Gaia Photometric Data: DR1 results and DR2 expectations

Dafydd Wyn Evans, Marco Riello, Francesca De Angeli, Giorgia Busso, Floor van Leeuwen, Laurent Eyer, Carme Jordi, Claus Fabricius, Josep Manel Carrasco, Michael

Weiler, Paolo Montegriffo, Carla Cacciari, Elena Pancino

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

Link with François Mignard

• Hipparcos• Photometry Working Group• François was in charge of photometry for FAST• Also on Hipparcos Science Team

Champery 1993 Darmstadt 1994

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

Gaia DR1

• Photometric contents• G-band mean fluxes and errors for over 1 billion sources

• van Leeuwen et al. A&A, 599, A32 (2017)

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

Gaia DR1

• Photometric contents (variables)• Epoch photometry for 599 Cepheid (43 new) and 2595 RR Lyrae (343 new)

• Eyer et al. 2017 (arXiv:1702.03295) • Clementini et al. A&A, 595, A133 (2016)double-mode pulsator

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

Source of G, BP, RP photometry

5

AF BP/RP

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

Large-scale calibration

ReferenceCatalogue

CalibrationsUncalibratedData

Internally Calibrated Photometry

(mean and epoch)

Small-scale calibration (effectively a 1d flat field)

Calibration unit - CCD, FoV, gate and window classShort time scaleModel of AC position and colour information

Calibration unit - group of 4 pixel columns, gate and window classLong time scale (months)Zero point only (configurable for the future)

Internal calibrationlarge number of sources

no external reference datacalibrate raw data to an average instrument

This only works if sources are observed under many different conditions,i.e. if there is a good level of mixing between the various calibration units!

Principles of calibration

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

External calibration

• Uses ~200 SpectroPhotometric Standard Stars• Pancino et al. MNRAS, 426, 1767 (2012)• Generates Vega and AB magnitude zeropoints• No passband information in DR1

• Various colour-colour relationships given in van Leeuwen (2017)

• For more details about the internal and external calibration see Carrasco et al. A&A, 595, A7 (2016)

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

Unexpected complexities

• Contamination• water from spacecraft depositing on mirrors

and CCDs• mitigated by periodic heating

(decontamination)• Stresses photometric initialization• Affects throughput (detection limit)• Most of contaminant now gone

• Straylight• More background light from Sun• Background-subtraction algorithms had to be

improved• At the faint end there was a loss of

performance• sky limited

© ESA 2017

Not in DR1

Gaia Collaboration, Prusti, et al. 2016

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

DR1 validation

• Error on mean photometry for sources with ~100 observations• Evans et al. A&A, 600, A51 (2017)

Nominal mission prediction

+3 mmag added in quadrature

0.4 slope (sky dominated)

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

DR1 validation

• External catalogue comparisons• Evans et al. A&A, 600, A51 (2017)• All catalogues have features - what is true?

• Internal consistency check• Arenou et al. A&A, 599, A50 (2017)• Compares BP and RP with G• Cannot indicate where difference lies

• Need additional information

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

Example science from DR1 photometry

• Hertzsprung progression• Classical Cepheids• Shape of light curve changes with period

• Observed that bump changes position• Probably linked to a shock wave

Plot prepared by V. Ripepi

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

Example science from DR1 photometry

• Innovative use of mean G photometry• uses errors and number of observations to flag for variability

• Belokurov et al. MNRAS, 466, 4711 (2017)• RR Lyrae• Combined data from GALEX

• Deason et al. MNRAS, 467, 2636 (2017)• Miras• Combined data from 2MASS and WISE

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

DR2 photometry (contents)

• Mean G fluxes and errors (as DR1)

• Colours• Mean BP and RP integrated fluxes (and errors)

• Epoch photometry and colours (G, BP, RP) for more variables (tens of thousands?)

• Full passband will be available (G, BP, RP) along with Vega/AB zeropoints

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

DR2 photometry (improvements)

• Better initial data for G band• Different image parameter determination

• LSF/PSF models more sophisticated• Saturation handled better• Less systematics

• Cross match improved• Less outlier observations

• Improved initialization of the photometric system• Uses only low contamination period• Less systematics

• Better handling of outliers• TGAS and other faint anomalies should not occur

• Recovery of extreme colour sources• Very blue and red sources will not be missing

• Probably more bright sources included

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

DR2 photometry (differences)

• New initialization implies a (slightly) different passband• initialization uses much less contaminated data• contamination is effectively a filter that changes the passbands

• New cross match implies changes to some Source IDs• not a simple one-to-one relationship

• some transits will have been distributed to different sources• some sources with the same ID might not have the same transits

• Moral: treat Gaia DR2 as a totally new catalogue

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

DR2 Validation

Cycle 2 over Cycle 1 Raw accumulations

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

DR2 ValidationAccumulations GBAND LS2 INIT C02

5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 1mag

-4 .50-4 .25-4 .00-3 .75-3 .50-3 .25-3 .00-2 .75-2 .50-2 .25-2 .00-1 .75-1 .50-1 .25-1 .00

log1

0(er

ror)

Nccd ~ 100

• After first iteration: more iterations to come and small-scale calibration• Calibration floor better than DR1, many bumps reduced

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

DR2 expected science

• Cluster HR diagrams will improve with use of DR2 colours

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

• RR Lyrae (AB type)• 71,464 identified from other surveys

• point colour indicates source of identification• Gaia magnitudes and colours (not corrected for extinction) show advantage

of having a homogeneous photometric system• Gaia Image of the Week 24/3/17 (Rimoldini et al.)

Sagittarius dwarf

DR2 expected science

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

• Coming to a data centre near you April 2018

• © ESA/Gaia/DPAC

Gaia DR2 in colour

Dafydd Wyn Evans IAUS 330 @ Nice, 28 April 2017

• Coming to a data centre near you April 2018

• © ESA/Gaia/DPAC

Gaia DR2 in colour