Unexpected Changes within the Most Predictable of Stars

Animation Pulsar Spinning Neutron Star

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Artist’s idea of a pulsar.

Pulsars, a category of neutron stars, are extraordinarily predictable stars. They are shaped from the hearts of large stars which have since collapsed in on themselves, not in a position to burn sufficient gasoline to fend off the crushing gravity the star possesses. If the circumstances are proper, the star will proceed to break down in on itself till what’s left is a remnant of what was there earlier than, normally solely in regards to the dimension of the Melbourne CBD, however 1-2 occasions as heavy as our Sun, making these a few of the densest objects within the Universe.

These stars don’t produce a lot seen mild, however from their magnetic poles, they emit surprisingly brilliant beams of radio waves. If we’re fortunate, because the star rotates, these beams will wash over the Earth and we observe ‘pulses’. While most pulsars spin round in a few second, there’s a subclass of those stars that spin round in just some thousandths of a second—they’re referred to as ‘millisecond’ pulsars.

Observing the pulses from these millisecond pulsars provides physicists clues to many questions, together with testing General Relativity and understanding the densest states of matter. But one of many foremost objectives of observing these extremely quick, dense stars is to detect ultra-long wavelength gravitational waves. And by long, we mean many light-years long. These gravitational waves distort space-time between us and the pulsars, causing the pulses to arrive earlier or later than expected. It’s likely that these gravitational waves come from a background produced by all the binary supermassive black holes in the Universe, which form from galaxies crashing into one another.

As part of OzGrav, we try and detect this gravitational wave background by looking at collections of the most predictable stars (called pulsar timing arrays) and measuring how they change over time. We did this by using the world’s most sensitive radio telescopes, including the Australian Murriyang telescope (also known as the Parkes telescope) and the ultra-sensitive MeerKAT array telescope in South Africa.

But it’s not quite that simple. From our observations with MeerKAT we found that the most precisely timed (read: predictable) pulsar, J1909-3744, was misbehaving. We found that the pulses were changing shape, with bright pulses arriving earlier and narrower than faint ones. This lead to greater uncertainty in its predicted emission. Fortunately, we were able to establish a method to account for this change and time tag the pulsar more precisely than ever before. This method could be of use for other pulsars and will be important when more advanced telescopes are available in the future.

​Written by OzGrav PhD student Matthew Miles, Swinburne University.