On December 9, 2021, NASA sent out a report entitled "NASA Launches New Mission to Explore Universe's Most Dramatic Objects." The report discussed the launching of the IXPE (Imaging X-ray Polarimetry Explorer) in which I joined. In Japan, RIKEN (Institute of Physical and Chemical Research) issued a related press release.^[1][2] In this article, I will explain the importance of X-ray polarimetry from celestial sources, which has been a long-awaited goal of X-ray astronomers, and the long journey of related technology up until this point.

An eye for observing extreme environments in the universe: X-ray astronomy

If you closely observe stars in the night sky, you will notice that each star has a different color. This color represents the surface temperature of the star, with red stars at approximately 4,000 degrees and blue stars at approximately 10,000 degrees. The human eye can only directly confirm the celestial sources within this temperature range in our universe. In reality, our universe is filled with an even wider variety of celestial sources and accompanying phenomena. For example, neutron stars, which are extremely small stars, have a strong magnetic field which is approximately 10 million to 10 billion times stronger than the steady magnetic field that can currently be created by human beings. Black holes have a density in which one spoonful of mass is equivalent to the Earth. When such celestial sources combine with stars and form a binary star system, it causes the generation of plasma which reaches a temperature range of several tens of millions of degrees, and intense X-rays are emitted. In order to explore such an extreme environment that is difficult for human beings to reproduce on Earth, it is necessary to observe the invisible cosmic X-rays generated in that environment. This is the field of X-ray astronomy. Universe's most dramatic objects as discussed at the beginning of this article refer to celestial sources that cause high-energy phenomena in such an extreme environment beyond human intelligence.

Compared to visible light astronomy, which observes light that is visible to the human eye, there is one major obstacle to observing X-rays coming from space. X-rays are blocked by the Earth's atmosphere and do not reach the Earth's surface. In other words, to observe cosmic X-rays, the observation device must be carried out of the Earth's atmosphere. Therefore, X-ray astronomers capture cosmic X-rays by attaching telescopes and observation devices to satellites and launching them, or by attaching observation devices to the International Space Station (ISS). Researchers at the Chuo University Astrophysics Laboratory also participated in the operation of the Japanese Monitor of All-sky X-ray Image (MAXI) that is installed in the ISS. On a daily basis, we study the extreme environment of the universe, including data from the visible light telescope installed in the Korakuen Campus Building No. 5.^[3][4]

Remaining challenges of X-ray astronomy: Polarimetry

In the method of astronomy, it is important to obtain four pieces of information from light (electromagnetic waves) in order to investigate the properties of a celestial source. The first is the change in brightness of the celestial source over time, the second is the color (spectrum), the third is the information on the shape of the celestial source obtained from imaging (such as the galaxy spiral), and the fourth is the polarization, which represents the bias of electromagnetic waves. For example, light reflected on a plane such as the water surface will be biased in a direction parallel to the water surface. The human eye is not able to detect this bias. However, when wearing polarized sunglasses designed to utilize this polarization property, only the reflected light on the water surface is cut, and the wearer becomes able to see the inside of the water. X-rays possess similar properties. For example, in a neutron star with a strong magnetic field that can never be reproduced on the Earth, the vacuum is distorted due to the strong magnetic field, so the X-rays passing through are also expected to be biased. Also, in the case of binary stars composed of stars and black holes, there is a high temperature plasma at tens of millions of degrees in the region where the space-time is thought to be distorted by the huge gravitational field around the black hole. The X-rays which come to Earth from binary stars are believed to include components which are reflected by the surrounding plasma. The polarization information has the potential to clarify the space-time structure around the black hole. In this way, X-ray polarization information is an important probe for exploring the properties of extreme environments with strong magnetic fields and strong gravity. However, X-ray polarization has been successfully observed only once from a single celestial source in the 1970s, about 50 years ago. Since then, no X-ray polarization from new celestial sources has been detected. This rarity is due to the difficulty of observing X-ray polarization.

Method of X-ray polarimetry

In order to measure polarization information in the visible light band, we obtain information by using a polarizing plate, which has a slit of the wavelength size (nanometer) for visible light. Many Japanese people have used polarizing plates in science classes during elementary school or junior high school. However, since the wavelength of X-rays is about the same size as an atom, it is difficult to make a slit of that size and it is not possible to directly measure the polarization. Therefore, to obtain polarization information of electromagnetic waves with a wavelength shorter than that of X-rays, we must measure the polarization degree of incident light by causing an interaction between the incident electromagnetic wave and something, and then measuring the secondary reaction occurring at that time. In the X-ray band, an interaction is caused between the incident X-rays and the gas, and the direction of the photoelectrons output due to the photoelectric effect depends on the polarization direction of the incident X-rays. By utilizing this phenomenon, it is possible to measure polarization by recording the direction of the output photoelectrons. In principle, this method has been known for a long time. However, utilizing this method in satellite orbit, not on the ground, and securing sufficient performance for observing cosmic X-rays coming from a location that is several thousand light years away involve great technical difficulty. Accordingly, researchers around the world have been working for many years to develop X-ray polarimeters. In Japan, researchers have selected the appropriate gas and continued to refine the gas electron multiplication foil necessary to obtain photoelectron tracks through efforts led by RIKEN. Japanese researchers have been waiting for the appropriate timing to mount this technology on a satellite.

Long road to installation in satellites

Even if development of the detector progresses, the creation of a satellite is a huge project, and there are few chances to mount the developed observation device on a satellite. NASA runs an open recruitment program for observation satellites called the SMEX (Small Explorers) Program. When open recruitment for SMEX is announced, proposals are submitted by many research institutes that had been waiting. The first step of the screening process is to narrow the field down to about three missions through the first selection. Next, after a preparation period of about one year, the second step of the screening process is performed, and a long and tough battle awaits until deciding upon one satellite mission. In fact, the GEMS (Gravity and Extreme Magnetism Small Explorer) satellite project, which was led by NASA's Goddard Space Flight Center (NASA/GSFC) and developed by RIKEN from Japan, won the review and was supposed to be launched in 2014. However, at NASA's final internal review held in 2012, an announcement was suddenly issued that the mission had been terminated. Personally, I was in the third year of my PhD program and had participated in the development of the X-ray polarimeter that was to be mounted on GEMS. For me, the announcement seemed to signal an end to my future academic path. However, both NASA/GSFC and our research group remained hopeful, and we decided to try again in the form of a new mission. Specifically, we sought to restart the project under the new mission name of PRAXyS (Polarimeter for Relativistic Astrophysical X-ray Sources), and we submitted a mission proposal for the next SMEX open recruitment. PRAXyS successfully passed the first step of screening in 2015. However, upon seeing the names of other missions that also passed the first screening, I was shocked once again. In the same category for X-ray polarization observation missions, there was the name of IXPE from the NASA Marshall Space Flight Center (NASA/MSFC), which is a separate organization from NASA/GSFC. It was unprecedented for two missions with the same purpose to pass the first step of screening. The difference between PRAXyS and IXPE was that from among the four important pieces of information in astronomy which I introduced above, PRAXyS chose to eliminate the imaging function and improve the sensitivity to polarization detection. On the other hand, IXPE chose to pursue an imaging function even if it meant reducing the sensitivity. In order to prepare for the upcoming second step of screening, our PRAXyS polarimeter team worked hard to calibrate the device.^[5] Ultimately, IXPE was selected as the result of the second screening step conducted in 2016. It is impossible to describe the disappointment felt by the Japanese team, which had been developing X-ray polarimeters for many years with GEMS and PRAXyS. Nevertheless, in order to create the X-ray polarization observation satellite which has been a long-awaited goal of X-ray astronomers, we offered to cooperate with the team engaged in IXPE mission, which had been our competitor and rival until just recently, with a promise of success of IXPE mission. Meanwhile, RIKEN provided a gas electron multiplication foil necessary for X-ray polarimeters and Nagoya University offered passive thermal control membrane filters used for X-ray telescopes. Institutions such as Yamagata University, Hiroshima University, Osaka University, and Chuo University decided to cooperate in the development of data analysis software and the holding of scientific research.

After this long journey, the IXPE satellite was finally launched into outer space on December 9, 2021 and successfully put into orbit. Since then, preparations have been made for starting observation equipment, and the observation is proceeding as planned. In the upcoming year, IXPE should provide us with information on extreme environmental physics in neutron stars and black holes that no one has seen thus far. As with many astronomical phenomena found so far, the results may be far beyond our imagination, and it is possible that astronomers around the world will be confounded. However, since we have been waiting for this data for so many years, the truth is that we would find it enjoyable to be perplexed.

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[1] NASA Launches New Mission to Explore Universe's Most Dramatic Objects
https://www.nasa.gov/press-release/nasa-launches-new-mission-to-explore-universe-s-most-dramatic-objects
[2] Exploring a new method of observing a black hole: Launch of IXPE (Imaging X-ray Polarimetry Explorer)
https://www.riken.jp/pr/news/2021/20211208_2/index.html
[3] Homepage of MAXI.
http://maxi.riken.jp/top/index.html
[4] Homepage of the Tsuboi Lab, Astrophysics Laboratory, Department of Physics, Faculty of Science and Engineering, Chuo University
https://tsuboi-lab.r.chuo-u.ac.jp/Top/
[5] "Performance of the PRAXyS X-ray polarimeter", W.B. Iwakiri et al., Nuclear Inst. and Methods in Physics Research, A, Volume 838, p. 89-95. (2016)

Wataru Iwakiri
Assistant Professor, Faculty of Science and Engineering, Chuo University
Areas of Specialization: Elementary Particles, Atomic Nuclei, Cosmic Rays, and Cosmic Physics

Wataru Iwakiri was born in Tokyo in 1984. He graduated from Koishikawa High School in Tokyo in 2003.
In 2008, he graduated from the Department of Physics, Faculty of Science, Saitama University.
In 2010, he completed the Master’s Program in the Graduate School of Science and Engineering, Saitama University.
In 2012, he completed the Doctoral Program of the same graduate school. He holds a PhD in science from Saitama University.
After serving as a postdoctoral fellow at the Japan Society for the Promotion of Science and as a special researcher under RIKEN’s program for Special Postdoctoral Researchers, he assumed his current position in 2018.

His specialty is X-ray astronomy. For X-ray transients such as X-ray bursts from neutron stars and stellar flares, he uses the Japanese Monitor of All-sky X-ray Image (MAXI) installed on the International Space Station (ISS) and the X-ray telescope NICER of the NASA Goddard Space Flight Center (also installed on the ISS) to conduct data analysis and to develop an X-ray polarimeter meter.

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