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HomeScienceGround-based telescope confirms that black hole shadow is "real"

Ground-based telescope confirms that black hole shadow is “real”


Scientists have revealed new details of a colossal black hole 53 million light-years away, first imaged 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 article published in January 18EHT scientists reported capturing details at the level of the giant’s event horizon (the limit beyond which light from the other side cannot reach the observer) showing the formation of a distinctive ring around it.

The EHT’s previous run had revealed the “shadow” of the black hole, a result of the gravitational effects of the event horizon and a signature of its presence. The image also helped constrain the black hole’s mass and discover that its size and shape align with predictions from the general theory of relativity. The image was acclaimed at the time as a cause for “amazement and amazement” for revealing “a part of the universe that was off limits.”

The new data, obtained after improving the coverage and resolution of the telescope, reiterated the size and shape of the shadow.

“The main finding is confirmation of the black hole ‘shadow’,” wrote Venkatessh Ramakrishnan, a postdoctoral researcher at the Finnish Center for Astronomy of the European Southern Observatory, in an email to The Hindu. “So this shows us, as a first step, that we are not predisposed to making the black hole appear.”

Dr. Ramakrishnan has been part of the EHT collaboration since 2017.

Many telescopes in one

The EHT is not a single telescope, but a global network of radio telescopes working together to study a common object in space. They benefit from a technique called very long baseline interferometry, where the data each telescope collects on the object is correlated with the others’ data using extremely precise clocks.

In this configuration, the maximum distance between the telescopes defines the resolution of the network.

In 2017, the EHT reported detecting a bright, asymmetric ring of light consistent with the presence of a supermassive black hole. Independent analyzes of EHT data also supported the presence of the ring.

Based on these observations, scientists improved the EHT by increasing its data recording speed, its ability to track spatial information, and adding the Greenland Telescope to the mix. The latter “improved the resolution of the EHT in the north-south direction,” Dr. Ramakrishnan said.

Joining the data

In the new campaign, the EHT had nine stations collecting data during six days of observation in April 2018, at four frequencies.

The scientists then correlated the data sets with each other to increase the signal-to-noise ratio. This task and its subsequent processing were carried out in centers in Germany and the United States.

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 the telescopes worked together during the 2018 observations differed from that in 2017. In this difference, scientists can track changes in the configuration or structure of the black hole.

The team also used general relativistic magnetohydrodynamics (GRMHD) simulations to create models of the M87 black hole. These models incorporate how the black hole’s gravity influences the space-time around it using Einstein’s theory of general relativity. By comparing the simulated black hole with the real one, scientists gained valuable information about the intricate dynamics near the event horizon, such as an effect called lensing.

gravitational lenses

The findings confirmed the presence of an asymmetric ring structure approximately 42 microseconds wide. This is like observing a grain of sand 25 kilometers away.

They also revealed that its diameter had not changed much between the 2017 and 2018 observations, meaning that the black hole’s gravity constantly deflected light over time to form the observed ring.

This is in line with a prediction from the general theory of relativity that the light around a black hole is strongly focused. Objects with a lot of mass curve space-time around them more. When light travels in this region, its path bends in the same way that a magnifying glass does. Images transmitted by light therefore appear larger than they really are, and this phenomenon is called lensing.

This is also why, as the team discovered, the southwest corner of the ring appears brighter than other parts. The black hole is spinning, dragging the space-time around it in the direction of its rotation and generating more light in some areas.

Technically speaking, the observations matched predictions of a shadow formed by lensing emission around a Kerr black hole (i.e., a rotating black hole) with a mass of about 6.5 billion times the 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 that spirals toward the black hole.

The scientists observed that parts of the accretion disk and jet appeared to shift by about 30 degrees between 2017 and 2018, meaning that these structures had changed their position or orientation over time.

“The 30-degree change can be attributed to the spin of the black hole,” said Dr. Ramakrishnan. “This cannot yet be firmly established from two images taken two years apart, but that expectation was already established in a paper published a few years ago.”

This change is involved in a complex interaction between the accretion disk and jet formation. For example, models such as the GRMHD suggest that the observed change could correspond to variations in the structure of the magnetic field. This, in turn, can generate observable changes in the radiation emitted by matter in the disk.

By studying these subtle changes, scientists gain valuable insight 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

Using the EHT to photograph black holes millions of light years away is a time-consuming task that requires meticulous observations.

The new results have reaffirmed the characteristics of the black hole reported in 2019, including a very stable ring formation process and other physical characteristics. The findings were also consistent across two observation periods and multiple frequencies, which speaks to their reliability.

Examining how we measure ring diameter shows that our observation techniques are continually improving. Image-based techniques for studying objects in space tend to underestimate diameters compared to direct modeling methods. And the more precise measurement of ring diameter by the EHT suggests that this gap between the two techniques is closing. Future work could further reconcile the discrepancy and the overall performance of the EHT.

Dr. Ramakrishnan also said the telescope collaboration is planning “a so-called ‘movie project’ in 2026, when we will track the black hole for a month or two to see the brightness changes across the black hole.”

Tejasri Gururaj is a freelance journalist and science writer.

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