New Method For Finding Stellar-sized Black Holes

New method for detecting black holes in orbit around other stars looks very promising.

Published on 27th Oct, 2018

Hello Space Fans and welcome to another edition of Space Fan News. In this episode, as you can imagine, black holes are hard to see. Considering that they are so dense that their gravity won’t let anything out, not even light, you can imagine that well… they are pretty black. Couple that with how black space is and it’s easy to conclude that black holes are hard to find. Well this week, astronomers have found an new technique for finding them. Until somewhat recently, black holes have been mathematical constructions. One of the first mathematicians to come up with the idea of black holes was the mathematician Pierre Simon-Laplace around the late 18th century, sometimes referred to as the Isaac Newton of France, but I can imagine the French not liking that characterization very much. Later in the early twentieth century, using Einstein’s General Relativity, Karl Schwartzchild characterized what black holes would be like if we ever found one. Black holes are strange things, they are objects that take up no space - they have zero volume - and yet have infinite density. Zero Volume. Infinite Density. Try the the new Miller Black Hole Beer. Sorry, bad joke. Black holes are actually a problem for physicists. Within the event horizon, the laws of nature do not work. Beyond the point where light cannot get out, physics as we understand it, stops working. Regardless of how uncomfortable that is for people studying the universe, black holes nonetheless are predicted by the laws of science that has served us everywhere else so well, and it has only been recently that we started to find them. We can’t see black holes directly because they don’t let any light out, so we do with black holes what we do with dark matter: we infer their existence by looking at the the effects they have on things we can see. For example, we can’t see black holes just sitting in the emptiness of space. We won’t see a black hole just bobbing around in the curvature of spacetime, they are too black. To see a black hole, it needs to be feeding on things that are close by: gas, dust and even better, stars. When a black hole is sucking in stuff, that material gets accelerated as it is sucked in and that material radiates in wavelengths we can see with our telescopes. The best possible situation to be in when trying to see a black hole is to find one orbiting a nearby star that it is sucking the material off of. These binary star systems consisting of a star and a black hole going around it, are essential laboratories for understanding black holes. So far, astronomers have found 60 candidates for this type of black hole in the Milky Way Galaxy. These binary systems give off a lot of X-ray eruptions that can be seen by space telescopes like Chandra. What’s interesting to me is that of these 60 candidates, only 17 have been confirmed. I always thought we knew about more black holes than this. I should point out that this research is oriented towards stellar-sized black holes, not the supermassive ones found in the centers of most galaxies. Those are much easier to see because of their size and the fact that they have an entire galaxy to feed on. Here, we’re talking about black holes much smaller, the ones created with a massive star dies. These are tens to hundreds of solar masses, not millions or billions like in galaxy centers. So this week astronomers from the Instituto de Astrofisica de Canarias, think they have a new way to confirm the existence of a black hole this size. By looking at the spectrum of a black hole binary in the red region of visible light called h-alpha, they can see a black hole by its gravity. When something with a strong gravitational field exists near a star, the visible light portion of the spectrum gets stretched out, and it is most visible in the red part of the spectrum, around 6562 angstroms. So when astronomers look at a binary they think has a black hole around it, and that spectral line is smeared out, or stretched, they can infer that there is probably a black hole there doing the stretching. And how wide the the h-alpha line is can be used as an indicator of the strength of the gravitational field so they can rule out neutron stars and stuff like that because they would have weaker lines. To demonstrate the their new method, they observed four systems with confirmed black holes using a set of special filters on ACAM, an instrument on the 4.2-meter William Herschel Telescope of the Isaac Newton group of Telescopes at the Roque de los Muchachos Observatory (Garafia, La Palma). The results were then compared with direct measurements of the width of the H-alpha line obtained with the ISIS spectrograph on the Gran Telescopio de Canarias (GTC). The result showed that it is practical to measure the width of the H-alpha line using photometric techniques, which opens the door to a more efficient detection of inactive black holes in binary systems. They estimate that an analysis of some 1000 square degrees (10 percent) of the zone of the galactic plane using this technique should detect at least 50 new objects of this type, which is three times the currently known population. This search could also yield a detailed census of other galactic populations, such as short period cataclysmic variables, X-ray binaries containing neutron stars, and ultra-compact binaries with period shorter than one hour. 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