Strange quasi-periodic flashes were found in the Sagittarius A black hole, flashing once every 30 minutes

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered quasi-periodic flickering in millimeter waves from Sagittarius (SGR) A*, the center of the Milky Way. The study shows that these flickers are caused by the rotation of radio spots around a supermassive black hole with an orbital radius smaller than Mercury. This is an interesting clue that can be used to study space-time with extreme gravity. The study was published in the journal Astrophysics. Yuhei Iwata, the lead author of the study and a graduate student at Keio University in Japan, said:

It is well known that the Sagittarius A* black hole sometimes flares in millimeter wavelengths. This time, we used ALMA to obtain high-quality data on the intensity changes of radio waves from the Sagittarius A* black hole for 10 days, 70 minutes a day, and then found two trends: quasi-periodic changes with a typical time scale of 30 minutes and slow changes of up to an hour. Astronomers speculate that a supermassive black hole with a mass of 4 million suns is located at the center of Sagittarius A*. The flares of Sagittarius A* are not only observed in the millimeter wave band, but also in infrared light and X-rays.

The changes detected with ALMA, however, are much smaller than those detected before, and these levels of small changes are likely to always occur in Sagittarius A**. The black hole itself does not produce any form of radiation, the source of emission is the hot disk of gas surrounding the black hole. The gas around the black hole does not directly enter the gravity well, but swirls around the black hole to form an accretion disk. The research team focused on changes on short timescales and found that the 30-minute period of change is equivalent to the orbital period of the innermost edge of an accretion disk with a radius of 0.2 astronomical units (1 astronomical unit is equivalent to the distance between the Earth and the Sun: 150 million kilometers).

By comparison, Mercury, the innermost planet in the solar system, orbits the sun at a distance of 0.4 AU. Given the huge mass at the center of the black hole, its gravitational effects are also extreme in the accretion disk. Tomoharu Oka, a professor at Keio University, said: "This emission may be related to some exotic phenomena happening near supermassive black holes. Hot spots form sporadically in the disk and rotate around the black hole, emitting strong millimeter waves. According to Einstein's special theory of relativity, radiation is greatly amplified when a light source moves towards the observer at a speed comparable to the speed of light. The rotation speed at the inner edge of the accretion disk is quite large, so this special effect occurs.

Astronomers believe this is the source of short-term variations in the millimeter-wave emission from Sagittarius A*. The team speculates that such variations could affect efforts to take images of supermassive black holes with the Event Horizon Telescope. In general, the faster something moves, the harder it is to take a picture of it, but variations in the emission itself provide compelling insights into the motion of the gas. Through a long-term monitoring campaign with ALMA, it might be possible to witness the moment when the black hole absorbs gas. The goal of the research is to extract independent information to understand the puzzling environment around supermassive black holes.

Bo Ke Yuan | Research/From: National Institute of Natural Sciences

The research was published in the journal Astrophysics

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