Gamma-Ray Bursts Size Up the Milky Way’s Spiral Arms

By: Govert Schilling July 20, 2026 0

Observations of remote cosmic explosions reveal that the outer spiral arms of the Milky Way might extend farther out than previously thought.

Based on measurements of X-ray light echoes created by three distant powerful explosions, a team of astronomers led by Beatrice Vaia (Scuola Universitaria Superiore IUSS of Pavia, Italy) concludes that the Outer and the Outer Scutum-Centaurus arms of our home galaxy may lie up to 10% farther away than earlier estimates.

artist's illustration of Milky Way — An artist’s concept showing the Milky Way Galaxy as seen from above, with the estimated positions of spiral arms based on previous data in blue. Overlaid on this is an updated view of the Milky Way showing different positions for the two outermost spiral arms, shown in red and bordered by dashed lines. Both arms may be more distant than previously thought, based on newly-processed X-ray data from Chandra and XMM.
*NASA / CXC / SAO / M. Weiss*

Determining the size and structure of the Milky Way is hard, since we live inside it. It’s like trying to draw up a map of a city when you’re stuck in a house in one of the suburbs.

The European Space Agency’s (ESA’s) Gaia mission has yielded precise distances for 2 billion stars in our galaxy, but only out to some 10,000 light-years. The galaxy’s disk stretches well beyond that horizon. So to map the Milky Way’s outer spiral arms, astronomers have had to rely on less direct methods.

Now, long gamma-ray bursts (GRBs) have come to the rescue. These gigantic blasts of gamma- and X-rays result from the dramatic collapse and subsequent explosion of very massive, rapidly rotating stars in distant galaxies.

Vaia and her colleagues studied three gamma-ray bursts: GRB 031203, GRB 160623A, and GRB 221009A. (The numbers refer to the dates they went off; GRB 221009A is the most luminous burst ever recorded.) These explosions happened to occur in places that put them close to the Milky Way’s central plane on the sky, called the galactic equator, so that we’re looking at them through the dust and gas of the disk. As a result, the high-energy radiation from the three blasts passed through one or more galactic spiral arms before arriving at Earth.

In the days after the explosions, NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton space telescope detected numerous expanding rings around each burst. Such light echoes are produced when radiation is scattered by intervening dust clouds in our galaxy.

Since the scattered X-rays follow a slightly longer path, they arrive a bit later than the instantaneous flash of the gamma-ray burst, just like an echo arrives after the initial sound because it took a longer path by bouncing off a surface before reaching the listener.

It turns out there’s a direct relation between this delay time, the size of a light echo on the sky, and the distance of the echo-producing dust.

“By measuring the radius of each ring in the available X-ray observations and using the elapsed time since the burst, we can identify the corresponding dust layer and determine its distance,” explains Vaia.

Composite data showing GRB light echoes — These images include X-ray data from Chandra (blue data shown in box outlined in white) and optical data from the Pan-STARRS telescope. The composite image shows X-ray rings generated by a gamma-ray burst (GRB), a bright X-ray source located outside our galaxy. In a phenomenon called light echoes, the X-rays from the GRB bounced off dust clouds in the spiral arms of our galaxy. The diameters of the rings in the Chandra data give the distances of the dust clouds to Earth, with larger rings being generated by dust clouds closer to us. The GRB is located at the center of the circles defining the rings, to the left of the X-ray data outlined by the white square.
*X-ray: NASA / CXC / INAF / B. Vaia et al.; Optical: Pan-STARRS; Image processing: NASA / CXC / SAO / N. Wolk & P. Edmonds*

Large dust complexes are located in our galaxy’s spiral arms, so this is a neat way to map the arms’ extent. In a paper published last month in Astronomy & Astrophysics, the team concludes that the Perseus Arm — the one just beyond our Local Arm — lies where we thought it did. But the Outer Arm beyond it and the Outer Scutum-Centaurus Arm beyond that may both be up to 10% more distant than earlier studies predicted.

In a press statement, team member Ilaria Fornasiero (University of Trento, Italy) says that “this could mean that astronomers have to revise estimates of the mass of the galaxy, because that affects how wide the arms stretch.”

With just three sightlines and no independent way of knowing where exactly the dust clouds are located in the spiral arms, the results are necessarily preliminary. Future observations of X-ray light echoes around new gamma-ray bursts that occur close to the galactic equator may further improve the distance estimates of the arms.