In April 2019, the international cooperation project “EHT (Event Horizon Telescope)” in which radio telescopes around the world collaborated,He succeeded in photographing a black hole directly for the first time in the world, announced. Many of you may know the news at the time, and many of you may be able to see the picture as a donut painted in red and yellow.
This black hole is located at the center of the elliptical galaxy M87, which is about 55 million light-years from Earth.M87*” he is called.Posted photoWhen you see it, many people think that the entire dark area like a donut hole is a black hole. But this is not exactly the case. The black hole’s body occupies only part of the dark area in the center of the cake, and the rest is only optically dark. How does this happen?
You may have often heard the explanation that a black hole is a celestial body with a very strong gravitational pull, and not even light can escape from it. To explain it more precisely, “there is a spherical region from the center of the black hole to a certain distance from which even light cannot escape.”
The radius defining this region isSchwarzschild radiusthe imaginary spherical surface that acts as a boundary between regions isevent horizonCall. The region bounded by the event horizon is what can be called the body of a black hole. It is possible to get inside from outside the event horizon, but, on the contrary, it is absolutely impossible to get inside out.
So, it is not necessarily that it is possible to move freely outside the event horizon.
outside the event horizonphoton ball There is a spherical circumferential surface called . The radius of the photon sphere is 1.5 times the event horizon for non-rotating black holes, but differs for rotating black holes such as M87*.
When the light is emitted from the outside of the photon ball inward, the light cannot escape to the outside of the photon ball, and eventually traces a spiral path and falls into the event horizon. On the other hand, if you shoot the light from inside the photon ball to the outside, depending on the angle, you can escape from the black hole forever. By the way, if the light is emitted exactly at the distance of the photon ball, then the light will orbit the photon ball forever and form a light ball. That is why it is called a photon ball.
However, due to gravitational fluctuations around a real black hole, the slight fluctuations cause the light to escape from the photon ball and either be absorbed into the black hole or escape. This allows us to detect light from the photon sphere.
With this interpretation, you might think that when you look at a black hole from the outside, it appears to shine like a photon ball, but this is not actually the case. This is because the photon sphere itself is strongly bent by the black hole’s intense gravitational pull. When a black hole is observed from Earth, the light emitted from the surface of the photon sphere on the back side of the black hole as seen from Earth comes forward, making it look like a ring, but the area around the center looks bleak. This is what we see as a donut-like appearance of a direct image of a black hole. What does the central dark region look likeblack hole shadowcircle around it,photon ringhe is called
The image of M87* from the EHT shows the photon ring, but the image is somewhat blurred. Even with EHT, which is comparable to taking pictures of a 1 yen coin on the Moon, the width of the photon ring is very narrow and falls below the resolution limit.
In addition, the observation data includes radiation from sources other than the photon sphere (mainly radiation from the accretion disk surrounding the black hole), which makes the picture as a whole hazy. It is estimated that only 10% to 30% of the light from the photon ring is in the donut-like optical ring.
However, unlike regular astronomical telescopes, the EHT visualizes black holes by analyzing data obtained by radio telescopes at eight locations around the world. Since the essence is in the data, if we change the analysis method, we can do detailed scientific research on the photon ring.
A research team led by Avery E. Broderick of the University of Waterloo attempted to reanalyze M87* data obtained by the EHT in 2017 to study the photon ring of M87*. By building a new image processing algorithm, we processed the data to distinguish photon rings from other emission. Since the photon ring profile is very narrow, the analysis was performed assuming that the radiation from the photon ring was concentrated in a finite region.
As a result, the research team succeeded in isolating the radiation from the photon ring whose radio intensity changes on a daily basis! Based on the diameter of the photon’s ring, the research team calculated the mass of M87* to be about 7.13 billion times the mass of the Sun. This value differs from the ~6.5 billion times that EHT showed with its first image release. This is due to differences in methods of analysis and differences in values assumed during analysis.
We also know that there is a stream of jets emitted from the black hole southwest of M87*. In this image analysis, correlations with planes were also verified.
As a result of the analysis, it was found that the daily changes in the photon ring observed in the radio wave intensities are related to the changes in the diffuse outward flux. This is consistent with what has been repeatedly shown in previous studies, and provides further evidence that M87* rotation and jetting force are related.
This analysis, in which the photon ring emission generated by M87* was isolated and examined, is the first case of light capture behind a black hole. In addition to estimating the mass of M87* in a new way, we were also able to understand the relationship between the photon ring and its flux activity. The method used this time shows that ‘radio interferometry’, which links several radio telescopes like the EHT, is effective as a way to deepen our understanding of black holes, which are difficult to observe in detail.
- Avery E. Broderick et al. – “M87’s Photon Ring *”. (Astrophysical Journal)
- Nadia Whitehead Close-up ready: New technology sharpens images of black holes. (Harvard and Smithsonian University Astrophysics Center)
- EHT Collaboration – “The first image of a black hole”. (European Southern Observatory)
Text: Riri Aya
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