FAST Pulsar Exploration

FAST stands for Five-hundred-meter Aperture Spherical radio Telescope, with a diameter of 500 meter at the crown opening. It is currently the world largest single dish telescope. FAST situated in a Karst depression in southwest China. This Karst depression is close to a sphere, which minimize the excavation. It is higher than the Karst depressions around, which makes a good shielding of radio interference.

There are several subsystems of FAST. This is the active reflector, which will form a parabola in observing. The feed cabin is suspended up in on the focus sphere. There are some instruments to measure and control the position and orientation of the feed cabin.

During the observation, a source will cross the sky, so the feed cabin will move accordingly, the reflector will deform accordingly.

Why bother to deform the reflector? Here you can see that the parallel light rays will be focused to a line by a spherical reflector.

In order to focus to a point, or an Airy dot, a parabola is needed.

Let’s watch a video to have a direct impression. The reflector is a parabola. You can see the balls all focus to the focal point. You can imagine the bell is the receiver.

This is the real receiver. On the left, it is the ultra-wideband receiver. On the right is the 19-beam receiver.

FAST have several science goals, including Galactic HI mapping, HI galaxy search, pulsar search, interstellar molecules.

Now let’s have a look at pulsar studies. The title of this talk is ‘cosmic lighthouse’. Let’s have a look at lighthouse. Lighthouses are once critical to navigation. Look closely, you can see a rotating light beam. That’s why a lighthouse is pulsating.

When the weather is cloudy all the time, and you do not have a GPS system, the most efficient way is building lighthouses. That’s the USSR have done in the 1960s.

When you know the position of two lighthouses, by measuring the angle, you can get a rough estimate of the distance.

Now we have GPS on a car, a ship and even in a cell phone.

The principle is similar. But here the key point is an accurate clock.

What is a clock? Something that is periodic can be a clock.

In principle, a periodic pulsating star can be used as a clock for navigation. The only thing is that most pulsating stars are not accurate enough.

But we have accurate clocks in the universe. They are pulsars.

Looks like a lighthouse, right?

Many people think that pulsars are neutron stars. Some people don’t agree.

A pulsar is a small object. It is no larger then a big city. It is however weighted to a solar mass. Typically, it rotates several rounds per second. You can imagine the material in a pulsar is extremely dense.

What happens? In normal matter, there are a lot of space in the atom. But for the matter in a pulsar (neutron star), the atoms have been squeezed together.

This is possible in a collapse at the death of stars.

Usually, stars rotate slowly. Have a look at our sun.

How can slowly rotating stars becomes rapidly rotating pulsars? The key point is compression.

The angular momentum is conserved, lower the moment of inertia, you get a higher rotating speed.

Try this experiment.

The first pulsar is discovered by Jocelyn Bell. She was just awarded the Breakthrough Prize.

Light will disperse in a lens.

The magnetic wave will also have dispersion in the interstellar medium. High frequency wave will arrive earlier.

This is the signal of the first pulsar discovered by Jocelyn Bell. Where is the high frequency signal?

Since we know how the wave is dispersed. We can extract the de-dispersed signal.

Then we added up the de-dispersed signal, and got a pulse. That’s how we search for pulars.

Now we have discovered several pulsars.

Have a look at this website if you are interested.

(http://crafts.bao.ac.cn/pulsar/fast_all_pulsar_list/)

What to do next? We may use pulsar for navigation and search for gravitational waves.