Image of the Week

First calibrated XP spectra

 

Figure 1: Mean and normalised spectra derived for a set of particular sources with different GBP - GRP colours taken as examples. The left panel shows the spectra derived from the Blue Photometer instrument, the right panel the mean spectra from the Red Photometer. Image credit: ESA/Gaia/DPAC

Gaia has proven to be a powerful tool providing very precise astrometry and photometry in the first releases of the mission (see Gaia DR2 and Gaia DR1). The third release and beyond (find here the current release scenario planning) will include some calibrated spectrophotometric data.

Spectrophotometry in Gaia is obtained using two prisms. Each of these prisms is able to divide the incoming light of the observed source into the different colours of the rainbow (similarly to what is represented in the iconic image of the cover of the Pink Floyd’s album “The dark side of the Moon”). One of these prisms is designed to detect blue light, called Blue Photometer or BP, covering a wavelength range from 330 to 680 nm. The second prism is, instead, designed for redder light, called Red Photometer or RP, covering the wavelength range from 640 to 1100 nm.

When the final Gaia catalogue will be available, it will constitute the most complete and homogeneous spectrophotometric catalogue of sources, containing more than one billion objects with homogeneous spectra obtained with the same instrument all across the sky.

This large catalogue of spectrophotometric data will be very useful to extract the astrophysical information for each source. In the case of stars, the BP and RP spectra provide temperatures, surface gravity, chemical composition and interstellar absorption. For galaxies and quasars, BP and RP spectra allow deriving the redshift. For asteroids one can determine their composition and types.

Right after the launch of the Gaia space telescope, and before starting the scientific operations, several tests were performed to assure the health of the payload. At that time, some raw observations obtained with the BP and RP spectrophotometers were published (see this image of the week of June 2014). Those spectra were uncalibrated single observations. Since then, we have accumulated observations in BP and RP for each source in the sky.

Before its publication in the Gaia catalogue, the team in charge of the photometric calibration in Gaia (Coordination Unit 5 or CU5) has calibrated the two spectrophotometers and combined the different observations of the same source to build a mean spectrum representing the average of all transits. These mean spectra will suffer less noise effects than every individual observation, providing more information than a single transit. The figures included here show some examples of these mean calibrated spectra derived by CU5.

In the absence of extinction, the colour of the stars is related with their temperature. Hot stars are bluer than cooler ones. Thus, hot stars have most of their flux in the BP instrument and cooler ones, instead, are brighter in RP. This is a gradual effect and intermediate temperatures will have a more balanced contribution in both wavelength ranges. This can be clearly seen in Figure 1, showing selected examples of the first calibrated mean BP and RP spectra for stars having different observed colours.

By integrating the spectra, one can derive two associated magnitudes, one for the BP and one for the RP instrument. We call these two magnitudes GBP and GRP, respectively. Taking the difference between these two magnitudes gives an indication of how blue or red a source is. Blue (hotter) stars have smaller GBP - GRP values than red (cooler) stars. In Gaia, there is also a third (and more precise) magnitude, obtained with the astrometric detectors in white light devoted to the astrometric measurements, called G, which provides a better estimate of the global apparent brightness of the star. The mean spectra in Figure 1 have been normalised by the total flux in G. The GBP - GRP colour of all sources is also given in the legend of the plot.

The abscissa in all plots is given in pseudo-wavelength. This is an internal wavelength scale, close to the actual sampling of the spectra, representing different absolute wavelenghts in BP and RP. Conversion to the absolute wavelength physical units will be provided with the released spectra (expected in Gaia DR3).

 

 

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​​​​​​​​​​​​​Figure 2. Epoch and mean spectra for an extragalactic source used in Gaia DR2 to derive the Gaia Celestial Reference Frame (Gaia-CRF). The titles in each plot include the magnitudes in the astrometric field (G) and each spectrophotometric instrument (GBP and GRP) and also the number of individual observations in each instrument N​​​​​​​obs. Image credit: ESA/Gaia/DPAC

 

Sources in Figure 1 have a magnitude of about G=13 mag. The signal to noise ratio observed at the maximum flux of the spectra at this magnitude is about 600 for BP and 1000 for RP.

Gaia also observes point-like sources other than stars. For example, extragalactic sources (far-away quasars) are used to establish the astrometric reference frame (Gaia Celestial Reference Frame, Gaia-CRF). For those sources, the spectral energy distribution in BP and RP differs drastically from those of stellar sources. Some of these quasars can have strong emission lines. Two of these type of sources are plotted in Figures 2 and 3.

 

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​​​​​​​Figure 3. The same as in Figure 2 but for another quasar. The flux-level variations between observations made at different epochs indicate the variable nature of the source. Image credit: ESA/Gaia/DPAC

 

These plots show both mean and epoch spectra overplotted, illustrating the process to build the mean spectra from different epoch observations. The different epochs in Gaia are measured in number of six-hour revolutions done by the satellite while surveying the sky. The source in Figure 2 has a very low flux level in one of its transits. The photometric processing is sufficiently robust to consider this measurement as an outlier, thereby not producing a deviating mean spectrum by including this bad observation. The source plotted in Figure 3 is a variable one showing a changing flux level at different epochs. The mean spectrum built is a weighted average of all the epoch observations. For variable sources, this means that although the mean spectra may not be representative of any particular epoch, they are useful to provide a hint of the average behaviour and mean characteristics of the source.

 

Credits: ESA/Gaia/DPAC, Coordination Unit 5, Josep Manel Carrasco, Francesca De Angeli, Dafydd Wyn Evans, Carme Jordi, Michael Weiler

[Published: 12/08/2020]

 

Image of the Week Archive

2020
29/10: Gaia EDR3 passbands
15/10: Star clusters are only the tip of the iceberg
04/09: Discovery of a year long superoutburst in a white dwarf binary
12/08: First calibrated XP spectra
22/07: Gaia and the size of the Solar System
16/07: Testing CDM and geometry-driven Milky Way rotation Curve Models
30/06: Gaia's impact on Solar system science
14/05: Machine-learning techniques reveal hundreds of open clusters in Gaia data
20/03: The chemical trace of Galactic stellar populations as seen by Gaia
09/01: Discovery of a new star cluster: Price-Whelan1
08/01: Largest ever seen gaseous structure in our Galaxy
2019
20/12: The lost stars of the Hyades
06/12: Do we see a dark-matter like effect in globular clusters?
12/11: Hypervelocity star ejected from a supermassive black hole
17/09: Instrument Development Award
08/08: 30th anniversary of Hipparcos
17/07: Whitehead Eclipse Avoidance Manoeuvre
28/06: Following up on Gaia Solar System Objects
19/06: News from the Gaia Archive
29/05: Spectroscopic variability of emission lines stars with Gaia
24/05: Evidence of new magnetic transitions in late-type stars
03/05: Atmospheric dynamics of AGB stars revealed by Gaia
25/04: Geographic contributions to DPAC
22/04: omega Centauri's lost stars
18/04: 53rd ESLAB symposium "the Gaia universe"
18/02: A river of stars
2018
21/12: Sonification of Gaia data
18/12: Gaia captures a rare FU Ori outburst
12/12: Changes in the DPAC Executive
26/11:New Very Low Mass dwarfs in Gaia data
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
2017
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
04/10: 1:1 Gaia model arrives at ESAC
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
27/08: Quest for the Sun's siblings
 
Please note: Entries from the period 2003-2010 are available in this PDF document.