How a supermassive black hole can be hidden

Ars Technica : In 2002 the Chandra X-Ray Observatory‘s imagery of NGC 5548 showed a typical galaxy with an active supermassive black hole at its core. One year later, significantly reduced levels of UV and x-ray emissions suggested that a cloud of gas was passing in front of the black hole. In 2012 Chandra saw a gradual increase in emissions, and then in 2013 the levels dropped again. That variability led to a joint effort using Chandra and a wide array of other observatories to try to understand the nature of the obscuring object. The work has revealed that the obscurer is located between 10¹⁴ and 10¹⁷ m away from the black hole and moving with a very high blueshift. It also appears to be pulling material from the black hole’s accretion disk and from nearby gas clouds. Although the observations don’t explain the presence of the obscurer, they do provide some insight into the nature of the areas surrounding active supermassive black holes.