Outlines

The international research team of the balloon-borne hard X-ray polarimetry mission XL-Calibur, including Associate Professor Hiromitsu Takahashi of the Graduate School of Advanced Science and Engineering, Hiroshima University, Professor Hironori Matsumoto of the Graduate School of Science, the University of Osaka, Assistant Professor Yoshitomo Maeda of the Institute of Space and Astronautical Science of JAXA, and Professor Hisamitsu Awaki of the Graduate School of Science and Engineering, Ehime University, conducted polarization measurements of hard X-ray radiation to gain a deeper understanding of the environment in which matter swirls and releases enormous amounts of energy before falling into a black hole.

The balloon-borne hard X-ray polarimetry mission XL-Calibur observed the black hole X-ray binary (BHXRB) Cygnus X-1 (Cyg X-1) during its nearly 6 day long-duration balloon flight from Sweden to Canada in July 2024. XL-Calibur has succeeded in observing polarization information (degree and angle) of the 15-60 keV X-rays emitted from Cyg X-1 with a sensitivity approximately 20 times higher than previous measurements, providing the most precise constraints to date. The results show that within 2000 km of the center of the 125 km diameter black hole, the brightly shining region (corona) is aligned vertically with the massive plasma jet that erupts billions of km from the black hole. These results revealed that the corona forms a flattened structure along the disk where material stolen from the companion star falls in a spiral pattern.

In the future, improved balloon experiments and results from satellite observations of X-ray polarization, photometry, and spectroscopy, as well as theoretical research, will reveal how matter being sucked into black holes of various masses (ranging from a few times to super-large size of 10 billion times the mass of the sun) is affected by gravity. This is expected to lead to a better understanding of the characteristics of the black hole at the center (its rotation speed) and the relativistic effects of black holes (the distortion of space-time).

In this mission, Japanese researchers were responsible for fabricating and calibrating the X-ray focusing mirror, which is the core component of the instrument. Japan's technological capabilities have played a key role in international observation.

Research Background

The matter that falls into the black hole is heated to extremely high temperatures (about 10 million degrees) by the strong gravity and glows brightly in X-rays. Therefore, if it is possible to clarify the physical state of the accreting matter near the black hole through X-ray observations, it is also expected to observe the physical quantities of the black hole itself at the center, as well as the general and special relativistic effects in a strong gravitational field. However, for many years, the state of the accreting material had remained stagnant, based solely on observations of time variations (photometry) and energy (spectroscopy). This is because those materials are so far away, they only appear as dots in the image, and their structure cannot be examined.

Unlike imaging, time fluctuations, or energy measurements, polarization observations can infer the geometric structure of matter, such as whether the photons emitted by high-energy particles arrived directly from the material or were reflected or scattered somewhere, from information on the polarization (the bias in the direction of vibration of the electric field) of the photons. This is a common technique for radio waves and visible light, but due to technical difficulties in the X-ray and gamma-ray bands, the only experiment that has been able to obtain polarization information in the hard X-ray band so far has been the balloon-borne X-ray polarimetry experiment PoGO+ conducted in 2016. (However, only if an upper limit is imposed)

Research Contents

In July 2024, an international collaborative research group, including a Japanese team, conducted new observations using the balloon-borne telescope XL-Calibur to clarify the extreme environment around the black hole. This mission is led by the University of Washington in the United States, with researchers from Hiroshima University, the University of Osaka, JAXA's Institute of Space and Astronautical Science, Ehime University, and other Japanese institutions playing a central role by providing the world's largest X-ray focusing mirror. The object of observation is Cygnus X-1 (Cyg X-1), which is located about 7,000 light-years from the Earth.

Discovered in 1964, it is the first X-ray astronomy in the Milky Way that is widely accepted as a black hole. The black hole has a mass about 21 times that of the sun.

The infalling and ejecting matter around a black hole is thought to form three components as follows:

1. Accretion disks: Material stolen from nearby stars falls into the black hole in a spiral shape.

2. Coronal plasma: High-temperature plasma energizes the light from the accretion disks, making it more energetic.

3. Plasma jet (outflow): The distortion of space-time and the strong magnetic field caused by the black hole's rotation causes some material to be ejected at high speed toward the poles.

XL-Calibur observations have provided particularly strong constraints on the shape, location, and origin of coronal plasma. Previous observations by PoGO+ only revealed that the polarization of hard X-rays was weak (an upper limit of <8.6%). However, with the latest XL-Calibur used in this study, the sensitivity has been improved approximately 20 times, allowing the researchers to measure a polarization degree of approximately 5.0%. The results demonstrated that within 2,000 km of the center of the 125 km diameter black hole, a bright corona is vertically aligned with a huge plasma jet that erupts billions of kilometers from the black hole.

Previous observations from the PoGO+ experiment only revealed that the corona is not compact and localized within 100 km of the black hole but rather exists in a spread-out form. The results from these observations by the XL-Calibur experiment clarified that the shape of the expanded corona is a flat structure that conforms to the disk.

Social Impact of the Study

The information obtained from this study, combined with other spectroscopic satellites such as NASA's IXPE polarimetric satellite (low energy 2–8 keV) and JAXA's XRISM, as well as the latest computer simulations, is expected to construct more precise physical models of black holes and their vicinity in the coming years. The XL-Calibur team now aims to fly from Antarctica to observe the polarization of other black holes and neutron stars with strong magnetic fields.

The XL-Calibur balloon experiment was made possible through international collaboration including more than 13 institutions, including the University of Washington, University of New Hampshire, the University of Osaka, Hiroshima University, the Institute of Space and Astronautical Science of JAXA, KTH Royal Institute of Technology, Sweden, NASA's Goddard Space Flight Center, and Wallops Flight Facility.

The mission leader is Professor Henric Krawczynski of the University of Washington.

Notes

The article, “XL-Calibur Polarimetry of Cyg X-1 Further Constrains the Origin of its Hard-state X-ray Emission,” was published in The Astrophysical Journal at DOI: https://doi.org/10.3847/1538-4357/ae0f1d

Fig. 1 The XL-Calibur balloon launched from Sweden on July 9, 2024.

(YouTube video: "NASA XL-CALIBUR Launch")

Credit: Hironori Matsumoto

Fig. 2 Aerial photo of the landing site taken on July 15 (NASA). The gondola has been safely recovered from the balloon and preparations are underway for the next Antarctic flight.
Credit: Hironori Matsumoto

Fig. 3 Observation results by XL-Calibur. It was discovered that the polarization direction of the hard X-rays emitted by the high-temperature corona near the black hole is aligned (parallel) with the direction of the giant jet (white) observed in radio waves. The soft X-ray observation results from the IXPE satellite are indicated in pink.
Credit: Hironori Matsumoto

Fig. 4 Artist's impression of the newly discovered corona (cross-section). The researchers were able to clarify that the corona forms a flat shape along the disk (extending perpendicular to the jet)

Credit: Hironori Matsumoto

Fig. 5 Mechanism of X-ray telescope　　　　
◎Nagoya University U-Lab X-ray Group ◎Research Project

Credit: Hironori Matsumoto

Fig. 6 Hard x-ray

Credit: Hironori Matsumoto

Fig. 7 Polarization

Credit: Hironori Matsumoto

Fig. 8 X-rays observed 40 km above the Arctic Circle (where only 0.3% of the Earth's atmosphere exists)

Credit: Hironori Matsumoto

Left: Cygnus X-1 observed in visible light (Digitized Sky Survey). The companion supergiant star appears pale blue. Right: Artist's impression of Cygnus X-1. https://chandra.harvard.edu/photo/2011/cygx1/

The dark area in the center on the left image is the black hole. The pale blue star on the right is a companion star (a supergiant star). The red disk is the accretion disk. The structure extending up and down is the plasma jet. The coronal plasma that was the subject of this study exists very close to the black hole.

Credit: Hironori Matsumoto

Fig. 9 X-ray focusing mirror (world's largest, made in Japan)

Credit: Hironori Matsumoto

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