Brief background: when a massive star explode, its core collapses. If it has enough mass, the core shrinks down into a black hole. If it doesn't have quite that much oomph (if it has about 1 - 2.8 times the mass of the Sun) it forms a weird object called a neutron star. As massive as a star but only a few kilometers across, a neutron star is incredibly dense, rapidly rotating, and has a magnetic field intense enough to give you an MRI from a million kilometers away.
OK, I made that last one up, but in fact it sounds about right. The point: neutron stars are seriously awesome, right on the edge of matter as we understand it.
The supercharged magnetic field channels a tremendously powerful flow of energy away from the star in twin beams like a lighthouse. And, like a lighthouse, as the star rotates these beams sweep around. If they're aimed at Earth we see a pair of pulses every time the star spins around once. So, duh, we call these special neutron stars pulsars. You can see a way cool animation of this on NASA's Conceptual Image Lab web page.
Usually, the beams from these pulsars contain light from all (or nearly all) across the electromagnetic spectrum. We seem them in radio waves, visible light, ultraviolet, even X-rays and some in gamma rays. The processes that create these beams are pretty fierce and weird, and the type of light emitted depends on the process. However, in general, if we see high energy light (like X- and gamma rays) from a pulsar, we tend to see it in lower energy light (optical and radio) as well.
But Fermi found an oddball! Located about 4600 light years away in the constellation of Cepheus, CTA-1 is a supernova remnant, the expanding debris from an exploding star. But that expanding junk is only from the outer layers of the detonated star: the core collapsed down into a neutron star, and that's what Fermi detected. This newly discovered gamma-ray-only pulsar spins three times per second -- think on that; an object with the mass of an entire star spinning at that rate! -- and is blasting out gamma radiation with 1000 times the Sun's entire energy output.
And all of it in super-high energy invisible gamma rays. The Hulk has nothing on this pulsar.
Actually, let's pause for just a sec. Is it sunny outside? Good. Go outside, and hold your hand up. Feel the warmth? That's just a bit of optical light warming your hand. Now think about how much energy is falling over the entire Earth itself, a gazillion times the size of your hand. Now think about how much energy the Sun is emitting in all directions; the entire Earth only intercepts about one-two billionths of that light. Now think about one thousand times that much energy. Now think of all that energy being only in the form of DNA-shattering gamma rays.
Yeah, now you're getting it. This object is seriously freaky.
We know of about 1800 pulsars, and all of them emit radio waves. All but this guy. It's a brand new category of object (well, a sub category, but still), a new character on the cosmic stage. But why does it only emit gamma rays? Hey, good question. I don't know the answer (and the press release doesn't say, in fact). I suspect the answer right now is, we don't know. This object was only discovered a little while ago, and worse, gamma rays are really difficult to study. That's why we launched Fermi in the first place! Worse even than that, without being able to look at this object in radio, optical, or any other form of light really hobbles our ability to study it.
For now, I think we'll have to rely on Fermi's observations and then look at theoretical models. I imagine there will be astronomers all over the world pouncing on this, trying to figure out how the magnetic fields of the star can be so choosy (maybe they're elitist).
But until then, as usual, I have to wonder: if we only just now found this object, what the heck else is floating around out there just waiting for us to find?
Pulsar image credit: NASA.