Radio telescope records a rare ‘glitch’ in a pulsar’s regular pulsing beat

The Vela pulsar makes about 11 complete rotations every
second, it also has a glitch. Credit: X-ray: NASA/CXC/Univ
of Toronto/M.Durant et al; Optical: DSS/Davide De Martin

Pulsars are rapidly rotating neutron stars and sometimes they
abruptly increase their rotation rate. This sudden change of
spin rate is called a “glitch” and I was part of a team that
recorded one happening in the Vela Pulsar, with the results
published today in Nature.


Approximately 5-6% of pulsars are known to . The Vela pulsar is perhaps the most
famous – a very southern object that spins about 11.2 times per
second and was discovered by scientists in Australia in 1968.

It is 1,000 light-years away, its supernova occurred about
11,000 years ago and roughly once every three years this pulsar
suddenly speeds up in rotation.

These glitches are unpredictable, and one has never been
observed with a radio telescope large enough to see individual
pulses.

To understand what the glitch may be, first we need to
understand what makes a pulsar.

Collapsing stars

At the end of a typical star’s life, one of three things can
happen.

A small star, similar to the size of our Sun, will just quietly
expire like a fire going out.

If the star is sufficiently large, a supernova will occur.
After this massive explosion the remains will collapse. If the
object is sufficiently large then its escape velocity will be
greater than the speed of light, and a black
hole
will be formed.

But if we have a Goldilocks-sized star that is large enough to
go supernova,
but small enough not to be a black hole, we get a neutron
star
.

The gravity is so strong that the electrons orbiting the atom
are forced into the nucleus. They combine with protons in the
nucleus to form neutrons.

These objects are estimated to have a mass of about 1.4 times
the mass of our Sun, and a diameter of 20km. The density is
such that a cupful of this material would weigh as much as Mt
Everest.

They also rotate quite quickly (and very gradually slow down
over time) as well as having a massive magnetic field, three
trillion times that of the Earth. Electromagnetic radiation
emits from both ends of this huge rotating magnet.

Now if one of the poles of this rotating magnet happens to
sweep past Earth, we see a brief “flash” in radio waves (and
other frequencies too) once every rotation. This is called a
pulsar.

The 26m antenna at the Mount Pleasant Radio Observatory.
Credit: University of Tasmania, Author provided

The hunt for a ‘glitch’

In 2014 I started a serious observing campaign with the
University of Tasmania’s 26m radio telescope, at the
Mount Pleasant Observatory
, with a goal to catch the Vela
Pulsar’s glitch live in action.

I collected data at the rate of 640MB for each 10 second file,
for 19 hours a day, for most days over nearly four years. This
resulted in over 3PB of data (1 petabyte is a million
gigabytes) that was collected, processed and analysed.

On December 12, 2016, at approximately 9:36pm at night, my
phone goes off with a text message telling me that Vela had
glitched. The automated process I had set up wasn’t completely
reliable – radio frequency interference (RFI) had been known to
set it off in error.

So sceptically I logged in, and ran the test again. It was
genuine! The excitement was incredible and I stayed up all
night analysing the data.

What surfaced was quite surprising and not what was expected.
Right as the glitch occurred, the pulsar missed a beat. It
didn’t pulse.

The pulse before this “null” was broad and weird. Nothing like
I’d ever seen or heard of before.

The two pulses following turned out to have no linear
polarisation which was also unheard of for Vela. This meant the
glitch had affected the strong magnet that drives the emission
that comes from the .

Following the null, a train of 21 pulses arrived early and the
variance in their timings was a lot smaller than normal – also
very weird.

The glitch explained, sort of

So what causes glitches? The hypothesis that is best supported
is that the neutron star has a hard crust and a superfluid
core. The outer crust is what slows down, while the superfluid
core rotates separately and does not slow down.

This is a very simplified explanation. What really happens is
quite complex and involves microscopic superfluid vortices
unpinning from the crust’s lattice.

After about three years the difference in rotation between the
core and crust gets too great and the core “grips” the crust
and speeds it up. The data seems to show that it took about
five seconds for this speed-up to occur. This is on the faster
end of the scale that the theorists had predicted.

All this and other information could help us understand what is
called the “equation of state” – how matter behaves at
different temperatures and pressures – in a laboratory that we
simply cannot create here on Earth.

It also gives us, for the first time, a glimpse into the inside
workings of a neutron star.

Explore further:

Accretion-powered pulsar reveals unique timing glitch

More information: Jim Palfreyman et al. Alteration of
the magnetosphere of the Vela pulsar during a glitch,
Nature (2018). DOI: 10.1038/s41586-018-0001-x

Be the first to comment

Leave a Reply

Your email address will not be published.


*