Jun Kataoka - Academia.edu (original) (raw)

Papers by Jun Kataoka

Research paper thumbnail of Citizen science observation of a gamma-ray glow associated with the initiation of a lightning flash

Gamma-ray glows are observational evidence of relativistic electron acceleration due to the elect... more Gamma-ray glows are observational evidence of relativistic electron acceleration due to the electric field in thunderclouds. However, it is yet to be understood whether such relativistic electrons contribute to the initiation of lightning discharges. To tackle this question, we started the citizen science “Thundercloud Project’, where we map radiation measurements of glows from winter thunderclouds along Japan sea coast area. We developed and deployed 58 compact gamma-ray monitors at the end of 2021. On 30 December 2021, five monitors simultaneously detected a glow with its radiation distribution horizontally extending for 2 km.The glow terminated coinciding with a lightning flash at 04:08:34 JST, which was recorded by the two radio-band lightning mapping systems, FALMA and DALMA. The initial discharges during the preliminary breakdown started above the glow, i.e., in vicinity of the electron acceleration site. This result provides one example of possible connections between electro...

Research paper thumbnail of FIRSTNuSTAROBSERVATIONS OF MRK 501 WITHIN A RADIO TO TeV MULTI-INSTRUMENT CAMPAIGN

The Astrophysical Journal, 2015

We report on simultaneous broadband observations of the TeV-emitting blazar Markarian 501 between... more We report on simultaneous broadband observations of the TeV-emitting blazar Markarian 501 between 2013 April 1 and August 10, including the first detailed characterization of the synchrotron peak with Swift and NuSTAR. During the campaign, the nearby BL Lac object was observed in both a quiescent and an elevated state. The broadband campaign includes observations with NuSTAR, MAGIC, VERITAS, the Fermi Large Area Telescope, Swift X-ray Telescope and UV Optical Telescope, various ground-based optical instruments, including the GASP-WEBT program, as well as radio observations by OVRO, Metsähovi, and the F-Gamma consortium. Some of the MAGIC observations were affected by a sand layer from the Saharan desert, and had to be corrected using eventby-event corrections derived with a Light Detection and Ranging (LIDAR) facility. This is the first time that LIDAR information is used to produce a physics result with Cherenkov Telescope data taken during adverse atmospheric conditions, and hence sets a precedent for the current and future ground-based gamma-ray instruments. The NuSTAR instrument provides unprecedented sensitivity in hard X-rays, showing the source to display a spectral energy distribution (SED) between 3 and 79 keV consistent with a log-parabolic spectrum and hard X-ray variability on hour timescales. None (of the four extended NuSTAR observations) show evidence of the onset of inverse-Compton emission at hard X-ray energies. We apply a single-zone equilibrium synchrotron self-Compton (SSC) model to five simultaneous broadband SEDs. We find that the SSC model can reproduce the observed broadband states through a decrease in the magnetic field strength coinciding with an increase in the luminosity and hardness of the relativistic leptons responsible for the high-energy emission.

Research paper thumbnail of SUZAKU WIDE BAND ANALYSIS OF THE X-RAY VARIABILITY OF TeV BLAZAR Mrk 421 IN 2006

The Astrophysical Journal, 2009

We present the results of X-ray observations of the well-studied TeV blazar Mrk 421 with the Suza... more We present the results of X-ray observations of the well-studied TeV blazar Mrk 421 with the Suzaku satellite in 2006 April 28. During the observation, Mrk 421 was undergoing a large flare and the X-ray flux was variable, decreasing by ∼ 50 %, from 7.8 × 10 −10 to 3.7 × 10 −10 erg s −1 cm −2 in about 6 hours, followed by an increase by ∼ 35 %. Thanks to the broad bandpass coupled with high-sensitivity of Suzaku, we measured the evolution of the spectrum over the 0.4-60 keV band in data segments as short as ∼ 1 ksec. The data show deviations from a simple power law model, but also a clear spectral variability. The time-resolved spectra are fitted by a synchrotron model, where the observed spectrum is due to a exponentially cutoff power law distribution of electrons radiating in uniform magnetic field; this model is preferred over a broken power law. As another scenario, we separate the spectrum into "steady" and "variable" components by subtracting the spectrum in the lowest-flux period from those of other data segments. In this context, the difference ("variable") spectra are all well described by a broken power law model with photon index Γ ∼ 1.6, breaking at energy ǫ brk ≃ 3 keV to another photon index Γ ∼ 2.1 above the break energy, differing from each other only by normalization, while the spectrum of the "steady" component is best described by the synchrotron model. We suggest the rapidly variable component is due to relatively localized shock (Fermi I) acceleration, while the slowly variable ("steady") component is due to the superposition of shocks located at larger distance along the jet, or due to other acceleration process, such as the stochastic acceleration on magnetic turbulence (Fermi II) in the more extended region.

Research paper thumbnail of Characteristic X‐Ray Variability of TeV Blazars: Probing the Link between the Jet and the Central Engine

The Astrophysical Journal, 2001

We have studied the rapid X-ray variability of three extragalactic TeV gammaray sources; Mrk 421,... more We have studied the rapid X-ray variability of three extragalactic TeV gammaray sources; Mrk 421, Mrk 501 and PKS 2155−304. Analyzing the X-ray light curves obtained from ASCA and/or RXTE observations between 1993 and 1998, we have investigated the variability in the time domain from 10 3 to 10 8 sec. For all three sources, both the power spectrum density (PSD) and the structure function (SF) show a roll-over with a timescale of the order of 1 day or longer, which may be interpreted as the typical timescale of successive flare events. Although the exact shape of turnover is not well constrained and the low-frequency (long timescale) behavior is still unclear, the high-frequency (short timescale) behavior is clearly resolved. We found that, on timescales shorter than 1 day, there is only small power in the variability, as indicated by a steep power spectrum density of f −2∼−3. This is very different from other types of mass-accreting black-hole systems for which the short timescale variability is well characterized by a fractal, flickering-noise PSD (f −1∼−2). The steep PSD index and the characteristic timescale of flares imply that the X-ray emitting site in the jet is of limited spatial extent; D ≥ 10 17 cm distant from the base of the jet, which corresponds to ≥ 10 2 Schwarzschild radii for 10 7−10 M black-hole systems.

Research paper thumbnail of Variability pattern and the spectral evolution of blazars

Astronomische Nachrichten, 1999

Research paper thumbnail of X-ray Emission Properties of Large Scale Jets, Hotspots and Lobes in Active Galactic Nuclei

Weexamineasystematiccomparisonofjetknots,hotspots,andradiolobesrecentlyobservedwithChandraand ASC... more Weexamineasystematiccomparisonofjetknots,hotspots,andradiolobesrecentlyobservedwithChandraand ASCA. This report discusses the origin of their X-ray emissions and investigates the dynamics of the jets. The data were compiled at well-sampled radio (5 GHz) and X-ray (1 keV) frequencies for more than 40 radio galaxies. We examinethreemodelsfortheX-rayproduction:synchrotron(SYN),synchrotronself-Compton(SSC),andexternal Compton (EC) on cosmic microwave background (CMB) photons. For the SYN sources—mostly jet knots in nearby low-luminosity radio galaxies—X-rayphotonsare produced byultrarelativistic electrons with energies 10– 100 TeV that must be accelerated in situ. For the other objects, conservatively classified as SSC or EC sources, a simple formulation of calculating the ‘‘expected’’ X-ray fluxes under an equipartition hypothesis is presented. We confirm that the observed X-ray fluxes are close to the expected ones for nonrelativistic emitting plasma velocities in the case of radio lo...

Research paper thumbnail of Physics Performance of the Large Size Telescope prototype of the Cherenkov Telescope Array

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

The Large-Sized Telescope (LST) prototype of the future Cherenkov Telescope Array (CTA) is locate... more The Large-Sized Telescope (LST) prototype of the future Cherenkov Telescope Array (CTA) is located at the Northern site of CTA, on the Canary Island of La Palma. It is designed to provide optimal performance in the lowest part of the energy range covered by CTA, observing gamma rays down to energies of tens of GeV. The LST prototype started performing astronomical observations in November 2019 during the commissioning of the telescope and it has been taking data since then. In this contribution, we will present the tuning of the characteristics of the telescope in the Monte Carlo (MC) simulations to describe the data obtained, the estimation of its angular and energy resolution, and an evaluation of its sensitivity, both with simulations and with observations of the Crab Nebula.

Research paper thumbnail of Development and performance verification of a 3-D position-sensitive Compton camera for imaging MeV gamma rays

Scientific Reports

In gamma-ray astronomy, the 1–10 MeV range is one of the most challenging energy bands to observe... more In gamma-ray astronomy, the 1–10 MeV range is one of the most challenging energy bands to observe owing to low photon signals and a considerable amount of background contamination. This energy band, however, comprises a substantial number of nuclear gamma-ray lines that may hold the key to understanding the nucleosynthesis at the core of stars, spatial distribution of cosmic rays, and interstellar medium. Although several studies have attempted to improve observation of this energy window, development of a detector for astronomy has not progressed since NASA launched the Compton Gamma Ray Observatory (CGRO) in 1991. In this work, we first developed a prototype 3-D position-sensitive Compton camera (3D-PSCC), and then conducted a performance verification at NewSUBARU, Hyogo in Japan. To mimic the situation of astronomical observation, we used a MeV gamma-ray beam produced by laser inverse Compton scattering. As a result, we obtained sharp peak images of incident gamma rays irradiatin...

Research paper thumbnail of Inverse Compton Scattering of Starlight in the Kiloparsec-scale Jet in Centaurus A: The Origin of Excess TeV γ-Ray Emission

The Astrophysical Journal

Centaurus A (Cen A) is the nearest active radio galaxy, which has kiloparsec-scale jets and giant... more Centaurus A (Cen A) is the nearest active radio galaxy, which has kiloparsec-scale jets and giant lobes detected by various instruments in radio and X-ray frequency ranges. The Fermi-Large Area Telescope and High Energy Stereoscopic System (HESS) confirmed that CenA is a very high-energy (VHE; >0.1 TeV) γ-ray emitter with a known spectral softening in the energy range from a few GeV to TeV. In this work, we consider a synchrotron self-Compton model in the nucleus for the broadband spectrum below the break energy and an external Compton model in kiloparsec-scale jets for the γ-ray excess. Our results show that the observed γ-ray excess can be suitably described by the inverse Compton scattering of the starlight photons in the kiloparsec-scale jets, which is consistent with the recent tentative report by HESS on the spatial extension of the TeV emission along the jets. Considering the spectral fitting results, the excess can only be seen in CenA, which is probably due to two factors: (1) the host galaxy is approximately 50 times more luminous than other typical radio galaxies and (2) the core γ-ray spectrum quickly decays above a few MeV due to the low maximum electron Lorentz factor of γ c =2.8×10 3 resulting from the large magnetic field of 3.8 G in the core. By the comparison with other γ-ray detected radio galaxies, we found that the magnetic field strength of relativistic jets scales with the distance from the central black holes d with B(d) ∝ d −0.88±0.14 .

Research paper thumbnail of Hard x-ray imager onboard Hitomi (ASTRO-H)

Journal of Astronomical Telescopes, Instruments, and Systems

The hard x-ray imaging spectroscopy system of "Hitomi" x-ray observatory is composed of two sets ... more The hard x-ray imaging spectroscopy system of "Hitomi" x-ray observatory is composed of two sets of hard x-ray imagers (HXI) coupled with hard x-ray telescopes (HXT). With a 12-m focal length, the system provides fine (1 0 :7 half-power diameter) imaging spectroscopy covering about 5 to 80 keV. The HXI sensor consists of a camera, which is composed of four layers of Si and one layer of CdTe semiconductor imagers, and an active shield composed of nine Bi 4 Ge 3 O 12 scintillators to provide low background. The two HXIs started observation on March 8 and 14, 2016 and were operational until 26 March. Using a Crab observation, 5 to 80 keV energy coverage and good detection efficiency were confirmed. The detector background level of 1 to 3 × 10 −4 counts s −1 keV −1 cm −2 (in detector geometrical area) at 5 to 80 keV was achieved, by cutting the high-background time-intervals, adopting sophisticated energy-dependent imager layer selection, and baffling of the cosmic x-ray background and active-shielding. This level is among the lowest of detectors working in this energy band. By comparing the effective area and the background, it was shown that the HXI had a sensitivity that is same to that of NuSTAR for point sources and 3 to 4 times better for largely extended diffuse sources. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Research paper thumbnail of Hitomi observations of the LMC SNR N 132 D: Highly redshifted X-ray emission from iron ejecta*

Publications of the Astronomical Society of Japan

We present Hitomi observations of N 132 D, a young, X-ray bright, O-rich core-collapse supernova ... more We present Hitomi observations of N 132 D, a young, X-ray bright, O-rich core-collapse supernova remnant in the Large Magellanic Cloud (LMC). Despite a very short observation of only 3.7 ks, the Soft X-ray Spectrometer (SXS) easily detects the line complexes of highly ionized S K and Fe K with 16-17 counts in each. The Fe feature is measured for the first time at high spectral resolution. Based on the plausible assumption that the Fe K emission is dominated by He-like ions, we find that the material responsible for this Fe emission is highly redshifted at ∼ 800 km s −1 compared to the local LMC interstellar medium (ISM), with a 90% credible interval of 50-1500 km s −1 if a weakly informative prior is placed on possible line broadening. This indicates (1) that the Fe emission arises from the supernova ejecta, and (2) that these ejecta are highly asymmetric, since no blueshifted component is found. The S K velocity is consistent with the local LMC ISM, and is likely from swept-up ISM material. These results are consistent with spatial mapping that shows the He-like Fe concentrated in the interior of the remnant and the S tracing the outer shell. The results also show that even with a very small number of counts, direct velocity measurements from Doppler-shifted lines detected in extended objects like supernova remnants are now possible. Thanks to the very low SXS background of ∼ 1 event per spectral resolution element per 100 ks, such results are obtainable during short pointed or slew observations with similar instruments. This highlights the power of high-spectral-resolution imaging observations, and demonstrates the new window that has been opened with Hitomi and will be greatly widened with future missions such as the X-ray Astronomy Recovery Mission (XARM) and Athena.

Research paper thumbnail of Glimpse of the highly obscured HMXB IGR J16318−4848 with Hitomi*

Publications of the Astronomical Society of Japan

We report a Hitomi observation of IGR J16318-4848, a high-mass X-ray binary system with an extrem... more We report a Hitomi observation of IGR J16318-4848, a high-mass X-ray binary system with an extremely strong absorption of N H ∼ 10 24 cm −2. Previous X-ray studies revealed that its spectrum is dominated by strong fluorescence lines of Fe as well as continuum emission. For physical and geometrical insight into the nature of the reprocessing material, we utilize the high spectroscopic resolving power of the X-ray microcalorimeter (the soft X-ray spectrometer; SXS) and the wide-band sensitivity by the soft and hard X-ray imager (SXI and HXI) aboard Hitomi. Even though photon counts are limited due to unintended off-axis pointing, the SXS spectrum resolves Fe Kα 1 and Kα 2 lines and puts strong constraints on the line centroid and width. The line width corresponds to the velocity of 160 +300 −70 km s −1. This represents the most accurate, and smallest, width measurement of this line made so far from any X-ray binary, much less than the Doppler broadening and shift expected from speeds which are characteristic of similar systems. Combined with the K-shell edge energy measured by the SXI and HXI spectra, the ionization state of Fe is estimated to be in the range of Fe I-IV. Considering the estimated ionization parameter and the distance between the X-ray source and the absorber, the density and thickness of the materials are estimated. The extraordinarily strong absorption and the absence of a Compton shoulder component is confirmed. These characteristics suggest reprocessing materials which are distributed in a narrow solid angle or scattering primarily with warm free electrons or neutral hydrogen. This measurement was achieved using the SXS detection of 19 photons. This provides strong motivation for follow-up observations of this and other X-ray binaries using the X-ray Astrophysics Recovery Mission, and other comparable future instruments.

Research paper thumbnail of Unifying X-ray Emission Properties of Large Scale Jets, Hotspots and Lobes in AGN

AIP Conference Proceedings

We examine a systematic comparison of jet-knots, hotspots and radio lobes recently observed with ... more We examine a systematic comparison of jet-knots, hotspots and radio lobes recently observed with Chandra and ASCA. This report will discuss the origin of their X-ray emissions and investigate the dynamics of the jets. The data was compiled at well sampled radio (5 GHz) and X-ray frequencies (1keV) for more than 40 radio galaxies. We examined three models for the X-ray production: synchrotron (SYN), synchrotron self-Compton (SSC) and external Compton on CMB photons (EC). For the SYN sources-mostly jet-knots in nearby low-luminosity radio galaxies-X-ray photons are produced by ultrarelativistic electrons with energies 10−100 TeV that must be accelerated in situ. For the other objects, conservatively classified as SSC or EC sources, a simple formulation of calculating the "expected" X-ray fluxes under an equipartition hypothesis is presented. We confirmed that the observed X-ray fluxes are close to the expected ones for nonrelativistic emitting plasma velocities in the case of radio lobes and majority of hotspots, whereas considerable fraction of jet-knots is too bright at X-rays to be explained in this way. We examined two possibilities to account for the discrepancy in a framework of the inverse-Compton model: (1) magnetic field is much smaller than the equipartition value, and (2) the jets are highly relativistic on kpc/Mpc scales.We also briefly discuss the other possibility, namely that the observed X-ray emission from all of the jet-knots is synchrotron in origin.

Research paper thumbnail of Long-term Variations in the Distribution of Radioactive Cs in Plant, Soil, Stream Bottom Sand in a Small Forest in Fukushima Prefecture

Research paper thumbnail of Performance and current status of Monitor of All-sky X-ray Image on the International Space Station

AIP Conference Proceedings

Monitor of All-sky X-ray Image (MAXI) will be launched and attached on the International Space St... more Monitor of All-sky X-ray Image (MAXI) will be launched and attached on the International Space Station (ISS) next year. MAXI scans the sky while ISS goes around the Earth and provides an all sky X-ray image every orbit. MAXI has two types of X-ray instruments: gas-proportional counters for 2-30 keV and CCD cameras for 0.5-10 keV. MAXI will be able

Research paper thumbnail of Evidence for a characteristic time-scale in the X-ray light curves of TeV blazars

AIP Conference Proceedings

We have studied the rapid X-ray variability of four TeV blazars, Mrk 421, Mrk 501, and Analyzing ... more We have studied the rapid X-ray variability of four TeV blazars, Mrk 421, Mrk 501, and Analyzing the X-ray light curves obtained from ASCA and/or RXTE observations, we have investigated the variability in the time domain from to sec. The structure functions show a roll-over ...

Research paper thumbnail of High Energy Observations of AGN Jets and Their Future Prospects

AIP Conference Proceedings, 2008

In next five years, dramatic progress is anticipated for the AGN studies, as we have two importan... more In next five years, dramatic progress is anticipated for the AGN studies, as we have two important missions to observe celestial sources in the high energy regime: GLAST and Suzaku. In this talk, I will summarize recent highlights in studies of AGN jets, focusing on the high-sensitivity X-ray observations that may shed new light on the forthcoming GLAST era. I will especially present some examples from most recent Suzaku observations of blazars, which provides important hints for the shock acceleration in sub-pc scale jets, as well as particle content in jets. Then I will focus on the neutral iron-line feature observed in some broad line radio galaxies, as a probe of jet launching and/or the disk-jet connection. Finally, I will discuss new results of large scale (kpc to Mpc) jets recently resolved with Chandra X-ray observatory. Simultaneous monitoring observations in various wavelengths will be particularly valuable for variable blazar sources, allowing the cross correlations of time series as well as detailed modeling of the spectral evolution between the X-ray and gammaray energy bands. Possible impacts of these new observations across the electromagnetic spectrum on various spatial scales are discussed to challenge the long-standing mystery of AGN jet sources.

Research paper thumbnail of Suzaku View of Powerful Gamma-Ray Quasars

Int J Mod Phys D, 2008

We present the results from multiwavelength campaigns of three powerful gamma-ray quasars, PKS 15... more We present the results from multiwavelength campaigns of three powerful gamma-ray quasars, PKS 1510-089, RBS 315 and Swift J0746.3+2548, recently organized with Suzaku. The Suzaku observation provided one of the highest S/N X-ray spectra ever reported between 0.3 and 50 keV. For these quasars, the X-ray spectrum is well represented by an extremely hard power-law with photon index Γ ≃ 1.2, but is augmented by an additional soft component apparently below 1 keV for PKS 1510-089, whereas a strong deficit of soft photons is observed in RBS 315. We model the broadband spectra of these powerful quasars and argue that the power of the jet is dominated by protons but with the number of electrons/positrons exceeding the number of protons by a factor ≃ 10. We also argue that an extremely hard X-ray spectra may result from a double power-law form of the injected electrons, with the break energy γ br ≃ 1000 corresponding to the anticipated threshold of diffusive shock acceleration.

Research paper thumbnail of ASCA Observation of X-ray/TeV Flare from Mkn42l

Blazar Continuum Variability, 1996

Research paper thumbnail of Suzaku Observations of Extreme MeV Blazar Swift J0746.3+2548

Astrophysical Journal, Dec 1, 2008

We report the Suzaku observations of the high luminosity blazar SWIFT J0746.3+2548 (J0746) conduc... more We report the Suzaku observations of the high luminosity blazar SWIFT J0746.3+2548 (J0746) conducted in November 2005. This object, with z = 2.979, is the highest redshift source observed in the Suzaku Guaranteed Time Observer (GTO) period, is likely to show high gamma-ray flux peaking in the MeV range. As a result of the good photon statistics and high signal-to-noise ratio spectrum, the Suzaku observation clearly confirms that J0746 has an extremely hard spectrum in the energy range of 0.3-24 keV, which is well represented by a single power-law with a photon index of Γ ph ≃ 1.17 and Galactic absorption. The multiwavelength spectral energy distribution of J0746 shows two continuum components, and is well modeled assuming that the high-energy spectral component results from Comptonization of the broad-line region photons. In this paper we search for the bulk Compton spectral features predicted to be produced in the soft X-ray band by scattering external optical/UV photons by cold electrons in a relativistic jet. We discuss and provide constraints on the pair content resulting from the apparent absence of such features.

Research paper thumbnail of Citizen science observation of a gamma-ray glow associated with the initiation of a lightning flash

Gamma-ray glows are observational evidence of relativistic electron acceleration due to the elect... more Gamma-ray glows are observational evidence of relativistic electron acceleration due to the electric field in thunderclouds. However, it is yet to be understood whether such relativistic electrons contribute to the initiation of lightning discharges. To tackle this question, we started the citizen science “Thundercloud Project’, where we map radiation measurements of glows from winter thunderclouds along Japan sea coast area. We developed and deployed 58 compact gamma-ray monitors at the end of 2021. On 30 December 2021, five monitors simultaneously detected a glow with its radiation distribution horizontally extending for 2 km.The glow terminated coinciding with a lightning flash at 04:08:34 JST, which was recorded by the two radio-band lightning mapping systems, FALMA and DALMA. The initial discharges during the preliminary breakdown started above the glow, i.e., in vicinity of the electron acceleration site. This result provides one example of possible connections between electro...

Research paper thumbnail of FIRSTNuSTAROBSERVATIONS OF MRK 501 WITHIN A RADIO TO TeV MULTI-INSTRUMENT CAMPAIGN

The Astrophysical Journal, 2015

We report on simultaneous broadband observations of the TeV-emitting blazar Markarian 501 between... more We report on simultaneous broadband observations of the TeV-emitting blazar Markarian 501 between 2013 April 1 and August 10, including the first detailed characterization of the synchrotron peak with Swift and NuSTAR. During the campaign, the nearby BL Lac object was observed in both a quiescent and an elevated state. The broadband campaign includes observations with NuSTAR, MAGIC, VERITAS, the Fermi Large Area Telescope, Swift X-ray Telescope and UV Optical Telescope, various ground-based optical instruments, including the GASP-WEBT program, as well as radio observations by OVRO, Metsähovi, and the F-Gamma consortium. Some of the MAGIC observations were affected by a sand layer from the Saharan desert, and had to be corrected using eventby-event corrections derived with a Light Detection and Ranging (LIDAR) facility. This is the first time that LIDAR information is used to produce a physics result with Cherenkov Telescope data taken during adverse atmospheric conditions, and hence sets a precedent for the current and future ground-based gamma-ray instruments. The NuSTAR instrument provides unprecedented sensitivity in hard X-rays, showing the source to display a spectral energy distribution (SED) between 3 and 79 keV consistent with a log-parabolic spectrum and hard X-ray variability on hour timescales. None (of the four extended NuSTAR observations) show evidence of the onset of inverse-Compton emission at hard X-ray energies. We apply a single-zone equilibrium synchrotron self-Compton (SSC) model to five simultaneous broadband SEDs. We find that the SSC model can reproduce the observed broadband states through a decrease in the magnetic field strength coinciding with an increase in the luminosity and hardness of the relativistic leptons responsible for the high-energy emission.

Research paper thumbnail of SUZAKU WIDE BAND ANALYSIS OF THE X-RAY VARIABILITY OF TeV BLAZAR Mrk 421 IN 2006

The Astrophysical Journal, 2009

We present the results of X-ray observations of the well-studied TeV blazar Mrk 421 with the Suza... more We present the results of X-ray observations of the well-studied TeV blazar Mrk 421 with the Suzaku satellite in 2006 April 28. During the observation, Mrk 421 was undergoing a large flare and the X-ray flux was variable, decreasing by ∼ 50 %, from 7.8 × 10 −10 to 3.7 × 10 −10 erg s −1 cm −2 in about 6 hours, followed by an increase by ∼ 35 %. Thanks to the broad bandpass coupled with high-sensitivity of Suzaku, we measured the evolution of the spectrum over the 0.4-60 keV band in data segments as short as ∼ 1 ksec. The data show deviations from a simple power law model, but also a clear spectral variability. The time-resolved spectra are fitted by a synchrotron model, where the observed spectrum is due to a exponentially cutoff power law distribution of electrons radiating in uniform magnetic field; this model is preferred over a broken power law. As another scenario, we separate the spectrum into "steady" and "variable" components by subtracting the spectrum in the lowest-flux period from those of other data segments. In this context, the difference ("variable") spectra are all well described by a broken power law model with photon index Γ ∼ 1.6, breaking at energy ǫ brk ≃ 3 keV to another photon index Γ ∼ 2.1 above the break energy, differing from each other only by normalization, while the spectrum of the "steady" component is best described by the synchrotron model. We suggest the rapidly variable component is due to relatively localized shock (Fermi I) acceleration, while the slowly variable ("steady") component is due to the superposition of shocks located at larger distance along the jet, or due to other acceleration process, such as the stochastic acceleration on magnetic turbulence (Fermi II) in the more extended region.

Research paper thumbnail of Characteristic X‐Ray Variability of TeV Blazars: Probing the Link between the Jet and the Central Engine

The Astrophysical Journal, 2001

We have studied the rapid X-ray variability of three extragalactic TeV gammaray sources; Mrk 421,... more We have studied the rapid X-ray variability of three extragalactic TeV gammaray sources; Mrk 421, Mrk 501 and PKS 2155−304. Analyzing the X-ray light curves obtained from ASCA and/or RXTE observations between 1993 and 1998, we have investigated the variability in the time domain from 10 3 to 10 8 sec. For all three sources, both the power spectrum density (PSD) and the structure function (SF) show a roll-over with a timescale of the order of 1 day or longer, which may be interpreted as the typical timescale of successive flare events. Although the exact shape of turnover is not well constrained and the low-frequency (long timescale) behavior is still unclear, the high-frequency (short timescale) behavior is clearly resolved. We found that, on timescales shorter than 1 day, there is only small power in the variability, as indicated by a steep power spectrum density of f −2∼−3. This is very different from other types of mass-accreting black-hole systems for which the short timescale variability is well characterized by a fractal, flickering-noise PSD (f −1∼−2). The steep PSD index and the characteristic timescale of flares imply that the X-ray emitting site in the jet is of limited spatial extent; D ≥ 10 17 cm distant from the base of the jet, which corresponds to ≥ 10 2 Schwarzschild radii for 10 7−10 M black-hole systems.

Research paper thumbnail of Variability pattern and the spectral evolution of blazars

Astronomische Nachrichten, 1999

Research paper thumbnail of X-ray Emission Properties of Large Scale Jets, Hotspots and Lobes in Active Galactic Nuclei

Weexamineasystematiccomparisonofjetknots,hotspots,andradiolobesrecentlyobservedwithChandraand ASC... more Weexamineasystematiccomparisonofjetknots,hotspots,andradiolobesrecentlyobservedwithChandraand ASCA. This report discusses the origin of their X-ray emissions and investigates the dynamics of the jets. The data were compiled at well-sampled radio (5 GHz) and X-ray (1 keV) frequencies for more than 40 radio galaxies. We examinethreemodelsfortheX-rayproduction:synchrotron(SYN),synchrotronself-Compton(SSC),andexternal Compton (EC) on cosmic microwave background (CMB) photons. For the SYN sources—mostly jet knots in nearby low-luminosity radio galaxies—X-rayphotonsare produced byultrarelativistic electrons with energies 10– 100 TeV that must be accelerated in situ. For the other objects, conservatively classified as SSC or EC sources, a simple formulation of calculating the ‘‘expected’’ X-ray fluxes under an equipartition hypothesis is presented. We confirm that the observed X-ray fluxes are close to the expected ones for nonrelativistic emitting plasma velocities in the case of radio lo...

Research paper thumbnail of Physics Performance of the Large Size Telescope prototype of the Cherenkov Telescope Array

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

The Large-Sized Telescope (LST) prototype of the future Cherenkov Telescope Array (CTA) is locate... more The Large-Sized Telescope (LST) prototype of the future Cherenkov Telescope Array (CTA) is located at the Northern site of CTA, on the Canary Island of La Palma. It is designed to provide optimal performance in the lowest part of the energy range covered by CTA, observing gamma rays down to energies of tens of GeV. The LST prototype started performing astronomical observations in November 2019 during the commissioning of the telescope and it has been taking data since then. In this contribution, we will present the tuning of the characteristics of the telescope in the Monte Carlo (MC) simulations to describe the data obtained, the estimation of its angular and energy resolution, and an evaluation of its sensitivity, both with simulations and with observations of the Crab Nebula.

Research paper thumbnail of Development and performance verification of a 3-D position-sensitive Compton camera for imaging MeV gamma rays

Scientific Reports

In gamma-ray astronomy, the 1–10 MeV range is one of the most challenging energy bands to observe... more In gamma-ray astronomy, the 1–10 MeV range is one of the most challenging energy bands to observe owing to low photon signals and a considerable amount of background contamination. This energy band, however, comprises a substantial number of nuclear gamma-ray lines that may hold the key to understanding the nucleosynthesis at the core of stars, spatial distribution of cosmic rays, and interstellar medium. Although several studies have attempted to improve observation of this energy window, development of a detector for astronomy has not progressed since NASA launched the Compton Gamma Ray Observatory (CGRO) in 1991. In this work, we first developed a prototype 3-D position-sensitive Compton camera (3D-PSCC), and then conducted a performance verification at NewSUBARU, Hyogo in Japan. To mimic the situation of astronomical observation, we used a MeV gamma-ray beam produced by laser inverse Compton scattering. As a result, we obtained sharp peak images of incident gamma rays irradiatin...

Research paper thumbnail of Inverse Compton Scattering of Starlight in the Kiloparsec-scale Jet in Centaurus A: The Origin of Excess TeV γ-Ray Emission

The Astrophysical Journal

Centaurus A (Cen A) is the nearest active radio galaxy, which has kiloparsec-scale jets and giant... more Centaurus A (Cen A) is the nearest active radio galaxy, which has kiloparsec-scale jets and giant lobes detected by various instruments in radio and X-ray frequency ranges. The Fermi-Large Area Telescope and High Energy Stereoscopic System (HESS) confirmed that CenA is a very high-energy (VHE; >0.1 TeV) γ-ray emitter with a known spectral softening in the energy range from a few GeV to TeV. In this work, we consider a synchrotron self-Compton model in the nucleus for the broadband spectrum below the break energy and an external Compton model in kiloparsec-scale jets for the γ-ray excess. Our results show that the observed γ-ray excess can be suitably described by the inverse Compton scattering of the starlight photons in the kiloparsec-scale jets, which is consistent with the recent tentative report by HESS on the spatial extension of the TeV emission along the jets. Considering the spectral fitting results, the excess can only be seen in CenA, which is probably due to two factors: (1) the host galaxy is approximately 50 times more luminous than other typical radio galaxies and (2) the core γ-ray spectrum quickly decays above a few MeV due to the low maximum electron Lorentz factor of γ c =2.8×10 3 resulting from the large magnetic field of 3.8 G in the core. By the comparison with other γ-ray detected radio galaxies, we found that the magnetic field strength of relativistic jets scales with the distance from the central black holes d with B(d) ∝ d −0.88±0.14 .

Research paper thumbnail of Hard x-ray imager onboard Hitomi (ASTRO-H)

Journal of Astronomical Telescopes, Instruments, and Systems

The hard x-ray imaging spectroscopy system of "Hitomi" x-ray observatory is composed of two sets ... more The hard x-ray imaging spectroscopy system of "Hitomi" x-ray observatory is composed of two sets of hard x-ray imagers (HXI) coupled with hard x-ray telescopes (HXT). With a 12-m focal length, the system provides fine (1 0 :7 half-power diameter) imaging spectroscopy covering about 5 to 80 keV. The HXI sensor consists of a camera, which is composed of four layers of Si and one layer of CdTe semiconductor imagers, and an active shield composed of nine Bi 4 Ge 3 O 12 scintillators to provide low background. The two HXIs started observation on March 8 and 14, 2016 and were operational until 26 March. Using a Crab observation, 5 to 80 keV energy coverage and good detection efficiency were confirmed. The detector background level of 1 to 3 × 10 −4 counts s −1 keV −1 cm −2 (in detector geometrical area) at 5 to 80 keV was achieved, by cutting the high-background time-intervals, adopting sophisticated energy-dependent imager layer selection, and baffling of the cosmic x-ray background and active-shielding. This level is among the lowest of detectors working in this energy band. By comparing the effective area and the background, it was shown that the HXI had a sensitivity that is same to that of NuSTAR for point sources and 3 to 4 times better for largely extended diffuse sources. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Research paper thumbnail of Hitomi observations of the LMC SNR N 132 D: Highly redshifted X-ray emission from iron ejecta*

Publications of the Astronomical Society of Japan

We present Hitomi observations of N 132 D, a young, X-ray bright, O-rich core-collapse supernova ... more We present Hitomi observations of N 132 D, a young, X-ray bright, O-rich core-collapse supernova remnant in the Large Magellanic Cloud (LMC). Despite a very short observation of only 3.7 ks, the Soft X-ray Spectrometer (SXS) easily detects the line complexes of highly ionized S K and Fe K with 16-17 counts in each. The Fe feature is measured for the first time at high spectral resolution. Based on the plausible assumption that the Fe K emission is dominated by He-like ions, we find that the material responsible for this Fe emission is highly redshifted at ∼ 800 km s −1 compared to the local LMC interstellar medium (ISM), with a 90% credible interval of 50-1500 km s −1 if a weakly informative prior is placed on possible line broadening. This indicates (1) that the Fe emission arises from the supernova ejecta, and (2) that these ejecta are highly asymmetric, since no blueshifted component is found. The S K velocity is consistent with the local LMC ISM, and is likely from swept-up ISM material. These results are consistent with spatial mapping that shows the He-like Fe concentrated in the interior of the remnant and the S tracing the outer shell. The results also show that even with a very small number of counts, direct velocity measurements from Doppler-shifted lines detected in extended objects like supernova remnants are now possible. Thanks to the very low SXS background of ∼ 1 event per spectral resolution element per 100 ks, such results are obtainable during short pointed or slew observations with similar instruments. This highlights the power of high-spectral-resolution imaging observations, and demonstrates the new window that has been opened with Hitomi and will be greatly widened with future missions such as the X-ray Astronomy Recovery Mission (XARM) and Athena.

Research paper thumbnail of Glimpse of the highly obscured HMXB IGR J16318−4848 with Hitomi*

Publications of the Astronomical Society of Japan

We report a Hitomi observation of IGR J16318-4848, a high-mass X-ray binary system with an extrem... more We report a Hitomi observation of IGR J16318-4848, a high-mass X-ray binary system with an extremely strong absorption of N H ∼ 10 24 cm −2. Previous X-ray studies revealed that its spectrum is dominated by strong fluorescence lines of Fe as well as continuum emission. For physical and geometrical insight into the nature of the reprocessing material, we utilize the high spectroscopic resolving power of the X-ray microcalorimeter (the soft X-ray spectrometer; SXS) and the wide-band sensitivity by the soft and hard X-ray imager (SXI and HXI) aboard Hitomi. Even though photon counts are limited due to unintended off-axis pointing, the SXS spectrum resolves Fe Kα 1 and Kα 2 lines and puts strong constraints on the line centroid and width. The line width corresponds to the velocity of 160 +300 −70 km s −1. This represents the most accurate, and smallest, width measurement of this line made so far from any X-ray binary, much less than the Doppler broadening and shift expected from speeds which are characteristic of similar systems. Combined with the K-shell edge energy measured by the SXI and HXI spectra, the ionization state of Fe is estimated to be in the range of Fe I-IV. Considering the estimated ionization parameter and the distance between the X-ray source and the absorber, the density and thickness of the materials are estimated. The extraordinarily strong absorption and the absence of a Compton shoulder component is confirmed. These characteristics suggest reprocessing materials which are distributed in a narrow solid angle or scattering primarily with warm free electrons or neutral hydrogen. This measurement was achieved using the SXS detection of 19 photons. This provides strong motivation for follow-up observations of this and other X-ray binaries using the X-ray Astrophysics Recovery Mission, and other comparable future instruments.

Research paper thumbnail of Unifying X-ray Emission Properties of Large Scale Jets, Hotspots and Lobes in AGN

AIP Conference Proceedings

We examine a systematic comparison of jet-knots, hotspots and radio lobes recently observed with ... more We examine a systematic comparison of jet-knots, hotspots and radio lobes recently observed with Chandra and ASCA. This report will discuss the origin of their X-ray emissions and investigate the dynamics of the jets. The data was compiled at well sampled radio (5 GHz) and X-ray frequencies (1keV) for more than 40 radio galaxies. We examined three models for the X-ray production: synchrotron (SYN), synchrotron self-Compton (SSC) and external Compton on CMB photons (EC). For the SYN sources-mostly jet-knots in nearby low-luminosity radio galaxies-X-ray photons are produced by ultrarelativistic electrons with energies 10−100 TeV that must be accelerated in situ. For the other objects, conservatively classified as SSC or EC sources, a simple formulation of calculating the "expected" X-ray fluxes under an equipartition hypothesis is presented. We confirmed that the observed X-ray fluxes are close to the expected ones for nonrelativistic emitting plasma velocities in the case of radio lobes and majority of hotspots, whereas considerable fraction of jet-knots is too bright at X-rays to be explained in this way. We examined two possibilities to account for the discrepancy in a framework of the inverse-Compton model: (1) magnetic field is much smaller than the equipartition value, and (2) the jets are highly relativistic on kpc/Mpc scales.We also briefly discuss the other possibility, namely that the observed X-ray emission from all of the jet-knots is synchrotron in origin.

Research paper thumbnail of Long-term Variations in the Distribution of Radioactive Cs in Plant, Soil, Stream Bottom Sand in a Small Forest in Fukushima Prefecture

Research paper thumbnail of Performance and current status of Monitor of All-sky X-ray Image on the International Space Station

AIP Conference Proceedings

Monitor of All-sky X-ray Image (MAXI) will be launched and attached on the International Space St... more Monitor of All-sky X-ray Image (MAXI) will be launched and attached on the International Space Station (ISS) next year. MAXI scans the sky while ISS goes around the Earth and provides an all sky X-ray image every orbit. MAXI has two types of X-ray instruments: gas-proportional counters for 2-30 keV and CCD cameras for 0.5-10 keV. MAXI will be able

Research paper thumbnail of Evidence for a characteristic time-scale in the X-ray light curves of TeV blazars

AIP Conference Proceedings

We have studied the rapid X-ray variability of four TeV blazars, Mrk 421, Mrk 501, and Analyzing ... more We have studied the rapid X-ray variability of four TeV blazars, Mrk 421, Mrk 501, and Analyzing the X-ray light curves obtained from ASCA and/or RXTE observations, we have investigated the variability in the time domain from to sec. The structure functions show a roll-over ...

Research paper thumbnail of High Energy Observations of AGN Jets and Their Future Prospects

AIP Conference Proceedings, 2008

In next five years, dramatic progress is anticipated for the AGN studies, as we have two importan... more In next five years, dramatic progress is anticipated for the AGN studies, as we have two important missions to observe celestial sources in the high energy regime: GLAST and Suzaku. In this talk, I will summarize recent highlights in studies of AGN jets, focusing on the high-sensitivity X-ray observations that may shed new light on the forthcoming GLAST era. I will especially present some examples from most recent Suzaku observations of blazars, which provides important hints for the shock acceleration in sub-pc scale jets, as well as particle content in jets. Then I will focus on the neutral iron-line feature observed in some broad line radio galaxies, as a probe of jet launching and/or the disk-jet connection. Finally, I will discuss new results of large scale (kpc to Mpc) jets recently resolved with Chandra X-ray observatory. Simultaneous monitoring observations in various wavelengths will be particularly valuable for variable blazar sources, allowing the cross correlations of time series as well as detailed modeling of the spectral evolution between the X-ray and gammaray energy bands. Possible impacts of these new observations across the electromagnetic spectrum on various spatial scales are discussed to challenge the long-standing mystery of AGN jet sources.

Research paper thumbnail of Suzaku View of Powerful Gamma-Ray Quasars

Int J Mod Phys D, 2008

We present the results from multiwavelength campaigns of three powerful gamma-ray quasars, PKS 15... more We present the results from multiwavelength campaigns of three powerful gamma-ray quasars, PKS 1510-089, RBS 315 and Swift J0746.3+2548, recently organized with Suzaku. The Suzaku observation provided one of the highest S/N X-ray spectra ever reported between 0.3 and 50 keV. For these quasars, the X-ray spectrum is well represented by an extremely hard power-law with photon index Γ ≃ 1.2, but is augmented by an additional soft component apparently below 1 keV for PKS 1510-089, whereas a strong deficit of soft photons is observed in RBS 315. We model the broadband spectra of these powerful quasars and argue that the power of the jet is dominated by protons but with the number of electrons/positrons exceeding the number of protons by a factor ≃ 10. We also argue that an extremely hard X-ray spectra may result from a double power-law form of the injected electrons, with the break energy γ br ≃ 1000 corresponding to the anticipated threshold of diffusive shock acceleration.

Research paper thumbnail of ASCA Observation of X-ray/TeV Flare from Mkn42l

Blazar Continuum Variability, 1996

Research paper thumbnail of Suzaku Observations of Extreme MeV Blazar Swift J0746.3+2548

Astrophysical Journal, Dec 1, 2008

We report the Suzaku observations of the high luminosity blazar SWIFT J0746.3+2548 (J0746) conduc... more We report the Suzaku observations of the high luminosity blazar SWIFT J0746.3+2548 (J0746) conducted in November 2005. This object, with z = 2.979, is the highest redshift source observed in the Suzaku Guaranteed Time Observer (GTO) period, is likely to show high gamma-ray flux peaking in the MeV range. As a result of the good photon statistics and high signal-to-noise ratio spectrum, the Suzaku observation clearly confirms that J0746 has an extremely hard spectrum in the energy range of 0.3-24 keV, which is well represented by a single power-law with a photon index of Γ ph ≃ 1.17 and Galactic absorption. The multiwavelength spectral energy distribution of J0746 shows two continuum components, and is well modeled assuming that the high-energy spectral component results from Comptonization of the broad-line region photons. In this paper we search for the bulk Compton spectral features predicted to be produced in the soft X-ray band by scattering external optical/UV photons by cold electrons in a relativistic jet. We discuss and provide constraints on the pair content resulting from the apparent absence of such features.