At the heart of our galaxy lurks a supermassive monster, which is currently slumbering – but it hasn’t always been so quiet. New observations reveal an X-ray “echo” from a time when the Milky Way’s central black hole awoke just 200 years ago, shining a million times brighter than today.
Like most galaxies, the Milky Way has a supermassive black hole at its core. With the mass of about four million Suns, this object, known as Sagittarius A* (Sgr A*), is relatively quiet compared to some of its peers we’ve seen in other galaxies.
But we know that wasn’t always the case. There’s evidence that Sgr A* fired off huge flares of radiation about six million and 3.5 million years ago, blasting away matter and leaving vast shock waves that are still visible today in certain wavelengths of light. But now, astronomers have discovered the smoking gun from a much more recent outburst that took place a mere 200 years ago.
Several X-ray space observatories, including IXPE, Chandra and XMM-Newton, have previously spotted giant molecular clouds near Sgr A* that are unexpectedly bright in X-rays. Astronomers used the IXPE satellite to measure the polarization of light coming from these clouds, which pointed the finger at the culprit.
When light is polarized, its waves are all pointing along the same plane. In this case, the angle of the polarization pointed towards Sgr A* as the source of the X-ray radiation, while the degree of the polarization revealed how far the clouds had traveled since they’d been belched from the black hole. This in turn allowed the researchers to work out when the flash took place – a little under two centuries ago.
From those details, the astronomers were then able to estimate how bright the original flash had been. It turns out, our local supermassive black hole briefly shone about a million times brighter in X-rays than usual. That would’ve put it on par with a Seyfert galaxy, which host a core that’s as bright as all the stars in that galaxy. And before you ask how 19th century astronomers missed such a show, this was long before X-ray telescopes were invented and pointed at the heavens.
Getting a better understanding of Sgr A*’s history of activity can help us predict its future. After all, in recent years it’s thrown off new flares of X-rays and near-infrared light, which could be harbingers of a new period of increased activity or just a few fireworks from wayward objects falling in.
The research was published in the journal Nature.
Source: University of Strasbourg