Vampire star

Contributed by
Sep 12, 2007
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On June 7, 2007, NASA's Swift observatory detected a blast of X-rays coming from near the center of the Milky Way. Swift is designed to see high-energy light from gamma-ray bursts, vast explosions usually coming from incredibly far away.

Because this one was near the Galactic center on the sky, it made astronomers Craig Markwardt and Hans Krimm suspicious that it might actually be from some much closer object, one inside the Milky Way. They decided to follow up on the object using RXTE, another high-energy satellite. What they found is nothing short of extraordinary.

The object turned out to be a pulsar, the leftover remnant of a supernova explosion. When the massive star's core collapsed in the explosion, the normal matter in it was crushed to unbelievable densities. Electrons combined with protons (and antineutrinos for those keeping score) to form neutrons, and the leftover ball had the mass of a star crushed into a sphere only a few miles across. It's essentially an atomic nucleus the size of a city, where one cubic centimeter -- the size of a sugar cube -- would have about as much mass as all the cars in America combined.

This neutron star spun madly, and had a strong magnetic field. A star like that can sweep up material around it, which then follows along the magnetic field lines of the star and slams into the magnetic poles. This heats up the star, causing it to glow in two spots (the north and south magnetic poles). As the star spins, we see these glowing spots blip on and off like a lighthouse as they pass into and out of our field of view. The star pulses, and so we call this kind of neutron star a pulsar.

In the case of the pulsar discovered by Swift in June, though, things are a little different. The star had a companion, a normal star that was originally maybe not too different than the Sun. It survived the supernova explosion, and aged. Fast forward a few billion years. The second star started to become a red giant, and its outer layers puffed up. The pulsar greedily started eating that material, pulling it right off the other star. It got incredibly hot as it did so, and a fierce pulsar wind started which pummeled the other star, literally eroding away much of the other star. In the meantime, the material falling on the pulsar sped up its rotation.

Fast forward a few more million years. So much material has been drained off the second star that it is a shell of its former self. It may have started with thousands of times the mass of Jupiter, but it has now been whittled down to a mere 7 Jupiter masses or so. It is stretched into a teardrop shape by the incredible tidal forces from the pulsar. A thin stream of matter connects it to the pulsar, as the tiny but dense cinder continues to drain the star's matter. Sometimes that stream becomes unstable, and a bigger blob of matter slams into the pulsar. When that happens, the pulsar erupts in a burst of X- and gamma rays, which is what Swift detected. The flare fades with time, and it was good planning that allowed the astronomers to detect this bizarre system before it faded back into obscurity.

The system is totally awe-inspiring. The pulsar spins 182 times per second; that's faster than a kitchen blender's blade. The second star was detected because as it orbits the pulsar the pulsar orbits it. This mutual gravitational dance causes the pulses to be slightly delayed or advanced as measured on Earth, and that allows astronomers to determine the size and orbit of the star. It turns out that the second star, the one being drained, orbits the pulsar at a distance of 230,000 miles -- the same distance the Moon orbits the Earth. However, instead of taking a month to orbit as the Moon does, this star orbits the pulsar once every 55 minutes!

Imagine: an object that is far more massive than Jupiter, being tossed around at well over one million miles per hour! Neutron stars are scary. Here's a scale diagram of the system:

In reality, the pulsar would be a microscopic dot on this scale, but you get the idea.

I used to work on the public outreach for Swift, and it's great to see it doing such amazing work (and to see my friend Aurore Simonnet still doing artwork for it; she drew the diagrams above). I think it's perhaps the single most successful NASA mission ever, and it's still going strong. You tend not to hear the success stories, but Swift ranks very near the top.

Sometimes I have to sit back and chuckle: it seems fantastic (literally, like a fantasy) that systems like this vampire pulsar can even exist. But it seems even more fantastic that just by carefully examining a handful of photons from it we can deduce so much information about it. The Universe unfolds before us, and really, all we need is the will to see it.