What is the Event Horizon Telescope?; NASA's SWIFT Finds a Star Falling in a Black Hole

Radio astronomers are ready to point their telescopes towards the center of our galaxy to directly image the supermassive black hole Sgr A* using the Event Horizon Telescope; and NASA’s SWIFT captures a star falling into a black hole.

Published by Tony Darnell on 24th Mar, 2017

Hello Space Fans and welcome to another edition of Space Fan News. Radio astronomers from around the globe are getting ready to point their telescopes towards the center of our galaxy in an attempt to directly image the event horizon of the supermassive black hole known as Sagittarius A* using a collection of telescopes collectively known as the Event Horizon Telescope; and NASA’s SWIFT space telescope captures a glimpse of the death throes of a star as it plunges into a black hole at the center of its host galaxy.

For many years, radio astronomers have been quietly assembling one of the largest collections of telescopes in the world in an attempt to do something extraordinary: directly image the event horizon of the supermassive black hole located at the center of the Milky Way, fondly known as Sagittarius A Star.

Nine of the world’s most powerful radio telescopes are coordinating plans in such a way that all of them will point towards the same patch of sky for several days in April, and when they are done they will have generated two petabytes of data each night they observe.

All nine of these observatories located around the world are collectively called the Event Horizon Telescope and they are all trying to do one seemingly impossible thing: to see a black hole. Astronomers hope to catch not only a glimpse of the supermassive black hole in our galaxy, but also one in a more distant galaxy known as M87*.

For a lot of reasons, black holes are very hard to see: they need to be feeding on gas and dust to be seen at all, which means they are usually embedded behind a lot of material that blocks a large percentage of electromagnetic radiation from reaching us, and well… even though they are huge in terms of mass, they are actually small in the sky. To actually see a black hole that is as far away as Sgr A*, we need a telescope that has more than 1000 times the resolution of the Hubble Space Telescope.

And Humanity has no such telescope yet, not even Hubble’s successor, the James Webb Space Telescope will have that resolution. And even if it did, it would be looking at the wrong wavelength because infrared radiation is blocked by all the gas and dust at the center of our galaxy.

Which is why the Event Horizon Telescope is made up of the best radio telescopes in the world. If we have any hope of seeing the black hole at the center of the Milky Way, then we need to look in the radio part of the spectrum, specifically around 1.3 or 0.87 millimeters, because that light will not be blocked by gas and dust.

But what about resolving the black hole? How do we get a telescope big enough to resolve it? A typical radio telescope ( say 100 meters) gas a resolution of 60 arc seconds. By comparison, the full moon is a half a degree or 1,800 arc seconds.

The problem is huge. Astronomers have a pretty good idea of how smaller black holes can form but as I mentioned in last week’s SFN, they still aren’t sure how the supermassive ones get so big, these are tens of millions to billions of solar masses.

So for a long time, astronomers just resigned themselves that they would probably never achieve the necessary resolution required to image them in any detail.

The resolution of a telescope depends primarily on its diameter and the wavelength of light trying to be captured. If you double the diameter of a telescope mirror, you can resolve details half as wide. The same goes for halving the wavelength. But at the wavelengths of 1.8 or 0.87 millimeters - which is the only wavelengths that do not get absorbed by the Earth’s atmosphere or scattered by interstellar dust and gas - astronomers calculated they would need a radio dish that was larger than the Earth to image Sgr A* or M87* (the black hole at the center of M87).

But there is hope, I’ve said many times astronomers are clever people. In the same way that Hubble was able to see galaxies farther away than it was designed for by using galaxy clusters as gravitational lenses, in the late 1990’s astronomers pointed out that the optical distortion caused by a black hole's gravity would act like a lens, magnifying Sgr A∗ by a factor of five or so.

This means that resolving the event horizon of Sgr A* might be within the reach of a radio dish with a diameter smaller than the Earth. And to get a radio dish that big, astronomers combined radio dishes from across the planet to act as one giant radio telescope the size of the Earth using a technique called Very Long Baseline Interferometry of VLBI.

So the biggest challenge: getting a telescope big enough to resolve it appears to be solved. But there are other things that are helping too.

The reason that there is any hope at all of imaging Sgr A∗, and the larger M87∗, is that they are surrounded by superheated plasma, possibly the residue of stars that did not get swallowed up outright but got torn apart under the intense gravitational stress. The gas forms a rapidly rotating 'accretion disk', with its inner parts slowly spiralling in. Astronomers reasoned that a VLBI network spread along the entire globe, and working at around 1 mm wavelength, should be just about sensitive enough to resolve the shadow cast by Sgr A∗ against the halo of gas of the accretion disk.

So a lot of things have come together to make this theoretically possible, but to make it a reality astronomers had to form a collaboration that included some of the world’s top radio observatories:

  • The Arizona Radio Observatory/Submillimeter-wave Astronomy Telescope
  • Atacama Pathfinder Experiment (APEX)
  • Atacama Submillimeter Telescope Experiment (ASTE)
  • Combined Array for Research in Millimeter-wave astronomy (CARMA)
  • Caltech Submillimeter Observatory
  • IRAM 30-Meter Telescope
  • The Large Millimeter Telescope
  • The Submillimeter Array
  • Atacama Large Millimeter/Submillimeter array (ALMA) -one of the most oversubscribed telescopes on the planet
  • The South Pole Telescope

Those last two: ALMA and the South Pole Telescope were the final components needed to make this worldwide telescope sensitive enough to make the observations.

In April, these behemoth telescopes will have a total of four, or possibly five night’s observing time with the limit being set primarily by ALMA. And they will combine their power by looking at the black hole in our galaxy and in the center of M87. To make things extra tense, the weather from all eight observatories will need to be perfect, from the Pyrenees to the South pole. And this means that the best observing window consists of only one week every year to attempt this.

As they collect signal, a GPS time signal will be collected to accurate time the observations, then the hard drives will be sent to a central location to combine all the signals to make an image and to do that, they need to correct for any time differences due to location and atmosphere (which is why the weather needs to be good, you don’t want too much difference between locations if you can help it).

Once the time signals have been precisely aligned, they can make an image hopefully showing the shadow of the event horizon against the background gas revealing details like the size and shape of the event horizon.

Sgr A* is estimated to be around 4 million solar masses, which is small as supermassive black holes go, but these observations promise to teach us more than we have ever learned about black holes before and test Einstein’s theory of relativity yet again on a scale of the Laser Interferometer Gravitational Wave Observatory (LIGO) did last year.

This effort is huge and won’t result in an image right away, there will be so much data to process that it might not be until 2018 before they have anything definitive. The racks of hard disks will be flown to two central locations where computer clusters will combine petabytes of data into one picture, that alone could take up to six months.

After that, then they begin the actual scientific study.

And of course, I will keep you posted.

Next, NASA’s SWIFT Telescope captured light from a star that travelled a little too close to a black hole in its host galaxy some 290 million years ago.

Swift was designed to quickly look for and measure gamma ray bursts, which last only a short time from a few milliseconds to a few minutes. It uses three telescopes which work together to provide rapid identification and rapid follow-up of GRB’s within 20 to 75 seconds.

In this case however, Swift was able to map out how and where different wavelengths occurred when a star fell into this black hole in an event called ASASSN-14li. ASASSN is an acronym for the All Sky Automated Survey for Supernovae which includes robotic telescopes in Hawaii and Chile.

ASASSN saw brightnesses in optical wavelengths that were also observed in X-rays by Swift and UV and Optical telescopes eight days later and continued every few days for the next nine months.

Astronomers think ASASSN-14li was produced when a sun-like star wandered too close to a 3-million-solar-mass black hole similar to the one at the center of our own galaxy.

When a star passes too close to a black hole with 10,000 or more times the sun's mass, tidal forces outstrip the star's own gravity, converting the star into a stream of debris. Astronomers call this a tidal disruption event. Matter falling toward a black hole collects into a spinning accretion disk, where it becomes compressed and heated before eventually spilling over the black hole's event horizon, the point beyond which nothing can escape and astronomers cannot observe. Tidal disruption flares carry important information about how this debris initially settles into an accretion disk.

This event show how interactions among the infalling debris could create the observed optical and UV emission.

Well that’s it for this week Space Fans, this series is supported solely by Patreon Patrons whose contributions each month make this possible so thank you, thanks to all of you for watching and as always, Keep Looking Up!

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