Remember that amazing ‘first image’ of the black hole Sagittarius A* (Sgr A) at the heart of the Milky Way? Well, that may not be entirely accurate, according to researchers at the National Astronomical Observatory of Japan (NAOJ).
Instead, the accretion disk around Sgr A* may be more elongated than the circular shape we first saw in 2022.
Scientists at NAOJ applied different analysis methods to the Sgr A* data obtained for the first time by the Event Horizon Telescope (EHT) team. The EHT data came from an array of eight ground-based radio telescopes. The original analysis showed a bright ring structure surrounding a dark central region.
The resulting reanalysis in a different form implies something about the motions and distribution of matter in the disk.
In fairness to both teams, radio interferometry data is extremely complex to analyze. According to NAOJ astronomer Miyoshi Mikato, the rounded appearance may be due to the way the image was constructed.
“We hypothesize that the ring image resulted from errors during the analysis of the EHT image and that part of it was an object, rather than the actual astronomical structure,” Miyoshi suggested.
Explaining the appearance of the black hole
So what does Sgr A* look like in the NAOJ reanalysis?
“Our image is slightly elongated in the east-west direction, and the eastern half is brighter than the western half,” Miyoshi said.
“We think this appearance means that the accretion disk surrounding the black hole is rotating at about 60 percent of the speed of light.”
The accretion disk is filled with superheated material that ‘surrounds the drain’ so to speak, pouring into the 4 million solar mass black hole. As it circulates through the accretion disk, friction and the action of magnetic fields heat the material. This makes it glow, mainly in X-rays and visible light, as well as emitting radio emissions.
Various factors also affect the shape of the accretion disk, including the spin of the black hole itself. Furthermore, the rate of accretion (ie, how much material falls on the disc), as well as the angular momentum of the material, all affect the shape.
The gravitational pull of the black hole also distorts our view of the accretion disk. This kind of ‘fun mirror’ distortion makes the image incredibly difficult. As it turns out, either view of the disk’s actual shape—the original circular EHT view or the elongated NAOJ view—can be correct.
So why the different views of the black hole?
How did the teams come up with two slightly different views of Sgr A* using the same data?
“No telescope can capture an astronomical image perfectly,” Miyoshi noted. For EHT observations, it turns out that interferometric data from widely coupled telescopes can have gaps. During data analysis, scientists must use special techniques to build a complete picture. This is what the EHT team did, resulting in the ’round black hole’ image.
Miyoshi’s team published a paper describing their results. In it, they propose that the ring structure in the 2022 image released by the EHT is an artifact caused by the bumpy point spread function (PSF) of the EHT data.
The PSF describes how an imaging system deals with a point source in the region it is viewing. It helps give a measure of the amount of blur that occurs due to imperfections in the optics (or in this case, gaps in the interferometric data). In other words, there were problems with ‘filling in’ the gaps.
The NAOJ team re-analyzed the data and used a different mapping method to bridge the gaps in the data. This resulted in an elongated shape for the Sgr A* accretion disk.
Half of the disc is brighter and they suggest this is due to a Doppler shift as the disc rotates rapidly. They suggest that the newly analyzed data and extended image show a part of the disk that extends several Schwarzschild radii away from the black hole, spinning extremely fast and seen from an angle of 40°-45°.
What’s next?
This reanalysis should aid in a better understanding of what the Sgr A* accretion disk actually looks like. The EHT survey of Sgr A* that resulted in the 2022 image release was the first detailed attempt to map the region around the black hole.
The EHT consortium is working on improvements to produce better and more detailed interferometric images of this and other black holes. Ultimately, this should result in more accurate views.
Follow-up studies should help fill in any gaps in observations of the accretion disk. In addition, detailed studies of the immediate environment around the black hole should provide more clues about the black hole hidden inside the disk.
This article was originally published by Universe Today. Read the original article.
