Let's enjoy the universe together through observation with XRISM!
We spoke with Kyoko Matsushita (Tokyo University of Science), a XRISM Project Scientist. As this is the first in a series of interviews, we begin with an introduction to XRISM.
What is unique about XRISM compared to other X-ray astronomical satellites?
The X-ray energy resolution of XRISM will be dramatically improved compared to previous X-ray astronomy satellites, thanks to the XRISM's microcalorimeter (Resolve) detector (*).
For example, conventional semiconductor detectors such as CCDs can barely detect the motion of gas emitting X-rays faster than about 1,000 km/s. However, Resolve enables us to measure the motion of about 100 km/s with a margin of error.
The rotation speed of our Milky Way Galaxy is about 200 km/s. Therefore, XRISM's accuracy can measure the gas motion on the scale of a galaxy. In other words, XRISM will detect the velocity of gas (galactic wind) blowing out from the galaxy.
XRISM also has another observation instrument. This Xtend X-ray CCD camera has a wide field of view and high-resolution images.
What are you trying to study with XRISM?
I specialize in studying galaxies and galaxy clusters, so I will talk from that perspective. One of the main themes in modern astronomy is determining how star formation occurs in galaxies and how it stops.
To form a star, a huge amount of cold gas is required. On the other hand, massive stars die and explode soon after birth. A massive star's powerful and luminous explosion is called a supernova explosion. When many supernova explosions occur, the gas in the galaxy is heated, changing the conditions for star formation; the gas is lost as galactic wind, stopping star formation, and elements produced by the star are scattered outside the galaxy.
It would be exciting if XRISM could observe elements from supernova remnants, galactic winds, and elements scattered outside the galaxy.
I also expect that we will be able to study how much a black hole affects its surroundings. A supermassive black hole is thought to exist at the center of a galaxy. When matter falls into a black hole, some of the gas is ejected outward as a high-speed jet, affecting the environment around the black hole. The velocity of such gas can also be observed with XRISM.
We are also hoping that XRISM will provide clues to the evolution and growth of galaxy clusters, which are large groups of galaxies and the largest objects in the universe, in the form of the motion of gas in galaxy clusters. Clusters of galaxies are objects finally formed by the gravitational gathering of dark matter over a very long time, from just after the birth of the universe to the present. Although it may seem like just a collection of galaxies when seen in visible light images, there is also gas of several tens of millions of degrees called "galaxy cluster gas."
Are there any difficulties or innovations in observation?
The most important thing for a satellite is to keep its solar array pointed at the sun at all times, so we can only observe celestial objects within the range that can be observed with the solar array pointed at the sun (i.e., at an angle of about 90 degrees away from the sun). Therefore, for most celestial objects, there are two observation periods of two or three months in a year. The "key" is efficiently observing the celestial objects you want to study.
XRISM is positioned as a replacement for ASTRO-H. What is the difference between XRISM and ASTRO-H?
XRISM is basically a satellite similar to ASTRO-H. Still, XRISM has improved the parts of ASTRO-H that caused accidents. Also, when ASTRO-H was launched, X-ray astronomy had already made more progress than when the ASTRO-H project started. The initial observations of ASTRO-H were also successful. In light of this, redefining the purpose of the satellite, we decided that XRISM would specialize in soft X-ray spectroscopic imaging observations.
ASTRO-H (Hitomi): An X-ray astronomy satellite launched on February 17, 2016. Communication anomalies occurred on March 26 of the same year due to satellite attitude anomalies, and operations were suspended on April 28.
You have been appointed Project Scientist for XRISM. What kind of work do you do?
When the project was launched, I led the study of what kind of satellite to operate and for what purpose. After the project was established in the fall of 2020, my primary task was to select the objects to be observed during the initial observation phase for the first six months after launch. We collected proposals from the team and selected 60 objects from among them. Still, we received more than that number of observation proposals from the team. Naturally, the volume of observation proposals was large, and reading and reviewing them all was difficult.
Once we selected the observational targets, the next step was to plan how to analyze the data.
XRISM also accepts observation proposals from researchers outside the project after the initial observations. We are also discussing how to handle observation proposals, including how to accept and review proposals.
How you came to be appointed as a project scientist?
I have experience assisting the work of the Science Team under Dr. Takaya Ohashi (Tokyo Metropolitan University), the Publication Board and Science Office Manager for ASTRO-H, and at XRISM. That is why I was appointed as a Principal Investigator (research organizer), Dr. Makoto Tashiro.
The ASTRO-H science team has all worked hard to create science results using ASTRO-H data. However, ASTRO-H was shut down just as we were about to start full-scale observations. I still have regrets. For this reason, I am now very eager to make science at XRISM a great success.
Enjoy the Universe!
Going off-topic, how did you first become interested in X-ray astronomy?
I am from the astronomy department and learned about stars and galaxies in my undergraduate classes. When I was going to graduate school, a doctoral student in our lab told me that inside galaxies and galaxy clusters, there is a lot of hot gas that emits X-rays and that we could find out things about it by studying it. I was surprised by what he said. I thought it sounded interesting, so I went into the field of X-ray astronomy.
It was just before the launch of ASCA (ASTRO-D, 1993-2001). Naturally, my master's and doctoral thesis were studies using ASCA data.
As a researcher, are there any particular topics that interest you?
I have been working on the chemical evolution of galaxies since graduate school, and the ability of XRISM to precisely determine X-ray energies means that we can detect even very weak characteristic X-rays emitted by various elements.
Suppose we can detect a tiny fraction of elements in the hot gas of a galaxy cluster or galaxy. In that case, we can determine the type of supernovae that produced those elements and how many such supernovae exploded. Through such research, we aim to comprehensively understand how supernovae heat the gas in galaxies to generate galactic winds and how various elements are ejected into space.
I enjoy working with students, so I hope to analyze XRISM data with them and the young people who have contributed to the project to produce results.
What would you like to say to those looking forward to XRISM?
Let's enjoy the universe together through observations with XRISM!
This article was translated from Japanese.
Date：August 30, 2021
Interviewer: Taro Nakano
Editors：Takashi Horiuchi & Chisato Ikuta