Image of the Week

 

Astrometric microlensing effect in the Gaia16aye event

 

An overview of the Gaia data for the Gaia16aye microlensing event. Top panel shows the photometry from the Gaia Photometric Science Alerts (black points) along with the follow-up observations (salmon points). Solid green line is the microlensing model which reflects the multiple spikes in the light curve caused by the rotating binary lens composed of two main sequence stars (Wyrzykowski et al. 2020). Middle panel: Gaia 1-D astrometric (black points) deviations from a mean position in milliarcseconds. Blue solid line is the standard 5-parameter astrometric model. Red solid line is the astrometric model which also includes the motion of the centre of light of microlensed images caused by the binary lens. Bottom panel: residuals of the two astrometric models presented on the middle panel. Astrometric microlensing signal is clearly present in the Gaia data and coincides with the spikes seen in the photometry. On the X-axis is shown the time in days. Credit: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgement: ESA/Gaia/DPAC/CU7-CU3-CU4-CU5, Krzysztof Rybicki, Łukasz Wyrzykowski (Warsaw University).

 

Gaia16aye was a binary microlensing event detected by Gaia in 2016 as one of the first transients of that kind reported by Gaia Photometric Science Alerts (Hodgkin et al. 2021). The event has been followed-up from the ground by a network of telescopes over the period of two years (see our image of the week story of october 2016 and the story featured on sci.esa.int). Dense coverage and exquisite photometric data revealed in very detail numerous features in the light curve, which could be reproduced by a microlensing model employing full Keplerian motion of a binary system composed of two Main Sequence stars (Wyrzykowski et al. 2020).

During a microlensing event the light of a background star (source) is split into multiple images due to the gravitational influence of a foreground object (lens). The angular size of the images is larger than the unlensed source and the surface brightness is preserved. Thus, the total flux during the event is increased. The individual images are hard to resolve as their separation is of order of 1 mas. Though this is much smaller than the resolving power of Gaia (which would actually require a telescope aperture of at least 60 m), it can however measure the centroid of the combined light with sub-mas precision.

In case of a single lens, the smaller projected lens-source separation on the sky, the larger the amplification, hence the source getting brighter. In a binary lens, there are more regions of high amplification, called caustics. When the source crosses the caustic, its amplification jumps dramatically, often by many magnitudes. In Gaia16aye there were multiple jumps due to rotation of the system of caustics, caused by the rotation of the binary lens system.

Crossing a caustic also means appearance or disappearance of additional images. This causes a dramatic change in the position of the centre of light.

Gaia has been observing the source star of Gaia16aye since 2014, collecting not only photometric measurements (which led to the alert on the anomalous increase of brightness, upper panel of the image), but also the positional astrometric data. The high precision Gaia astrometric measurements are one-dimensional along the scanning direction. Due to the movement of Gaia’s spin axis (precessing around the Sun at 45 deg angle every 63 days), the scanning angle passing over a source is changing with time, allowing for measurement of 5 astrometric parameters (2D positions, 2D proper motions, parallax).

A portion of Gaia 1D astrometric data from Cycle 3 (until May 2017) for this microlensing event was investigated within DPAC, as part of a procedure of validating the epoch astrometry and preparation for the microlensing events detection pipeline. The data (shown in the middle panel) was obtained with a preliminary astrometric solution (AGIS 3.1). The blue line denotes the standard 5-parameter astrometric model. As can be seen in the residuals at the lowest panel, the blue model follows the data only at the very beginning, and departs significantly during the microlensing event.

The photometric model of this complex light curve contains all the information about the microlensing event apart from one - the size of the Einstein Radius, which is the angular scale of the microlensing event and the radius of the ring-shaped image that would form in an ideal case of perfect alignment of the point source, point lens and the observer. In case of Gaia16aye it was eventually measured thanks to the detailed photometric coverage of the caustic crossings captured by follow-up observations (salmon markers in the top panel of Figure 1). This allowed measuring the size of the source in the units of Einstein Radius, which in turn was compared with the physical size of the source as measured from source colour and spectroscopy. But if there was no caustic crossing present, like in case of single lensing, the Einstein Radius would be the missing crucial bit of information which allows us to compute the mass of the lens, hence tell anything on its nature.

The shift of the centroid of light affected by microlensing is a linear function of the Einstein Radius. Combining the knowledge of the binary system from photometry with the astrometric Gaia data yielded an independent determination of the value of the Einstein Radius in Gaia16aye. The 6-parameter astrometric model that include lensing (with Einstein Radius) is shown as a red line. The uncertainty of the data is shown as a grey band in the residual plot. Now we are finally able to reproduce Gaia 1-D astrometric measurements correctly! The value of the Einstein radius derived from Gaia astrometry is also in perfect agreement with the one derived from photometry and is about 3 mas.

Microlensing events in their classical photometric form have been being detected for over 30 years now and tens of thousands of events were found. However, their astrometric signature has not been so easily detectable and so far only a couple of detections were made in extreme cases using Hubble Space Telescope or ESO/VLTI. Here we see a foretaste of Gaia’s capability to provide astrometric time-series for all microlensing events which occur during its operations (e.g. Rybicki et al. 2018, Kluter et al. 2019). This method gives direct access to the mass of lenses for fairly standard events, including ’invisible’ lenses like neutron stars and stellar black holes.

 

The animation, available here, illustrates the motion of the source, the binary lens and the caustics as well as the trajectory of the centre of light.

Time evolution of the Gaia16aye microlensing event, with the "data representation" on the left and more intuitive "geometric representation" on the right. The light curve (top left) shows how the source brightness has been changing during the event. Middle panel contains the one-dimensional (in the direction of Gaia scanning motion) offset of the light centroid from the reference position, along with residuals. Majority of the centroid motion is due to parallax and proper motion of the source star (blue model), but these two effects do not describe all the data properly. The impact of astrometric microlensing is relatively small, but clearly visible, especially on the residuals panel (bottom left), which shows both lensing (red) and non-lensing (blue) models, with respect to the data. The non-lensing model fails to reproduce the Gaia measurements especially during the caustic crossings, where the light centroid position changes vastly due to strong variations in the configuration of the source images. Since Gaia astrometry is one-dimensional, it is difficult to imagine the geometry of the event using only the astrometric data presented on the left. We reinforce the readers imagination with the right panel, which shows the trajectory of the source (dashed, blue line), the light centroid (red) and orbital motion of the lensing system, along with the caustic (dark gray) that gradually changes its shape and rotates due to the motion of the lens' components. Black filled circles mark epochs of Gaia astrometric measurements. Trajectories shown on the right panel are relative to the mass center of the lensing system. The light green vertical line on the left panels marks the ongoing epoch displayed on the right, to help indentifying what is happening during the event and navigate between the "data representation" and "geometric representation". For example, one can easily see the critical changes happening during caustic crossings - whenever the source trajectory intersects with the caustic, one can see the flux spike (top left panel), but also a stronger astrometric deviation (bottom left).

Credit: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgement: ESA/Gaia/DPAC/CU7-CU3-CU4-CU5, Krzysztof Rybicki, Łukasz Wyrzykowski (Warsaw University).

 

Further reading:

 

 

Credits: ESA/Gaia/DPAC/CU7-CU3-CU4-CU5, Krzysztof Rybicki, Łukasz Wyrzykowski, Katarzyna Kruszyńska (University of Warsaw), Berry Holl, Laurent Eyer, Nami Mowlavi, Isabelle Lecoeur-Taibi (University of Geneva). We thank Przemek Mróz for his extensive help in the modelling of the light curve.

Published: 24/09/2021

 

Image of the Week Archive

2021

01/12: Observation of a long-predicted new type of binary star

24/09: Astrometric microlensing effect in the Gaia16aye event

22/09: the power of the third dimension - the discovery of a gigantic cavity in space

16/09: An alternative Gaia sky chart

25/08: Gaia Photometric Science Alerts and Gravitational Wave Triggers

09/07: How Gaia unveils what stars are made of

23/06: Interviews with CU3

27/04: HIP 70674 Orbital solution resulting from Gaia DR3 processing

30/03: First transiting exoplanet by Gaia

26/03: Apophis' Yarkovsky acceleration improved through stellar occultation

26/02: Matching observations to sources for Gaia DR4

2020

22/12: QSO emission lines in low-resolution BP/RP spectra

03/12: Gaia Early Data Release 3

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.