Science Highlights

The following sections show selected science highlights for GRAVITY after five years of operation. This includes a broad range of science opportunities for astrophysical research inside and beyond the Milky Way. We will start with a series of exciting discoveries around the Galactic Center’s supermassive black hole, which is followed by breakthrough studies in active galactic nuclei, young stellar objects, and exoplanet systems.

Galactic Center

The Galactic Centre is by far the closest and best-studied galactic nucleus home to a supermassive black hole. Here, we summarize several breakthroughs that GRAVITY observations have achieved toward testing general relativity at an unprecedented level. This includes measurements of the gravitational redshift and the Schwarzschild precession of the star S2, the orbital motion of the accretion flare, and the discovery of new stars close to the black hole.

Imaging new stars

On a quest to find even more stars close to the black hole, we use an advanced, Bayesian image reconstruction technique for the GRAVITY data, allowing us to detect stars fainter and closer to Sgr A* than what possible before. The faintest secure detection is the star S301 with a magnitude of K = 19.3. So far, five new faint objects have been spotted, labelled S300 to S304.

At any moment on time, there appears to reside a handful of stars in the central 100mas, i.e. within the field of view of a single GRAVITY pointing. We have traced interferometrically not only the fly-by of S2 in 2018, but also those of S24, S29, S38, S42 and S55.

In 2023, we discovered the new record-holder in terms of how close to Sgr A* a star comes. The above-mentioned star S301 revolves on a 8.5 year orbit, with an eccentricity larger than e = 0.98. At (the unobserved) pericenter it passed Sgr A* at only 140 Schwarzschild radii – still safe from tidal disruption, but entering a regime where the spin of Sgr A* might alter its orbit on the time scale of a decade sufficiently to be detectable with GRAVITY.

These annotated images, obtained with the GRAVITY instrument on ESO’s Very Large Telescope Interferometer (VLTI) between March and July 2021, show stars orbiting very closely to Sgr A*, the black hole at the heart of the Milky Way. One of these stars, S29, was observed as it was making its closest approach to the black hole at 13 billion kilometers, just 90 times the distance between the Sun and Earth. Another star, S300, was detected for the first time in the new VLTI observations.

These annotated images, obtained with the GRAVITY instrument on ESO’s Very Large Telescope Interferometer (VLTI) between March and July 2021, show stars orbiting very closely to Sgr A*, the black hole at the heart of the Milky Way. One of these stars, S29, was observed as it was making its closest approach to the black hole at 13 billion kilometers, just 90 times the distance between the Sun and Earth. Another star, S300, was detected for the first time in the new VLTI observations.

The orbit of the newly discovered star S301. It is visibly subject to general relativistic precession

The orbit of the newly discovered star S301. It is visibly subject to general relativistic precession

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Related press release: https://www.mpe.mpg.de/7808727/news2021214?c=260780

Orbit of S2

Obscured by thick clouds of absorbing dust, the closest supermassive black hole to the Earth lies 26,000 light-years away at the center of the Milky Way. This gravitational monster, which has a mass four million times that of the Sun, is surrounded by a small group of stars orbiting around it at high speed. This extreme environment – the strongest gravitational field in our galaxy – makes it the perfect place for exploring gravitational physics, and particularly for testing Einstein’s general theory of relativity.

This picture is an artist’s impression of the gravitational redshift of S2 when passing the supermassive black hole at the center of the Milky Way.

This picture is an artist’s impression of the gravitational redshift of S2 when passing the supermassive black hole at the center of the Milky Way.

New infrared observations from the exquisitely sensitive GRAVITY, SINFONI, and NACO instruments, developed under the lead of the Max Planck Institute for Extraterrestrial Physics, have now allowed astronomers to follow one of these stars, called S2, as it passed very near the black hole during May 2018. At its closest point, this star was at a distance of less than 20 billion kilometers from the black hole, moving at a speed of over 25 million kilometers per hour – almost three percent of the speed of light.

For the first time, observations have revealed that a star orbiting the supermassive black hole at the center of the Milky Way moves just as Einstein’s theory of general relativity predicted. Its orbit is shaped like a rosette and not, according to Newton’s theory of gravity, like an ellipse. This effect, known as the Schwarzschild precession, had never been measured before for a star near a supermassive black hole. This artist’s impression illustrates the precession of the star’s orbit, exaggerating the effect for a better visualization.

For the first time, observations have revealed that a star orbiting the supermassive black hole at the center of the Milky Way moves just as Einstein’s theory of general relativity predicted. Its orbit is shaped like a rosette and not, according to Newton’s theory of gravity, like an ellipse. This effect, known as the Schwarzschild precession, had never been measured before for a star near a supermassive black hole. This artist’s impression illustrates the precession of the star’s orbit, exaggerating the effect for a better visualization.

The team compared the position and velocity measurements from GRAVITY and SINFONI respectively, along with previous observations of S2 using other instruments, with the predictions of Newtonian gravity, general relativity, and other theories of gravity. The new results are inconsistent with Newtonian predictions and in excellent agreement with the predictions of general relativity. Two effects of general relativity are observed for the first time around a supermassive black hole: the gravitational redshift and the Schwarzschild precession. Light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton's theory of gravity.

The figure on the left shows data points for the orbit of S2 around Sgr A* (black cross at (0, 0)), which were collected by different instruments with the Very Large Telescope over the course of 27 years. Even though the star’s orbit appears almost closed in this image, the small Schwarzschild precession is significantly detectable and corresponds to the theoretical predictions of general relativity. The figure on the right shows that the positions of the star (turquoise dots) agree with the theoretical predictions of general relativity (red line) within the measurement inaccuracy. The Newtonian prediction (blue dashed line) is clearly excluded.

The figure on the left shows data points for the orbit of S2 around Sgr A* (black cross at (0, 0)), which were collected by different instruments with the Very Large Telescope over the course of 27 years. Even though the star’s orbit appears almost closed in this image, the small Schwarzschild precession is significantly detectable and corresponds to the theoretical predictions of general relativity. The figure on the right shows that the positions of the star (turquoise dots) agree with the theoretical predictions of general relativity (red line) within the measurement inaccuracy. The Newtonian prediction (blue dashed line) is clearly excluded.

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Flares from the black hole

The Galactic Center black hole is surprisingly faint – its average luminosity is only about 0.1 millionth of the Eddington luminosity, emitted predominantly at radio to submillimeter wavelengths. On top of this quasi-steady component, there is variable emission in the X-ray and IR bands. Some of this comes as flares, typically a few times per day, and lasts for about one to two hours, reaching the brightness of massive main-sequence stars. The three most plausible explanations for the origin of these flares are a jet with clumps of ejected material, hot spots orbiting a black hole, or statistical fluctuations in the accretion flow.

Recent GRAVITY observations caught several flare events of the Galactic Center and revealed that the flares are emissions from clumps of gas swirling around at about 30% of the speed of light on a circular orbit just outside the innermost stable orbit of a black hole of four million solar mass. The motion of the three flares observed in the Galactic Center can be explained by a simple orbit model with a radius of three to five times the event horizon. Thus, these observations confirmed the theoretical predictions for such hot spots orbiting at the innermost edge of stable orbits.

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Related press release:

https://www.mpe.mpg.de/6954912/news20181031

Top: A bright flare from Sgr A*. Left: View of the central arcsecond before the flare, obtained in 2009 with NACO in the L-band (3.8µm). Right: During its peak Sgr A* outshines the stars in its vicinity. Bottom left: Observed astrometric flare motion, overlaid onto the EHT image of Sgr A*. The flares orbit Sgr A* at around 4.5 Schwarzschild radii, with a period of around an hour. The enclosed mass is around 4.3 million solar masses, consistent with the mass measurements further out from the S2 or S301 data.

Top: A bright flare from Sgr A*. Left: View of the central arcsecond before the flare, obtained in 2009 with NACO in the L-band (3.8µm). Right: During its peak Sgr A* outshines the stars in its vicinity. Bottom left: Observed astrometric flare motion, overlaid onto the EHT image of Sgr A*. The flares orbit Sgr A* at around 4.5 Schwarzschild radii, with a period of around an hour. The enclosed mass is around 4.3 million solar masses, consistent with the mass measurements further out from the S2 or S301 data.

Resolving the Broad-Line Region of AGN

This optical image of the quasar 3C 273 was obtained with the Hubble Space Telescope. The quasar resides in a giant elliptical galaxy in the constellation of Virgo at a distance of about 2.5 billion light-years. It was the first quasar ever to be identified.

This optical image of the quasar 3C 273 was obtained with the Hubble Space Telescope. The quasar resides in a giant elliptical galaxy in the constellation of Virgo at a distance of about 2.5 billion light-years. It was the first quasar ever to be identified.

This map shows the best-fitting model for the velocity of the clouds in the broad-line region. Red-colored clouds are moving away from the observer, blue-colored clouds are moving toward the observer. It is easy to distinguish a rotation, and the rotation axis coincides with the direction of the jet inside the model error bars.

This map shows the best-fitting model for the velocity of the clouds in the broad-line region. Red-colored clouds are moving away from the observer, blue-colored clouds are moving toward the observer. It is easy to distinguish a rotation, and the rotation axis coincides with the direction of the jet inside the model error bars.

The broad atomic emission lines are an observational hallmark of active galactic nuclei (AGN), clearly indicating the extragalactic origin of the source. So far, the size of the broad-line region is measured mainly by a method called “reverberation mapping.” Brightness variations of the black hole accretion cause a light echo once the radiation hits clouds further out – the larger the size of the system, the later the echo. In the best cases, the motions of the gas can also be identified, often implying a disk in rotation. This result, derived from timing information, can now be confronted with spatially resolved observations with GRAVITY. Since the first observation of 3C 273, GRAVITY has yielded in total three measurements of the broad-line region of nearby AGN. The resulting sizes and black hole masses of the broad-line region are consistent with the results from reverberation mapping. Thus, GRAVITY provides both a confirmation of the main method used previously to determine black hole masses in AGN and a new and highly accurate, independent method to measure such masses. It thereby promises to provide a benchmark for measuring black hole masses in thousands of other AGN.

Currently published GRAVITY measurements of broad line region sizes (red stars) are consistent with the results of reverberation mapping (black and gray), which reveal the relation between size and luminosity of AGN’s broad-line regions.

Currently published GRAVITY measurements of broad line region sizes (red stars) are consistent with the results of reverberation mapping (black and gray), which reveal the relation between size and luminosity of AGN’s broad-line regions.

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Related press release:

https://www.mpe.mpg.de/6966658/news20181129

Inner Region of Young Stellar Objects

Astronomers believe that young stars acquire matter via their magnetic fields, and that this material falls toward the surface at supersonic velocities. This process has never been directly observed before, despite the fact that it occurs on scales equivalent to a few solar radii. While that may seem like a large distance, the problem is that the nearest young star is so far away that it requires high angular resolution and sensitivity that were not feasible before GRAVITY.

TW Hydrae is located very close to Earth, at a distance of only about 200 light-years. The disk of material surrounding the star is almost face-on. This makes it the ideal candidate to probe how matter from a planet-forming disk is channeled on to the stellar surface. GRAVITY observations of TW Hydrae reveal that the material is guided by magnetic fields and comes from the disks surrounding these stars, the same disks that eventually give rise to planets.

This picture shows an artist’s impression of the streams of hot gas that builds up stars. Matter from the surrounding protoplanetary disk, the birthplace of planets, is channeled onto the stellar surface by magnetic fields shocking the surface at supersonic velocity.

This picture shows an artist’s impression of the streams of hot gas that builds up stars. Matter from the surrounding protoplanetary disk, the birthplace of planets, is channeled onto the stellar surface by magnetic fields shocking the surface at supersonic velocity.

Here, you can see a sketch of the inner-disk region of TW Hydrae. The main features of the inner disk are represented: the dusty disk (brown), the dust sublimation radius located at about 7.5 R⁎, the inner gaseous disk (blue), truncated by the stellar magnetosphere (red) at about 3.5 R⁎, along with the Brγ line-emitting region, which probably traces the width of the accretion columns.

Here, you can see a sketch of the inner-disk region of TW Hydrae. The main features of the inner disk are represented: the dusty disk (brown), the dust sublimation radius located at about 7.5 R⁎, the inner gaseous disk (blue), truncated by the stellar magnetosphere (red) at about 3.5 R⁎, along with the Brγ line-emitting region, which probably traces the width of the accretion columns.

Related press release: https://www.mpe.mpg.de/7485655/news20200827?c=260780

Exoplanet astrometry and spectroscopy

GRAVITY has been used to constrain the accurate astrometry and K-band spectroscopy of exoplanets whose position is precisely pinpointed by other methods. The planet HR 8799e was discovered in 2010. The GRAVITY observation of HR 8799e was the first to use optical interferometry to reveal details of an exoplanet, and the new technique furnished an exquisitely detailed spectrum of unprecedented quality – ten times more detailed than earlier observations. This method revealed a complex exoplanetary atmosphere with clouds of iron and silicates swirling in a planetwide storm. The technique presents unique possibilities for characterizing many of the exoplanets known today.

This picture is an artist’s impression of the observed exoplanet, which goes by the name HR 8799e. The GRAVITY observations revealed a complex exoplanetary atmosphere with clouds of iron and silicates swirling in a planetwide storm.

This picture is an artist’s impression of the observed exoplanet, which goes by the name HR 8799e. The GRAVITY observations revealed a complex exoplanetary atmosphere with clouds of iron and silicates swirling in a planetwide storm.

β Pictoris c is the second planet found to orbit its parent star. It was originally detected by the so-called “radial velocity,” which measures the drag-and-pull motion on the parent star due to the planet’s orbit. β Pictoris c is so close to its parent star that even the best telescopes have not been able to directly image the planet so far. With GRAVITY observation, it is the first planet to be detected and confirmed with both methods, radial velocity measurements and direct imaging. In addition to the independent confirmation of the exoplanet, astronomers can now combine the knowledge from these two previously separate techniques and measure both the brightness and mass of the planet at the same time.

These schematic images show the geometry of the β Pictoris system: the image on the left shows both the star and the two planets embedded in the dusty disk in the orientation as visible from the vantage point of the solar system. This view was constructed using the information from real observations. The middle panel is an artist’s impression of the disk/planet system. The image on the right shows the dimensions of the system when viewed from above and previous observations of β Pictoris b (orange diamonds and red circles) and the new direct observations of β Pictoris c (green circles). The exact orbit of planetβ Pictoris c is still somewhat uncertain (fuzzy white area).

These schematic images show the geometry of the β Pictoris system: the image on the left shows both the star and the two planets embedded in the dusty disk in the orientation as visible from the vantage point of the solar system. This view was constructed using the information from real observations. The middle panel is an artist’s impression of the disk/planet system. The image on the right shows the dimensions of the system when viewed from above and previous observations of β Pictoris b (orange diamonds and red circles) and the new direct observations of β Pictoris c (green circles). The exact orbit of planetβ Pictoris c is still somewhat uncertain (fuzzy white area).

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