The colossal black hole at the centre of the Milky Way hasn鈥檛 always been so quiet.
Our galaxy鈥檚 supermassive black hole, called Sagittarius A* (Sgr A*), is relatively inactive now, but observations of the gas clouds around it hint that it may have released a powerful blast of X-rays about 205 years ago.
When a black hole puts out an聽X-ray flare, that radiation can bounce off any gas in the area before spreading outwards to the rest of the galaxy. It is similar to how city lights illuminate a cloudy sky 鈥� even from afar, even if you can鈥檛 see the city itself, you can tell that it is there. In this case, though, the enormous distances the light travels between the black hole, the gas clouds and observers on Earth means that there is a delay between the flare being released and the light bouncing off the clouds and reaching us.
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聽at the University of Strasbourg in France and his colleagues observed the centre of the galaxy using the Imaging X-ray Polarimetry Explorer (IXPE) space observatory. They found that the聽clouds near the galactic centre聽were glowing with X-rays, and the radiation was consistent with having originally come from Sgr A* about 205 years ago, give or take about 30 years.
They found that it is most likely that the flare lasted less than 1.6 years, during which time it released about 1040 Joules of energy 鈥� about 1.5 times the total energy contained in the mass of Earth鈥檚 moon.
Such a flare would be at least 100,000 times brighter than Sgr A*鈥檚 current X-ray output, says聽 at the University of Paris, who wasn鈥檛 involved in the work. 鈥淰ery different from the faint and short bursts [Sgr A*] is experiencing nowadays.鈥�
As powerful as it was, an outburst like this wouldn鈥檛 have affected Earth at all, Terrier says. Dissipation of the energy over such a vast distance would have weakened the flare significantly by the time it reached our planet.
鈥淭he distance to the galactic centre is more than a billion times larger than the distance to the sun,鈥� he says. 鈥淭he flux reaching Earth of a huge Sgr A* X-ray outburst would therefore be relatively modest compared to the sun鈥檚 X-ray luminosity.鈥�
There are several ideas for what could have caused the flare. Such flares are often caused by a black hole聽ripping apart and eating another object, such as a planet or star. It could also be due to a star falling apart as it is pulled towards Sgr A*, or a huge explosion nearby. IXPE looked at galactic centre for a little under 11 days for this study, so longer observations could help illuminate what caused the flare.
arXiv
