The reawakening of a magnetar, a city-sized star named XTE J1810-197, is incredibly violent.
After a decade of silence, one of the most powerful magnets in the universe suddenly came back to life in late 2018. The reawakening of this “magnetar”, a city-sized star named XTE J1810-197, born from a supernova explosion, is a major event. Incredible violence.
The breaking and unraveling of entangled magnetic fields releases large amounts of energy, such as gamma rays, X-rays and radio waves.
By capturing such magnetar outbursts, astronomers are beginning to understand what causes their erratic behavior. We also discovered a potential link to mysterious radio light seen from distant galaxies, called fast radio bursts.
In two new studies published in Nature Astronomy, we used three of the world’s largest radio telescopes to capture a series of never-before-seen changes in the radio waves emitted by these rare objects in unprecedented detail.
Magnet monster
Magnetars are young neutron stars with magnetic fields billions of times stronger than our most powerful Earth magnet. The slow decay of their magnetic fields creates huge stresses in their hard shells until they eventually break. This distorts the magnetic field, releasing large amounts of high-energy X-rays and gamma rays as it unfolds.
These bizarre stars were first discovered in 1979, when a spacecraft traveling through the solar system captured an intense gamma-ray burst from one of the stars. Since then, 30 more magnetars have been discovered, the vast majority of which have been detected only as X-ray and gamma-ray sources. However, it was later discovered that a very small number also emitted flashes of radio waves.
The first “radio loud” magnetar was named XTE J1810-197. Astronomers originally discovered it as a bright X-ray source following a 2003 outburst, then discovered that it spins every 5.54 seconds, emitting bright pulses of radio waves.
Unfortunately, the intensity of the radio pulses dropped rapidly, and within two years it completely disappeared from view. XTE J1810-197 has been in this state of radio silence for over a decade.
A shaky start
On December 11, 2018, astronomers using the 76-meter Lovell Telescope at the University of Manchester’s Jodrell Bank Observatory noticed XTE J1810-197 emitting another bright radio pulse. This was quickly confirmed by the 100-meter Eiffelsberg radio telescope of Germany’s Max Planck Institute and the 64-meter Parkes radio telescope Murriyang of Australia’s CSIRO.
Following the confirmation, all three telescopes began intense activity to track how the magnetar’s radio emission evolved over time.
Reactivated radio pulses from XTE J1810-197 were found to have a highly linear polarization, appearing to oscillate up and down, side to side, or some combination of the two. Careful measurements of the polarization direction allow us to determine the direction of the magnetar’s magnetic field and rotation relative to Earth.
Our diligent tracking of the polarization direction revealed something remarkable: The direction of the star’s rotation was slowly oscillating. By comparing the measured wobbles with simulations, we were able to determine that the magnetar’s surface became slightly bumpy as a result of the outburst.
The lumps were small in size, just a millimeter or so short of a perfect sphere, and gradually disappeared over the three months after XTE J1810-917’s awakening.
distorted light
Typically, magnetars emit only very small amounts of circularly polarized radio waves, which propagate in a spiral pattern. Unusually, we detected a large amount of circular polarization in XTE J1810-197 during the 2018 outburst.
Our observations of Murriyang indicate that normally linearly polarized radio waves are being converted into circularly polarized waves.
This “linear-to-circular conversion” has long been predicted to occur when radio waves pass through a soup of superheated particles in the magnetic field of a neutron star.
However, theoretical predictions about how the effect changes with observation frequency do not match our observations, although we are not too surprised. The environment around an exploding magnetar is a complex place and can have many effects that cannot be explained by relatively simple theories.
piece them together
The discovery of the slight wobble and circular polarization in the XTE J1810-197 radio emission represents an exciting leap forward in our study of radio-strong magnetar bursts. It also paints a more comprehensive picture of the 2018 outbreak.
We now know that ruptures in a magnetar’s surface cause it to twist and wobble for short periods of time, while the magnetic field is filled with super-hot particles whizzing at nearly the speed of light.
Combined with other observations, the amount of wobble can be used to test our theories about how matter should behave at densities much higher than we hope to replicate in laboratories on Earth. On the other hand, the linear-to-circular conversion is inconsistent with theory, prompting us to devise more complex ideas about how radio waves escape their magnetic fields.
What’s next?
While XTE J1810-197 remains active to this day, it has since settled into a more relaxed state, with no sign of further wobbles or linear-to-circular transitions. However, there are indications that both phenomena may have appeared in past observations of other radio-loud magnetars and may be common features of their outbursts.
Just like cats, it’s impossible to predict what a magnetar will do next. But with current and future upgrades to telescopes in Australia, Germany and North America, we are now more ready than ever to pounce the next time we decide to wake up.
(author:Marcus Lower, Postdoctoral Fellow, CSIRO; Gregory Desvignes, Postdoctoral Fellow, Fundamental Physics of Radio Astronomy, Max Planck Institute for Radio Astronomy; Patrick Weltevrede, Lecturer in Pulsar Astrophysics, University of Manchester)
(Disclosure Statement:Gregory Desvignes received funding from the European Research Council (ERC) Synergy Grant “BlackHoleCam” Grant Agreement No. 610058. Patrick Weltevrede received funding from the Science and Technology Facilities Council (STFC). Marcus Lower does not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant relationships beyond his academic appointment)
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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