Our local monster black hole had a massive belch 200 years ago

July 10, 2023   Issue #588

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Personal Stuff

Because I’m a person

As I write this (and maybe as you read it) I’m in Minneapolis, Minnesota with my wife. We’re visiting our daughter, the first part of our multi-legged trip across the US midwest and east coast, with the eventual destination of Virginia. Even then we can’t move into our new house until September, so for a few months we are peripatetic, stopping by here and there to visit friends and family. I don’t know how suited I am to this lifestyle, so good thing it’s just for a couple of months. We drove a brain-fossilizing amount of time to get here and I’m still a little dazed, but I’m thankful past-Phil wrote some articles that I can use for the newsletter! That past-Phil was a great guy. I wonder whatever happened to him? Anyway, enjoy today’s newsletter, which talks about huge violent cosmic events which nonetheless are so dimmed by distance we didn’t even notice them until centuries later.

Astronomy News

It’s a big Universe. Here’s a thing about it.

It’s easy to feel removed from most astronomy stories, since they’re about weird goings-on with objects so far away you get a headache just trying to picture the scale.

Then there are stories I read and a slow chill starts in the back of my head and crawls down my neck, making the hairs stand up. Yes, this is going to be about weird goings-on in some distant cosmic object, but it’s not all that far away, hinting that a sleeping monster in our neighborhood sometimes wakes up.

Sgr A* (pronounced “Sagittarius A star”) is the black hole in the center of our Milky Way galaxy. At four million times the mass of the Sun it’s what we call a supermassive black hole, for obvious enough reasons.

We’re pretty sure every big galaxy in the Universe has a black hole like this in its core; they form along with the galaxy itself and can have some pretty profound effects on the growth and evolution of the galaxy over time.

In some galaxies, these black holes can be actively feeding, meaning clouds of gas and dust are falling into them. As they fall toward the black hole from the galaxy’s core, this material forms a flat disk that can be many light-years across. Called an accretion disk, the inner parts close to the black hole are screaming around it at a respectable fraction of the speed of light, while material farther out is moving more slowly. The friction between them is enormous, so much so that the material can be heated to millions of degrees — imagine rubbing your hands together at, say, a billion kilometers per hour (literally) and imagine how much they’d warm up.

The energy generated in this disk is purely terrifying; it can blast light that can outshine the rest of the entire galaxy, by a lot. When we look at some of these active galaxies, that light is all we see because it totally drowns out even the combined light of billions of stars. And a lot of that energy is high-energy, like X-rays, so it’s not something you want to be anywhere near.

Which brings us back to Sgr A*. Our local supermassive monster is quiescent, meaning its not feeding right now. There is some evidence that small dust clouds fall in every now and again; we see flares (bright flashes) from it in infrared light, for example, but nothing too dramatic from our nosebleed seats 26,000 light-years away in the galactic suburbs.

But that’s not always been the case.

We can be pretty sure that our galaxy was an active one in its youth, billions of years ago; the natural course of growth in a galaxy like ours means colliding and merging with other galaxies, and when that happens a lot of fuel gets dumped into the galactic core where the black hole eagerly awaits. We see a lot of such galaxies in the early Universe.

But nowadays not so much.

Except…

New observations of dust clouds surrounding Sgr A* shows they’re sending high-energy X-rays our way, something cold dust clouds cannot do on their own. How can this be?

In the new work [link to paper], the astronomers give pretty convincing evidence that they’re just reflecting these X-rays. The actual source is Sgr A*, our supermassive black hole. The details are complicated, involving polarized light.

Light is a series of waves, and from most sources is emitted in randomly rotations. Some come at you vertically, some horizontally, and at all angles in between. But in some cases, especially when light is reflected off an object these waves can line up, becoming polarized. Water and metal are really good at doing this, polarizing sunlight reflected off them. Polarized sunglasses only let through light at a certain range of angles, so most of the polarized light doesn’t reach your eye. If you wear a pair and rotate your head while looking at glare reflected off a pond or a car hood you’ll see the light brighten and dim.

Black holes are lonely. Why not share this newsletter issue with a friend and brighten its day?

X-rays reflected off dust clouds do the same thing, and the angle at which they’re polarized can tell you the direction to the source. So the astronomers measured the polarization angle of the X-rays seen from these clouds, and guess what? That angle points very close to the direction of Sgr A*. Given the distance to these clouds from the black hole, this flare must have happened about 200 years ago (well, 26,000 + 200 years, given its distance, depending on how relativistically you think).

OK, that means Sgr A* blarted out some X-rays back in the 17 or 1800s. So what?

Well, yeah, here’s the flinchy bit. The brightness of the X-rays coming from the clouds means that the flare form Sgr A* was, um, big. Like, holy crap big: The total energy emitted was about 10^40 Joules. That’s a lot. It’s equivalent to the amount of energy the Sun emits in three million years.

Three. MILLION. Years.

Given various other quantities they measured it looks like this flare lasted no more than 1.6 years, and may have been significantly shorter. It’s hard to nail down the flare’s luminosity — how much energy it emitted per second — but they found a range of a million to 100 billion times brighter than the Sun at its peak.

I mean. Yikes. That’s the equal to your average active galaxy, something I prefer to be millions or billions of light-years away, not tens of thousands.

Now before you start panic screaming and running around in circles, keep in mind that this happened two hundred years ago and we’re still here. This didn’t vaporize the Earth or cause rampant mutations in our DNA or anything like that. Hell, no one even noticed. Thomas Jefferson was a smart guy, but he didn’t have an X-ray telescope handy. I’ll also note smaller flares happen all the time; here’s video showing a few we saw in 2019:

The flares from 200 years ago likely blasted out a lot of visible light as well, but there’s so much dust and junk floating in space between us and the black hole that this light was likely absorbed, so it never got to us. Even if someone had been looking they wouldn’t have seen anything. This may have sucked for any alien civilization near the galactic core, but for us all we can see is the far fainter X-ray echo.

But what caused this flare? The black hole clearly ate something, but what exactly isn’t certain. The energy is consistent with it having torn apart a star or a white dwarf, or possibly a huge pile of gas. This sort of thing doesn’t happen too often for a given supermassive black hole, so again it’s not something we need to worry about.

Unless you’re an astronomer. And then it’s not so much worry as it is salivate over the idea that we can see evidence of an active supermassive black hole up close, kinda, without having to worry about having our planet sterilized by the radiation. Even the closest actually active galaxies (ones with sustained activity for millions of years or more) are millions of light-years away, and the really scary ones are clear across the observable Universe from us. That’s good from a safety point of view but it does make it tough to figure out what they’re doing.

Having one in our own back yard, especially one that’s quiet most of the time, is a great opportunity to understand these colossal beasts far better than we do now.

As a scientist and a science communicator it would certainly be exciting for it to flare up with all our modern equipment ready to be aimed that way. I’m not saying I hope Sgr A* eats another star any time soon, but to be honest I’m not not saying that.

Et alia

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