Chiharu Tanihata

Hard X-ray Detector (HXD) on board Suzaku

The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from 10 keV to 600 keV by combination of silicon PIN diodes and GSO scintillators. The HXD is designed to achieve an extremely low in-orbit back ground based on a combination of new techniques, including the concept of well-type active shield counter. With an effective area of 142 cm^2 at 20 keV and 273 cm2 at 150 keV, the background level at the sea level reached ~1x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 30 keV for the PI N diodes, and ~2x10^{-5} cts s^{-1} cm^{-2} keV^{-1} at 100 keV, and ~7x10^{-6} cts s^{-1} cm^{-2} keV^{-1} at 200 keV for the phoswich counter. Tight active shielding of the HXD results in a large array of guard counters surrounding the main detector parts. These anti-coincidence counters, made of ~4 cm thick BGO crystals, have a large effective area for sub-MeV to MeV gamma-rays. They work as an excellent gamma-ray burst monitor with limited angular resolution (~5 degree). The on-board signal-processing system and the data transmitted to the ground are also described.

Characteristic X-Ray Variability of TeV Blazars: Probing the Link between the Jet and the Central Engine

We have studied the rapid X-ray variability of three extragalactic TeV γ-ray sources: Mrk 421, Mrk 501, and PKS 2155-304. Analyzing the X-ray light curves obtained from ASCA and/or Rossi X-Ray Timing Explorer observations between 1993 and 1998, we have investigated the variability in the time domain from 103 to 108 s. For all three sources, both the power spectrum density (PSD) and the structure function (SF) show a rollover 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 ≥ 1017 cm distant from the base of the jet, which corresponds to ≥102 Schwarzschild radii for 107-10 M☉ black hole systems.

RXTE observations of 3C 273 between 1996 and 2000: variability time-scale and jet power

We present the results of a long-look monitoring of 3C 273 with RXTE between 1996 and 2000. A total of 230 observations amounts to a net exposure of 845 ks, with this spectral and variability analysis of 3C 273 covering the longest observation period available at hard X-ray energies. Flux variations by a factor of 4 have been detected over 4 yr, whereas flux variations of less than 30 per cent have been observed for individual flares on time-scales of ∼ 3d . Two temporal methods, the power spectrum density (PSD) and the structure function (SF), have been used to study the variability characteristics of 3C 273. The hard X-ray photon spectra generally show a power-law shape with a differential photon index of � � 1.6 ± 0.1. In 10 of 261 data segments, exceptions to power-law behaviour have been found: (i) an additional soft excess below 4 keV; and (ii) a broad Fe fluorescent line feature with EW ∼ 100‐200 eV. Our new observations of these previously reported X-ray features may imply that 3C 273 is a unique object whose hard X-ray emission occasionally contains a component that is not related to a beamed emission (Seyfert-like), but most hard X-rays are likely to originate in inverse Compton radiation from the relativistic jet (blazar-like). Multifrequency spectra from radio to γ -rays are presented in addition to our RXTE results. The X-ray time variability and spectral evolution are discussed in the framework of the beamed, synchrotron self-Compton picture. We consider the ‘power balance’ (both radiative and kinetic) between the accretion disc, the sub-parsec-scale jet and the 10-kpc-scale jet.

Rapid Synchrotron Flares from BL Lacertae Detected by ASCA and RXTE

We report the variable X-ray emission from BL Lacertae detected in the ASCA ToO observation conducted during the EGRET and RXTE pointings, coincident with the 1997 July outburst. The source showed a historically high state of X-ray, optical, and γ-ray emission, with its 2-10 keV flux peaking at ~3.3 × 10-11 ergs cm-2 s-1. This is more than 3 times higher than the value measured by ASCA in 1995. We detected two rapid flares that occurred only in the soft X-ray band, while the hard X-ray flux also increased, but decayed with a much longer timescale. Together with the requirement of a very steep and varying power law dominating the soft X-ray band in addition to the hard power law, we suggest that both the high-energy end of the synchrotron spectrum and the hard inverse Compton spectrum were visible in this source during the outburst. We discuss the possible origins of the observed variability timescales, and interpret the short timescales of the soft X-ray variability as reflecting the size of the emission region.

Preflight performance of the Astro-E hard X-ray detector

The hard x-ray detector (HXD) is one of the three experiments of the Astro-E mission, the fifth Japanese X-ray Satellite devoted to studies of high energy phenomena in the universe in the x-ray to soft gamma-ray region. Prepared for launch at the beginning of 200 via the newly developed M-V launch vehicle of the Institute of Space and Astronomical Science, the Astro-E is to be thrown in to a near-circular orbit of 550 km altitude, with an inclination of 31 degrees. The flight model has been finished assembled this year, and we carried out various tests to verify the performance. We acquired the background spectrum at sea level, and confirmed that our system is operating effectively in reducing the background level. The HXD will observe photons in the energy range of 10-600 keV, and the calculations based on the preflight calibration suggest that the HXD will have the highest sensitivity ever achieved in this energy range. We also verified that our electronic system will maintain its performance against charged particle events expected in orbit.

Fabrication of the ASTRO-E hard-x-ray detector

The Hard X-ray Detector (HXD) is one of the three instruments on the fifth Japanese cosmic X-ray satellite ASTRO-E, scheduled for launch in January 2000. The HXD covers a wide energy range of 10-600 keV, using 16 identical GSO/BGO phoswich-counter modules, of which the low-energy efficiency is greatly improved by adding 2 m-thick silicon PIN diodes. Production of the HXD has been completed and pre-flight calibration is now in progress. The design concept of the HXD sensor, detail of the production process, and a brief summary of the measured performance is reported.

Verification of the Astro-E Hard X-ray Detector based on newly developed ground support equipment

We report the first results of the ground test of the Hard X-ray Detector (HXD) on board the Astro-E mission, by means of the newly developed Ground Support Equipment (GSE). Astro-E will be launched in 2000 by a Japanese M-V rocket. In order to verify the detector system during the limited time before launch, fast and versatile GSE is necessary. For this, we have developed a flexible test system based on nine VME I/O boards for a SUN workstation. These boards carry reconfigurable Field Programmable Gate Arrays with 50,000 gates, together with 1 Mbyte SRAM devices tightly coupled to each FPGA device. As an application of using this GSE, we have tested the performance of a phoswitch unit of the Flight Model of the HXD. In this paper, we present a schematic view of the GSE highlighting the functional design,and the result of our ground test of the HXD-sensor under the high count rate environment expected in orbit.