XMM-Newton helps revise distance to outer spiral arms

Written by Copernical Team Wednesday, 01 July 2026 11:00

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The European Space Agency’s XMM-Newton and NASA’s Chandra X-ray space telescopes have spotted the aftermath of three bright explosions echoing through the outer spiral arms of our galaxy, the Milky Way. By measuring the distance to these echoes, they find the outer arms to be up to 10% further away than we thought.

Perhaps surprisingly, we don’t know much about the structure of our galaxy’s outer regions. It’s difficult to observe our galaxy from the inside; the Solar System is well embedded in its disc, preventing a bird’s eye view, and many regions are obscured by thick clouds of cosmic dust.

But this is changing: we’ve learnt a huge amount since the launch of ESA’s star-surveying Gaia space telescope. Using data collected by Gaia, scientists are currently mapping the Milky Way galaxy in more detail than ever before by measuring precise distances to its stars. Before Gaia, we weren’t even sure if our galaxy had two or four spiral arms (we now know the answer to be four).

Now, another of ESA's missions has found a new way to map the extremities of our galaxy. “We usually model the Milky Way's outer arms indirectly based on what we know of how our galaxy rotates, but doing it this way leaves room for error," says Beatrice Vaia of Istituto Nazionale di Astrofisica (INAF), Italy, who led the research as part of her PhD.

“Instead, we did something new: we looked at the aftermath of three cosmic explosions that took place in far more distant galaxies. These explosions flung out X-rays that echoed through several of the Milky Way’s outer arms – and we measured the distances to these echoes directly.”

X-ray light was thrown out by three bright explosions known as gamma-ray bursts (GRBs). The X-rays bounced around and were scattered by dust grains within the Milky Way galaxy’s spiral arms, forming bright rings that were then picked up by XMM-Newton and Chandra.

By studying how these ring-shaped echoes slowly expanded over time, Beatrice and colleagues were able to pinpoint the distance of the scattering dust grains. As these lie in clouds within the arms of our galaxy, the team could directly measure the distance of the arms. Besides confirming the known distance to the Perseus arm, the scientists found that two of the Milky Way galaxy’s arms – Outer Scutum-Centaurus Arm and Outer Arm – lie up to 10% further away than we thought.