Studies were made of ASCA spectra of seven ultra-luminous compact X-ray sources (ULXs) in nearby spiral galaxies; M33 X-8 (Takano et al. two sources in NGC 4565 (Mizuno et al. 1999). With the 0.5-10 keV luminosities in the range 10 39−40 ergs s −1 , they are thought to represent a class of enigmatic X-ray sources often found in spiral galaxies. For some of them, the ASCA data are newly processed, or the published spectra are reanalyzed. For others, the published results are quoted. The ASCA spectra of all these seven sources have been described successfully with so called multi-color disk blackbody (MCD) emission arising from optically-thick standard accretion disks around black holes. Except the case of M33 X-8, the spectra do not exhibit hard tails. For the source luminosities not to exceed the Eddington limits, the black holes are inferred to have rather high masses, up to ∼ 100 solar masses. However, the observed innermost disk temperatures of these objects, T in = 1.1 − 1.8 keV, are too high to be compatible with the required high black-hole masses, as long as the standard accretion disks around Schwarzschild black holes are assumed. Similarly high disk temperatures are also observed from two Galactic transients with superluminal motions, GRO 1655-40 and GRS 1915+105. The issue of unusually high disk temperature may be explained by the black hole rotation, which makes the disk get closer to the black hole, and hence hotter.

High-sensitivity wide-band X-ray spectroscopy is the key feature of the Suzaku X-ray observatory, launched on 2005 July 10. This paper summarizes the spacecraft, in-orbit performance, operations, and data processing that are related to observations. The scientific instruments, the high-throughput X-ray telescopes, X-ray CCD cameras, non-imaging hard X-ray detector are also described.

High-resolution X-ray spectroscopy with Hitomi was expected to resolve the origin of the faint unidentified » E 3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the Hitomi first-light observation of the Perseus cluster. The emission line expected for Perseus based on the XMM-Newton signal from the large cluster sample under the dark matter decay scenario is too faint to be detectable in the Hitomi data. However, the previously reported 3.5 keV flux from Perseus was anomalously high compared to the sample-based prediction. We find no unidentified line at the reported high flux level. Taking into account the XMM measurement uncertainties for this region, the inconsistency with Hitomi is at a 99% significance for a broad dark matter line and at 99.7% for a narrow line from the gas. We do not find anomalously high fluxes of the nearby faint K line or the Ar satellite line that were proposed as explanations for the earlier 3.5 keV detections. We do find a hint of a broad excess near the energies of high-n transitions of S XVI (  E 3.44 keV rest-frame)-a possible signature of charge exchange in the molecular nebula and another proposed explanation for the unidentified line. While its energy is consistent with XMM pn detections, it is unlikely to explain the MOS signal. A confirmation of this interesting feature has to wait for a more sensitive observation with a future calorimeter experiment.

Due to the appearance of SN 1993J, the region of M81 (NGC 3031) was observed with ASCA ten times between 1993 April and 1995 April. Data from eight of these observations have been used to study the 0.5–10 keV spectral and temporal properties of the source designated X-5, coincident with the M81 nucleus. The 2–10 keV luminosity of X-5 varied by a factor of 1.7 over a period of two years, around a mean of ∼ 2 × 1040 erg s−1 . Variations by ∼ 20% on a time scale of one day were also detected. The average spectrum of X-5 can be represented in the 2–10 keV range by a power-law continuum of the photon index, Γ = 1.85± 0.04, and a relatively low intrinsic absorption column of NH ∼ 1 × 1021 cm−2 . In the soft (< 2 keV) energy range, an additional thermal component with a temperature of 0.6–0.8 keV has been detected. An apparently broad or complex Fe-K emission line centered at 6.6–6.7 keV was also detected at an equivalent width of 170 ± 60 eV. These results indicate that the M81 nucleus is a low-luminosity active galactic nucleus, and suggest that similar objects are relatively numerous. A comparison is made between the nucleus of M81 and those of Seyfert galaxies.

Abstract. The ASCA satellite made a quick followup observation of GRB 970828 and detected its fading X-ray afterglow at the position localized by the All-Sky Monitor on the Rossi XTE satellite. Because of the quickness, the afterglow was still rather bright (∼ 4 10 −13 ergs cm −2 s −1 : 2 − 10 keV), and ASCA obtained the data of good quality. We found the spectral features of the absorption which implies a column density of 4.1 10 21 cm −2 at z = 0, together with an emission line at ∼ 5 keV. The line could be red shifted Fe K α line. Hence, it suggests a relatively small red shift z ≈ 0.33 for this GRB.