The Dish Helps Study Nearby Magnetar

In 1963 at the height of the Cold War, the United States and the Soviet Union both signed a treaty which prohibited the detonation of nuclear weapons in the atmosphere, in outer space, and under water. The treaty was known as the Partial Test Ban Treaty (PTBT), but with suspicions between the superpowers still high, the United States went ahead with the development of orbital satellites able to detect nuclear testing beyond Earth’s atmosphere. The project’s name was Project Vela, and just one week after the PTBT had been signed the first of six pairs of Vela satellites was launched into a 118,000-km orbit. Over more than 20-years of operation the satellites didn’t have a single confirmed detection of the detonation of a nuclear weapon, but they did detect something coming from outer space.

On 2 July 1967, the Vela 3 and 4 satellites detected a six second flash of gamma rays with a cosmic origin, the first ever detection of a gamma-ray burst. Incredibly it took another two years before the event was even found in the satellites’ data, and by that time several other cosmic gamma-ray bursts had been identified by other Vela satellites. The discovery sparked a flurry of activity in the scientific community, as astronomers tried to uncover the mystery behind the origin of the gamma-ray bursts. Over 50 years later we are still trying to understand this unusual phenomenon, but now with NASA’s Neils Gehrels Swift observatory, which recently celebrated 15 years of service, we have a dedicated satellite that can alert the astronomical community to newly detect X-ray and gamma-ray bursts almost instantly.

And some of the data coming in from the observatory is almost as strange as those first gamma-ray bursts were back in the 1960s. Earlier this year it detected a burst of radiation from half-way across the Milky Way, and when the data was analysed it was found to have come from a rare object known as a magnetar. Magnetars are a type of slowly spinning neutron star with extremely high magnetic fields on their surfaces, and around 30 have so far been found, but this object was even more special. Swift had found the fifth of a type of magnetar just 16,000 light years away whose radiation pulses at regular intervals. And this one, known as Swift J1818.0-1607, might be only about 240 years old making it the youngest magnetar ever discovered.

There is a caveat, however. Magnetars, like other neutron stars, are formed through the core-collapse of massive stars in supernovae explosions. Such a young magnetar should have a supernova remnant associated with it, but nothing has yet been found near Swift J1818.0-1607. Also, its age was measured by observing the rate at which its spin slowed, and this can be quite variable after an outburst of radiation. Further measurements of Swift J1818.0-1607 in quiescence are going to be needed for astronomers to get a better estimate of its age, not to mention further radio and X-ray imaging of its position in the sky to search for any signs of a recent supernova. But the properties that have been observed are leading astronomers to speculate that Swift J1818.0-1607 may have been born as a pulsar.

Marcus Lower, a PhD student at the Swinburne University of Technology, led a study carried out by OzGrav, Australia’s ARC Centre of Excellence for Gravitational Wave Discovery, that used the CSIRO Parkes Radio Telescope in NSW to look at J1818.0-1607 across a range of frequencies. By doing so they were able to better understand the environment it lives in and make some inferences about how this odd ball magnetar was born. Their observations, which were made only a couple of weeks after J1818.0-1607 was first spotted by NASA’s Neil Gehrels Swift Observatory, led them to speculate that it may have been born a pulsar and become a magnetar over time. As Marcus explained, “this can happen if the magnetic and rotational poles of a neutron star rapidly become aligned, or if supernova material fell back onto the neutron star and buried its magnetic field”.

But why use the Parkes radio telescope, better known as The Dish? Marcus Lower: “Parkes was recently equipped with a new ultra wide-band radio receiver that allowed us to detect radio pulses from this magnetar from 704 MHz to 4032 MHz. No other telescope on Earth can currently observe pulsars with a bandwidth that covers these frequencies simultaneously.” From the data they obtained, Marcus and his collaborators discovered that J1818.0-1607 lives in a fairly boring environment, but more importantly they found that it was much brighter at low frequencies than high frequencies, an unusual quality in this type of star.

Unusual, but not unique. Swift J1818.0-1607’s radio signature was actually similar to that of another pulsar, PSR J1119-6127, that had undergone a magnetar-like outburst back in 2016. If both pulsars have similar power sources, J1818.0-1607 should slowly start to look like other observed radio magnetars. But even if it doesn’t, there will be something to learn. Perhaps the reason we don’t see pulses from all magnetars is that, like J1818.0-1607, they are very faint at high frequencies. And this could have implications for the nature of another one of the universe’s many mysteries, the fast radio burst.

Marcus Lower described the link between magnetars and fast radio bursts. “There is a growing body of evidence that fast radio bursts come from magnetars in distant galaxies. If magnetars like J1818.0-1607 emit fast radio bursts, then the radio spectrum we detected might explain why we haven’t seen many fast radio bursts at frequencies above 8 GHz. They could be too faint at those frequencies.” More observations of Swift J1818.0-1607 over many years will be required to test these theories, but Australia’s much-loved Dish is the one place in the world that is perfectly suited to the task.

The study’s authors acknowledge the Wiradjuri people as the traditional owners of the Parkes radio telescope Observatory site.