Hubble is spying on what could be a rogue state just 5,000 light-years away

Using a combination of massive telescopic measurements on the ground and the sharp vision of the Hubble Space Telescope, two teams of astronomers have found what could be: be able to be — a rogue black hole that hurtles through space about 5,000 light-years from Earth.

The science of this is cool, and what’s even more fun is that the two teams disagree on some very fundamental aspects of this. So it is not clear what is happening here.

First of all, a note: I wrote about this in February 2022 when one of the teams put their then not yet peer-reviewed paper online. However, things have changed with the publication of the second article, which is described below. [Link to paper 1, link to paper 2]

The discovery is based on what is called gravity lensing† I’ve written about this before:

Any object with mass – a galaxy, a star, you, me – bends space, literally distorts it. We perceive this bending as gravity. When you shoot a rocket past the moon, the moon’s gravity bends that rocket’s path.

This also happens with light. Like a car following a bend in the road, a photon (a particle of light) traveling through the universe will bend its path slightly in this way as it passes by massive objects. The more massive the object (and the closer the photon flies past it), the more the light is bent. We call objects like these gravitational lenses because a lens is an object that bends light.

Galaxies are huge and can bend the light from galaxies even further away. That is large-scale lensing. But smaller objects like stars and planets can do this too, and we call them microlenses† The stronger the gravity, the more intense the event, meaning they can be seen even better from things like neutron stars and black holes.

These events are somewhat rare, so the more distant stars you can see at once, the more likely you are to see one. Over the years, several studies have been conducted into places such as the center of our Milky Way Galaxy, where stars are quite close together.

In 2011, two of these studies saw a lens event; One is called OGLE, for the Optical Gravitational Lens Experiment, and the other MOA, for Microlensing Observations in Astrophysics. They gave it different names — MOA-2011-BLG191 and OGLE-2011-BLG-0462 — so I just call it “the event.”

The lensed star in the event is a faint, reddish star likely located between 20,000 and 26,000 light-years from Earth, between us and the galactic center. It was found to brighten by a factor of 15 to 100, depending on the color measured and the observatory used, and then faded. The whole thing lasted about 250-300 days, which is much longer than a regular lens. Given how sharply the brightness peaks, this implies that it was a very small source with very strong gravity. hmm.

Microlens diagram

Light from an object, such as a star, is bent on its way to Earth by a massive object such as a planet, star, or black hole, making it appear as if it is a slightly different place in the sky than it actually is. This is called gravitational microlensing and while the change may be small, it can be measurable. Photo: NASA, ESA, STScI, Joseph Olmsted

Importantly, the star did not change color during the event. If the lens object had been a star, its own color would have been added to the color of the background star, changing it. Since no color change was seen, it strongly means that the lens object was not emitting light.

Crucially, the mass, distance and speed of the foreground lens object can be found by looking at how the star’s brightness changes over time and how much the light was diffracted by the lens. The studies give the former characteristics, but the deflection is so small that Hubble had to be used to measure it. Even then there are problems.

The two teams of astronomers used slightly different methods to look at the data; in fact, in 2021 one team used some of Hubble’s observations that the other team didn’t. They agree on many general basics of the event: how long it lasted, for example, how far away the lens object is — about 5,000 light-years — and the fact that it’s a compact stellar-mass object.

Blackhole Microlens Hst

Blackhole Microlens Hst

The microlensing event OGLE-2011-BLG-0462 was seen toward the center of the galaxy (large image). During 2011/2012, a star was observed to brighten and change position very slightly, giving keys to the mass, distance and speed of the lens object, which may be a small black hole. Photo: SCIENCE: NASA, ESA, Kailash Sahu (STScI) IMAGE PROCESSING: Joseph DePasquale (STScI)

But then they deviate.

One team gets a mass for the object of 7.1 ± 1.3 times the mass of the sun. The only thing that could have that much mass is a black hole, and in fact they say their detection of the black hole is “unambiguous”.

However, the other team gets a lower mass range of 1.6 – 4.4 times that of the Sun. On the high side, yes indeed that is a black hole. But at the lower end of that range, the object could be a neutron star! They can’t say it somehow.

So which one is it? We do not know† So it’s impossible to say for sure at this point.

Getting worse. By using Hubble to view the deflection of the star’s apparent position due to the lens distortion, they not only get different numbers, but they see it moving in different directions. Making it a bit too simple, one team saw the star’s position move slightly east, the other west. That’s disturbing. The motion is incredibly small, about 1 milli arcsecond, which is very difficult to measure even with Hubble and subject to uncertainties.

They also get different velocities for the lens through space; one team clocked it at 45 kilometers per second, the other at less than 30.

I have to admit that these numbers are contradictory enough to make my skeptical feeling tingle. Not that I think the event isn’t real; it sure is. But the precise features of what the lens did is something I think we need to be a little more careful about. It could be a black hole, but it could also be a neutron star.

One team wants to use an extra set of Hubble sightings to see if they can tighten things up, which is good. The problem is, you can’t repeat an event like this, so more observations made now won’t necessarily help. The exception would be if a very deep X-ray observation were made; black holes and neutron stars emit X-rays in different ways, and it’s possible, though not certain, that this could help distinguish the two.

So for now, this remains a mystery, albeit an exciting one. And the good news is, it shows that multiple observations with multiple observatories can theoretically be used to track down what’s causing these events. It also shows that it is not easy. Hopefully, future events will be able to make firmer statements.

Resident Alien Season 2

Resident Alien Season 2

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