Astronomers working with Fermi — a mission that is mapping the sky in gamma rays — have just released a new catalog of objects detected by the spacecraft. They’ve re-analyzed two years worth of data and have found nearly 2000 objects blasting out this super-high-energy form of light.

Here is the all-sky map they made from that data:

[Click to enhulkenate, and see a labeled version.]

The map is set up in galactic coordinates, so the Milky Way itself runs across the center. There are a lot of gamma-ray sources in our galaxy, most of which are bright simply because they’re close. Others are actually luminous sources like the Crab Nebula, various pulsars, and other violent objects. The map is very similar to one released by Fermi a while back, but this new one is more sensitive, and can see fainter objects.

About half the detected sources are active galaxies: distant galaxies with supermassive black holes at their hearts, actively gobbling down matter and spewing out vast amounts of energy in the process (black holes are sloppy eaters). The folks at Goddard Space Flight Center put out a nice, short video explaining this:

(more…)

On June 11, 2008 — three years ago today — NASA launched the Gamma-ray Large Area Space Telescope into orbit:

Fermi — as it was renamed once it reached orbit, after the great Italian scientist Enrico Fermi — is designed to observe gamma rays, the highest energy flavor of light. Gamma rays are only emitted from the most violent events in the universe: black holes gobbling down matter, exploding stars, antimatter particles annihilating each other, and so on. Fermi surveys the sky day after day, returning gobs of data to waiting scientists.

I was involved with Fermi when it was still called GLAST. Long before launch, I signed on to do education and public outreach for GLAST at Sonoma State University. Along with our team, I wrote web pages and helped create educational activities — including classroom lessons, a card game, a paper model of GLAST, a planetarium show, a PBS NOVA episode… we even built a small observatory near the University to augment GLAST observations! You can find all this on the SSU Fermi website.

Fermi has been a very successful mission, and I’m proud to have done my small part for it. And I guess I’m still doing it; technically, writing this blog post is EPO. So happy birthday, Fermi! You’ll always be GLAST in my heart.

- What is GLAST?
- Fermi sees the gamma-ray sky for the first time
- Pulsar SMASH!
- GORT bags a burst
- The hulking sky
- Fermi smooths out space

Randall Munroe, who draws the geekerrific xkcd webcomic, has created a really good chart showing relative radiation doses absorbed by humans doing various activities.

I’ve put a piece of it here, the section with the lowest doses. I like this! A lot of folks don’t understand what radiation is — light is radiation, for example — or that just by existing on the surface of our planet you absorb a certain amount all the time: from the ground, from space, from things you eat. Wikipedia actually has an excellent rundown of what radiation is, and the critical distinction between ionizing and non-ionizing radiation (there’s also electromagnetic versus subatomic particle radiation, but that’s less of a concern here).

In the chart, Russel deals with doses from ionizing radiation. This is the kind that can cause damage… but only in sufficiently high doses. For example, bananas are a natural source of gamma rays due to the decay of an isotope of potassium (⁴⁰K). It’s a pretty weak source — a few years back I had access to a gamma-ray detector and we could barely detect a banana’s emission — and it doesn’t affect you in any real way. Potassium iodide is a common salt that’s also a gamma-ray emitter, but again you’d need a lot of it for it to be dangerous… and if you ate that much you’d have worse issues!

The average amount of radiation you absorb in a year is about 3 – 4 milliSieverts, depending on where you live. At higher elevations — like, say, Boulder, where I live — cosmic radiation puts you on the higher end of that scale. I’ll note that cancer risk is not really higher living up here than at sea level (lung cancer rates are lower than average here, probably due to the healthy lifestyles most people follow in Boulder, but skin cancer rates are slightly higher than average, probably due to a combination of people being outside more than average together with the thinner air blocking less UV).

In general, you can actually absorb a much higher than usual radiation dose (up to a point, of course) without ill effects, since your body can heal some amount of damage (just like it heals from a cut). Too many such doses too close together, or too big a dose all at once, can do too much tissue damage and be fatal (I guess, again, like a cut). For example, I like to point out that the Apollo astronauts got roughly a year’s worth of radiation absorption in their tissue while voyaging to the Moon and back, but didn’t suffer any ill effects.

Obviously, this is a complicated issue, but the xkcd chart looks like a pretty good way to eyeball where things fall on a scale of "nothing to worry about" to "AIEEEEPANICPANICPANIC".

A thousand years ago, and 6500 light years away from Earth, a high mass star exploded. An octillion tons of gas blasted outwards at speeds of thousands of kilometers per second, forming tendrils and wisps as it raced away. At the center of the conflagration, the core of the star had collapsed into an ultradense object called a neutron star. It has the mass of the Sun crammed into a ball only 20 – 30 km (12 – 18 miles) across, and is spinning at a rate of 30 times per second.

All this happened a long time ago. The debris is what we now call the Crab Nebula, and is one of the best-studied objects in the sky. And that’s a good thing, because even now the Crab is capable of throwing tantrums… and we can see it when it does!

This image is a brand spanking new shot of the heart of the Crab Nebula taken by the Hubble Space Telescope. And by new, I mean it was taken on Saturday, October 2! It’s a bit hard to see what’s going on, so I created an annotated version:

The pulsar is labeled. It’s sitting right at the center of the gas cloud, which extends way beyond the edges of this picture. As the pulsar spins, it emits a fast stream of particles that act like a wind, compressing the gas in the nebula and creating those circles of light. They look elliptical because the whole system is tilted, and you’re seeing it like a DVD held at an angle. From what I can tell, the bottom left is the side toward us, and the upper right is farther away, as if we’re looking down on it.

In mid-September, just a couple of weeks ago, several orbiting observatories noted that there was an increased amount of gamma rays coming from this part of the sky. Gamma rays are the highest energy form of light, and there aren’t many sources in the sky that can create them at all, let alone in quantities that can be seen. The Crab is the brightest continuous gamma-ray source we know, and so it was immediately put on the Most Wanted list.

(more…)

It’s a (Bruce) banner moment for NASA’s new Fermi satellite: it’s found a pulsar that emits only gamma rays.

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.