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gaia data release 3 documentation

5.8 The Gaia Andromeda Photometric Survey

5.8.3 Data description and known issues

This data set contains epoch photometry in G, GBP and GRP for 1 257 319 sources within 5.5 of M31 (RA=00h 42m 44s and DEC= +41d 16m 07s). Note that the G value is averaged over a field of view giving only one G value per transit. Details about the fields provided can be found in the epoch_photometry table in the Gaia DR3 archive. This data is available from the Massive Data service (VO Datalink protocol) and not via the main archive TAP service. Details about this can be found in Chapter 18 and Gaia demos and tutorials.

The source data and what further information is available for each GAPS source can be found in the main gaia_source archive table using the query in_andromeda_survey = 't'.

For very faint sources, the error distribution of the epoch fluxes is effectively Gaussian. However, when they are transformed to magnitudes, this property is lost. Also, negative fluxes would have undefined magnitudes. These negative fluxes should ideally have been published in this survey since they are as equally valid as the positive ones cf. negative parallaxes. These were filtered out in the processing of the data for the archive. Care should be taken when using the magnitudes, since any flux values close to zero will transform to very large magnitudes, well beyond the nominal detection limits of Gaia. This is especially true for GBP and GRP.

The following list contains the identified issues with the Gaia EDR3 mean photometry described in Section 8 of Riello et al. (2021). Each item describes briefly how these issues affect the GAPS epoch photometry.

  • Overestimated mean GBP flux for faint red sources
    This issue only effects the mean source photometry and was caused by filtering fluxes smaller than 1 e- s-1 from the calculation. For the epoch photometry, negative fluxes were filtered from the archive as described above.

  • Sources with poor SSCs
    1 250 from the original list are within the area covered by GAPS. The mean G photometry for these sources can be found in the the Known Issues web page for Gaia EDR3, but not in the main section of the archive. The epoch photometry for these sources is very unreliable since they have been calibrated using unreliable SSC values.

  • Systematics due to use of default colour in the IPD
    The correction for this systematic (Section 8.3 of Rowell et al. (2021)) has been applied to both the mean and epoch G photometry in Gaia DR3.

  • G-band magnitude term for blue and bright sources
    Only eleven GAPS sources, with G<13 and GBP -GRP <-0.1, are affected by this effect. This small magnitude term on G is linked to PSF/LSF calibration issues (Rowell et al. 2021).