An international research team reports that the activity of black holes can be observed as visible light during outbursts, and that flickering light emerging from gases surrounding black holes is a direct indicator of this. The team’s results, published in Nature, indicate that optical rays and not just X-rays provide reliable observational data for black hole activity. Credit: Eiri Ono/Kyoto University
Black hole activity can be observed via visible rays. All you need is a 20 cm telescope to observe a nearby, active black hole. An international team reports the activity of black holes can be observed as visible light during outbursts, and that flickering light emerging from gases surrounding black holes is a direct indicator of this. New results indicate that optical rays and not just X-rays provide reliable observational data for black hole activity.
“We now know that we can make observations based on optical rays – visible light, in other words – and that black holes can be observed without high-spec X-ray or gamma-ray telescopes,” explains Mariko Kimura, a master’s student at Kyoto University.
Once in several decades, some black hole binaries undergo “outbursts,” in which enormous amounts of energy – including X-rays – are emitted from substances that fall into the black hole. Black holes are commonly surrounded by an accretion disk, in which gas from a companion star is slowly drawn to the hole in a spiral pattern. Activities of black holes are typically observed through X-rays, generated in the inner portions of accretion disks where temperatures reach 10 million deg K or more.
V404 Cygni, one of the black hole binaries thought to be nearest to Earth, “woke up” after 26 years of dormancy on 15 June 2015 as it underwent such an outburst. Led by astronomers from Kyoto University, the team succeeded in obtaining unprecedented amounts of data from V404 Cygni, detecting repetitive patterns having timescales of several minutes to a few hours. The optical fluctuation patterns, the team found, were correlated with those of X-rays.
Based on optical and X-ray data, JAXA, national lab RIKEN, and Hiroshima University showed that the light originates from X-rays emerging from the innermost region of the accretion disk around a black hole. These X-rays irradiate and heat the outer region of the disk, making it emit optical rays and thus becoming visible to the human eye.
The study also revealed that these repetitive variations occur at mass accretion rates lower than 1/10 of that previously thought. This indicates that the quantity of mass accretion rate isn’t the main factor triggering repetitive activity around black holes, but rather the length of orbital periods.
http://www.eurekalert.org/pub_releases/2016-01/ku-bh010416.php
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