A magnetar parallax
Astronomers have just made the most accurate distance measurements yet to the ultra-magnetised star XTE J1810-197 – and at a distance of about 8,000 light-years, the rare magnetar is one of the closest to us, and is quite a bit closer than we previously thought.
The research was led by OzGrav astronomers and made use of the OzSTAR supercomputer at Swinburne University of Technology.
In addition to getting perhaps the most precise distance to a magnetar to date, astronomers were also able to hypothesise about the magnetar’s genesis. A supernova remnant, located rather too close by to be coincidental, may be the remains of a former companion star of XTE J1810-197.
Magnetars are a special and particularly terrifying variety of neutron star; stars composed nearly entirely of neutrons that spin at insane rates, up to hundreds of times per second. Their magnetic fields are stronger than anything else known.
The magnetar XTE J1810-197 was discovered in 2003 by the Rossi X-Ray Timing Explorer (RXTE) as it was observing another magnetar, a soft gamma repeater, known as SGR 1806-20. At the time it was discovered, XTE J1810-197 was furiously emitting X-rays, but gradually faded away until 2018 when it became active again.
Magnetars are rather rare, with less than 30 known examples in our galaxy. The magnetar XTE J1810-197 is rarer still, a special class of neutron star that has the properties of both magnetars and pulsars. Recent evidence suggests that neutron stars may go through different stages of evolution, first as a pulsar then as a magnetar (or maybe the other way around), but there is still a lot to learn about stars like XTE J1810-197. Knowing with some accuracy how far away they are is a good start.
Led by OzGrav PhD student Hao Ding from Swinburne University, a collaboration between researchers in Australia, the USA and South Africa have taken measurements of the parallax of XTE J1810-197 over a period of a little more than a year. Previously thought to be 10,000 light-years away, they found it to be substantially closer at about 8,000 light-years.
‘In this work, we measure the positions of the magnetar with respect to two quasars that are quasi-linear to the magnetar. The technique we used can also be used to measure the parallaxes of radio-bright stars within about 10 kpc [kilo-parsecs, a commonly used unit of measurement amongst astronomers] distance. Beyond the distance limit, a parallax would be too small to detect.’
According to Hao Ding, having an accurate distance for XTE J1810-197, as well as an understanding of its motion across the sky (known as its proper motion), will benefit researchers trying to understand the properties of these fascinating stars. ‘Precise proper motion and parallax measurements would benefit long-term pulsar timing of magnetars, and, in particular, lead to a more reliable characteristic age’.
Collaborator and OzGrav PhD student Marcus Lower is also from Swinburne and has an affiliation with the CSIRO. ‘Having an accurate distance measurement to the magnetar is extremely useful. For instance, we can now accurately measure the temperature of its surface based on how bright it appears in X-rays. It also allows us to measure the distance to blobs of hot gas between us and the magnetar based on the twinkling of its radio pulses.’
There’s also the question of whether there is any link between the nearby supernova remnant (SNR) – the remains of stars blown apart in supernovae explosions – and XTE J1810-197. While the two are separated by some distance, ‘our precise proper motion points back to the central region of the SNR called G11.0-0.0 at about 70,000 years ago,’ says Hao Ding.
The magnetar XTE J1810-197 was the first one observed emitting radio pulses, and over 15 years later it is still giving up its secrets in the biggest science lab there is – the universe.
Extracted from the feature article on Space Australia written by Dan Lambeth
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