Gamma-ray burst in M82 may actually have been an 'extragalactic magnetar giant flare'

A split-second flash from the nearby starburst galaxy M82 has sparked a careful rethinking of what we call a short gamma-ray burst.

The signal was so brief and intense that it first looked like a compact-object merger far across the universe, yet the evidence points to a different engine close to home.

In a new study, researchers analyze GRB 231115A and show why it matches the profile of an extragalactic magnetar giant flare.

The analysis was led by Aaron C. Trigg of the Department of Physics & Astronomy at Louisiana State University (LSU).

Magnetar flare or GRB in M82?

On November 15, 2023, detectors caught a hard, fast flash from the direction of galaxy M82, about 11.4 million light years away. The timescale was tens of milliseconds, shorter than a blink.

Astronomers quickly pinpointed the origin of the flash to the nearby starburst galaxy M82 using data from several space-based gamma-ray observatories and follow-up analyses.

The position lined up within a tight margin of error, confirming the burst came from that galaxy rather than from deep space.

That finding shifted the focus from estimating its distance to understanding the nature of the explosion itself.

If it truly came from M82, it was far closer and more energetic than any ordinary stellar flare, yet too short to match a supernova or a merger event.

The team reports rapid evolution in the spectrum during the spike, a hallmark of nearby magnetar outbursts. As energy rose and fell, the emission hardened and softened in step.

How a magnetar flares

A magnetar is an ultra-magnetic neutron star whose crust can crack and release a trapped reservoir of energy.

When that happens, the magnetar can throw off a pulse of high-energy light that peaks in gamma rays.

Past events show a sharp flare followed by a softer tail that can last minutes and pulse with the star’s spin. At the distance of M82, that tail is too faint for most instruments.

The physics favors a narrow, relativistic outflow that sweeps across our line of sight. As the beam passes, intensity and spectral hardness climb together, then fall together.

What the detectors saw

The Fermi Gamma-ray Burst Monitor is an orbiting instrument  with 12 sodium iodide and 2 bismuth germinate detectors spanning from hard X-rays to tens of MeV. That coverage captured the fast, multi-peaked structure.

The team’s time-resolved fits found a high peak energy and a clean correlation between brightness and spectral hardness.

The minimum variability timescale sat near 1 millisecond, which points to a compact, rapidly changing emission region.

The authors describe the burst as short and spectrally hard, with clear evolution across just a few tens of milliseconds. That pattern is difficult to square with a cosmological merger but natural for a nearby magnetar.

Clues pointing to a magnetar flare

Short, structured spikes set tight limits on the size and speed of the emitting region. When a source brightens and hardens over mere milliseconds, the flow must be relativistic.

In this case, the highest-energy photons and the fast rise imply a modest lower bound on the Lorentz factor, the measure of how close the flow is to light speed.

The bound is consistent with expectations for magnetar outflows rather than the much larger values often inferred for distant, merger-driven bursts.

Ruling out a distant merger

Merger afterglows tend to appear across the spectrum if the event is nearby and bright. Follow-up X-ray observations after this burst did not turn up a new source within the error region down to deep limits.

A past event showed the same kind of fast, high-energy pattern. In 2020, scientists detected a burst called GRB 200415A that came from the nearby Sculptor galaxy, NGC 253.

Careful follow-up revealed it behaved more like an intense flare from a magnetar than a distant gamma-ray burst caused by colliding stars.

Gravitational-wave detectors were operating around the event time, yet no candidate signal was found in the relevant window.

Radio searches also set strict limits, reducing the odds that an accompanying fast radio burst (FRB) occurred at the same moment.

Why this burst is different

Short gamma-ray bursts from mergers arrive from far-flung galaxies and often show afterglows days to weeks later. Here, the position in M82 and the millisecond-scale evolution argue for a local, single-star origin.

The spectrum prefers a cut-off power-law form that evolves across the pulse. That evolution tracks the beam as it swings across our view, a behavior seen in other extragalactic magnetar candidates.

“This burst exhibits distinctive temporal and spectral characteristics, including a short duration and a high peak energy, consistent with known MGFs,” wrote Aaron C. Trigg.

The authors emphasize that the source behaves like the initial spike of a magnetar giant flare in another galaxy.

Why the tail matters

The smoking gun for a magnetar giant flare is a soft, pulsating tail that echoes the spin of the star. Catching it at the distance of M82 requires very fast repointing and low instrumental background.

Computer models suggest that telescopes such as Swift or NICER could spot the faint afterglow of a magnetar flare from M82 if they manage to turn toward it within a few minutes.

Catching it that quickly is difficult, but automated systems are getting fast enough to make it possible.

A robust tail detection would not just clinch the magnetar origin. It would also pin the exact location inside M82, letting astronomers study the star’s neighborhood with arcsecond precision.

Identifying magnetar giant flares

This event narrows the gap between short GRBs and magnetar flares by showing how often the former can masquerade as the latter. A small fraction of the short-GRB catalog likely hides similar nearby outbursts.

“The detection and analysis of GRB 231115A provide compelling evidence of an extragalactic MGF originating from the starburst galaxy M82,” wrote Trigg.

Better coordination across gamma-ray, X-ray, radio, and gravitational-wave facilities will raise the odds of catching both spike and tail next time.

The study is published in Astronomy & Astrophysics.

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