Hubble witnesses flaring in black hole jet

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 16 April, 2009

A flare of matter blasting out from a monster black hole is outshining even the core of its host galaxy, M87.

The outburst is emanating from a localised blob of matter – known as HST-1 – embedded in a jet of hot gas spewing out of the supermassive black hole. “The plasma jet is like a flowing river that has white water at several locations,” describes Juan Madrid of McMaster University in Hamilton, who lead the Hubble observations of HST-1. “These sites of white water are called knots. Charged particles in the jet (mostly electrons) are accelerated in these knots, sometimes through shocks, instead of flowing smoothly. This acceleration makes the charged particles radiate photons away.”

These images taken in ultraviolet light by Hubble reveal the brightening of a jet of gas blasting from the core of the gigantic elliptical galaxy M87 over a period of seven years. The core of M87 is located at lower left in the images. HST-1 is the bright blob at centre. The glowing material at far right is part of a stream of particles in the jet that speed up and glow in the ultraviolet. Image: NASA, ESA, and J. Madrid (McMaster University).

The gas knot is located 214 light years from the galaxy’s core, and over the last seven years, telescopes such as the Hubble Space Telescope and Chandra X-Ray Observatory have seen the glowing knot brighten, fade, and now brighten again, causing astronomers to scratch their heads.

“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,” says Madrid. “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?”

Two hypothesis compete for the explanation of this flare.
“If the flowing jet is heating something like a dust cloud it would create the flaring,” Madrid tells Astronomy Now. “The jet also has magnetic fields, and if the magnetic field lines get
close together and touch a flare can happen as well.”

Noticeable brightening first began between 1999 and 2001, continuing through to 2005 when it became 90 times brighter than the first observations. After May 2005 the flare began to fade, but it intensified again in November 2006, although the second outburst was fainter than the first.

Madrid hopes that future observations of HST-1 will reveal the cause of the mysterious activity. “We hope the observations will yield some theories that will give us some good explanations as to the mechanism that is causing the flaring,” he says. “Astronomers would like to know if this is an intrinsic instability of the jet when it ploughs its way out of the galaxy, or if it is something else.”

The study’s results are published in the April 2009 issue of The
Astronomical Journal.

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