Topic: Space

A team of astronomers used Japan's XRISM space telescope to solve a 50-year-old mystery about a star visible to the naked eye. They found that the star's extreme X-rays are caused by a white dwarf orbiting it.

The star γ Cas, located in the constellation Cassiopeia, has been puzzling astronomers for decades. It produces X-rays much hotter and stronger than expected from a typical massive star. New observations using the Resolve instrument on Japan's XRISM space telescope have finally linked these emissions to a white dwarf orbiting the star.

γ Cas was first classified as a Be-type star in 1866 by Italian astronomer Angelo Secchi. These massive stars spin rapidly and regularly eject material into space, forming a disc around the star that can be detected through specific features in its optical spectrum.

In the 1970s, scientists realized that γ Cas emits X-rays about forty times stronger than similar stars. The plasma responsible reaches temperatures above 100 million degrees and changes rapidly. Over the following two decades, space observatories found around twenty stars with similar behavior, now known as 'γ Cas analogues'. Astronomers at the University of Liège played a major role in identifying more than half of these objects.

Several theories had been proposed to explain this emission. One involved local magnetic reconnection between the surface of the Be star and its disc. Others suggested X-rays were linked to a companion, whether a star stripped of its outer layers, a neutron star, or an accreting white dwarf. Researchers had already ruled out stripped stars and neutron stars because observations did not match theoretical predictions.

To resolve the mystery, the team carried out a series of observations using Resolve, a high-precision microcalorimeter on board XRISM that is transforming high-energy astrophysics. Data were collected in December 2024, February 2025, and June 2025, covering the full 203-day orbit of the system.

The spectra revealed that the signatures of the high-temperature plasma change velocity between the three observations, following the orbital motion of the white dwarf rather than that of the Be star. This shift was measured with high statistical reliability. It is, in fact, the first direct evidence that the ultra-hot plasma responsible for the X-rays is associated with the compact companion, and not with the Be star itself.

The measurements also provide insight into the nature of the white dwarf. The spectral features have a moderate width (of the order of 200 km/s), which rules out a non-magnetic white dwarf. Instead, the results indicate a magnetic white dwarf, where the disc is cut off and the magnetic field directs incoming material toward its poles.

These findings show that γ Cas and similar stars belong to a class of Be + white dwarf binary systems that had long been predicted but never clearly observed. Researchers at ULiège also identified two key traits of this group. It mainly involves massive Be stars and represents about 10% of them.

Why It Matters

Understanding the behavior of stars like γ Cas can help us better understand the evolution of binary star systems, which are crucial for our understanding of the universe.

Key Facts

• The star γ Cas produces X-rays much hotter and stronger than expected from a typical massive star.
• The X-rays are caused by a white dwarf orbiting the star.
• γ Cas was first classified as a Be-type star in 1866 by Italian astronomer Angelo Secchi.
• The plasma responsible for the X-rays reaches temperatures above 100 million degrees and changes rapidly.
• The findings show that γ Cas and similar stars belong to a class of Be + white dwarf binary systems.

Key Terms

Be-type star

A massive star that spins rapidly and regularly ejects material into space.

Implications

Understanding the behavior of stars like γ Cas can help us better understand the evolution of binary star systems, which are crucial for our understanding of the universe.