Free-Floating Black Hole Discovered

Yes, a free floating black hole has been discovered. Maybe.

I qualify many of my cosmology stories with 'maybe'. We never really know do we?

Anyway, this is another story in my occasionally attempted light-hearted series about black hole research and related discoveries. I’ll try to keep it brief because my remaining time on this planet is dilating rapidly.

If you don’t want to bother with my inexpert blathering then you can go straight to the research article in The Astrophysical Journal Letters.

The news

The study by a team at Berkeley was the first exhaustive attempt to identify black holes that do not have a companion star . Such ‘free-floating objects’ are thought by some physicists to be abundant throughout the Milky Way — and I guess, by extension, in other galaxies.

Crazy headlines

One British newspaper reported the find with the headline ‘Black Hole Bombshell’. That would be funny of it wasn’t so sad. But all the headlines have focused on the words ‘black hole’ when, in fact, the researchers are not sure whether it is a black hole or a neutron star.

Or at least one team is not sure, the other is certain.

What did they find?

Multiple images of background stars. The images were distorted by the gravity of a massive object estimated roughly at 2–5 solar masses between the Earth and the star. This object is estimated to be about 5,000 light years away. However, the data leads other astronomers to suggest that the nearest might actually be about 80 light years away (NASA). Compare that with the distance to our nearest stellar neighbour other than the Sun — Proxima Centauri is 4.24 light years away.

How did they find it?

By analysing images of stars captured by that great warhorse, the Hubble Telescope (currently 32 years old and still delivering). You’ve got to hand to the team that designed it. It’s fighting hard not to be eclipsed by the new James Webb Space Telescope.

Anyway, it was a lucky find, because the technique — a micro-lensing survey — which made it possible depended on gravitational micro-lensing, which when combined with the Hubble images revealed the existence of the object. The stars had to be in alignment so to speak (it’s my turn for red-top journalese now).

Here’s a video from the Berkeley team which explains the find. It’s not too heavy on the technical side.

Another team

For the record — very broadly similar results (mass estimates differ quit a lot) were produced by another team using a slight different technique, for which they should get credit, although they are certain it is a black hole:

Notably, a competing team from the Space Telescope Science Institute (STScI) in Baltimore analyzed the same microlensing event and claims that the mass of the compact object is closer to 7.1 solar masses and indisputably a black hole. A paper describing the analysis by the STScI team, led by Kailash Sahu, has been accepted for publication in The Astrophysical Journal.

— Phys.org

How many are there?

Two.

Not really — each team has given the same object a different designation. Because both microlensing surveys caught the same object, it has two names: MOA-2011-BLG-191 and OGLE-2011-BLG-0462, or OB110462, for short.

Things are becoming confusing.

But really, how many? This is a controversial question, because many astronomers believe that the twenty plus black holes already definitively identified in our galaxy are in binary systems — that is, paired up with a companion star — and are not representative of the 200 million black holes thought to exist just in our Milky Way galaxy. The recent findings support the estimates of the theorists.

The numbers are staggering to think about and as I look up right now into the cloudless Australian night sky from my boat, I’m wondering how many of these black holes — or neutron stars — I’m looking at.

Hang on, I’ll just try a little lensing through the bottom of my whisky glass. No, no, I still can’t see any.

But if they’re black holes I wouldn’t se them would I? Well, there is Hawking radiation. Or is there? The black hole paradox rears its invisible head.

If these free-floaters are that close and there are so many then the risks of meeting one may be greater, as I wrote about in an earlier story about that probability.

And now I’m going to top up my glass and see if more amber liquid will help my micro-lensing or just make me cross-eyed and consign me to a whirling pit.

Fact check controversy?

It could be the amber nectar, but the confusion continues as something made me question the distances quoted to the object. I’m sure that I read 2 to 6 light years in the Physics World article, my first read. That seemed awfully close. So I checked:

• NASA said 5,000 light years.
• Physics World said 2–6 light years
• Phys.org said 2,200–6,200 light years (roughly)
• Berkeley said 2,200–6,200 light years (roughly)

The Astrophysical Journal Letters (the source paper) said ‘1.47–1.92 kpc’ — I used an online converter to convert kiloparsecs to light years and that comes up with (for 1.5 kpc) 4.89 x 10³ light years — i.e. 5,000 light years

So, it looks like Physics World has got it wrong and dropped a factor of 1,000.

Oops. Not the kind of thing I expect from them. Further away is definitely better than nearer.

I took a screenshot for posterity, not that I’m pedantic:

We wouldn’t be here if it had been as close as 2–6 light years, we’d probably be through a wormhole somewhere.

You just can’t believe everything you read these days.

Hairy black holes?

---

This story was first published on Medium on 25 July 2022