HEAPOW: Looking Hard to Find Black Holes (2021 Jul 05)

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HEAPOW: Looking Hard to Find Black Holes (2021 Jul 05)

Black holes are good at hiding. If they are not actively feeding, the only practical way to find them is by the way they warp the spacetime around them as they spin, or the gravitational waves they produced when they collide (or, theoretically, by the burst of radiation they produce when they evaporate). Black holes that are actively feeding (that is, swallowing vast amounts of material near them) can be recognized by the enormous X-ray emission produced as the material being swallowed spirals through a flat disk and finally into the black hole. But many feeding black holes evidently don't like to be watched while they eat. They can be surrounded by a thick ring of obscuring material which hides the black hole, and its accretion disk, from the X-ray view of telescopes like the Chandra X-ray Observatory or XMM-Newton. But the NuSTAR hard X-ray observatory comes to the rescue. NuSTAR can detect very high energy X-rays that can penetrate through even the thickest black-hole shrouding rings. A new study using NuSTAR observations of 19 nearby, X-ray faint galaxies showed that, in reality, almost all of them contained actively feeding supermassive black holes near the center of the galaxy, but appear faint due to obscuration. The NuSTAR study was not only able to identify the actively feeding blacks in these systems, but helps us gain a fuller picture of the overall supermassive black hole population in the Universe.

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Author: Dr. Michael F. Corcoran

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Uncovering the Hidden Black Holes
NASA | JPL-Caltech | NuSTAR | 2021 Jun 25

Artist's illustration depicting a black hole wrapped in a toroidal, or donut-shaped cocoon of gas and dust. (Credit: NASA/CXC/M.Weiss)

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How many fireflies are in your backyard? Can you count them all? It’s a challenge. Their flashes are short. More distant fireflies are fainter – but could fainter ones also be younger, older, or smaller fireflies? How often does a firefly flash? And imagine if there were mist or smoke from a camp fire, making them even harder to see.

One solution is not to try to count all of the fireflies in the entire yard, but to watch for a long time in just a small square of the yard, closer to you. Later, if you add up how many squares of that size are in your yard, you can figure out the total number of flies. At least if there is no fog or smoke. In that case, you would want a special camera that could see through the fog.

This is the challenge that astronomers have when they want to count how many black holes are in the universe, particularly the supermassive black holes that reside in the centers of large galaxies. Like fireflies, black holes are usually quiet, with no matter falling in on them. But when matter does fall on them, that accreting matter heats up and shines brightly. However, like mist or smoke in a yard, gas and dust in a galaxy can hide the accreting black holes – and NASA’s NuSTAR mission has the special camera that can see through the gas and dust, and thus provide an accurate count of black holes. NuSTAR is sensitive to high-energy X-rays, similar to the X-rays used by doctors and airport security, enabling it to detect energetic photons from accreting supermassive black holes that penetrate the enshrouding gas and dust. ...

A Hard Look at Local, Optically Selected, Obscured Seyfert Galaxies ~ E. S. Kammoun et al

• Astrophysical Journal 901(2):161 (2020 Oct 01) DOI: 10.3847/1538-4357/abb29f
• arXiv.org > astro-ph > arXiv:2007.02616 > 06 Jul 2020 (v1), 24 Aug 2020 (v2)