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Astronomer here- I’m actually the 3rd author on this paper, and am quoted in this press release! It means I helped discover the second closest black hole to Earth, just 3800 light years from us! Paper available here

Now first thing to clarify is, this is truly the lead author's discovery (Kareem El-Badry), who is an amazing astronomer. What he's been doing is going into the Gaia catalog (which carefully tracks the precise movement of billions of sources) and being great at finding "needle in a haystack" type things. In this case, the thing was a red giant star, about the same mass as our sun, orbiting an unseen companion that we've concluded must be a black hole, named Gaia BH2.

How do you do this? Well as you might recall, orbital mechanics state that if you have two objects in space gravitationally bound, they will orbit a common point of interest. When this happens, you'll see the objects "wobble" in their movement back and forth over the course of their mutual orbit (which is how we find many exoplanets, in fact!) What Kareem did, strictly speaking, was find a star with a weird "wobble" in the data... and that "wobble" indicated the star's orbit was in a period of P= 1277 days, and the companion it was orbiting would be a compact object ~9x the mass of the sun.

Now, a star 9x the mass of the sun would be stupid bright, and very obvious bc this visible star is pretty bright on its own (12th magnitude). Definitely nothing there in follow-up observations, so it's not a star. So basically at this point, the argument is "if only we knew of something that was very massive, so massive light doesn't escape it... oh yeah, a black hole!"

Now the trick is some black holes do emit at low levels, thanks to accreting dust onto them- this happens in closer star- black hole pairs, called X-ray binaries. This emission is basically created as particles get close to the event horizon of the black hole, "feeding" it, and how we can spot them usually in radio and X-rays. And, well, we know this star pretty well because we can see it, and every star will have some amount of particles coming off of it in a stellar wind (like the sun does, and how we get the aurora), which is pretty well understood for stars of this type. So then the question is- is Gaia BH2 emitting at any wavelength?

Now this is where I come in, in my role of someone who knows a thing or two about how to get radio observations of weird black holes. :) Kareem is in my institute and came in to tell me about this object a few months ago, and that he'd discovered the closest period in its ~3.5 year orbit was happening this past month! (Yes, that's a bit of luck- in science it's good to be lucky sometimes!) So if you want to detect particles interacting with the black hole, your best chance of seeing it is basically now. Also, it was a very southern hemisphere object, so not just any telescope can look at it.

So, what I did was file for emergency time to use the MeerKAT telescope in South Africa, the best telescope on Earth to do this observation, asking for a several-hour observation of Gaia BH2. Luckily, they agreed and granted the time, so we took a look a few weeks ago! (And I have now officially hung up my shingle as a "black hole consultant" btw- my rates are very reasonable! :) )

Now, the bad news is, we did not detect any radio emission from Gaia BH2 (nor did the Chandra X-ray telescope.) You can see the details in Figure 10 of the paper linked at top. But the good news is this is actually massively helpful, because there is so much we don't understand about black holes! For example, how does this accretion process work for emission from black holes? Our data is good enough that we can say most of those stellar wind particles never reach the event horizon- maybe there are strong winds blowing them away, or similar. Not as exciting as a detection, but still really useful!

Anyway, moving on from that, Gaia BH2 is exciting because as the name implies, it's the second such Gaia black hole- the first being Gaia BH1. This discovery happened a few months ago (press release if you missed it then), and that one happens to be the closest black hole to Earth that we know of (and why Gaia BH2 is second- this one has the largest orbit known for a black hole though). This is super exciting because it now implies that these black holes in orbits are actually rather common in space- more common than ones where the black hole and star are closer at this rate!- and the trouble is detecting them. (It's also not clear how they form, so some nice work for theorists to do.) Well, for now- the good news is Gaia is still taking data, and its next data release (in ~2025) will have a lot more of these stars with mystery black hole companions in it! So, guess there will be a lot more to do!

This guy black holes !!!

Congratulations on this exciting discovery, and thank you for sharing!

How did MeerKAT admin respond when they heard about what you observed?

• How common is it to have enough observations of a star to be able to detect this kind of wobble in its orbit?

• Does this affect our estimate for the overall prevalence of black holes, if the first one discovered by this method isn’t emitting the x-rays we’ve used to find the other currently-known ones?

Thanks! But not a guy! :)

Hah, can't say we told them. That's not common when using telescopes.

1. Not common at all, that's why it's literally only happened two times. Happens much more commonly when searching for exoplanets however.

2. It does! It implies the rate of black holes is much higher than previously thought!

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Dude, if I was a technician, I would love to know the results of my work!

And it’s literally named meerkat. They are standing up at attention, waiting for interesting things to happen.

Haha, don't worry, well when we apply in May for normal time, they get an update on what all happened to the data we asked for in the previous year. Guess they'll hear then!

I have always wondered what happens to a black hole when there is basically nothing around for it to suck up… like yeah maybe stray light particles but no matter. Would it just chill out suspended until it finds something to eat? Dissipate? Interesting that it’s even possible for solar winds to out power a black hole like that if true. Crazy stuff!

> This is super exciting because it now implies that these black holes in orbits are actually rather common in space- more common than ones where the black hole and star are closer at this rate!- and the trouble is detecting them.

If it turns out that galaxies contain far more black holes than previously assumed, what ramifications might that have, e.g. for understanding galaxies' masses, rotations, dark matter complements, etc.?