Active shielding is an effective technique to reduce background signals in hard X-ray detectors and to enable observing darker sources with high sensitivity in space. Usually the main detector is covered with some shield detectors made of scintillato
r crystals such as BGO (Bi$_4$Ge$_3$O$_{12}$), and the background signals are filtered out using anti-coincidence among them. Japanese X-ray observing satellites Suzaku and ASTRO-H employed this technique in their hard X-ray instruments observing at > 10 keV. In the next generation X-ray satellites, such as the NGHXT proposal, a single hybrid detector is expected to cover both soft (1-10 keV) and hard (> 10 keV) X-rays for effectiveness. However, present active shielding is not optimized for the soft X-ray band, 1-10 keV. For example, Bi and Ge, which are contained in BGO, have their fluorescence emission lines around 10 keV. These lines appear in the background spectra obtained by ASTRO-H Hard X-ray Imager, which are non-negligible in its observation energy band of 5-80 keV. We are now optimizing the design of active shields for both soft and hard X-rays at the same time. As a first step, we utilized a BGO crystal as a default material, and measured the L lines of Bi and K lines of Ge from it using the X-ray SOIPIX, XRPIX.
Magnetars are a special type of neutron stars, considered to have extreme dipole magnetic fields reaching ~1e+11 T. The magnetar 4U 0142+61, one of prototypes of this class, was studied in broadband X-rays (0.5-70 keV) with the Suzaku observatory. In
hard X-rays (15-40 keV), its 8.69 sec pulsations suffered slow phase modulations by +/-0.7 sec, with a period of ~15 hours. When this effect is interpreted as free precession of the neutron star, the object is inferred to deviate from spherical symmetry by ~1.6e-4 in its moments of inertia. This deformation, when ascribed to magnetic pressure, suggests a strong toroidal magnetic field, ~1e+12 T, residing inside the object. This provides one of the first observational approaches towards toroidal magnetic fields of magnetars.
We searched for evidence of line emission around 4keV from the northwestern rim of the supernova remnant RX J0852.0-4622 using Suzaku XIS data. Several papers have reported the detection of an emission line around 4.1keV from this region of the sky.
This line would arise from K-band fluorescence by Sc, the immediate decay product of 44Ti. We performed spectral analysis for the entire portion of the NW rim of the remnant within the XIS field of view, as well as various regions corresponding to regions of published claims of line emission. We found no line emission around 4.1keV anywhere, and are able to set a restrictive upper limit to the X-ray flux: 1.1x10^-6 s^-1 cm^-2 for the entire field. For every region, our flux upper limit falls below that of the previously claimed detection. Therefore, we conclude that, to date, no definite X-ray line feature from Sc-K emission has been detected in the NW rim of RX J0852.0-4622. Our negative-detection supports the recent claim that RX J0852-4622 is neither young (1700--4000 yr) nor nearby(~750 pc).
We report on the wide band spectra of SN 1006 as observed by Suzaku. Thermal and nonthermal emission are successfully resolved thanks to the excellent spectral response of Suzakus X-ray CCD XIS. The nonthermal emission cannot be reproduced by a simpl
e power-law model but needs a roll-off at 5.7$times 10^{16}$ Hz = 0.23 keV. The roll-off frequency is significantly higher in the northeastern rim than in the southwestern rim. We also have placed the most stringent upper limit of the flux above 10 keV using the Hard X-ray Detector.
