Alisa Rachubo, who is an undergrad student of Carnegie Mellon University in US, visitedMakishima-Nakazawa Group during her summer break in July-August 2009. As a short-term workshop, she was working to write a report on activities of our group by interviewing individual members about their current task and what is interesting in his/her research. In spite of limited working time, she has accomplished a well-written report with which people outside the high energy astrophysics community can grasp the atmosphere and research activities of our group.
She kindly provided the material allowing us to open it online. We uploaded it here with great thank to Alisa.
High Energy Astrophysicists in Action
I was lucky enough for my summer as an undergraduate Freshman to visit the Makishima-Nakazawa Group and learn about what each member was doing through interviews. It was a very good experience, with everyone being very kind to spare some time from their busy lives to explain their current research activities. I gained more understanding of what really is done on a daily basis for astrophysics research and came to appreciate the amount of effort that each member of the Makishima-Nakazawa Group put into their research.
Now as an undergraduate Sophomore starting my first astrophysics class, I remember the inspirational people I met at the Makishima-Nakazawa Group and their dedication to high energy astrophysics. I would like to thank them for their time and letting me visit the lab. I wish them all success with their current tasks and the many more they will do in the future!
Alisa Rachubo, August 2009
Dr. Kazuo Makishima has many roles in the Makishima-Nakazawa Group. As a professor he has teaching duties, he supervises the graduate students in his group, he has administration duties to fulfill, he does some data analysis himself, and he also supervises another lab in RIKEN where he spends a third of his time.
As the supervisor of the Makishima-Nakazawa Group, he has to have knowledge on topics covered by his students. Although he writes only one or two papers a year, he spends a lot of time and effort for his students’ papers, reading each page three times for maximum revision.
Dr. Makishima has done a lot of work with satellites from the past. He had important roles with the Ginga spacecraft launched in 1987, the Yohkoh spacecraft in 1991, the ASCA spacecraft in 1993, the ASTRO-E satellite in 2000 (that unfortunately had a launch failure), and the Suzaku satellite in 2005. Currently, Dr. Makishima and others are working on upgrading the HXD (Hard X-ray Detector) for the next satellite launched in 2013, ASTRO-H.
His interests in astrophysics are reflected by his students in his group. For example, he shows interest for black holes, ULXs, magnetized neutron stars, white dwarfs, and galaxy clusters. However, Dr. Makishima approaches these astrophysics features in a rather unique way: he questions the currently held common view. For instance, many believe that magnetized neutron stars contain electromagnets. On the other hand, Dr. Makishima believes that instead they contain permanent magnets (due to aligned neutrons) from his observations since 1990, and is working on proving this conjecture. Similarly, Dr. Makishima has a conjecture that there are strong interactions between moving galaxies and hot plasma (gas) in galaxy clusters, although most think there is no interaction.
Dr. Makishima’s main motivation is connecting fundamental physics with x-ray astrophysics. For the conjectures he creates, he puts his grounds in fundamental physics (for example with magnetized neutron stars ferro-magnetism is fundamental physics). With his challenging attitude to various theories in x-ray astrophysics and his goal to connect fundamental physics with experimental x-ray astrophysics, I believe he is a great example for students in his group to constantly look for new views in x-ray astrophysics and to strive to connect to the fundamentals of physics that is so important in science.
Dr. Nakazawa works on two components of the Astro-H satellite: the HXI (Hard X-ray Imager) and the SGD (Soft Gamma-ray Detector). In fact, he is the sub-leader for the HXI. Working on Astro-H is important for Dr. Nakazawa’s study on galaxy clusters: a wider range of x-rays is needed to observe galaxy clusters, and Astro-H will provide this.
Dr. Nakazawa is a very passionate man, astounded by the many mysteries of the universe, such as how it evolved into what it is today (and what the future holds). In order to unlock these mysteries of the universe, Dr. Nakazawa believes that studying galaxy clusters are vital. Galaxy clusters are the largest astronomical objects in the universe – in fact, the majority of the “normal matter” of the universe (4% of the composition, together with 22% of dark matter and 74% of dark energy) is in a form of the gas trapped in galaxy clusters. However, three fundamental features of galaxy clusters are not known exactly: the temperature distribution (although the temperature of the gas is somewhat known, the temperature throughout the cluster is not known), the magnetic field, and the acceleration of particles. The roles of these features may be small, but even the importance of their roles in galaxy clusters are not known yet. For instance, more than 90% of the energy of blast-waves in supernova remnants is used for acceleration of particles, as opposed to heat – i.e. heat is observed, but not as much as expected. Similar phenomena may take place in galaxy clusters when an event of merger takes place. Dr. Nakazawa’s and others’ work on Astro-H will allow more data on galaxy clusters to be obtained and more aspects of galaxy clusters will be revealed.
Observations using x-rays are absolutely necessary for galaxy clusters. Dr. Nakazawa believes that when x-rays started being used for observations of the universe, a new view of the universe was obtained. With more modes of observations, our current view of the universe may change altogether. Dr. Nakazawa believes that as a scientist, his goal is to gain new observations to create a new outlook on the universe and with this new outlook understand the mysterious universe even better.
Takao Kitaguchi is a Postdoctoral fellow in the Makishima-Nakazawa Group. The main subject of his research is to see if observing nearby relaxed clusters by hard X-rays would be any different than watching them with soft X-rays.
After doing spectral analysis from data obtained by the Suzaku satellite, he found that hard X-ray emission from relaxed clusters can be accounted for by the optically thin thermal plasma Bremsstrahlung model determined by soft X-rays. As a result, he concluded that there is no difference between hard and soft X-ray spectra. He expects that the ASTRO-H satellite will be able to achieve a breakthrough in hard X-ray observation of clusters due to the improved HXD that will be on the satellite.
Diane Sonya Wong
Her interest for ULXs stemmed from working at NRAO (National Radio Astronomy Observatory), including the VLA (Very Large Array) where the movie “Contact” was filmed! She worked mainly with radio astronomy, and her first encounter with ULXs was when her advisor mentioned it to her.
No one knows for sure what ULXs are still and how they are formed, but there are four possibilities for what they can be. Firstly, ULXs can be Intermediate Mass Black Holes (IMBH): these are black holes whose masses are between stellar black holes and supermassive black holes. For the formation of these IBMHs, the question is when the merging happens. Some believe the theory that the stars themselves merge and then form an IMBH. Diane’s theory is that the merging may happen before the star forms, and then the large star turns into an IMBH. Another possibility for what ULXs are is that they can be beamed black hole binaries (where the black hole is stellar-mass). Other possibilities for the identity of ULXs are supernovae and background objects.
Diane uses data from the world’s largest ground optical telescope in Hawaii. Using a catalog of ULXs, she finds those that can be observed in the optical range and then does follow up. She analyzes wavelength lines that are in the optical range of the EMR spectrum that show high ionization. She is currently running simulations to compare her observations with theory. She is looking forward to finishing her current optical paper, after which she will move to using the x-ray range of the EMR spectrum to study ULXs.
Akira Okumura is a 5th year Doctorate student and this is currently Okumura’s second year at the Makishima-Nakazawa Group. Previously he worked at the Institute for Cosmic Ray Research, University of Tokyo in Kashiwa for five years where he observed astronomical objects with gamma rays and visible light.
Okumura is a unique figure in the Makishima-Nakazawa Group in two ways: firstly, he is the only student that is married! Secondly, Okumura is the only person to specialize in gamma rays – everyone else uses X-rays. Due to this he works closely with Stanford. Also, he uses data from the Fermi Gamma-ray Space Telescope in contrast to others who all mainly use the Suzaku telescope.
Okumura’s main research focus is observing molecular clouds using gamma rays. This is possible as molecular clouds emit high energy gamma rays after cosmic rays interact with the nuclei.
Just recently Okumura submitted his Ph.D thesis: ‘Gamma Ray Observations of the Orion Molecular Clouds using the Fermi Large Area Telescope’. To create this thesis he did a lot of data analysis, and programming to make software that actually performed the data analysis. If all goes well, he will graduate in September. He plans to continue with this work, and is seeking Post-Doctorate positions.
To begin with, neutron stars are special type of stars, with a rather small radius (about ten kilometers) for its mass of about 1.4 solar masses. Furthermore, neutron stars rotate very fast (with periods of equal to and less than a second), though the rotation slows down over time. Enoto studies particular neutron stars with very strong magnetic fields, also known as magnetars. Magnetars emit x-rays by the decay of the magnetic fields. Using the Suzaku satellite, Enoto uses x-ray observations to observe magnetars, and is trying to find out why hard x-rays are emitted. One hypothesis is that soft x-rays are emitted directly from the surface, and these soft x-rays combine with an electric current flowing from the distorted magnetic field, thus creating hard x-rays.
Enoto’s other project (though he mostly worked on it when he was a Masters student) is GROWTH (Gamma-Ray Observation of Winter Thunderclouds). Two radiation detectors were placed at the coastal area of Japan Sea in Niigata prefecture, and successfully measured high-energy gamma rays during a winter thunderstorm in 2007. It is believed that electrons accelerating to relativistic velocities in the electric fields of the thunderclouds produced the high-energy gamma rays. The detectors are still working today, collecting data so more about this phenomenon can be known.
Suzaku’s data is currently available for the whole world, as it has been past one year since Suzaku’s launch date. Yamada calibrates Suzaku’s data so the proper signals are found against the background signals. Yamada must keep in mind that the environment in space is very different from the environment on earth: it is a high radiation environment in space due to cosmic rays, and also it is a vacuum. Yamada tests an identical copy of a part of the Suzaku on earth, for instance by creating signals that would be produced in the high radiation environment in space. By doing this, Yamada can see how he has to calibrate the real data coming from Suzaku. Calibration of the data seems to be a bit of a tedious task, as Yamada has to continuously renew his calibration.
Perhaps balancing out this tedious task, Yamada studies one of the most exciting phenomena in the universe: black holes. There are many reasons to study black holes, such as to learn the evolution of stars and galaxies, and to study high-energy astrophysics. Yamada’s main motivation, however, is from his interest in general relativity. General relativity is connected strongly to physics in strong gravity situations, and can be the key in having a more definite confirmation of black holes’ existence. Currently, Yamada is observing the Cygnus X-1 black hole candidate using the Suzaku satellite’s x-ray observations. Cygnus X-1 in fact belongs to a binary system that also includes a star, which is a crucial factor. The star’s gas flows towards Cygnus X-1 and then jets are released. These jets emit x-rays and visible light that can allow Cygnus X-1 to be observed (or instead of jets it can be accretion flow as it is still unclear which of the two it is). What Yamada wants to find out is how this mechanism occurs, specifically how the conversion of gravitational energy to light occurs.
Takayuki Yuasa is a second year Doctorate student. He is currently using x-ray astronomy to study binary systems that contain white dwarfs (very dense stars having approximately the size of the earth and the mass of the sun).
When a binary system consists of a white dwarf and another star, there is mass transfer from the star to the white dwarf: this is called the Roche-lobe overflow. As the gas flows onto the white dwarf’s surface, there is hot plasma created near the surface – this hot plasma emits x-rays that can be observed by a satellite. The temperature of this hot plasma can be found from the data of the x-ray observations, and this allows the free fall velocity of the gas to be found, as these two properties are proportional to each other. Through calculations that involve the free fall equation, the mass of the white dwarf can then be found.
As the white dwarfs’ masses increase, there are higher energy x-rays emitted and the plasma’s temperature increases. As it turns out, the masses found by x-ray observations fits with what was found using optical observations. Using x-ray astronomy is more advantageous than only using the visible range of the EMR spectrum, as x-ray observations provide information that optical observations cannot provide, such as the temperature of the hot plasma, which can reach up to 3x108K (the sun’s surface is only 6000K in comparison). X-ray astronomy is good for obtaining observations of fast, high temperature objects, as this case with the binary systems illustrate. It is still not yet known how many binary systems there are in the Milky Way galaxy, but it is possible to find this by x-ray astronomy.
In addition to his research on binary systems containing white dwarfs, Yuasa also worked with creating the SpaceWire ADC (analog-to-digital converter) Box. SpaceWire is the standardized network interface created for satellites. Components created around the world can use this single interface, so the time and costs for the satellite’s development can be reduced greatly.
One of his experimental projects is to figure out how to support the x-ray detector component (for the Astro-H satellite that is aiming for a launch date in 2013) against large forces up to 100G experienced by the satellite due to massive vibrations. As Nakajima just started this project, he is currently running simulations to reduce stress on the BGO crystal shields surrounding the x-ray detector. For example, corners of the shield are prone to most stress as the area is small, so the stress on a flattened corner is compared with the normal sharp corner.
His other experimental project is to create an efficient collimator for the x-ray detector component. The collimator allows only x-rays that are parallel to the wall of the collimator to be detected, and blocks the other background rays from other stars that come in non-parallel to the detector. Nakajima is currently investigating gold and tungsten as possible materials to create the most efficient collimator with. The absorption of background rays is proportional with the atomic number of the element – however, with a higher atomic number activation occurs, where the cosmic rays cause the element to change into different elements, emitting radiation such as x-rays and gamma rays through the process. Nakajima is going through experiments that collect data of gold’s and tungsten’s radiation from activation, so it can be subtracted from the data collected from the detector.
Finally, Nakajima does data analysis on galaxy clusters, specifically on cluster mergers. This merging of clusters is caused by infinitely strong gravity pulling the clusters together. By using x-ray observations on merging clusters, he can find out if what is observed is a cluster merger or the aftermath of the merge. X-rays only allows the ICM (intracluster medium) of clusters to be seen. When there is a cluster merger, high temperature and an odd shape of the clusters merging are observed. On the other hand, after the merger a constant temperature over one big cluster is found. The Suzaku satellite is the provider of the x-ray data, as it allows a wider range of x-rays for observation.
Hiroyuki Nishioka is a first year Masters student, working hard on an experimental project of creating an efficient shield for the x-ray detector component for the Astro-H satellite that is aiming for a launch date in 2013.
Nishioka is currently working with a BGO crystal active shield that acts as a scintillator. Active shields are not as heavy as passive shields (for instance like lead that only blocks rays), so it is preferred over. Stray rays that are not needed for data collection are found by the fact that the BGO shield reacts to the ray (by producing visible light) along with the main detector reacting to the ray. Nishioka changes various aspects of the BGO shield to make it most efficiently pass all photons made when a ray hits the BGO shield to the detector. Thus, stray rays are detected as data as they hit the detector, but they also hit the shield. By not adopting the data in which the detector and shield are hit together, true signals can be distinguished from other signals.
For example, Nishioka alters the shape of the BGO shield, such as into a trapezoid shape instead of a rectangular box, to find how the light reflects inside it as it is passed onto the detector. Another example is by making the surface of the BGO shield smooth or rough to see how much light is reflected or absorbed. Of course, not all sides have to be the same – some sides can be smooth while the others are rough. Additionally, Nishioka is finding out if it is more efficient to stick the walls of the shield together or apart. Both have downsides: if the walls are together light can mix up inside, but if the walls are apart light can escape.
Along with collecting experimental data, Nishioka runs simulations as well. These simulations allow Nishioka to understand the physical process when photons travel in the BGO shield – for example, the number of times the photons are reflected, and where in the shield the photons are absorbed. By understand this physical process, the most effective BGO shield design can be found efficiently. Altering the BGO shield for various shapes is not only expensive but time-consuming as well; thus, running simulations are vital for Nishioka.
The currently widely accepted view of what AGNs are is extreme Kerr black holes. By using the Suzaku satellite’s hard X-ray observing abilities, hard X-ray excess components were found. This indicates strong reflections from the AGN, which in turn points to bright reflections. Following this path of logic, AGNs are thought to be Kerr black holes.
However, when Noda himself did some data analysis on a particular AGN (MCG -6-30-15), he found two issues that caused him to wonder if AGNs were really extreme Kerr black holes. One issue is that the reflections are too strong. Another one is that the observed variations are not like variations that are expected. Furthermore, along with these two issues, an “independent variable component” was found. By the “independent variable component” being found, Noda believes that the data from this AGN (MCG -6-30-15) is not by the broad Fe line but rather by continuum – if this is true then these two issues can be solved. To prove this conjecture and to further find out if this can be applied to other AGNs, Noda is continuing to observe and study AGNs.
Shunsuke Torii is a first year Masters student. He is currently working on an experimental project of creating a Compton Camera, which is part of the SGD (Soft Gamma-ray Detector) that will be carried in the satellite Astro-H that will be launched in 2013.
The Compton Camera consists of layers of semiconductors on top of each other. As an x-ray hits a semiconductor, Compton scattering occurs, and then the scattered gamma ray is absorbed. The two energies related to these two events can be used to calculate the angle that indicates where the source ray was from.
A collimator is set on top of the Compton Camera. This alignment makes it possible to ignore background rays, as the angles calculated from the background rays would be different from the angle calculated from the target ray.
Torii is specifically working on making the semiconductors of the Compton Camera.
Ozden Sengul is a Research Associate from Turkey who joined the Makishima-Nakazawa Group in April 2009. She is currently aiming to enter the graduate school here at the University of Tokyo, hoping to start her term from April 2010. In preparation for entering graduate school, presently she mostly studies for the upcoming GRE.
Before coming here, she did some research with x-ray astronomy, more specifically with galaxy clusters. Although at first she was a student in the physics education department of her university, she gained interest in x-ray astronomy after taking an astronomy course. Since then she took more x-ray astronomy courses, made a presentation at a student conference about x-ray astronomy, and did some data analysis of galaxy clusters.
After getting accepted into graduate school she hopes to continue on with x-ray astronomy, especially about galaxy clusters. Although galaxy clusters are observable by optical observations, it is preferable to use x-ray observations as some information about the structure and properties of galaxy clusters can be obtained using x-ray observations as x-rays have higher energy.