-15 | 2.46E-15 |

As in the case of Pickles library, the UV part of the spectra for cool stars is not reliable.

A calibration observing campaign is currently being carried out with OM in order to obtain the most reliable conversion factors for each stellar type. The users are invited to check periodically the content of this page, to ensure that the most updated calibrations are always being employed.

The AB system is defined for OM as:

mAB(filter) = Zero_point(filter) - 2.5 log10(Count_Rate)

where the Zero points for each filter come from the original definition of AB magnitude as:

Zero_point(filter) = -48.60 - 2.5 log10(1/n_phot)

with n_phot being the count rate measured in a filter for a constant incoming flux of one erg/sec/cm2/hz. The calculated zero points for the OM AB system are then:

Filter |
Zero-point |

V | 17.923 |

B | 19.081 |

U | 19.189 |

UVW1 | 18.566 |

UVM2 | 17.412 |

UVW2 | 16.572 |

The inverse of n_phot provides a count rate to flux conversion factor (in frequency space) for each filter:

Filter |
Conversion factor(erg/cm/cm/hz/cnt) |

V | 2.46E-27 |

B | 8.47E-28 |

U | 7.66E-28 |

UVW1 | 1.36E-27 |

UVM2 | 3.94E-27 |

UVW2 | 8.54E-27 |

These conversion factors are obtained by folding a constant flux in frequency space of one erg/sec/cm2/hz with the in-flight response curves of the OM.

**
**

- determine the source+background counts within an aperture of 12
pixels radius (unbinned) for U, B and V filters. For UVW1, UVM2 and
UVW2, the counts measured within the 12 pixels aperture are
extrapolated to 35 by using the PSF definition. Users doing this "by
hand" should take an aperture of 35 pixels. In case of binned images
(2x2) aperture sizes shoudl be reduced by half.

- determine the background counts within an annulus of radii 14 and 25 pixels (unbinned) for U, B and V filters (8 and 13 in binned images). For UVW1, UVM2 and UVW2 the inner radius should be at least 37
- apply the coincidence loss and deadtime corrections to the source+background counts as specified in the XMM-Newton User Handbook
- subtract the background counts
- correct for time sensitivity degradation as described in these Evergreen tips and
tricks

- calculate the instrumental magnitude, using the formula:
mag = -2.5 log10 (count rate) + zero-point where the zero-points are reported in the following table:**Filter****Zero-point**V 17.963 B 19.266 U 18.259 UVW1 17.204 UVM2 15.772 UVW2 14.867

This is the way SAS uses to derive the
magnitude of any detected star in an OM image. The use of SAS is
strongly recommended.

- Vega is used as a reference to determine the magnitude to flux
conversion. The flux of Vega should correspond to a magnitude
0.03 in OM B,V filters, and 0.025 in OM U and UV filters. The Vega
fluxes in the OM filters are listed in the following Table:
**Filter****Effective wavelength (nm)****Flux (erg/cm2/s/A)**V 543 3.70E-09 B 450 6.40E-09 U 344 3.20E-09 UVW1 291 3.68E-09 UVM2 231 4.33E-09 UVW2 212 5.03E-09

The quoted fluxes for Vega have been
obtained by interpolation in its calibrated spectrum provided by the
HST Calspec database.

- apply the following formula for a given filter
mag (Vega) - mag (source) = -2.5 log [Flux(Vega)/Flux(source)]

Any question about