Object 81: PSR J2124-3358
Podcast release date: 05 September 2022
Right ascension: 21:24:43.8
Corresponding Earth location: About 1350 km east of Uruguay in the Atlantic Ocean
PSR J2124-3358 is a pulsar. Pulsars are the remnants of dead, massive stars that form when stars much larger than the Sun (more than ~8 times, but we don’t know the exact details), explode as supernovae at the end of their lives and the stellar core collapses to such an extent that protons and electrons fuse together to form an object composed primarily of free neutrons, roughly 10 kilometres across, but with a mass close to that of the Sun. This object is called a neutron star. Neutron stars are extremely strongly magnetised and that magnetic field gives rise to beams of radiation, usually in the form of radio waves, that emanate from the magnetic pole of the star. As the star rotates about its rotation axis, those beams sweep around the sky, somewhat like a lighthouse, and if the Earth, happens to be located along the path of that beam, we see a little pulse of radio waves, with our radio telescopes, every time the star rotates and the beam crosses the Earth. For that reason we refer to these objects as pulsars.
This particular radio pulsar J2124-3358, was discovered as part of the Parkes Southern Survey of 1997 which detected 101 new radio pulsars using the Parkes radio telescope in New South Wales, Australia . 2124 is located in the constellation of Microscopium at a distance of 400 pc, or 1300 ly, or 8 quadrillion miles  (where 1 quadrillion is 1 million billions).
The star rotates roughly once every 5 ms (or ~200 rotations per second). The fact that this pulsar rotates so quickly puts it into a special class of pulsar, known as millisecond pulsars. We believe these stars have acquired their rapid rotation rates by being part of binary systems in which the pulsar gravitationally draws material from a companion star, causing the pulsar to spin-up to these millisecond periods. However, 2124 does not have a companion star. This alone gives us interesting clues as to the pulsar’s evolutionary history, because any companion star it did have must have either undergone its own supernova explosion, in such a way that the binary system was disrupted and the stars were separated or the combination of the gravitational attraction of stellar material from the companion and the highly energetic radiation from the pulsar, has cause the companion to evaporate. These types of systems are an active area of research at Jodrell Bank.
This is in fact one of around 850 pulsars we routinely observe at Jodrell Bank, using the Lovell telescope. We observe this source approximately once every two months in order to track its rotation. The Parkes observatory also time this source routinely, as part of the International Pulsar Timing Array . Millisecond pulsars such as 2124 have such remarkable rotational stability that we are exploiting them in order to search for a low frequency gravitational wave background from coalescing supermassive black hole binaries. As a gravitational wave passes over the Earth the distances to other objects changes because the spacetime between us and those objects is lengthening and contracting and these small perturbations in space-time cause the rotation rates of pulsars to appear to change. Now if we were just seeing these perturbations in a single pulsar, we wouldn’t be able to conclude that gravitational waves passing the Earth were causing it, as it could be related to processes in the pulsar itself. Instead, this same gravitational wave signal should be measurable in many pulsars, if the Earth is being affected by the passage of gravitational waves . And 2124 is one of these crucial highly stable pulsars that we are using as a Galactic scale gravitational wave detector to search for the signature of the Earth being affected by perturbations in space-time caused by distant merging black holes.
This pulsar isn’t just observable at radio wavelengths. We see X-ray pulses from the star, these were first observed by the German ROSAT instrument in 1999 . The emission comes primarily from particles streaming down the magnetic field, impacting the surface of the star, giving rise to hotspots near the polar cap. The X-ray emission from 2124 is routinely observed by the Swift X-ray telescope . It was also detected as a gamma ray pulsar during a survey by the Fermi Large Area Space Telescope in 2009 . Perhaps most interestingly, 2124 produces observational manifestations at optical wavelengths. This isn’t the pulsations that we’re seeing, although there are a very small number of pulsars that do exhibit optical pulsations, rather it’s the material surrounding the pulsar that is glowing at optical wavelengths. 2124 is moving through space at a few 10s of km/s . As it does so it experiences ram pressure from the free particles in the interstellar medium that it encounters as it moves. This has the effect of confining the outflowing wind from the pulsar, generating a bow shock wave that glows in the optical, specifically at the Hydrogen alpha wavelength of 656.3 nm . Observations of these shocks provide us with valuable insights into how pulsars interact with their immediate environments. In 2017, the Hubble Space Telescope also detected 2124’s shock in the optical and in the far-ultraviolet . This is only the second time such a shock has been seen in the far UV and this suggests that such shocks may be detectable around other pulsars where no optical shock has been detected because there are no free hydrogen atoms upstream of the shock to cause the optical emission, and so 2124 is providing a useful signpost to study shocks from pulsar winds at other wavelengths.
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