Researchers capture high-frequency oscillations in massive neutron star eruption

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Dec. 27 2021

(Nanowerk News) An international scientific group has succeeded in measuring for the first time the oscillations of the brightness of a neutron star –magnetar– during its most violent moments. In just a tenth of a second, the magnetar released energy equivalent to that produced by the Sun in 100,000 years.

The observation was carried out automatically, without human intervention, thanks to the Artificial Intelligence of a system developed at the Image Processing Laboratory (IPL) of the University of Valencia.

Among neutron stars, objects that can contain half a million times the mass of the Earth in about twenty kilometers in diameter, stands out a small group with the most intense magnetic field known: the magnetars. These objects, of which only about thirty are known, undergo violent eruptions which are still little known because of their unexpected nature and their duration of barely a few tenths of a second. Detecting them is a challenge for science and technology.

Artistic representation of a magnetar.

An international scientific team recently published in the journal Nature (“Very high frequency oscillations in the main peak of a giant magnetar eruption”) the study of the eruption of a magnetar in detail: they succeeded in measuring oscillations – impulses – of the brightness of the magnetar during its most violent moments. These episodes are a crucial part of understanding giant magnetar eruptions. It is a question long debated over the past 20 years that has an answer today, whether there are high frequency oscillations in magnetars.

The work has the contribution of six researchers from the University of Valencia and a strong Spanish participation – 15 scientists out of a total of 41. “Even in the inactive state, magnetars can be a hundred thousand times brighter than our Sun, but in the case of the flash that we studied – the GRB2001415 – the energy which was released is equivalent to that which our Sun radiates in a hundred thousand years ”, underlines the principal researcher Alberto J. Castro-Tirado, of the IAA -CSIC.

“The explosion of the magnetar, which lasted about a tenth of a second, was discovered on April 15, 2020 in the midst of a pandemic”, explains Víctor Reglero, professor of astronomy and astrophysics at UV, researcher at the Treatment Laboratory of images. (IPL), co-author of the article and one of the architects of ASIM, the instrument on board the International Space Station that detected the eruption. “Since then, we have developed very intense data analysis work, since it was a 10 ** 16 Gauss neutron star located in another galaxy. A real cosmic monster! », Remarks Reglero.

The scientific community believes that eruptions in magnetars may be due to instabilities in their magnetosphere or some sort of “earthquake” produced in their crust, a rigid, elastic layer about a kilometer thick. “Whatever the trigger, it is created in the magnetosphere of the star – the Alfvén – a type of waves which are well known in the Sun and which interact with each other by dissipating energy”, explains Alberto J. Castro-Tirado.

According to the study published today in Nature, the oscillations detected in the eruption are consistent with the emission produced by the interaction between the Alfvén waves, the energy of which is rapidly absorbed by the crust. Thus, in a few milliseconds the magnetic reconnection process ends and therefore also the pulses detected in GRB2001415, which disappeared 3.5 milliseconds after the main burst. The analysis of the phenomenon made it possible to estimate that the volume of the eruption was similar or even greater than that of the neutron star itself.

Algorithms capture it without human intervention

The eruption was detected by the ASIM instrument, which is on board the International Space Station (ISS). The ASIM, in which the University of Valencia participates, was the only one of the seven telescopes capable of recording the main phase of the eruption in its entire energy range without undergoing saturation. The science team was able to resolve the temporal structure of the event, a truly complex task that involved over a year of analysis for just two seconds during which the data was collected.

The Atmosphere Space Interactions Monitor (ASIM) is an ESA mission developed by Denmark, Norway and Spain, which has been operational in the ISS since 2018 under the supervision of researchers Torsten Neubert (Technical University of Denmark) , Nikolai Ostgaard (University of Bergen, Norway) and Víctor Reglero (University of Valencia, Spain), who form the scientific team of the ASIM Facility.

The objective of ASIM is to monitor the violent phenomena in the Earth’s atmosphere from optical rays to gamma rays at 40 MeV, an activity that the telescope has been carrying out since June 2018, having already detected 1000 gamma-ray flares. “Since these phenomena are unpredictable, ASIM decides completely autonomously when something has happened and sends the data to the different Science Data Center in Copenhagen, Bergen and Valencia,” explains Víctor Reglero.

The detection of quasi-periodic oscillations in GRB2001415 has been quite a challenge from a signal analysis point of view. “The difficulty lies in the brevity of the signal, the amplitude of which decreases rapidly and becomes embedded in the background noise. And, since it’s correlated noise, it’s hard to distinguish its signal, ”Reglero explains. The intelligence of the system that we have developed at the University of Valencia is what made it possible, with sophisticated data analysis techniques, to detect this spectacular phenomenon.

Although these eruptions have already been detected in two of the thirty known magnetars of our galaxy and in some other neighboring galaxies, GRB2001415 would be the most distant magnetar eruption captured to date, being in the Sculptor group of galaxies of about thirteen. millions of light years. “Seen in perspective, it was as if the magnetar wanted to indicate to us its existence from its cosmic solitude, singing in kHz with the force of a Pavarotti of a billion suns,” says Reglero.

According to the authors of the article now published in Nature, this eruption provided a crucial element in understanding how magnetic stresses are produced in and around a neutron star. Continuous monitoring of magnetars in nearby galaxies will help understand this phenomenon, and will also pave the way for a better understanding of rapid radio bursts, currently one of the most enigmatic phenomena in astronomy.

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