The Universe Seen in X-rays: Exploring the Violent Activity of Celestial Objects

The Universe hosts a wide variety of X-ray-emitting objects and phenomena, such as stellar winds, supernova remnants, galaxy clusters, and the environments around black holes. X-rays are an essential observational probe of hot, high-energy cosmic plasmas: their spectra allow us to determine the temperature, elemental composition, ionization state, and velocity structure of the plasma. In this talk, I will give an overview of the basics of X-ray astronomy and the techniques of X-ray spectroscopy, and introduce astronomical satellites capable of high-resolution X-ray spectroscopy.

Then, taking as an example a binary system in which two massive stars orbit each other, I will show what high-resolution X-ray spectroscopy can reveal. In massive-star binaries, the stellar winds blowing from the two stars collide, forming hot plasma at temperatures reaching tens of millions of kelvin. By examining in detail the X-rays emitted from this collision region, we can probe the temperature, elemental composition, ionization state, and velocity structure of the colliding-wind plasma. Finally, I will present observational results from the X-ray astronomy satellite XRISM and discuss what information high-resolution X-ray spectroscopy provides for understanding the physical phenomena occurring around massive stars.