Image of the Week

Visualising Gaia Photometric Science Alerts

   
  Example of a data visualisation chart developed to aid in reconstructing the environment of transient candidates. Click on the image for a PDF version.  
 

The discovery of ESA Gaia's first supernova, Gaia14aaa, in September 2014 marked the first Gaia photometric science alert (PSA) published. As of mid-May 2015, more than 240 alerts of transient candidates have been published, including supernovae, cataclysmic variables, active galactic nuclei and M-dwarf flares among others (see Gaia UK website).

Gaia observes some 50 million objects per day of operations, this data being downloaded to ground on a daily basis. Rapid processing is carried out through the Initial Data Treatment (IDT) pipeline, an element of the Data Processing and Analysis Consortium (DPAC) Gaia pipeline. IDT generates a preliminary astrometric and photometric catalogue of all the sources detected by Gaia during that observing period.

Finding the few genuine transients amongst these observations can resemble searching for 'a needle in a haystack'. Putting in place an effective detection pipeline is vital in rapidly detecting and publishing reliable photometric science alerts. DPAC's Photometric Science Alerts team has developed several of these filtering and selection processes that are able to identify potential PSAs.

Although the detection of transient candidates is a largely automated process, a visual inspection is still required as a final step, aiming to weed out spurious alerts. Gaia data alone provides valuable information on the position, brightness and bulk spectral properties of each source. Ancillary catalogues provide additional information about the environment around the transient candidates.

The figure above is an example of a data visualisation chart developed to aid in reconstructing the environment of the transient:

- The top left plot shows all Gaia observations (filled circles) for Gaia15aaj and the nucleus of its host galaxy (filled stars) found within a 10 arcseconds radius around the alerted position. The colour shows the magnitude of each observation. A small scatter has been added to the position of Gaia observations for visualisation purposes. The background shows an image from the SDSS catalogue and the positions for the sources found in the catalogue (empty squares) within a 10 arcseconds radius (large red circle). The dashed green circle shows the apparent angular size of the galaxies (PSF FWHM) in the SDSS "r" band.

- The top right plot is a close up of the Gaia observations only, colour coded by their observed time.

- The plot in the centre shows the light curve of the transient, which allows the evolution of the magnitude (brightness) of the detected sources to be tracked. Some of the observations are separated by a few hours, so they are seen as a single point in the lightcurve.

- The plot at the bottom shows the BP-RP colour for each observation.

The figure describes the following scenario. In mid-October 2014, the galaxy was observed by Gaia. Only the nucleus was detected. The colour of the observation was red and the position agreed with the position of an SDSS known galaxy. At the beginning of January 2015, the galaxy was observed again. In this case, along with the galaxy nucleus, Gaia detected a new blue source at approximately 5 arcseconds away from the previous observation. An alert was raised and published as supernova candidate Gaia15aaj. After the alert, Gaia observed the candidate on two additional occasions, proving that the object was fading with time.

Young supernovae usually display hot emission at around ~10,000K, which provides its characteristic blue colour. As the ejected material in the supernova expands, the detected light becomes dimmer and cooler. This process can be observed in the progressively redder colour of the supernova transits in the BP-RP diagram.

It is expected that, along with a better coverage of the sky and improved filtering techniques, the reliability of transient detections will increase. Visual inspection then will no longer be required to discard false positives, but will rather aid in characterising the environment of each transient. This contextual information will provide additional value to the community, enabling a better assessment to be made as to the scientific value of each published alert.
 

Credits: ESA/Gaia/DPAC/Photometric Science Alerts Team. The data visualisation work described here was carried out by Nadia Blagorodnova, as part of her GREAT-ITN PhD project, within the context of the DPAC work at the IoA, Cambridge, UK. The photometric science alerts pipeline builds on the Initial Data Treatment pipeline (developed by the DPAC team in Barcelona, with contributions from the teams in Leiden, Edinburgh and Lund) at DPCE and the data ingestion facilities at DPCI.

Acknowledgement: EU FP7 Grant Agreement 264895

[Published: 19/05/2015]

 

Image of the Week Archive

2017
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
 
Please note: Entries from the period 2003-2010 are available in this PDF document.