Institute of Cosmos Sciences - University of Barcelona Gaia Photometry Institute for Space Studies of Catalonia and Institute of Cosmos Sciences - University of Barcelona Claus Fabricius On behalf of DPAC CU5

Photometric data in Gaia DR1 Phot_g_mean_flux [e– /s] Phot_g_mean_flux_error s.d./ 𝑛 Phot_g_mean_mag [mag] Vega system Phot_variable_flag string mostly N/A Phot_g_n_obs – # CCD transits Flux_error is a coarse uncertainty measure Zero-point (DR1) Vega: 25.525 AB: 25.696 Small set of variables

Airbus DS

Gaia passbands Jordi et al. 2010

Gaia focal plane

Observation strategy Sky Mapper detection: magnitude Exposure time can be restricted by gates: 16, 126, 252, 503, 1006, 2013, 2850 ms G ≲ 12 mag 4417 ms G ≳ 12 mag Only small windows are read out G < 13 mag G: 13 – 16 mag G: 16 – 21 mag

Windows read out around stars 0.7 arcsec G = 14.9 2.1 arcsec Scan direction Windows measure 12 × 12 pixels (typically) Pixels are binned on chip before reading Lower readout noise Less telemetry

One sample masked (saturation) Recent obs. Bright star 2D window 1006 ms exp. One sample masked (saturation) Not DR1 ! G = 10.8

Recent obs. Bright detect 2D window 252 ms exp. Binary Poor centroid ~ 0.25 ̋ sep. Poor centroid G = 9.4

Simulated BP / RP spectra Jordi et al. 2010

BP spectra G = 17.5 600 400

RP spectra G = 16.7

Windows in case of conflict 0.7 arcsec Fainter detection Truncated window – not used for DR1 2.1 arcsec Brighter detection Complete window – with contamination Window for brighter detection is the winner Window for the fainter detection will lose Truncated windows are not used in Gaia DR1

Conflicts between spectra Brighter detection Complete window 3.5 arcsec 2.1 arcsec 4.1 arcsec Fainter detection Truncated window Window for brighter detection is the winner We need at least one “good” spectrum for a source Fainter source of a pair closer than 2 ̋ is lost in DR1

A dense field from DR1 Drop at 4 arcsec separation Very few below 2 arcsec separation

A sparse field from DR1 No drop at 4 arcsec separation Small peak of binaries

AF CCD flat field – pre launch Response non-uniformities wavelength & gate dependent 400 nm 550 nm 900 nm Carrasco et al. 2016

Simulated AC de-centring flux loss Carrasco et al. 2016

Calibration model Response variation Large scale Small scale Gate FoV & CCD level Small scale CCD column level, ~ 500 per CCD Gate 8 levels of exposure time (ranges of pixel rows) Colour 6 bands Time Strong variations: mirror contamination Large scale: calibration valid for 1 day Small scale: single calibration for DR1

Calibration “units” Concerns Scale CCDs Strategy Telescopes N_AC N_time N_CU AF LS 62 10 2 1 420 529 200 SS 492 309 960 BP/RP 7 6 35 280 20 664 Concerns Enough observations for the shorter gates ? Enough sources observed with different gates ? Enough sources observed with different window classes ?

Spectral shape coefficients 𝑪 𝟑 𝑪 𝟐 𝑪 𝟏 𝑪 𝟒 𝑪 𝟓 𝑪 𝟔 Carrasco et al. 2016

Flux extraction PSF fitting (G < 13 mag) Insensitive to AC de-centring LSF fitting (G > 13 mag) Wide windows (G 13-16 mag) Narrow windows (G 16-21 mag) For BP & RP Simple diaphragm photometry NB! Not included in DR1

Calibration model 𝑓𝑜𝑏𝑠=𝑓𝑖𝑛𝑠𝑡 𝐿 𝑆 𝐿 = 1 6 𝐴 𝑚 𝐶 𝑚 + 0 2 𝐵 𝑗 𝜇 𝑗 𝑓𝑜𝑏𝑠=𝑓𝑖𝑛𝑠𝑡 𝐿 𝑆 𝐿 : large scale (CCD level) 𝑆 : small scale (CCD column level) 𝐿 = 1 6 𝐴 𝑚 𝐶 𝑚 + 0 2 𝐵 𝑗 𝜇 𝑗 𝐶 𝑚 : spectral shape coefficients 𝜇 : CCD column 𝑆 : one coefficient every 4 columns Coefficients 𝐴 𝑚 , 𝐵 𝑗 , 𝑆 : valid over a calibration unit Carrasco et al. 2016

Response monitoring using Tycho-2 decontaminations M. Hauser 2016

Monitoring vs LargeScale calibration Evans et al. 2016

Small Scale calibr., three time ranges Evans et al. 2016

Scatter of individual CCD observations Gaia science performance Predicted accuracy Evans et al. 2016

Magnitude zeropoint Spectro-photometric standard stars, SPSS Nominal passband Calculate synthetic G magnitudes for SPSS Compare with instrument G magnitudes Details: Carrasco et al. 2016, A&A, in press Gaia DR1 on-line documentation

Zeropoint, Vega-system Why colour dependence ? Passband ? PSF ? Aperture ? Carrasco et al. 2016

Comparison to APASS Carrasco et al. 2016

G-RP-f(colour) versus G Arenou et al. 2016

Completeness of TGAS Missing bright stars Several stars too faint for Tycho-2

A very faint TGAS stars Tycho epoch Gaia epoch

A couple of RR Lyr: LMC / MW Clementini et al. 2016

Some limitations of DR1  Processing Simple cross match: source duplication Saturated samples not excluded No aperture correction (astrometry not yet known) Dense fields: few faint stars Extreme colours excluded (fuzzy limit) Many poorly scanned areas

Coming to you in DR2  Processing Dense fields: more faint stars Much improved cross match Saturation library enabled Aperture correction using preliminary DR2 astrometry Dense fields: more faint stars Extreme colours included Few poorly scanned areas