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A jet of gas spewing from a huge black hole has mysteriously brightened, flaring to 90 times its normal glow.
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For seven years the Hubble Space Telescope has been watching the jet, which pours out of the supermassive black hole in the center of the M87 galaxy. It has photographed the strange phenomenon fading and then brightening, with a peak that even outshines M87's brilliant core.
Scientists have dubbed the enigmatic bright blob HST-1, and are so far at a loss to explain its weird behavior.
"I did not expect the jet in M87 or any other jet powered by accretion onto a black hole to increase in brightness in the way that this jet does," said astronomer Juan Madrid of McMaster University in Hamilton, Ontario, who conducted the Hubble study. "It grew 90 times brighter than normal. But the question is, does this happen to every single jet or active nucleus, or are we seeing some odd behavior from M87?"
Many supermassive black holes have jets of material that spray out perpendicularly from the donut-shaped ring of matter falling onto the black hole. These beams of hot gas are thought to result from magnetic field lines that are twisted by the black hole's mass, and propel charged particles outward.
But most rays do not appear to blaze up with such extreme intensity as HST-1. Scientists aren't sure if it is an exceptional case, or if it represents a normal event for black hole jets, which are still not very well understood. In this case, the bright knot of HST-1 is about 214 light-years from the M87 galaxy's core
To learn more about this bizarre light show, Madrid analyzed the seven years' worth of Hubble images of the jet in ultraviolet light to capture changes in HST-1's behavior over time. He also compared the Hubble data to photos of the jet taken in X-rays by the Chandra X-ray Observatory and in radio by other telescopes.
Madrid found that between 1999 and 2005, the blob continually brightened. By May 2005, HST-1 was 90 times brighter than it was in 1999. After that, it seemed to fade, and then intensified again in November 2006.
"By watching the outburst over several years, I was able to follow the brightness and see the evolution of the flare over time," Madrid says. "We are lucky to have telescopes like Hubble and Chandra, because without them we would see the increase in brightness in the core of M87, but we would not know where it was coming from."
More data will be needed to solve the mystery of why HST-1 acts the way it does.
"We hope the observations will yield some theories that will give us some good explanations as to the mechanism that is causing the flaring," Madrid says. "Astronomers would like to know if this is an intrinsic instability of the jet when it plows its way out of the galaxy, or if it is something else."
This strange case could provide a unique opportunity to learn more about black hole jets in distant galaxies, which are difficult to study because they are so far away. M87 is located 54 million light-years away in the Virgo Cluster.
Madrid's study is detailed in the April 2009 issue of the Astronomical Journal.
[link to www.space.com]
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