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\t\t\t\t\t\t01\/07\/2026<\/span> The European Space Agency\u2019s XMM-Newton and NASA\u2019s 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.<\/p>\n<\/div>\n Perhaps surprisingly, we don\u2019t know much about the structure of our galaxy\u2019s outer regions. It\u2019s difficult to observe our galaxy from the inside; the Solar System is well embedded in its disc, preventing a bird\u2019s eye view, and many regions are obscured by thick clouds of cosmic dust.<\/p>\n But this is changing: we\u2019ve learnt a huge amount since the launch of ESA\u2019s 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\u2019t even sure if our galaxy had two or four spiral arms (we now know the answer to be four).<\/p>\n Now, another of ESA’s missions has found a new way to map the extremities of our galaxy. \u201cWe 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.<\/p>\n \u201cInstead, 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\u2019s outer arms \u2013 and we measured the distances to these echoes directly.\u201d<\/p>\n 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\u2019s spiral arms, forming bright rings that were then picked up by XMM-Newton and Chandra.<\/p>\n 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\u2019s arms \u2013 Outer Scutum-Centaurus Arm and Outer Arm \u2013 lie up to 10% further away than we thought.<\/p>\n<\/p><\/div>\n While ESA\u2019s Gaia has revolutionised our understanding of the Milky Way galaxy, the distance measurements available so far from the telescope are less precise for the outer arms. Using X-rays to probe the distances to dust clouds, as XMM-Newton and Chandra did here, is highly accurate out to longer distances, allowing the research team to revise the map of the outer Milky Way galaxy.<\/p>\n \u201cThis finding is a great example of how ESA\u2019s longer-standing missions \u2013 such as XMM-Newton, which launched in 1999 \u2013 still have a hugely important role to play in exploring the Universe,\u201d says Erik Kuulkers, ESA XMM-Newton project scientist.<\/p>\n \u201cNow in its third decade, XMM-Newton continues to return a steady stream of groundbreaking science on everything from the brightest-ever GRB, to stars being shredded by black holes, to X-ray snapshots of Mars. It\u2019s even more exciting when missions team up, as they did here. Together, they can reveal huge amounts about the skies around us.\u201d<\/p>\n What we know of our home galaxy will continue to grow in coming years. Alongside ever more detailed data from Gaia\u2019s fourth and fifth data releases (planned for December 2026 and after the end of 2030, respectively), ESA\u2019s next generation X-ray observatory NewAthena is poised to transform X-ray astronomy, and enable scientists to explore far fainter X-ray echoes in the outskirts of our galaxy.<\/p>\n<\/p><\/div>\n The researchers combined observations of GRBs 221009A (spotted in 2022), 160623A (2016), and 031203 (2003); the bright, dust-scattered, ring-shaped echoes of each were observed by XMM-Newton, Chandra, or both between December 2003 and November 2022.<\/p>\n \u2018Accurate distances of the Galactic spiral arms from dust-scattered X-ray emission of gamma-ray bursts\u2019 by B. Vaia et al. was published on 29 June in\u00a0Astronomy\u00a0<\/i>& Astrophysics<\/i>.\u00a0<\/p>\n \nFor more information, please contact:<\/b>
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