NASA Missions Unmask Magnetar Eruptions in Nearby Galaxies

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Universities Space Research Association

Jan 13, 2021, 12:15 ET

COLUMBIA, Md., Jan. 13, 2021 /PRNewswire/ -- On April 15, a brief burst of high-energy light swept through the solar system, triggering many space-based instruments, including those aboard NASA and European missions. Now, multiple international science teams conclude that the blast came from a supermagnetized stellar remnant known as a magnetar located in a neighboring galaxy.

This finding confirms long-held suspicions that some gamma-ray bursts (GRBs) – cosmic eruptions detected somewhere in the sky almost daily – are in fact powerful flares from magnetars relatively close to home. Papers analyzing different aspects of the event and its implications were published on Jan. 13 in the journals Nature and Nature Astronomy and presented at a meeting of the American Astronomical Society. University Space Research Association's Oliver Roberts analyzed the data from the Fermi Gamm-ray Burst Monitor (GBM) and Swift's Burst Alert Telescope (BAT).

This finding "has always been regarded as a possibility, and several GRBs observed since 2005 have provided tantalizing evidence," said Kevin Hurley, a Senior Space Fellow with the Space Sciences Laboratory at the University of California, Berkeley. "The April 15 event is a game changer because we found that the burst almost certainly lies within the disk of the nearby galaxy NGC 253."

GRBs, the most powerful explosions in the cosmos, can be detected across billions of light-years. At least some of those lasting less than about two seconds, called short GRBs, occur when a pair of orbiting neutron stars – both the crushed remnants of exploded stars – spiral into each other and merge.

Magnetars are neutron stars with the strongest-known magnetic fields, up to a thousand times the intensity of those of typical neutron stars and more than 10 trillion times the strength of a refrigerator magnet. Modest disturbances to the magnetic field can cause magnetars to erupt with sporadic X-ray bursts for weeks or longer.

Rarely, magnetars produce enormous eruptions called giant flares that produce gamma rays, the highest-energy form of light.

Most of the 29 magnetars now cataloged in our Milky Way galaxy exhibit occasional X-ray activity, but only two have produced giant flares. The most recent event, detected on Dec. 27, 2004, produced measurable changes in Earth's upper atmosphere despite erupting from a magnetar located about 28,000 light-years away.

Shortly before 4:42 a.m. EDT on April 15, a brief, powerful burst of X-rays and gamma rays swept past Mars, triggering the Russian High Energy Neutron Detector aboard NASA's Mars Odyssey spacecraft, which has been orbiting the planet since 2001. About 6.6 minutes later, the burst triggered the Russian Konus instrument aboard NASA's Wind satellite, which orbits a point between Earth and the Sun located about 930,000 miles (1.5 million kilometers) away. After another 4.5 seconds, the radiation passed Earth, triggering instruments on NASA's Fermi Gamma-ray Space Telescope, as well as on the European Space Agency's INTEGRAL satellite and Atmosphere-Space Interactions Monitor (ASIM) aboard the International Space Station.

The pulse of radiation lasted just 140 milliseconds – as fast as an eye blink or a finger snap.

The eruption occurred beyond the field of view of Swift's BAT, so its onboard computer did not alert astronomers on the ground. Thanks to a new capability, called the Gamma-ray Urgent Archiver for Novel Opportunities (GUANO), the Swift team can beam back BAT data when other satellites trigger on a burst. Analysis of this data provided additional insight into the event.

The Fermi, Swift, Wind, Mars Odyssey and INTEGRAL missions all participate in a GRB-locating system called the InterPlanetary Network (IPN). The IPN has operated since the late 1970s using different spacecraft located throughout the solar system. Because the signal reached each detector at different times, any pair of them can help narrow down a burst's location in the sky. The greater the distances between spacecraft, the better the technique's precision.

The IPN placed the April 15 burst, called GRB 200415A, squarely in the central region of NGC 253, a bright spiral galaxy located about 11.4 million light-years away in the constellation Sculptor. This is the most precise sky position yet determined for a magnetar located beyond the Large Magellanic Cloud, a satellite of our galaxy and host to the first giant flare ever detected, in 1979.

A detailed analysis of data from Fermi's Gamma-ray Burst Monitor (GBM) and Swift's BAT provides strong evidence that the April 15 event was unlike any burst associated with mergers, noted Oliver Roberts, an associate scientist at Universities Space Research Association's Science and Technology Institute in Huntsville, Alabama, who led the study.

In particular, this was the first giant flare known to occur since Fermi's 2008 launch, and the GBM's ability to resolve changes at microsecond timescales proved critical. The observations reveal multiple pulses, with the first one appearing in just 77 microseconds – about 13 times the speed of a camera flash and nearly 100 times faster than the rise of the fastest GRBs produced by mergers. The GBM also detected rapid variations in energy over the course of the flare that have never been observed before.

"Giant flares within our galaxy are so brilliant that they overwhelm our instruments, leaving them to hang onto their secrets," Roberts said. "For the first time, GRB 200415A and distant flares like it allow our instruments to capture every feature and explore these powerful eruptions in unparalleled depth."

Giant flares are poorly understood, but astronomers think they result from a sudden rearrangement of the magnetic field. One possibility is that the field high above the surface may become too twisted, suddenly releasing energy as it settles into a more stable configuration. Alternatively, a mechanical failure of the magnetar's crust – a starquake – may trigger the sudden reconfiguration.

Roberts and his colleagues say the data show some evidence of seismic vibrations during the eruption. The highest-energy X-rays recorded by Fermi's GBM reached 3 million electron volts (MeV), or about a million times the energy of blue light, itself a record for giant flares.

The researchers say this emission arose from a cloud of ejected electrons and positrons moving at about 99% the speed of light. The short duration of the emission and its changing brightness and energy reflect the magnetar's rotation, ramping up and down like the headlights of a car making a turn. Roberts describes it as starting off as an opaque blob – he pictures it as resembling a photon torpedo from the "Star Trek" franchise – that expands and diffuses as it travels.

The torpedo also factors into one of the event's biggest surprises. The satellite's main instrument, the Large Area Telescope (LAT), also detected three gamma rays, with energies of 480 MeV, 1.3 billion electron volts (GeV), and 1.7 GeV – the highest-energy light ever detected from a magnetar giant flare. What's surprising is that all of these gamma rays appeared long after the flare had diminished in other instruments.

Nicola Omodei, a senior research scientist at Stanford University in California, led the LAT team investigating these gamma rays, which arrived between 19 seconds and 4.7 minutes after the main event. The scientists conclude that this signal most likely comes from the magnetar flare. "For the LAT to detect a random short GRB in the same region of the sky and at nearly the same time as the flare, we would have to wait, on average, at least 6 million years," he explained.

And Eric Burns, an assistant professor of physics and astronomy at Louisiana State University in Baton Rouge, led a paper studying additional suspects that will appear in The Astrophysical Journal Letters.

Related Resources:

Fermi LAT paper, "High-Energy Emission from a Magnetar Giant Flare in the Sculptor Galaxy." The direct link is here: https://www.nature.com/s41550-020-01287-8.
NASA.gov
https://www.youtube.com/watch?v=yXYvhYXBeP0

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