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Hubble spies on what could be a rogue black hole just 5,000 light-years away

Hubble spies on what could be a rogue black hole just 5,000 light-years away

Using a combination of huge telescopic ground surveys and the keen vision of the Hubble Space Telescope, two teams of astronomers have found what could be – may being – a rogue black hole traversing space about 5,000 light-years from Earth.

The science of this is cool, and even more fun is that the two teams disagree on some very fundamental aspects of this. So what is happening here is unclear.

First, a note: I wrote about this in February 2022 when one of the teams uploaded their paper which was not yet peer-reviewed. With the publication of the second article, however, things changed, which is detailed below. [Link to paper 1, link to paper 2]

The discovery is based on what is called gravitational lens. I have already written about this:

Any object with mass – a galaxy, a star, you, me – bends space, literally distorts it. We perceive this bending as gravity. If you fire a rocket past the Moon, the Moon’s gravity deflects the trajectory of that rocket.

It also happens with light. Like a car following a curve in the road, a photon (a particle of light) traveling through the Universe will have its trajectory slightly curved back and forth as it passes through massive objects. The more massive the object (and the closer the photon passes to 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 massive and can deflect galaxy light even further. It is a large scale lens. But smaller objects like stars and planets can also do this, and we call them microlenses. The stronger the gravity, the more intense the event, meaning they can be seen even better from neutron stars and black holes.

These occurrences are quite rare, so the more distant stars you can see, the better the chance of seeing one. Over the years, several surveys have been conducted in places like the center of our galaxy, the Milky Way, where the stars are quite tightly packed.

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

The star crystallized in the event is a faint, reddish star probably between 20,000 and 26,000 light-years from Earth, between us and the galactic center. It has been seen to lighten by a factor of 15 to 100, depending on the color measured and the observatory used, and then fade. The whole thing took about 250-300 days, which is much longer than a typical goal. Given the sharpness of the brightness peaks, this implies that it was a very small source with very strong gravity. Hmmm.

Microlens Diagram

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 to be a place slightly different in the sky than it actually is. This is called gravitational microlensing, and although the change may be small, it may 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 added to the color of the background star, changing it. Since no color change was observed, this strongly implies that the lensed object was not emitting light.

Critically, the mass, distance, and velocity of the foreground lens object can be found by examining how the star’s brightness changes over time as well as the amount of light deflected by the lens. The readings give the first characteristics, but the deviation is so small that it was necessary to use Hubble to measure it. Even then, there are problems.

The two teams of astronomers used somewhat different methods to examine the data; in fact, one team used some Hubble observations in 2021 that the other team did not. They agree on many general bases of the event: the time it took, for example, the distance to which the lenticular object is – roughly 5,000 light-years – and the fact that it is a compact object of stellar mass.

Microlens Blackhole Hst

Microlens Blackhole Hst

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

But then they diverge.

One team obtains 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, their detection of the black hole is “unambiguous.”

However, the other team gets a lower mass range of 1.6 to 4.4 times that of the Sun. In the high end, yes indeed it is a black hole. But at the lower end of this range, the object could be a neutron star! They can’t tell one way or another.

So which one is it? We do not know. It is therefore impossible to make a firm decision at this time.

It’s getting worse. Using Hubble to look at the deviation in the star’s apparent position due to lens distortion, they not only get different numbers, but they see it moving in different directions. Simplifying a bit, one team saw the star’s position move slightly to the east, the other to the west. It’s troubling. The movement is incredibly small, around 1 milliarcsecond, which is very difficult to measure even with Hubble and subject to uncertainties.

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

I have to admit, these numbers are discordant enough to make my skeptical sense tingle. Not that I think the event isn’t real; it definitely is. But I think we need to be a little more circumspect about the precise characteristics of what made the lens. It could be a black hole, but it could be a neutron star.

One team is looking to use an extra set of Hubble observations to see if they can narrow things down, which is good. The problem is that you can’t repeat an event like this, so more observations made now won’t necessarily be helpful. The exception would be if a very deep X-ray observation was made; black holes and neutron stars emit X-rays in different ways, and it’s possible, but by no means certain, that this can help differentiate the two.

So, for now, it remains a mystery, even if it is exciting. And the good news is that this shows that multiple observations with multiple observatories can in theory be used to search for the cause of these events. It also shows that it really is not easy. Hopefully, future events seen can make more firm statements.

Resident Alien Season 2

Resident Alien Season 2

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