High-Energy X-Ray Astrophysics Group

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Assistant Professor of Physics (2012-present)
Physics & Astronomy Department
James Madison University

KITP Scholar (2015-2017)
Kavli Institute for Theoretical Physics
University of California, Santa Barbara

Research Associate (2008-2012)
University of Maryland, Baltimore County
NASA Goddard Space Flight Center, MD

NASA Postdoctoral Fellow (2006-2008)
NASA Goddard Space Flight Center, MD

Montana State University, Bozeman MT (PhD)

Hi. I am an assistant professor in the Physics and Astronomy Department at James Madison University (Harrisonburg, VA). I am from Osaka, Japan.

My research expertise is high-energy X-ray astrophysics. I am studying physics of compact objects particularly focusing on various energetic phenomena produced by magnetized gas (plasma) near accretion disks under strong gravity of central black holes from a theoretical standpoint. By constructing semi-analytic models I have been investigating the physical properties of accreting plasma as well as outflows including their geometry and observational signatures that can in principle be spectroscopically detected in UV/X-ray band. My research is relevant for both galactic black hole binaries (i.e. small black holes) in our Milky Way Galaxy as well as active galactic nuclei (AGNs) at the center of many luminous galaxies (i.e. supermassive black holes) beyond our own Miky Way.

To this end I utilize the well-established theoretical framework called general relativistic magnetohydrodynamics (GRMHD) that describes the physical property of plasma in the presence of background magnetic fields under strong gravity. By numerically solving the MHD equations along with other conditions appropriate under the extreme environment, one can obtain interesting insights into its observational consequences from such plasma in "distorted" spacetime. While spending more time building models from the first principle, I also take advantage of the state-of-the-art data obtained with a number of space-born X-ray observatories as international missions; namely, Suzaku, Chandra, XMM-Newton. Finally, I also collaborate with observers across the globe to further develop physically more reliable models. For that matter, rigorous comparison with the latest data is indeed crucial for theoretical work.

To be specific, I am currently interested in the following projects from a theoretical standpoint;

[1] Magnetically-driven accretion-disk wind in X-ray

It's been well-known for the past decades that more than ~50% of active galactic nuclei (AGNs), both Seyfert galaxies and quasars, exhibit a series of rich absorption features in the observed UV/X-ray spectra (e.g. between ~1-9 keV; kilo-electron volts). These spectroscopic signatures are clearly blueshifted (in wavelength) in most of the sources indicating that the "absorbers" must be expelled from their central engine (i.e. supermassive black hole) outflowing towards us. In a number of data obtained with a number of space-based X-ray observatories (e.g. Chandra, Suzaku, XMM-Newton…etc.) many absorption lines from various ions have been identified and detected due to atomic processes (primarily electric photoionization). By measuring these line transitions of rich spectroscopic features, one can learn and constrain the physical nature of the X-ray absorbers both locally and globally. While one of the most promising scenarios suggests the presence of an accretion disk wind around a black hole, the exact geometry and their launching mechanism(s) are yet to be understood to date. Our group has been proposing a magnetically-driven disk-wind in which accreting plasma is eventually expelled by the action of a global magnetic field through the Lorentz force.

This work is under progress being extended also to the galactic binary systems.

[2] X-ray soft-excess from relativistic accreting plasma in black hole magnetosphere

The observed X-ray spectra of AGNs in general contain a number of rich spectral components. Among others, especially from the so called narrow-line Seyfert 1 galaxies — a sub-class of Seyfert AGNs, it's been noted that the soft X-ray band exhibits a pronounced level of "excess" photons" known as "soft excess" at around 0.1-0.2 keV. While obvious in X-ray data, it's been puzzling as to what causes this feature. Here in our research, we are proposing a scenario in which accreting plasma, plunging from an accretion disk, slows and gets compressed developing a shock front at some point before reaching the event horizon. The downstream flow is then sufficiently heated providing an ideal geometrical site where the electrons can be efficiently accelerated accordingly. These energetic electrons can then Compton up-scatter the incoming thermal photons from the accretion disk producing a "hot" spectral feature similar to the detected soft excess.

[3] Broad Fe line reverberation from a black hole accretion disk

An accretion disk around a black hole (whether stellar-mass black holes in galactic binaries or supermassive black holes in AGNs) is believed to be the very physical site where a number of interesting spectroscopic features are observed. Among those is Fe (iron) fluorescent emission line. Iron atoms in the accretion disk is photoionized by some external hard X-ray source(s) with an electron being transitioned up to a higher-energy level. Since the configuration is energetically unstable, it is followed by emitting line photons at ~6.4 keV for neutral Fe atom and higher line photons for ionized Fe atom. This is called Fe fluorescence and it is known well that the process can imprint a well-defined spectroscopic signature. By studying the observed Fe line, one can probe the physics of the innermost accretion disk in the vicinity of the black hole via strong general relativistic effects under the curved spacetime.

[4] Implementation of model spectra into xspec software package

With the calculated theoretical models in hand, we are also developing model spectra that can be implemented into the X-ray software tool, xspec, widely used in the X-ray astrophysics community originally developed at NASA/GSFC. This will provide an observational realization of what theoretical models predict and it is extremely useful for understanding the underlying physics that would otherwise be inaccessible from direct observations.

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