Scientists have unveiled new details of a colossal black hole 53 million lightyears away first photographed by the earth-wide Event Horizon Telescope (EHT) in 2017. The feat provided the first visual evidence that black holes exist, confirming a fundamental prediction of general relativity.
In a new paper published on January 18, EHT scientists have reported capturing details at the level of the behemoth’s event horizon – the boundary beyond which light from the other side can’t reach an observer – showcasing the formation of a distinct ring around it.
The EHT’s previous run had revealed the black hole’s ‘shadow’, a result of the event horizon’s gravitational effects and a signature of its presence. The picture also helped constrain the black hole’s mass and find that its size and shape align with the predictions of the general theory of relativity. The image was hailed at the time as a matter of “astonishment and wonder” for revealing “a part of the universe that was off limits”.
The new data, obtained after improving the telescope’s coverage and resolution, reiterated the shadow’s size and shape.
“The primary finding is the confirmation of the black hole ‘shadow’,” Venkatessh Ramakrishnan, a postdoctoral researcher at the Finnish Centre for Astronomy with the European Southern Observatory, wrote in an email to The Hindu. “So this shows us as a first step that we are not biased towards making the black hole appear.”
Dr. Ramakrishnan has been part of the EHT collaboration since 2017.
Many telescopes as one
The EHT is not a single telescope but a worldwide network of radio telescopes that work together to study a common object in space. They benefit from a technique called very-long baseline interferometry, where the data each telescope collects about the object is correlated with data from the others using extremely precise clocks.
In this setup, the maximum distance between the telescopes defines the network’s resolution.
In 2017, the EHT reported detecting a bright, asymmetric ring of light consistent with the presence of a supermassive black hole. Independent analyses of EHT data also supported the ring’s presence.
Based on these observations, scientists improved the EHT by increasing its data-recording rate, its ability to track spatial information, and by adding the Greenland Telescope to the array. The last one “improved the resolution of the EHT in the north-south direction,” Dr. Ramakrishnan said.
Piecing the data together
In the new campaign, the EHT had nine stations gathering data across six observation days in April 2018, in four frequencies.
Then the scientists correlated the datasets with each other to increase the signal-to-noise ratio. This task and subsequent processing happened at centres in Germany and the U.S.
When comparing the 2017 observations with the new results, they observed significant changes in the closure phase – a measure of the relationship between three telescopes in an array.
Specifically, they found that the way telescopes worked together during the 2018 observations differed from that during the 2017 observations. In this difference, scientists can track shifts in the configuration or structure of the black hole.
The team further used general relativistic magnetohydrodynamic (GRMHD) simulations to create models of the M87 black hole. These models incorporate the way the black hole’s gravity influences spacetime around it using Einstein’s theory of general relativity. By comparing the simulated black hole with the actual one, scientists gained valuable insights into the intricate dynamics near the event horizon, such as an effect called lensing.
Gravitational lensing
The findings confirmed the presence of an asymmetric ring structure approximately 42 microarcseconds wide. This is like observing a grain of sand from 25 km away.
They also revealed its diameter hadn’t changed much between observations in 2017 and 2018 – meaning the black hole’s gravity bent light consistently over time to form the observed ring.
This is in line with a prediction from the general theory of relativity that light around a black hole is strongly lensed. Objects with a lot of mass bend spacetime more around them. When light travels in this region, its path is bent in the same way a magnifying glass does. Images carried by the light thus appear to be larger than they really are, and this phenomenon is called lensing.
This is also why, as the team found, the ring’s southwest corner appears brighter than other parts. The black hole is rotating, dragging the spacetime around it along the direction of its rotation and rendering more light in some areas.
In technical parlance, the observations matched predictions for a shadow formed by lensed emission around a Kerr black hole (i.e. a rotating black hole) with a mass of around 6.5 billion times that of the Sun.
The M87 galaxy is also associated with a prominent jet of high-energy particles extending from the black hole into space. Scientists believe the jet is connected to the black hole’s accretion disk, a belt-like structure of matter spiralling into the black hole.
Scientists observed that parts of the accretion disk and the jet appeared to be shifted by about 30 degrees between 2017 and 2018 – meaning these structures had changed their position or orientation over time.
“The 30-degree change can be attributed to the spin of the black hole,” Dr. Ramakrishnan said. “Now this can’t be firmly established yet from two images taken two years apart, but such an expectation was already established in a paper published a few years ago.”
This shift is involved in a complex interplay between the accretion disk and the formation of the jet. For example, models like the GRMHD suggest the observed shift could correspond to variations in the magnetic field structure. This in turn can render observable changes in the radiation emitted by matter in the disc.
By studying these subtle changes, scientists gain valuable insights into the hidden physics that controls the relationship between the accretion disk, the jet, and the magnetic environment around the black hole.
Room for more
Running the EHT to photograph black holes millions of lightyears away is a time-consuming task requiring meticulous observations.
The new results have reaffirmed the features of the black hole reported in 2019, including a very stable ring-formation process and other physical characteristics. The findings also were consistent across two observation periods and multiple frequencies, speaking to their reliability.
Examining how we measure the ring’s diameter shows that our observational techniques are continually improving. Image-based techniques to study objects in space tend to underestimate diameters compared to direct modelling methods. And the EHT’s finer measurement of the ring’s diameter suggests this gap between the two techniques is closing. Future work could further reconcile the discrepancy and the EHT’s overall performance.
Dr. Ramakrishnan also said the telescope collaboration is planning “a so-called ‘movie project’ in 2026, when we will track the black hole over a month or two to see the brightness changes along the black hole”.
Tejasri Gururaj is a freelance science writer and journalist.
- Scientists have unveiled new details of a colossal black hole 53 million lightyears away first photographed by the earth-wide Event Horizon Telescope (EHT) in 2017
- The image was hailed at the time as a matter of “astonishment and wonder” for revealing “a part of the universe that was off limits”
- The EHT is not a single telescope but a worldwide network of radio telescopes that work together to study a common object in space
