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SYFY WIRE Bad Astronomy

Chinese radio telescope finds hundreds of new pulsars

By Phil Plait
Artwork depicting a millisecond pulsar, sped up to phenomenal rotation rates by material it's eating from its companion star. Credit: NASA / GSFC SVS / Dana Berry

After the collapse of the Arecibo radio telescope platform last year, the largest single-dish radio telescope in the world is China's Five-hundred-meter Aperture Spherical radio Telescope (or FAST), and it's been producing some interesting science.

One of its key projects is to survey the sky and map out faint pulsars: Spinning neutron stars that send out beams of energy as they spin. Like a lighthouse, if the beam sweeps over Earth we see a blip, a pulse of energy from them.

This guy has a whole video on how that works:

FAST is so big it's sensitive to fainter pulsars than have ever been seen before. The survey scans the plane of the Milky Way to look for them — high mass stars that explode as supernovae and form neutron stars tend to be pretty much right along the disk of the galaxy. We are inside the galaxy, so we see this disk as a thick line across the sky (called, confusingly, the Milky Way) and so that's the best place to look for pulsars.

Roughly 3,000 are known already, but the survey found over 200 new ones while searching only about 5% of the galaxy's plane. That's impressive. If it continues like this it will more than double the known pulsars.

It found some interesting ones, too. 40 of the new found pulsars are what we call millisecond pulsars, spinning faster than about 300 times per second.

Yes, per second. It's hard to imagine something 20 or so kilometers across with twice the mass of the Sun spinning as rapidly as a kitchen blender blade, but the Universe does sometimes make terrifying things. Only about 400 such millisecond pulsars are known, so again this survey is likely to vastly increase how many are found.

One thing that's unusual is the distance determination for some of them. When the radio waves move through the galaxy, they pass through ionized gas. The radio waves interact with the electrons floating around out there, and the waves undergo dispersion: Their frequency gets spread out such that we get the higher frequency radio waves a little bit before the lower ones. The more stuff they pass through the bigger the dispersion. This can be turned around to find a pulsar's distance: We have a model of the density of material in the galaxy based on countless observations, so by measuring the amount of dispersion we can, in principle, get the distance to a pulsar.

But for some of them the dispersion is way too high, implying either they passed through a lot more (or denser) material, or they're much farther away than expected. If the latter, they'd be outside the galaxy itself, which seems unlikely. That means instead we don't understand the density of material in the galaxy as well (or in as much detail at small scales) as we thought. Neither choice is great, but at least one way or another we'll learn something once we figure out what's what.

And again, this survey has looked at only a small fraction of the region of the sky it's targeting. Hopefully as more pulsars are tagged it'll be easier to see what categories they fall in, and better statistics will be available. Mind you, there are likely millions of neutron stars in our galaxy — it's had a long time to make massive stars that go boom and leave behind neutron stars — and this survey will still only find a tiny fraction of them.

The Milky Way is very, very big. It's 120,000 light years wide — 1.2 quintillion kilometers — and has several hundred billion stars in it. There's a whole lot more out there to discover, and this is a solid step toward that.