An international scientific group with outstanding Valencian participation
has managed to measure for the first time oscillations in the brightness of
a magnetar during its most violent moments. In just a 10th of a second, the
magnetar released energy equivalent to that produced by the sun in 100,000
years. The observation was carried out without human intervention, thanks to
an artificial intelligence system developed at the Image Processing
Laboratory (IPL) of the University of Valencia.
Among neutron stars, objects that can contain a half-million times the mass
of the Earth in a diameter of about 20 kilometers, are magnetars, a small
group with the most intense magnetic fields known. These objects, of which
only 30 are known, suffer violent eruptions that are still little known due
to their unexpected nature and their duration of barely 10ths of a second.
Detecting them is a challenge for science and technology.
Over the past 20 years, scientists have wondered if there are high frequency
oscillations in the magnetars. The team recently published their study of
the eruption of a magnetar in the journal Nature. They measured oscillations
in the brightness of the magnetar during its most violent moments. These
episodes are a crucial component in understanding giant magnetar eruptions.
The work was conducted by six researchers from the University of Valencia
and Spanish collaborators.
"Even in an inactive state, magnetars can be 100,000 times more luminous
than our sun, but in the case of the flash that we have
studied—GRB2001415—the energy that was released is equivalent to that which
our sun radiates in 100,000 years," says lead researcher Alberto J.
Castro-Tirado, from the IAA-CSIC.
"The explosion of the magnetar, which lasted approximately a 10th of a
second, was discovered on April 15, 2020 in the midst of the pandemic," says
VÃctor Reglero, professor of Astronomy and Astrophysics at the UV,
researcher at the Image Processing Laboratory (IPL), co-author of the
article and one of the architects of ASIM, the instrument aboard 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 and located in another galaxy. A true cosmic monster," says
Reglero.
Scientists think that eruptions in magnetars may be due to instabilities in
their magnetospheres or to a kind of "earthquake" produced in their crust, a
rigid and elastic layer about a kilometer thick. "Regardless of the trigger,
a type of wave is created in the star's magnetosphere—the Alfvén—which are
well known in the sun, and which interact with each other, dissipating
energy," explains Alberto J. Castro-Tirado.
According to the study, the oscillations detected in the eruption are
consistent with the emission produced by the interaction between Alfvén
waves, whose energy is rapidly absorbed by the crust. Thus, in a few
milliseconds, the magnetic reconnection process, and therefore also the
pulses detected in GRB2001415, end, disappearing 3.5 milliseconds after the
main burst. The analysis of the phenomenon has 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, which is on board the
International Space Station (ISS). ASIM was the only one of the seven
telescopes capable of registering the main phase of the eruption in its full
energy range without suffering saturations. The scientific team was able to
solve the temporal structure of the event, a truly complex task that
involved more than 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
VÃctor Reglero (University of Valencia, Spain), who form the ASIM Facility
Science Team.
ASIM's objective is to monitor violent phenomena in the Earth's atmosphere
from optical to gamma wavelengths at 40 MeV, an activity that the telescope
has been carrying out since June 2018. It has already detected 1,000
gamma-ray eruptions. "Given that these phenomena are unpredictable, ASIM
decides completely autonomously when something has happened and sends the
data to the different centers of the Science Data Centre in Copenhagen,
Bergen and Valencia," explains VÃctor Reglero.
The detection of quasi-periodic oscillations in GRB2001415 was quite a
challenge from the point of view of signal analysis. "The difficulty lies in
the brevity of the signal, whose amplitude rapidly decays and becomes
embedded in background noise. And, as it is correlated noise, it is
difficult to distinguish its signal," says Reglero. The artificial
intelligence system, together with sophisticated data analysis techniques,
allowed the researchers to detect this spectacular phenomenon.
Although these eruptions had already been detected in two of the 30 known
magnetars in the galaxy and in other, nearby galaxies, GRB2001415 is the
most distant magnetar eruption captured to date, located in the Sculptor
group of galaxies about 13 million light years away. "Seen in perspective,
it has been as if the magnetar wanted to indicate its existence to us from
its cosmic solitude, singing in the kHz with the force of a Pavarotti of a
billion suns," says Reglero.
According to the authors of the paper, the eruption provides a crucial
component for understanding how magnetic stresses are produced in and around
a neutron star. Continuous monitoring of magnetars in nearby galaxies will
help to understand this phenomenon, and will also pave the way to a better
understanding of fast radio bursts, currently among the most enigmatic
phenomena in astronomy.
Reference:
A. J. Castro-Tirado et al, Very-high-frequency oscillations in the main peak
of a magnetar giant flare, Nature (2021).
DOI: 10.1038/s41586-021-04101-1
Tags:
Space & Astrophysics