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.
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.
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.
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.
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.
Related press releases:
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.
- GRAVITY Collaboration et al. 2018, A&A, 618L, 10
- GRAVITY Collaboration et al. 2020, A&A, 635A, 143
- GRAVITY Collaboration et al. 2020, A&A, 638A, 2
Related press release:
Imaging new stars
On a quest to find even more stars close to the black hole, the team uses different methods to obtain the deep image of the Galactic Center. In 2021, they detected a star of a magnitude of K = 18.9, previously known as S62, from GRAVITY data. The team recently developed a new analysis technique that has allowed them to discover a star, named S300, which had not been spotted before. This shows how powerful this method is to find very faint objects close to Sgr A*.
With the latest suite of observations conducted between March and July 2021, the team focused on making precise measurements of the paths of stars as they approached the black hole. This includes the record-holding star S29, which made its nearest approach to the black hole in late May 2021 at a stunning speed of 8,740 km/s, passing at a distance of 13 billion kilometers, just 90 times the distance between the Sun and Earth. No other star has ever been observed to pass this closely to or travel this fast around the black hole.
Related press release: https://www.mpe.mpg.de/7808727/news2021214?c=260780
Resolving the Broad-Line Region of AGN
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.
- GRAVITY Collaboration et al. 2018, Nature, 563, 657
- GRAVITY Collaboration et al. 2020, A&A, 643A, 154
- GRAVITY Collaboration et al. 2021, A&A, 648A, 117
Related press release:
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.
Related publication: GRAVITY Collaboration et al. 2020, Nature, 584, 547
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.
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.
Related press releases: