<p>
http://www.bu.edu/blazars/BLLac.html</p><p>Here's an excerpt taken from the original souce of the press release:</p><p>The supermassive black hole (with a mass of about 200 millions times that of the Sun, which corresponds to an event horizon that is about twice as large as the size of the Earth's orbit around the Sun) is just a very tiny black dot at the center. Surrounding it is an accretion disk of gas and dust from interstellar space that is slowly falling into the black hole while rotating around it in nearly circular orbits. (Despite the black hole's strong gravity, the gas has too much inertia because of the rotation of the disk to fall directly into the black hole. Instead, it swirls around until a process similar to friction slows it enough to fall past the event horizon.) The accretion disk contains a magnetic field that is twisted by the rotation (which is faster closer to the black hole). A coiled magnetic field creates a pinching force that focuses the plasma (charged particles that move together) into a narrow jet as it flows away from the black hole. (The magnetic field itself is probably kept from expanding excessively by pressure from a wind that we think surrounds the jet.) </p><p>We think that outbursts of radiation from blazars are triggered near the black hole, where some explosive event (such as "reconnection" of magnetic field in places where oppositely directed magnetic fields come in contact) shoots extra energy down the jet. This probably forms a shock wave that moves down the jet along a spiral path. The jet flow velocity increases with distance from the black hole, driven by magnetic forces. As the speed approaches the speed of light, the radiation is beamed more and more in the forward direction, similar to focusing a halogen flashlight. Since the jet points almost in our direction, the radiation from the shock wave gets most intense when the velocity reaches its maximum value, 0.98 times the speed of light. This actually creates an illusion that the bright knot of material made by the shock is moving 5 times faster than light travels. A few weeks later, after the emission has faded as the material in the shock expands and cools, we see a second brightening when the material is compressed by a stationary shock wave created by a pressure difference between the jet and the gas of the surrounding galaxy. </p> <p>The crucial aspect is what happened during the first flare. The polarization - which indicates the direction of the magnetic field - made about 1.5 rotations. So, that means that the blob passed through a coiled magnetic field. (In order to break the symmetry so that the polarization from different parts of the coil don't cancel, the blob needs to cover less than 100% of the width of the jet and it needs to follow a loose spiral path.) We surmise that the reason for the rise in brightness is the acceleration of the blob as it passes through the region of coiled magnetic field. All of this is exactly what was predicted by theorists, especially Nektarios Vlahakis (U. Athens, Greece) and Arieh Konigl (U. Chicago). But previous observations did not have frequent enough VLBA imaging combined with polarization observations and closely spaced measurements of the radio, visible, and X-ray brightness to fill in enough pieces of the puzzle to determine what is happening physically in the jets.</p><p> </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
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