Yasushi Ogasaka

A population of faint galaxies that contribute to the cosmic X-ray background

The origin of the cosmic X-ray background radiation, has remained mysterious since its discovery thirty-five years ago. Investigation of its origin has been difficult because instruments have had insufficient resolution to distinguish small, faint sources in the hard X-ray band (above 2u2009keV) that dominates the background. Until now, only three per cent of the flux in the 2–10u2009keV band could be attributed to individual sources. Here we report the results of a survey 100 times more sensitive than previous studies in the 2–10u2009keV band. We find many faint resolved sources, whose integrated flux accounts for 30 per cent of the X-ray background in this energy range. The average spectrum of the resolved sources is harder than those of nearby bright active galactic nuclei and is close to the spectrum of the X-ray background radiation. This means that a new class of sources, with hard X-ray spectra, dominate the sky at photon energies above 2u2009keV.

X-ray telescope onboard Astro-E: optical design and fabrication of thin foil mirrors

X-ray telescopes (XRTs) of nested thin foil mirrors are developed for Astro-E, the fifth Japanese x-ray astronomy satellite. Although the launch was not successful, the design concept, fabrication, and alignment procedure are summarized. The main purpose of the Astro-E XRT is to collect hard x rays up to 10 keV with high efficiency and to provide medium spatial resolution in limited weight and volume. Compared with the previous mission, Advanced Satellite for Cosmology and Astrophysics (ASCA), a slightly longer focal length of 4.5-4.75 m and a larger diameter of 40 cm yields an effective area of 1750 cm2 at 8 keV with five telescopes. The image quality is also improved to 2-arc min half-power diameter by introduction of a replication process. Platinum is used instead of gold for the reflectors of one of the five telescopes to enhance the high-energy response. The fabrication and alignment procedure is also summarized. Several methods for improvement are suggested for the reflight Astro-E II mission and for other future missions. Preflight calibration results will be described in a forthcoming second paper, and a detailed study of images will be presented in a third paper.

X-ray telescope onboard Astro-E. II. Ground-based x-ray characterization

X-ray characterization measurements of the x-ray telescope (XRT) onboard the Astro-E satellite were carried out at the Institute of Space and Astronautical Science (Japan) x-ray beam facility by means of a raster scan with a narrow x-ray pencil beam. The on-axis half-power diameter (HPD) was evaluated to be 1.8?-2.2?, irrespective of the x-ray energy. The on-axis effective areas of the XRTs for x-ray imaging spectrometers (XISs) were approximately 440, 320, 240, and 170 cm(2) at energies of 1.49, 4.51, 8.04, and 9.44 keV, respectively. Those of the x-ray spectrometer (XRS) were larger by 5-10%. The replication method introduced for reflector production significantly improved the imaging capability of the Advanced Satellite for Cosmology and Astrophyics (ASCA) XRT, whose HPD is ~3.6?. The increase in the effective area by a factor of 1.5-2.5, depending upon the x-ray energy, compared with that of the ASCA, was brought about by mechanical scale up and longer focal lengths. The off-axis HPDs were almost the same as those obtained on the optical axis. The field of view is defined as the off-axis angle at which the effective area becomes half of the on-axis value. The diameter of the field of view was ~19? at 1.49 keV, decreasing with increasing x-ray energy, and became ~13? at 9.44 keV. The intensity of stray light and the distribution of this kind of light on the focal plane were measured at the large off-axis angles 30? and 60?. In the entire XIS field of view (25.4 mm x 25.4 mm), the intensity of the stray light caused by a pointlike x-ray source became at most 1% of the same pointlike source that was on the optical axis.

Development of balloonborne hard x-ray telescopes using a multilayer supermirror

We present new results in the development of high throughput hard x-ray telescopes with multilayer supermirror coatings. Basic techniques to make the supermirror are at first developed in deposition of constant d spacing multilayers. The reflector of multilayer on the float glass achieves high reflectivity, limited by the surface roughness of substrate, while, we need more improvement in getting comparable reflectivity of multilayers on the replica foil. We put the mask just in front of sample, it reduced the nonuniformity of the thickness less than 2%. In order to maximize the effective area and field of view, we have optimized the supermirror parameters; d spacing, number of layer pairs, and thickness ratio of heavy and light element. Multiblock method is introduced to design the supermirror, and it gives high reflectivity of approximately 30% in the 25 - 40 keV band. A test supermirror sputtered on a glass sample exhibits reasonable reflectivities of about 20 - 30%. We designed a telescope system with 45 cm diameter, 20 cm mirror length in two stage, and focal length of 8 m. The effective area of four such telescopes is 320 cm2 for the x rays between 25 and 40 keV. We plan the application of this type of telescope to the balloon experiment named InFOC(mu) S to reveal hard x-ray images of clusters of galaxies or supernova remnants.

Asca Observations Of Luminous Infrared Galaxies

One of the most important results of the IRAS survey is the discovery of a class of “Luminous Infrared Galaxies” (LIGs), which emit most of the energy in the infrared and are the dominant population in the local universe at luminosities above 1011 L☉ (e.g., Sanders & Mirabel 1996). All LIGs appear to be extremely rich in molecular gas, and many of them show evidence of recent interacting/merging activities. Hence it is now accepted that strong interactions of gas-rich galaxies triggers large central concentration of molecular gas, and makes optimal conditions for both enormous nuclear starbursts and building and/or fueling AGN. Actually, various observations show evidence of starburst activity as well as that of AGN in many LIGs (Sanders & Mirabel 1996).

An ASCA attempt of GRB observation in x‐ray range

We are attempting to search GRBs from the extended halo around nearby galaxies in x‐ray band with the Gas Imaging Spectrometer on board the ASCA satellite which was launched in February, 1993. GIS/ASCA has the imaging capability and can detect x‐rays in 0.5 keV to 10 keV band in which GRBs commonly emit the radiation. GIS is sensible to bursts with SX≊10−10 ergu2009cm−2 which correspond to ∼1042 erg GRBs occurring in M31. We have performed a search on the ASCA PV phase data of M31, M33, LMC and SMC region, however we could not get any positive detection so far.

ASCA Observations of Luminous Infrared Galaxies: Evolution from Starburst to AGN?

We made a series of ASCA observations of Ultra Luminous Infrared Galaxies (ULIRGs), to reveal the origin of their luminosities and to test a proposed evolutionary scenario of ULIRGs. We discovered embedded AGN in some galaxies, but the relative contribution of AGN activity to the total luminosity varies significantly from galaxy to galaxy and has no correlation with the evolutionary sequence as mergers.

Optical Follow-Up Observations of ASCA Lynx Deep Survey

Since the bulk of the energy density of the Cosmic X-ray Background (CXB) resides in the harder energy band than that of the ROSAT band (0.5–2 keV) and since the X-ray sources identified in the ROSAT band have X-ray spectra softer than that of the CXB, investigation of nature of the X-ray sources at the harder energy band is indispensable to solve the origin of the CXB. However, only 2–3% of the CXB in the hard band (2–10 keV) had been resolved into discrete sources (Piccinotti et al. 1982, ApJ 253, 485). We present our preliminary results of optical follow-up observations of the ASCA Lynx deep survey.