Category Archives: Fermi/GLAST

Exploring the Mighty Blazar

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.

The H.E.S.S. telescope in Namibia.
The H.E.S.S. telescope in Namibia.

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.)

GLAST=Fermi

What it Means

NASA, the Stanford Linear Accelerator Group, and Sonoma State University jointly announced yesterday that their mission, the Gamma-ray Large Area Space Telescope (formerly known as GLAST) is now going by the name Fermi, and it has a nifty new logo to celebrate. I think the incorporation of a stylized active galactic nucleus and a spiraling jet is quite clever.

The spacecraft’s new name honors THE pioneer in high-energy physics, quantum theory, and particle physics, Italian scientist Enrico Fermi, who lived from 1901 to 1954.  He was the first person to figure out how cosmic particles could be accelerated to the high speeds that take them across the universe, and if he had lived to see the telescope that bears his name, he would have been delighted to study the pulsars in our galaxy and the gamma-ray signals from supermassive black holes at the cores of galaxies. His work set the stage for understanding the phenomena we see at these cosmic objects.

Fermi the spacecraft was launched two months ago and has been in testing and calibration mode since then. To celebrate the renaming, Fermi scientists used the Large Area Telescope onboard the spacecraft to “take” an all-sky image showing the glowing gas of the Milky Way, blinking pulsars, and a flaring galaxy billions of light-years away. It’s the result of 95 hours of the instrument’s “first light” observations. By comparison, when NASA’s now-defunct Compton Gamma-ray Observatory, did the same image, it took years of observations to create.

This Fermi image shows gas and dust in the plane of the Milky Way glowing in gamma rays due to collisions with accelerated nuclei called cosmic rays. The famous Crab Nebula and Vela pulsars also shine brightly at these wavelengths. These are fast-spinning neutron stars, which form when massive stars die.  The Crab and Vela pulsars were originally discovered by their radio emissions. The third pulsar shown here, named Geminga and located in Gemini, is not a radio source. It was discovered by an earlier gamma-ray satellite. Fermi is expected to discover many more radio-quiet pulsars, providing key information about how these exotic objects work.

A fourth bright spot in the LAT image lies some 7.1 billion light-years away, far beyond our galaxy. This is 3C 454.3 in Pegasus, a type of active galaxy called a blazar. It’s now undergoing a flaring episode that makes it especially bright. Another instrument onboard the spacecraft, the GBM, has already been recording gamma-ray bursts that occur when massive stars die or neutron stars merge.

At the rate Fermi is studying the sky, I don’t think it will be too long before we start seeing a new set of cosmic images that will bring us new views of the universe in much the same way that the Compton Gamma-ray Observatory did in its heydey.