Why does this galaxy light up every 114 days?

Frequent and regular “eruptions” have been spotted in a galaxy. A team of scientists observed these light episodes to determine the event that could have caused them.

Every 114 days or so, a galaxy ” lights up like fireworks », described The NASA. The origin of these strange eruptions could be explained, the space agency announced on January 12, 2021. One possibility is that a supermassive black hole located in the center of the galaxy could cause these light bursts, by partially swallowing a star giant in orbit.

The discovery, presented at an online conference of the American Astronomical Union, is the subject of a study accessible in its pre-published version on arXiv. Its authors are studying an event, named ASASSN-14ko, first detected at the end of 2014. The event occurred in the active galaxy ESO 253-3, located more than 570 million years ago -light of us. In their work, the scientists are looking at 20 repeated eruptions of this event, which was first considered to be a supernova (the destruction of a star).

Nevertheless, the data ” show that rashes occur at regular intervals “, Noted the researchers: the hypothesis of the supernova does not hold.

Using the Swift (Neil Gehrels Swift Observatory) and TESS (Transiting Exoplanet Survey Satellite) space telescopes, the researchers were able to obtain detailed images of the flares, in different wavelengths of light. It is rare to be able to witness such eruptions, in several wavelengths, so frequent and predictable in a galaxy.

Periodic flashes of light

Some galaxies are considered particularly active, and are classified as active galaxy nuclei. One of their characteristics is the intense luminosity emitted by these structures, a brilliance whose origin is not stellar, but would rather be explained by the presence of a central black hole agglomerating a lot of matter (gathered in what we call an accretion disk). This can cause random flashes of light.

Finding such galaxies, but where eruptions occur at regular intervals, is of great interest to scientists. ASASSN-14ko is one of the best examples identified in this regard: in 6 years of data collected, the event has proven to be very consistent in the periodicity of the eruptions. Each glow is separated by about 114 days, and itself lasts for about 5 days before its brightness gradually fades.

3 hypotheses, one of which is more convincing

In total, 3 scenarios are mentioned to explain the situation:

• There would be interactions between two accretion disks, linked to two supermassive black holes located in the center of the galaxy. However, this scenario should be qualified, because the two discs might not be close enough to each other,
• A star would pass through the disc of a black hole during its orbit. However, the flares would have to be asymmetrical, since the star would cross the disk twice, on either side of the black hole. However, here, all the eruptions are similar,
• The most likely scenario would therefore be for a star to pass a little too close to the black hole and for the gravitational forces exerted on it to be such that a flow of gas escapes from the star (part of star n ‘is therefore not aspirated). With each passage, the star would lose the equivalent of three times the mass of Jupiter in gas.

If the latter scenario is the correct one, one would expect the rashes to stop eventually. But difficult to predict when, since we do not know the initial mass that this star could have. In any case, future eruptions were anticipated by scientists, at the end of 2020, when their article was published: they then specified that they had planned to observe them, hoping to better understand the nature of these events.

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