Brightest Gamma-ray Burst Shines Light on Milky Way Structure

The brightest gamma-ray burst ever seen in 2022 still puzzles astronomers.

The more researchers look at a recent record-setting event, the stranger it gets.

The story begins on the evening of October 9th, 2022, when NASA’s Neil Gehrels Swift orbiting observatory detected a strong X-ray outburst. The source was in the direction of the constellation of Sagitta the Arrow along the galactic plane, suggesting a source in our own Milky Way galaxy. Follow-up observations from NASA’s Fermi Gamma-Ray Space Telescope and the Earth-based European Southern Observatory’s Very Large Telescope however, soon revealed that the source was much more distant, emanating from a gamma-ray burst lying beyond our galaxy. This outburst only appeared to have happened along our line of sight as seen through the plane own galaxy from our Earthbound perspective.

Astronomers soon realized they had an unprecedented opportunity to witness a rare event, and exploit it for science. Dubbed GRB 221009A, the outburst added a full gigawatt of power—equivalent to the output of a terrestrial power station—to the Earth’s upper ionosphere. Other spacecraft across the solar system to include the joint NASA/ESA Solar Heliospheric Observatory (SOHO) mission, Gaia, ESA’s Solar Orbiter, BepiColombo and ESA’s XXM-Newton were all witnesses to the event across the spectrum.

In fact, GRB 221009A was briefly bright enough for amateurs to catch it, though there’s no evidence that any lucky imagers had indeed spotted the outburst. The afterglow actually lingered in the x-ray and gamma-ray spectrum for some time.

GRB location
The location of GRB 221009A in the sky. Stellarium

Follow-up on GRB 221009A

Now, astronomers have released further observations of the event, and recently revealed findings from observations of the event on March 28, 2023 at the 20th Meeting of the High Energy Astrophysics Division of the American Astronomical Society.

“This has been a very eye-opening event,” says Alicia Rouco Escorial (ESA- Research Fellow) in a recent press release. “We have been very lucky to witness it.”

Redshift ultimately nailed the GRB’s tremendous distance at z=0.151, pegging the burst at a current distance of 2.4 billion light years away, having occurred 1.9 billion years ago. For context, this was at the tail-end of the Great Oxidation Event on Earth in the Proterozoic Eon. The discrepancy between dating such extreme cosmological distances occurs because the Universe itself is expanding during the intervening time.

A Bizarre Gamma-ray Burst

But what was it? Gamma-ray bursts are highly energetic events. Gamma-ray bursts were first detected when the U.S. launched the Vela satellite series starting in 1963, meant to detect terrestrial bursts from nuclear testing. The best working theory is that GRBs are emitted from supernovae or neutron star mergers. An event as energetic as GRB 221009A is thought to only happen once every few thousand years, leading astronomers to refer to it as a BOAT—Brightest of All Time.

Fermi GRB
NASA’s Fermi Gamma-ray Space Telescope sees GRB 221009A. NASA

The burst actually illuminated 20 dust clouds in our own galaxy, giving astronomers a one-time chance to estimate the distance and dust grain composition of these dust clouds along our line of sight. The burst energy from GRB 221009A traveled at the speed of light for nearly two billion years before encountering the first dust cloud in our Milky Way Galaxy ‘only’ 60,000 years ago, and interacting with the last one about a 1,000 years ago. These distance interaction can be seen in the perspective of the concentric rings in XMM-Newton observations.

GRB 221009A: looking back through time. ESA

But things get stranger still. Hubble and the James Webb Space Telescope have scoured the region of the outburst and thus far… turned up nothing beyond the afterglow. If GRB 221009A was a supernova explosion, the resulting black hole quickly devoured any remaining debris.

HST view
Hubble’s view of the afterglow (circled) from GRB 221008A. NASA/HST/STScI

“That’s weird,” says astronomer Andrew Levan (Radbound University-Netherlands) in a recent press release. “…and it’s still not totally obvious what it means.”

Though the mystery remains, events such a GRB 221009A will continue to shed light on the cutting edge of modern astrophysics.