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The Rigakubu News
Published in The Rigakubu News November 2024

The Frontiers of Research for Undergraduates>

Searching for the Identity of the Guest Stars Recorded in the Azuma Mirror

Ko Takatoshi, Department of Astronomy, 2nd Year Doctoral Student
Toshikazu Shigeyama, Professor, Department of Astronomy

Stars like the Sun have a finite life span and eventually become white dwarfs, which have a mass about the size of the Sun and a density as high as that of the Earth.
Since most stars form binary systems, some of them are expected to merge and explode after becoming white dwarfs.
However, the magnitude of the explosion was unknown.
Explosion phenomena of celestial objects in the galaxy are sometimes described in history books as "guest stars" that suddenly brighten.
In 1181 A.D., a celestial object recorded in history books such as Azuma Kagami as a guest star was found to consist of a peculiar white dwarf star and an expanding nebula surrounding it, according to recent observations of multi-wavelength electromagnetic waves.
This was thought to be the result of the merger of two white dwarfs. We reanalyzed the X-ray data and deduced what happened at that time.

The guest star was identified as WD J005311, a white dwarf at the center of an expanding nebula discovered near Cassiopeia in 2019 that shines brightly in infrared and X-rays (the distance to this object is approximately 7.1 × 10¹⁶ km). The key factors were the coincidence of the azimuthal direction and the fact that the formation time of the nebula, derived from the expansion rate and size of the nebula, coincides with the appearance time of the guest star.

We estimated the magnitude of the explosion of this star by estimating the energy and mass of the nebula based on the results of observations made in 2021 by the X-ray observation satellite XMM-Newton. According to the observations, the X-ray emission has a spherically extended component (radius 4.5 × 10¹⁶ m) and a point-like component at its center, as shown in the left panel of the figure. We hypothesize that the broadening component is caused by the collision of the expanding nebula with the surrounding thin gas, and that the shock wave heats the gas and emits radiation (right side of the figure). In order to explain the observed X-ray intensity, the spread, and the 840 years (= 2021-1181) elapsed since the explosion, theoretical calculations indicate that the nebula has a kinetic energy of (0.77-1.1)×10⁴¹ J and a mass of 0.18-0.53 times that of the Sun. Theoretical calculations indicate that its mass is 0.18-0.53 times that of the Sun. Compared to Type Ia supernovae, which are thought to originate from white dwarfs, its energy is about three orders of magnitude lower and its mass is only 1/8 to 1/3 of that of the Sun. This guest star was a small explosion for a supernova. This is consistent with the record at the time that the brightness was about the same as that of Saturn.

The left figure shows the intensity of X-rays in colors (intensity increases from blue to red to yellow), and the infrared intensity is shown by contours. The right panel shows the location of the shock wave produced by the explosion in 1181 and the shock wave produced by the stellar wind blowing from WD J005311 at present.

Reanalysis of the data obtained by the X-ray observation satellite Chandra, which has better visibility than XMM-Newton, revealed that the central point-like X-ray emission region is emitted from a region with a radius of 2.8 × 1014 m or larger. This is much larger than the 107 m radius of the white dwarf. Therefore, we hypothesized that the stellar wind blowing from the white dwarf collides with the nebula and heats the nebula to a high temperature of about 10⁷ K, resulting in the X-ray emission (right side of the figure). The presence of stellar winds with a velocity of 5% of the speed of light was already known from the broad emission lines in the visible light spectrum. A few years ago, we proposed a numerical model of the stellar wind from a heavy white dwarf formed by the merger of two white dwarfs, which has a strong magnetic field and rotates at a high speed. The stellar evolution model indicates that the white dwarf must have a mass 1.1-1.3 times that of the Sun and a period of 12-30 s. According to stellar evolution models, this large mass implies that the elemental composition of the white dwarf is dominated by oxygen and neon. The elemental composition can also be estimated from the observed X-ray and visible light spectra, which are rich in elements synthesized by carbon fusion reactions, consistent with theoretical predictions. Thus, this object became the first example of what might happen after the merger of a white dwarf.

However, if the stellar winds are blowing at such high speeds immediately after the merger of the white dwarf, the X-ray emitting region would be larger than observed. To avoid this, it is necessary to assume that this stellar wind started blowing about 30 years ago. This request will be confirmed by further observations.

This is a multi-laboratory collaboration, and the results have been published in T. Ko et al.

(Press release, July 5, 2024)