Looking into the Active Heart of a Galaxy

In the cosmic zoo of interesting things “out there”, blazars are right up there with neutron stars and gamma-ray bursters as astrophysically interesting objects. What are these blazars? Think of galaxies out there that have active cores — those regions are often referred to as active galactic nuclei. Such a place is busily pouring out radiation at nearly every wavelength and some are particularly bright in the x-ray, radio, and gamma-ray regimes. This is  happening because there’s a supermassive black hole at the center, gobbling up material and belching out radiation and emitting a jet that threads its way through an intensely twisted magnetic field.

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Blazars are very compact (tightly squeezed into a comparatively small area of the galaxy), they appear to be highly variable in their output, and as it turns out, their jets are pointing in our general direction.  So, when we look at a blazar, we are essentially looking along the long axis of the jet back “down” toward its source — which is presumably toward the black hole and its accretion disk.

Astronomers want to look at blazars in various wavelength “regimes” to understand the structure of these cosmic power plants. Different structures and activities radiate at different wavelengths. Recently an international group of astronomers looked at the galaxy PKS 2155-304, which is about 1.5 billion light-years away (relatively close, for a galaxy) and is a regular source of faint gamma-ray signals. Now, if you see gamma rays, you know there’s something really active going on, and when you see a gamma-ray source brighten and then dim down, you know you’ve got something interesting happening there. So, when PKS 2155-304 brightened up in 2006, the astronomers took a look it with optical (visible-light), x-ray, and gamma-ray telescopes to capture its “light signature” in as many wavelengths as they could.

Between August 25 and September 6, 2008, astronomers used several telescopes to monitor PKS 2155-304 as it was quiet and giving off no flares. They used the  Large Area Telescope (LAT) aboard NASA’s orbiting Fermi Gamma-ray Space Telescope to look for gamma-ray emissions. X-ray emissions were detected using NASA’s Swift and Rossi X-ray Timing Explorer (RXTE). Rounding out the wavelength coverage was the H.E.S.S. Automatic Telescope for Optical Monitoring, which recorded the galaxy’s activity in visible light.

What they found out about PKS during both its flaring and quiet states tells them something about the central engine. But what? During flaring episodes of this and other blazars, the x- and gamma-ray emission rise and fall together. However, when PKS 2155-304 is in its quiet state, the same two emission regimes do not seem to rise and fall together. Why this is is till a mystery. What’s even stranger is that the galaxy’s visible light rises and falls with its gamma-ray emission. One of the scientists on the team, Berrie Giebels, described it like this:  “It’s like watching a blowtorch where the highest temperatures and the lowest temperatures change in step, but the middle temperatures do not.”

So, the black hole engine at the heart of PKS 2155-304 is doing something, and the next step is to find out what. Clearly there’s something periodic going on as it gobbles up material in the accretion disk. Are there clumps in the accretion disk? Is there something that periodically affect the jet in some way?  Whatever it is gets “telegraphed” out in the radiation we’re seeing as the jets stream out from the action at the heart of the active galaxy. It’s not likely this will stay a mystery for TOO long, since continued observations over longer periods of time will eventually help astronomers uncover what’s going on in the middle of this blazar. (For more information on this study, surf over to NASA’s Fermi mission site.)