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\t\t\t\t\t\t24\/03\/2026<\/span> An invisible companion consuming material from the naked-eye star gamma-Cas has been revealed as the culprit for curious X-rays coming from the stellar system. This closes the case on a mystery that has puzzled astronomers for more than fifty years.\u00a0<\/p>\n<\/div>\n The star gamma-Cas (\u03b3-Cas) is visible to Europeans every cloudless night. It makes up the central \u2018point\u2019 of the distinctive \u2018W\u2019-shaped constellation Cassiopeia.\u00a0<\/p>\n Despite its prominence in the night sky, it has been shrouded in mystery since 1866 when Italian astronomer Angelo Secchi noticed something odd in its light signature. Its hydrogen \u2018fingerprint\u2019 was bright, whereas in stars like our own Sun this normally shows up as a dark line.<\/p>\n This weird feature inaugurated a new class of stars, called \u2019Be\u2019 stars, merging the \u2018B\u2019 associated with hot blue-white massive stars with the \u2018e\u2019 from the peculiar hydrogen emission.<\/p>\n It took several decades before astronomers understood that these emissions were coming from a rotating disc of matter ejected by the fast-spinning star. Such discs can build and disperse over time, resulting in variations in the star\u2019s brightness. This makes it a popular target for amateur astronomers still today.<\/p>\n As telescope observations became more refined, monitoring gamma-Cas\u2019s motion was possible, revealing that it must have a low-mass companion star. Since the companion remains invisible to spot directly with telescopes, astronomers think it might be a white dwarf \u2013 a compact object with the mass of the Sun but the size of Earth.<\/p>\n Then, in the mid-1970s, a new mystery emerged: gamma-Cas was discovered to shine in unusual high-energy X-rays. Further studies found the origin of this X-ray glow to be mostly coming from extremely hot 150-million-degree plasma, shining with a brightness some 40 times greater than typically expected for such massive stars.<\/p>\n With the dawn of X-ray space telescopes including ESA\u2019s XMM-Newton, NASA\u2019s Chandra and the Germany-led eROSITA, astronomers have found around two dozen gamma-Cas-type stars with similar, unusual X-ray emission, making them a special group among Be stars in general.<\/p>\n<\/p><\/div>\n Over the years, the explanation for the high-energy X-rays boiled down to two competing theories. Could the star\u2019s local magnetic fields be interacting with that of its surrounding disc, producing the hot material? Or, are X-rays generated by the Be star\u2019s disc material falling onto the white dwarf companion?<\/p>\n Finally, an instrument exists with high enough precision to solve the mystery: XRISM\u2019s high-resolution spectrometer Resolve. In a dedicated observation campaign XRISM revealed that the signatures of the hot plasma follow the orbital motion of the otherwise invisible companion star. In other words, the white dwarf companion consumes material from gamma-Cas, emitting X-rays as it does so.<\/p>\n \u201cThe previous work using XMM-Newton really cleared the way for XRISM, enabling us to eliminate numerous theories and prove which of the last two competing theories was correct,\u201d says Ya\u00ebl. \u201cIt\u2019s extremely satisfying to have direct evidence to solve this mystery at long last!\u201d<\/p>\n Understanding that gamma-Cas objects are Be type stars paired with a white dwarf that\u2019s accreting material, solves the X-ray mystery. But it also opens up another curiosity in terms of how the wider population of this type of binary systems forms and evolves.<\/p>\n Such pairs were long expected to be common, mainly among low\u2011mass stars. However, new research shows they are rarer than predicted and instead tend to occur in high\u2011mass Be stars.\u00a0<\/p>\n \u201cWe think the key is in understanding how exactly the interactions take place between the two stars,\u201d says Ya\u00ebl.\u00a0\u201cNow that we know the true nature of gamma-Cas, we can create models specifically for this class of stellar systems, and update our understanding of binary evolution accordingly.\u201d<\/p>\n \u201cIt\u2019s incredible to see how this mystery has slowly unfolded over the years,\u201d says Alice Borghese, an ESA Research Fellow specialising in the field of high-energy astrophysics. \u201cXMM-Newton did so much of the groundwork in ruling out various theories about gamma-Cas. And now with the next generation of advanced instrumentation, XRISM has brought us over the finish line.\u201d<\/p>\n \u201cThis wonderful result underlines the strong collaboration between XRISM\u2019s Japanese, European and American teams,\u201d adds Matteo Guainazzi, ESA\u2019s XRISM Project Scientist. \u201cThis international team combines the technical and scientific expertise needed to solve the X-ray Universe\u2019s biggest mysteries and open new avenues for research.\u201d<\/p>\n<\/p><\/div>\n Notes for editors<\/b><\/p>\n \u2018Orbital motion detected in \u03b3 Cas Fe K emission lines\u2019 by Y. Naz\u00e9 et al is published in Astronomy & Astrophysics. <\/i><\/p>\n XRISM (pronounced krizz-em) was launched on 7 September 2023. It is a mission led by the Japan Aerospace Exploration Agency (JAXA) in partnership with NASA and ESA. It carries two instruments: an X-ray calorimeter called Resolve capable of measuring the energy of individual X-ray photons to produce a spectrum at unprecedented level of \u2018energy resolution\u2019 (the capability of an instrument to distinguish the X-ray \u2018colours\u2019), and a large field-of-view X-ray CCD camera to image the surrounding field called Xtend.<\/p>\n<\/p><\/div>\n
\n\t\t\t\t68<\/span> views<\/small><\/span>
\n\t\t\t\t\t\t\t\t\t\t1<\/span> likes<\/small><\/span><\/p>\n<\/div>\nA mystery steeped in history<\/h2>\n
\n\t\t\t\t\t\t\t\t<\/figcaption><\/figure>\nThe final two theories<\/h2>\n