The black holes supermassive, which is found in the center of almost all galaxys, are so called by their immeasurable mass (between a few million and a few billion solar masses). Such black hole exists in the Milky Way, our galaxy: Sagittarius A * (or Sgr A *). It takes its name from the constellation Sagittarius in which it is located. It is not classified as an active nucleus. However, it now appears that the X-ray beams emitted from this black hole have increased significantly in recent years.
Several previous studies point out that the center of our galaxy is becoming more and more agitated. A Franco-Belgian team of researchers, led by astrophysicist Emmanuelle Mossoux from the University of Liège, wanted to confirm the conclusions of a 2017 study revealing that the rate of X-ray eruptions observed at Sgr A level *, had tripled since August 2014.
Brighter eruptions
Concretely, between 1999 and 2015, the XMM-Newton, Chandra and Swift space observatories detected a hundred light jets in total (107 precisely). Study of the data revealed that the brightest X-ray flares had increased significantly after August 2014; at the same time, the weakest jets appeared to have decreased since August 2013. The E. team. Mossoux wanted to verify the phenomenon by adding new radiographic data: that collected by the three telescopes between 2016 and 2018.
Capture of an illuminating jet on Sagittarius A * by the NuSTAR X-ray telescope, in 2012. Credit: NASA / JPL-Caltech
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Over this period, they identified 14 new illuminating jets, then re-studied all of the eruptions observed since 1999 by applying a new calculation algorithm to determine the intensity and duration of the different light flows. Their calculations enabled them to define a new detection threshold for each of the three telescopes concerned.
The overall analysis of the data revealed that in reality, the “low” emission rate had not decreased, but remained stable over the entire period. On the other hand, the increase in the most luminous and energetic eruptions has been confirmed: they have been three times more frequent since August 2014.
An origin still unknown
Namely, these X-ray flares are not the only evidence of unusual Sgr A * activity. Last year, the black hole emitted near infrared radiation 75 times brighter than usual, the brightest ever observed in these wavelengths.
To confirm once again the activity of the black hole – which is located at 26,000 light years from our planet – Mossoux’s team also looked at data collected last year. Result: 4 light jets, the largest number ever observed in a single observation campaign! The black hole is therefore always active.
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Finally, the activity of Sgr A * seems to have increased in several wavelengths, but the origin of this revival of activity remains unknown. Among the hypotheses, the experts evoke accretion – matter that would fall into the black hole – or disturbances due to the passage of asteroids …
To go further in their research, they are now studying near infrared and radio wave spectra on the one hand, corroborating their observations and on the other hand, looking for clues that could explain the phenomenon.
Sources: Astronomy & Astrophysics, E. Mossoux
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