Ancient Distance Measuring Technique Applied to Magnetar

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 distance measurements required the use of the Very Long Baseline Array (VLBA), a system of ten radio telescopes spread across the United States of America and operated by the National Radio Astronomy Observatory (NRAO). But the research was primarily carried out by astronomers here in Australia, and made use of the OzSTAR supercomputer at the Swinburne University of Technology in Melbourne.

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. Get close enough to one, and you won’t be home for dinner.

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.

A lot of what we understand about the universe depends on having the best possible estimates of the distances to faraway cosmic phenomena, and the gold standard of distance measurement is parallax.

Measuring distances by parallax requires only geometry; there is no need to invoke physics at all. To measure the distance to a star using parallax, all that is needed are two or more observations of the star from different locations so that the difference in its position relative to the background stars is evident. That involves observations being made at different times of the year as the Earth moves through its orbit around the Sun.

It wasn’t until Friedrich Bessel in the 19^th century that the first stellar parallax measurement was made (to the star 61 Cygni), but the technique had been used as far back as 200 BC when Hipparchus’ observations of a solar eclipse from two different locations gave him a distance to the Moon in terms of Earth radii. The results they got were quite close to the modern day distances, and the technique of distance by parallax remains the most fundamental measurement of distance on the so-called cosmic distance ladder.

The problem is that this technique only works for relatively close objects. For stars, galaxies, and other cosmic phenomena that are further from us, different distance measuring techniques need to be used, each relying on the one before – these are the rungs on the ladder. And because of the reliance on lower rungs of the ladder, it is pretty important to start off with measurements that are as accurate as possible.

All the magnetars currently known either reside in the Milky Way galaxy, or in either the Large or Small Magellanic Clouds, our close neighbours. The Neutron Star Interior Composition Explorer (NICER), an instrument installed on the International Space Station, allows us to study their magnetic fields and probe their internal structures. But studies of magnetars are usually limited by the uncertainty in their distances.

Led by PhD student Hao Ding of the Swinburne University of Technology, 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 at over 3 kpc (kilo-parsecs, a commonly used unit of measurement amongst astronomers) or 10,000 light-years away, they found it to be substantially closer at around 2.5 kpc or 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 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 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) 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 theory goes something like this. About 70,000 years ago, the progenitor of XTE J1810-197 had a companion star. The two separated when XTE J1810-197 underwent a supernova explosion, transforming as it did so into a neutron star, and the companion continued to evolve on its own. Within the last 3,000 years, the companion died its own fiery death, and SNR G11.0-0.0 is all that remains.

The idea is intriguing, and while plausible, it is also possible that what astronomers are seeing is simply a chance alignment. But it does give researchers a better idea of where to search for another SNR candidate associated with XTE J1810-197.

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.