A flashing mystery unfolds in the center of the Milky Way



Most galaxies seem to contain a supermassive black hole, and ours acts.

Most galaxies seem to contain a supermassive black hole, and ours acts. (Pixabay /)

Something weird is happening in the center of the Milky Way, and astronomers are scrambling for answers.

At the heart of our galaxy, a black hole covers about four million times the mass of the sun in an area about the size of Earth's orbit. Known as Sagittarius A * (or "Sagittarius A star") or Sgr A *, it creates a tumultuous environment, whipping the stars millions of kilometers at the hour and shredding all the asteroids close to gravity . . The beast now seems to act even more aggressively than usual, flashing twice as brightly as astrophysicists have ever seen it before. The researchers spotted the outbreak this spring and assume it comes from a heap of unidentified material that unexpectedly falls into the black hole.

"It was so brilliant that you could see it in real time," said Andrea Ghez, the Galactic Center Group's lead investigator at UCLA, who recorded the activity. "It was very clear that something was happening this year."

The black holes themselves can not emit as much as a photon (they have not received their name for nothing), but the surrounding matter is not under such a ban. Hot gases are thought to swirl around Sgr A *, forming an "accretion" disk of material for sticking to the surface of the black hole. The object also hosts a formidable magnetic field, and it is this force that drives the charged particles like electrons spiraling inward, making them shine. The dust prevents visible light from reaching the Earth, approximately 26,000 light-years into the disc, but the infrared radiation can bypass it.

When Ghez and his team began monitoring the center of the galaxy two decades ago, their main goal was to prove the existence of the supermassive black hole, which they did indirectly by observing the buzzing stars. around the nucleus of the galaxy, like asteroids around the sun.

Over the last 10 years, the Keck Observatory in Hawaii has allowed the group to study direct emissions from the vicinity of the black hole. Nowadays, the system can take a picture per minute and create a stop-motion movie that they can watch from Hawaii or UCLA. Sgr A * sparkles like a candle, depending on the momentary hazards of its consumption and the behavior of its magnetic field, but during four non-consecutive nights this spring, this flicker has become a flash bright enough to be recognized at the time. naked eye.

"Oh, what's going on," recalls Ghez. "I do not think we've ever seen that bright before."

During their observations, the brightness was multiplied by 75, which amounts to looking back between a 100 watt bulb and a projector. On a particular day, May 13, Sgr A * shone twice as loud as ever, compared to hundreds of observations spanning 20 years (new analytical techniques allowed the group to Retroactively extract approximate measures from the first 10 years). And the brightness actually decreased when they turned on the camera, indicating that the peak of the event was probably even brighter. A statistical test suggests that the chance to catch four nights as bright as they were was only 1 in 2,000.

"Given the behavior of the past 10 years," says Ghez, "you should not see this happen."

As far as this unprecedented activity is concerned, nobody really knows it. Deducting a particular interaction between gas, stars, magnetism and a black hole caused by a lightning bolt in a dusty corner of the sky is a big step forward, but the team has some enlightened assumptions.

The ideal candidate is a change in the process of accretion (read: eat) because more matter flowing to the black hole means that more electrons are illuminated by the magnetic field. One star in particular, S0-2, could have bypassed equipment during a close approach last year, where a local block of gas could have come too close.

Alternatively, Sgr A * may not be on unusual things. Maybe its magnetic field has broken, triggering a colossal rocket similar to that of our sun. Follow-up observations, especially in X-rays and radio waves, will help limit the possibilities.

Not knowing how long the push will last, the Ghez team is rushing to publish its findings, which went live in August and will soon be published in the press. The letters of the astrophysical journalfaster than any previous job. "One of the goals of the quick publication of this document is to encourage others to look at it before things change too much," she said.

And other researchers take note. Enmanuelle Mossoux, an astrophysicist at the University of Liège in Belgium, described their light rocket as "very interesting" and said that it corresponded to irregular activity patterns observed during the radiography, including a growing number large eruptions since 2014. It's We would like to see a more in-depth analysis comparing recent years to see if this year's surge is part of a broader upswing.

In both cases, astronomers will have to act quickly. Unlike galaxies and other cosmic entities before humanity that will go nowhere, no matter what Sgr A * does, he may not stick to it long. "We generally think that the universe is a very static place," says Ghez, "and we can see here in real time things that are changing before our eyes."