Research Topics

Compact Object : black holes, neutron stars, CVs, etc.

Blackhole Binaries

— The standard model of blackhole binaries and beyond — 

Blackhole binaries ( a system of a several solar mass black hole and a companion star ) is one of the strong X-ray emitters. It is because the accreting matter into a black hole releases its gravitational energy via X-ray.Through the X-ray analysis, we have confirmed the existence of a black hole.We reported that when its mass accretion rate becomes higher and are getting close to its Eddington luminosity,the standard picture (High/Soft state) of blackhole binaries breaks, and the Comptonized accretion disk emission (Very High state) and “slim disk”  appears (cf. Kubota et al. 2004).

On the other hand, when its mass accretion rate becomes lower, it is believed that accretion disk becomes truncated and thermal Compotonizing cloud appears (Low/Hard state). However, the geometry and origin of the seed photon input into the cloud is still unknown. To solve these issue, we analyzed Cygnus X-1 observed with Suzaku and successfully obtained its high-quality data. As a result, we revealed the spectral and timing property during Low/Hard state, and compiled the results (Makishima, Takahashi, Yamada et al. 2008).

The red spetrum is time-averaged one, while the green is the differential one.

The Intermediate Mass Black holes

— How was it created ? —

We can see several abnormally bright point sources in a spiral galaxy. We named it as ULX (Ultra-Luminous X-ray Sourece) at the first time, and presented the possibility that such a ULX can be interpreted as a heavy black holes with its mass ~ 10-100 solar. However, the temperature of the accretion disk is higher than a typical one. Thus, we regard its cause as a blackhole spin. Even if it is true, the birth of such a heavy blackhole is still mystery.

The Massive Mass Black holes or Active Galactic Nuclei

— one monster in one house —

The wide-band capability of Suzaku has allowed detailed spectral and timing studies of the iron emission line and the reprocessed 20–40 keV hump in accreting black holes. In particular, Miniutti et al. (2007) apparently reconfirmed the previous ASCA detection of the broad Fe-K line from the Syefert galaxy MCG –6-30-15, and interpreted the broadening as due to relativistic effects when line-emitting materials are located close to the gravitational radius Rg around an extreme Kerr black hole. Furthermore, they argued that the implied large equivalent width ( ∼ 320 eV) of the broad Fe-K line is consistent with the strong reflection requiring Ω/2π ≡ R 3. However, a very sophisticated model (“light bending model”) must be invoked in order to explain the large value of R that exceeds 2, and to reconcile the implied very close location of the reprocessing material with observed lack of variability in the Fe-K line and hard-hump intensities. Trying to find a more natural alternative to explain these properties of MCG –6-30-15 and similar ob jects, we here questioned the basic assumption, that the hard X-ray (> 10 keV) excess above an appropriately determined power-law is entirely due to a reflection component. For this purpose, we analyzed 30 archival Suzaku datasets of 18 AGNs, mainly Seyfert 1’s, focusing on their spectral variability. (Uehara’s Master thesis 2009) 


— 雷雲電場で加速された電子からの制動放射ガンマ線の観測 —

宇宙物理学や天文学、あるいは地球惑星物理学にまたがる重要なテーマのひとつに、 高エネルギー粒子が「どこで」「どのように」「どれくらい」加速されたのかとい う謎があります。牧島研究室、中澤研究室がこれまで取り組んできた、宇宙最大の構造を誇る銀河 団や、極限環境のブラックホールや中性子星、白色矮星などでは、荷電粒子の一部が 加速され、わたしたちの地球にも降り注ぐ宇宙線になっていると考えられています。 このような遥か彼方を目指す巨大科学の一方で、わたしたちにとって身近な雷や雷雲 といった現象にも、宇宙の遥か彼方の極限環境で起きているような粒子加速が、実は 隠れていることが明らかになってきました。

中澤研究室の前身、牧島研究室では2006年度から、理化学研究所の牧島宇宙放射線研究室と合同で、日本 海側に放射線検出器を設置し、冬季雷雲に伴って報告されていた「謎の放射線現象」 の解明に着手しました。シベリア寒気団からの風が日本海上空を通過する際に形成す る冬季雷雲は、世界的に見てもエネルギーの大きな雷を生じ、雷雲の高度も低いとい う面白い特徴を持ちます。この雷雲に同期して環境放射線が短期間に増大したように 見える現象がこれまで報告されてきましたが、その正体はよくわかっていませんでした。

東大と理研の合同チーム(GROTH Collabolation)は、2006年度冬季の観測中、その冬で 最も巨大な低気圧が発達した2007年1月7日に、雷放電が生じる70秒ほど前におよそ40 秒ほどにわたって、上空から10MeVに至るガンマ線が到来している現象を検出することに 成功しました。この雷雲からのガンマ線は、雷雲内部の強い電場領域において、電子が 相対論的な領域まで加速された際の制動放射ガンマ線だと考えられ、地球表面から数km 足らずの雷雲内部において、実際に電子が加速されている証拠を掴んだものと考えられ ます。2007年度の冬季にも観測を続け、すでに新たな雷雲ガンマ線現象を一例観測して います。雷雲ガンマ線現象が実際に存在することが明らかになったいま、次には小型の 放射線検出器を多数並べるなど、最新の観測技術を取り入れつつ、さらなる観測を続け ていきます。

Terrestrial Gamma-ray Counter (TGC) for lightning observatory satellite SPRITE-sat 

— A mini Gamma-ray detector system for a mini satellite —
TGCs (the long aluminum boxes) mounted at the bottom of SPRITE-SAT

From the year 2007, we are involved in the development of Terrestrial Gamma-Ray Counter (TGC) mounted onboard SPRITE-sat mini-satellite (two long box mounted in the bottom of the satellite: see figure). The 50 kg weight mini-satellite launched in January 2009 is lead by Tohoku University. Members from our group and JAXA/ISAS, with collaboration with Tohoku university, are leading the TGC development, utilizing our long experiment on space-born X and gamma-ray detector development.  

Development of Network I/F for Satellite : SpaceWire

Small Sized Computer : SpaceCube

In a spacecraft, we put many instruments and modules so that the satellite can control its attitude, maintain battery, measure its temperature, and observe the space. For example, our X-ray detectors outputs data of X-ray photons, and they have to be transfered to onboard Data Recorder and Data Processor for further transmission to the ground. On the other hand, the detector needs to be controlled by scientists, e.g. changing gain parameters of amplifiers or trigger threshold levels, to improve sensitivity or efficiency of observations. Those controls are done via Command sent from the ground control center. Thus, the data transfer channel plays very important role inside a spacecraft.

ESA, NASA, JAXA and other space institutes have been continuing the standardization of a network interface “SpaceWire” which reduces satellite development cost, shortens development time, and makes integration test easier by providing the same interface for individual modules developed by, for example, companies in different country. USB and Ethernet are very good analogy used in our everyday life. Unfortunately, it is not easy to use SpaceWire’s these companions in space because of limited resources (power, area) and strong cosmic radiation. SpaceWire standard is very simple and deterministic. Those features mean that it can be implemented within a small-size logic circuit and that tests cannot be complicated.

SpaceWire Interface Circuit BoardWe have been involved in the SpaceWire development together with Takahashi&Kokubun Group at JAXA/ISAS and Nomachi Group at Osaka University since 2006. Right figure shows a SpaceWire-based Digital Input/Output Board which we developed with Shimafuji Electric. It can be used as a front-end circuit for a radiation detector (also developed by us), and SpaceCube computer (shown above) reads data stored in the board via SpaceWire. We also developed SpaceWire/RMAP Library for the computer, and are now using them to develop detectors planned to be flown in 2013 on ASTRO-H satellite.

One of our product “Open SpaceWire IP Core” can be obtained via Shimafuji’s website without any registration or payment!

Features of SpaceWire

  • Simple Interface and Simple Protocol (mean low power consumption)
  • Flexible network topology (“loop” is allowed for redundancy)
  • Wide transfer speed of 2-400Mbps
  • Infinite Packet Size

Features of SpaceWire

SpaceWire – Home at ESA
SpaceCube by Shimafuji Electoric