Image of the Week


Did Gaia find its first neutron star?


Figure 1. Overview of known neutron stars, showing both recycled and non-recycled neutron stars. The colour red refers to recycled neutron stars, these are neutron stars that have accreted material after birth. The colour cyan refers to non-recycled neutron stars and this group consists mainly of double neutron star binaries with a few neutron star + massive white dwarf binaries. Gaia's potential first neutron star, Gaia NS1, clearly stands out with a high mass and a large period. Credits: Figure 11 of Kareem El-Badry, et al. 2024.


With the publication of Gaia’s non-single star catalogue, a massive follow-up started of potentially interesting cases. Scientists were especially eager to follow up on candidate dark companions like black holes and neutron stars.

Kareem El-Badry and other scientists across the world scheduled high-resolution spectroscopic follow-up of interesting candidates with dedicated observatories like Keck, Magellan, La Silla, and Whipple. While many follow-up campaigns are still ongoing, the first discoveries were published and led to the finding of two black holes: Gaia BH1 and Gaia BH2. A third black hole, Gaia BH3, was recently announced based on pre-release data from Gaia DR4. Now, also Gaia’s potential first neutron star is found, dubbed Gaia NS1.


Figure 2. Overview of black holes found by gravitational-wave observatories in distant galaxies complemented with the black holes found closer to home in our Milky Way through electromagnetic radiation. Since recently, this overview contains new entries found in a different way, through astrometry! Shown in green are the electromagnetically non-interacting systems, also known as dormant black holes. Newest addition can be found in pink: Gaia NS1. The Gaia mission plays an important role in finding new systems through astrometry. The original plot was adapted by several groups, and the additions show up as circles with different colours. These circles have a size that is not on the same scale as the original plot created for the LIGO-VIRGO-KAGRA results. Credits: Adapted from LIGO-VIRGO-KAGRA, Aaron Geller, Northwestern / L. Wyrzykowski, Warsaw by ESA/Gaia/DPAC.


Gaia NS1 is the dark companion that is part of a binary system with a luminous, low-metallicity star near the main-sequence turnoff. The dark companion has a mass of ~1.9 solar masses which is massive enough to be outside the white dwarf range but also a low enough mass to point to a neutron star rather than a black hole.

Since Gaia’s data releases, thousands of white dwarfs in astrometric and spectroscopic binaries were found. White dwarfs are the stellar cores left behind after a star died. They are no longer fueled by nuclear reactions and have expelled their outer layers. Typically, white dwarfs have masses ranging from 0.17 to 1.33 solar masses. It is believed that no stable white dwarfs can exist beyond a mass of ~1.4 solar masses, a limit referred to as the Chandrasekhar limit.

Neutron stars are much more massive than white dwarfs and are formed from the collapse of massive supergiant stars. They are also denser than any other stellar object, except for black holes. Gaia NS1 with its 1.9 solar masses places well beyond the Chandrasekhar limit, its full mass contained within a sphere of ~10 km radius.


Figure 3. Overview of known stellar mass black holes and neutron stars (excluding results found in distant galaxies through gravitational-wave observatories). To allow both black hole and neutron star populations to be shown in the same plot, please note the non-linear axes on the left. From this plot it is clear that the new additions found by Gaia all have wider orbits with larger orbital periods. Credits: Kareem El-Badry - CC BY-SA 3.0 IGO.


At first, Gaia NS1 was thought to be a black hole candidate. From Gaia data alone, the computed mass for the dark companion was found in the range of 2.2 to 4.3 solar masses, which points to a low-mass black hole of stellar origin. But these results assumed that the companion star was metal-rich, like the Sun.

The high-resolution spectroscopic follow-up uncovered that the companion star is actually metal-poor. It is a main-sequence star formed more than 12 billion years ago and belonging to the halo. Located near the main-sequence turnoff, it has already started to inflate and is expected to become a red giant in less than a billion years.

The fact that the companion star is metal-poor changes the estimate for the mass of this star quite significantly, from an assumed 1 to 1.4 solar masses to an estimated 0.8 solar masses. The mass of the dark companion is derived from the astrometric orbit as measured by Gaia, and if the bright companion’s mass changes significantly, this influences the derived mass of the dark companion, dropping it severely and moving it firmly into the neutron star range at 1.9 +/- 0.04 solar masses.

One might wonder if this dark companion couldn’t perhaps be a very low-mass black hole. So far there has not been evidence for such low mass black holes yet, but it cannot be excluded. It would be more likely though that the dark companion consists of a combination of two massive white dwarfs in a close binary themselves. With current stellar evolution models however, there is no evidence pointing to the creation of such a system. Leaving the simplest explanation: the dark companion is probably a neutron star.


Figure 4. The black lines show the best-fit orbit using both Gaia DR3 data and the Radial Velocity data from follow-up observations. Predicted observations times of Gaia NS1 photocentre by Gaia using the Gaia Observation Forecast Tool are shown in red. The predicted scan times show that the orbit is well sampled. Credits: Figure 4 of Kareem El-Badry, et al. 2024.


There is more to this discovery. The binary system has an orbital period of 731 days, with a low eccentricity orbit, making it almost circular. The eccentricity is so low that the dark companion is most likely formed with little mass loss and only a weak natal kick. A stronger natal kick would have increased the eccentricity of the orbit or even destroyed the binary system, while another formation scenario is very unlikely due to the tight orbit of the system. It makes Gaia NS1 the most massive neutron star formed without accretion.

But Gaia NS1 does not stand on its own. As seen already in Figure 3, many other candidates exist. A set of 21 astrometric binaries containing solar-type stars and dark companions with masses of about 1.4 solar masses are being followed-up. For several of them the luminous component seems to be a metal-poor star, and all of these metal-poor stars seems to be strongly enhanced in lithium, as is also the case for the luminous component of Gaia NS1.


Figure 5. On the left: Observed spectrum of the luminous component of Gaia NS1 (red) as compared to a reference star from the GALAH survey with a spectrum that best matches the one for Gaia NS1 system. On the right: Gaia NS1 is clearly an outlier in lithium abundance and has a higher Lithium value than any similar star from the GALAH survey. Credits: Figure 6 from Kareem El-Badry et al., 2024.


This observational evidence, along with the earlier findings on Gaia’s black holes, again challenges current binary evolution models. Lots of work remains for astronomers to improve these models and make them match Gaia’s observations of the exotic systems in our Milky Way. Gaia’s next data releases promise to bring a wealth of new dark companions to be studied, expanding on both mass ranges and binary periods. While most of the dark companions found with Gaia data are relatively close to Earth, with a longer time span of observations, the discovery space will expand further out into the Milky Way.


This animation shows the orbits of the luminous star and the neutron star around their common centre of mass. The red dots shown are the expected locations of Gaia observations of the luminous star as obtained with the Gaia Observation Forecast Tool. It is clear that there can be many different angles at which such systems are observed. Gaia NS1 is depicted in the bottom left. Credits: Kareem El-Badry - CC BY-SA 3.0 IGO.


Further reading:


Story written by T.Roegiers.


Credits: ESA/Gaia/DPAC, Kareem El-Badry

[Published: 24/05/2024]

Image of the Week Archive


25/06: Dynamical masses across the Hertzsprung-Russell diagram

28/05: Did Gaia find its first neutron star?

26/04: A textbook solar eruption

22/04: Gaia's contribution to discovering distant worlds

16/04: Gaia spots Milky Way's most massive black hole of stellar origin

02/04: The Gaia Cataclysmic Variable hook


19/12: 10 Science topics to celebrate Gaia's 10 years in space

31/10: Gaia observes cosmic clock inside a heavenly jewel

10/10: Gaia Focused Product Release stories

27/09: Does the Milky Way contain less dark matter than previously thought?

22/09: Mass-luminosity relation from Gaia's binary stars

13/09: Gaia DPAC CU8 seminars

13/06: Gaia's multi-dimensional Milky Way

18/05: Mapping the Milky Way

15/05: Goonhilly station steps in to save Gaia science data

25/04: The Gaia ESA Archive

05/04: Dual quasar found to be hosted by an ongoing galaxy merger at redshift 2.17

21/03: GaiaVari: a citizen science project to help Gaia variability classificaton

09/02: Missing mass in Albireo Ac: massive star or black hole?

31/01: Gaia reaches to the clouds – 3D kinematics of the LMC

25/01: Meet your neighbours: CNS5 - the fifth catalogue of nearby stars

18/01: A single-object visualisation tool for Gaia objects


25/11: 100 months of Gaia data

23/11: The astonishment

09/11: Gamma-Ray Burst detection from Lagrange 2 point by Gaia

04/11: Gaia's first black hole discovery: Gaia BH1

26/10: Are Newton and Einstein in error after all?

21/10: Gaia ESA Archive goes live with third data release

06/10: Mapping the interstellar medium using the Gaia RVS spectra

26/09: Gaia on the hunt for dual quasars and gravitational lenses

23/09: Gaia's observation of relativistic deflection of light close to Jupiter

13/06: Gaia Data Release 3

10/06: MK classification of stars from BP/RP spectrophotometry across the Hertzsprung-Russell diagram

09/06: BP/RP low-resolution spectroscopy across the Hertzsprung-Russell diagram

27/05: Cepheids and their radial velocity curves

23/05: The Galaxy in your preferred colours

19/05: GaiaXPy 1.0.0 released, a tool for Gaia's BP/RP spectra users

11/05: Systemic proper motions of 73 galaxies in the Local group

28/03: Gaia query statistics

16/03: Gaia's first photo shooting of the James Webb Space Telescope

08/03: Gaia's women in science - coordination unit 8

25/02: Not only distances: what Gaia DR3 RR Lyrae stars will tell us about our Galaxy and beyond

11/02: Gaia's women in science

31/01: Astrometric orbit of the exoplanet-host star HD81040

12/01: The Local Bubble - source of our nearby stars

05/01: A Milky-Way relic of the formation of the Universe


23/12: Signal-to-Noise ratio for Gaia DR3 BP/RP mean spectra

22/12: The 7 October 2021 stellar occultation by the Neptunian system

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


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


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

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
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
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
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
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
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
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
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
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.