Telescopes around the world
have joined forces to try to provide us with our first photograph of a black
hole. That's still a few months off, at least, but all that staring at a black
hole is already starting to produce results.
In 2013, the Atacama
Pathfinder Experiment (APEX) radio telescope in Chile joined the global
telescope collective that makes up the Event Horizon Telescope (EHT), to take
observations of Sagittarius A*, the supermassive black hole at the centre of
our galaxy.
And it almost doubled the
longest baseline length in the array, leading to observation of the finest details
yet of the space right around the event horizon of this black hole.
Now, we can't actually see
black holes. They're thought to be incredibly dense collapsed stars with a
gravitational pull so strong that nothing, not even light, can escape - and if there's
nothing coming out of them, we can't detect them.
What we can detect is the
space around them, along with matter as it falls into the black hole's
gravitational tug, forming a swirling accretion disc that glows with the
intense heat of friction. However, at a certain distance from the black hole,
there is no escape, and everything falls in. Even light. This point of no return,
where the escape velocity is higher than light speed, is called the event
horizon, and this is what the Event Horizon Telescope is trying to photograph.
Some black holes are absolutely
immense, like Sagittarius A*, which is millions of times the mass of the Sun.
These are way bigger than any stars that we know of, and astronomers are not
quite sure how they form. But a bigger object means it's easier to see - and
that's why the EHT is looking at the closest one we know.
The 2013 observations of
Sagittarius A* narrowed down the resolution to just three Schwarzschild radii -
one of which equals the radius of the event horizon, or the hypothetical size
of the black hole - revealing details as small as 36 million kilometres.
This may seem huge - after
all, Earth is 150 million kilometres from the Sun - but it's smaller than the
expected size of the accretion disc, and has allowed astronomers to start
figuring out the event horizon's structure by working out models and applying
them to the data.
"We started to figure
out what the horizon-scale structure may look like, rather than just draw
generic conclusions from the visibilities that we sampled," explained
astronomer Ru-Sen Lu of the Max Planck Institute for Radio Astronomy.
"It is very encouraging
to see that the fitting of a ring-like structure agrees very well with the data,
though we cannot exclude other models, e.g., a composition of bright
spots."
So it's looking like the
structure of the space around the event horizon of Sagittarius A* could be
shaped like a giant doughnut around the black hole. Which could explain why it
gets so hungry. Of course, the data isn't
detailed enough yet to draw any firm conclusions. Future observations by the
EHT will get more data in order to build up a more comprehensive picture. But,
so far, this is pretty exciting stuff.
"The results are an
important step to ongoing development of the Event Horizon Telescope,"
said Sheperd Doeleman from the Harvard-Smithsonian Center for Astrophysics and
director of the EHT.
"The analysis of new
observations, which since 2017 also include ALMA, will bring us another step
closer to imaging the black hole in the centre of our Galaxy."
The research has been
published in The Astrophysical Journal.
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