Scientists have found the first clues to understand the violent short-lived eruptions of a rare-class compact star called the magnetar located thirteen million light-years away. These compact stars with the most intense magnetic field known, of which only thirty have been spotted so far in our galaxy, undergo violent eruptions still little known because of their unexpected nature and their short duration. Scientists have long been puzzled by such short and intense bursts – transient x-ray pulses with energies many times greater than those of the Sun and lasting from a fraction of a few milliseconds to a few microseconds.
When massive stars like supergiant stars with a total mass of between 10 and 25 solar masses collapse, they can form neutron stars. Among neutron stars, there is a small group with the most intense magnetic field known: magnetars. These objects, of which only thirty are known to date, undergo violent eruptions that are still little known because of their unexpected nature and their short duration, barely a few tenths of a second.
A scientific group led by Professor Alberto J. Castro-Tirado of the Andalusian Institute of Astrophysics (IAA-CSIC) studied an eruption in detail: successfully measuring different oscillations, or impulses during moments of higher energy, which are a crucial element in understanding giant magnetar eruptions. Dr Shashi Bhushan Pandey of the Aryabhatta Research Institute of Observational Sciences (ARIES), an institute of the Department of Science and Technology, worked closely with Professor Alberto Castro Tirado and other members of the group in this research which has was published in the journal Nature. It is the first extragalactic magnetar studied in detail.
Even in the inactive state, magnetars can be several thousand times brighter than our Sun. But in the case of the flash we studied, GRB2001415, which occurred on April 15, 2020 and only lasted ‘About a tenth of a second, the energy that has been released is equivalent to the energy our Sun radiates in one hundred thousand (100,000) years. Observations revealed multiple pulses, with a first pulse appearing only a few times. tens of microseconds, much faster than other extreme astrophysical transients, ”said Alberto J. Castro-Tirado, IAA-CSIC and lead author.
It is believed that eruptions in magnetars may be due to instabilities in their magnetosphere or some kind of “earthquake” (“starquakes”) produced in their crust, a rigid and elastic layer of about a kilometer. thick. “Whatever the trigger, in the magnetosphere of the star a type of waves will be created. These well-known waves in the Sun are called Alfvén waves and by bouncing between points at the base of its magnetic field lines, they interact with each other by dissipating energy, ”says Castro-Tirado.
The oscillations detected during 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. So within a few milliseconds the magnetic reconnection process was completed and hence also the pulses detected in GRB200415, 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.
The eruption was detected by the ASIM instrument (Atmosphere-Space Interactions Monitor), on board the International Space Station. The scientific team was able to resolve the temporal structure of the event, analyzing the tiny scale of the data for more than a year. “Although several papers have been published on the event, because ASIM was the only mission that detected the main burst phase across the full photon energy range without saturation, this places the ASIM instrument in a unique position for unveil some of the secrets surrounding magnetars, ”said Nikolai Østgaard of the University of Bergen in Norway, the second author.
“The detection of oscillations in GRB 200415 has been a challenge due to the shortness of the signal, the amplitude of which decreases rapidly and is embedded in the background noise. We therefore owe this achievement to sophisticated data analysis techniques that have been applied independently But it is also undoubtedly a technological achievement due to the excellent quality of the data provided by the ASIM instrument on board the International Space Station ”, underlines Javier Pascual, researcher of the IAA- CSIC who participated in the work.
“Understanding these oscillations can shed light on the structure of these mysterious objects,” says Michael Gabler (University of Valencia, Spain).
These eruptions had been detected in two of the thirty known magnetars of our galaxy, the Milky Way, but also in two others located in other galaxies. GRB2001415 is believed to be the most distant magnetar eruption captured to date, found in the Sculptor group of galaxies (NGC 253) some thirteen million light-years away.
“Detections of giant eruptions from magnetars are extremely rare. This eruption provided a crucial element in understanding how magnetic stresses are produced in and around a neutron star,” concludes Castro-Tirado. “Continuous monitoring of magnetars in nearby galaxies will help understand this phenomenon, and will also pave the way for learning more about rapid radio bursts, one of the most enigmatic phenomena in astronomy today,” said Dr Shashi Bhushan Pandey, one of the co-authors of this article.
(With inputs to GDP)