田代 信

Using the ASCA observatory, X-ray observations were made of the spiral galaxy NGC 1097, which exhibits four optical jets. Hard X-ray emission from a point-like source at the nucleus was detected at a 2-10 keV luminosity of 1 x 10(41) erg s(-1) for an assumed distance of 24 Mpc. The 2-10 keV spectrum was well fitted with a power-law of photon index similar to 1.8. Although no X-ray variability was detected during a similar to 40 ks pointing, the observed X-ray to optical luminosity ratio of NGC 1097 is too. high for the X-ray emission to be explained by an assembly of discrete X-ray sources. From our results and optical activity, the existence of a low-luminosity active galactic nucleus (LLAGN) is strongly suggested. In the soft X-ray band (0.5-2 keV) a thin thermal emission with a temperature kT similar to 0.6 keV was also detected, which is understood to be emission from hot gas associated with starburst activity.

The Gas Imaging Spectrometer (GIS) system on board ASCA is described. The experiment consists of 2 units of imaging gas scintillation proportional counters with a sealed-off gas cell equipped with an imaging phototube. The performance is characterized by the high X-ray sensitivity (from 0.7 keV to over 10 keV), good energy resolution (7.8% FWHM at 6 keV following E(-0.5) as a function of X-ray energy E), moderate position resolution (0.5 mm FWHM at 6 keV with E(-0.5) dependence), fast time resolution down to 61 mu s, and an effective area of 50 mm diameter. The on-board signal processing system and the data transmitted to the ground are also described. The background rejection efficiency of the GIS is reaching the level achieved by the non-imaging multi-cell proportional counters.

The in-orbit performance and calibration of the Gas Imaging Spectrometer (GIS), located on the focal plane of the X-ray astronomy satellite ASCA, are described. An extensive in-orbit calibration has confirmed its basic performance, including a position resolution of 0.6 mm (FWHM) and an energy resolution of 7.8% (FWHM), both at 6 keV. When combined with the X-ray telescope, the GIS sensitivity range becomes 0.7-10 keV. The in-orbit non X-ray background of the GIS has been confirmed to be as low as (5-9) x 10(-4) cs(-1) cm(-2) keV(-1) over the 1-10 keV range. The long-term detector gain has been stable within a few ro for nearly 3 years. Extensive observations of the Crab Nebula and other sources have provided accurate calibrations of the position response, photometric capability, dead time, and timing accuracy of the GIS. Furthermore, the overall energy response, including the temporal and positional gain variations and the absolute gain scale, has been calibrated to similar to 1%. Thus, the GIS is working as an all-round cosmic X-ray detector, capable of X-ray imagery, fine X-ray spectroscopy, X-ray photometry With a flux dynamic range covering more than 5 orders of magnitude, and fast X-ray photometry with a time resolution up to 60 mu s.

Using the ASCA observatory, X-ray observations were made of the spiral galaxy NGC 1097, which exhibits four optical jets. Hard X-ray emission from a point-like source at the nucleus was detected at a 2-10 keV luminosity of 1 x 10(41) erg s(-1) for an assumed distance of 24 Mpc. The 2-10 keV spectrum was well fitted with a power-law of photon index similar to 1.8. Although no X-ray variability was detected during a similar to 40 ks pointing, the observed X-ray to optical luminosity ratio of NGC 1097 is too. high for the X-ray emission to be explained by an assembly of discrete X-ray sources. From our results and optical activity, the existence of a low-luminosity active galactic nucleus (LLAGN) is strongly suggested. In the soft X-ray band (0.5-2 keV) a thin thermal emission with a temperature kT similar to 0.6 keV was also detected, which is understood to be emission from hot gas associated with starburst activity.

We present data collected during a simultaneous GeV gamma-ray (Compton Gamma Ray Observatory EGRET) and X-ray (ASCA) observation of BL Lac object AO 0235+164 (z = 0.94); we also present ROSAT PSPC X-ray data, obtained 6 months earlier. The X-ray spectra from both ROSAT and ASCA confirm a substantial photoelectric absorption beyond the value expected from our Galaxy, which is most likely caused by heavy elements in the inerstellar medium of an intervening galaxy at z = 0.524; it is possible to measure the elemental abundances in it module the hydrogen spin temperature T-S. By comparing the absorption in our data to the 21 cm absorption, we can limit TS to at most similar to 700 K. The ROSAT data imply a rapid (similar to 3 day) doubling of the soft X-ray flux and a subsequent quasi-exponential decay, whereas no variability is apparent in the ASCA or EGRET data. We detect a significant spectral change of the BL Lac X-ray continuum between the ROSAT and ASCA observations; ASCA data show that the spectrum is substantially harder (with the energy power-law index alpha similar to 1 vs. similar to 2) and fainter than in the ROSAT data. The simplest interpretation of this spectral variability is that the ROSAT data are dominated by a bright, soft tail of a synchrotron component, which disappeared during the ASCA observation, uncovering a harder Compton component. The EGRET data imply a lower GeV flux than in the previous observations, with no apparent change of the GeV spectrum. The ratio of the gamma-ray to the X-ray flux [in E x F(E)] for this radio-discovered object is roughly 30, which is very substantially higher than that inferred for X-ray-discovered BL Lac objects, suggesting an important difference in the structure between the two subclasses of BL Lac objects.

We present data collected during a simultaneous GeV gamma-ray (Compton Gamma Ray Observatory EGRET) and X-ray (ASCA) observation of BL Lac object AO 0235+164 (z = 0.94); we also present ROSAT PSPC X-ray data, obtained 6 months earlier. The X-ray spectra from both ROSAT and ASCA confirm a substantial photoelectric absorption beyond the value expected from our Galaxy, which is most likely caused by heavy elements in the inerstellar medium of an intervening galaxy at z = 0.524; it is possible to measure the elemental abundances in it module the hydrogen spin temperature T-S. By comparing the absorption in our data to the 21 cm absorption, we can limit TS to at most similar to 700 K. The ROSAT data imply a rapid (similar to 3 day) doubling of the soft X-ray flux and a subsequent quasi-exponential decay, whereas no variability is apparent in the ASCA or EGRET data. We detect a significant spectral change of the BL Lac X-ray continuum between the ROSAT and ASCA observations; ASCA data show that the spectrum is substantially harder (with the energy power-law index alpha similar to 1 vs. similar to 2) and fainter than in the ROSAT data. The simplest interpretation of this spectral variability is that the ROSAT data are dominated by a bright, soft tail of a synchrotron component, which disappeared during the ASCA observation, uncovering a harder Compton component. The EGRET data imply a lower GeV flux than in the previous observations, with no apparent change of the GeV spectrum. The ratio of the gamma-ray to the X-ray flux [in E x F(E)] for this radio-discovered object is roughly 30, which is very substantially higher than that inferred for X-ray-discovered BL Lac objects, suggesting an important difference in the structure between the two subclasses of BL Lac objects.

Extended X-ray emission was detected with ASCA from the synchrotron double lobes of the radio galaxy Fornax A. This is thought to be the same emission as was detected with ROSAT. The excess X-rays exhibit very hard spectra, described by a power law of energy index 1.2 +/- 0.5. Since this index is consistent with the synchrotron. radio index of 0.9 +/- 0.2, the X-rays are thought to arise when the radio-emitting relativistic electrons make inverse-Compton scattering off the cosmic microwave photons. Comparison of the X-ray and radio fluxes yields the lobe magnetic field intensity of 2-4 mu G. The relativistic electrons in the lobes are inferred to have an energy density close to that of the magnetic fields.

Extended X-ray emission was detected with ASCA from the synchrotron double lobes of the radio galaxy Fornax A. This is thought to be the same emission as was detected with ROSAT. The excess X-rays exhibit very hard spectra, described by a power law of energy index 1.2 +/- 0.5. Since this index is consistent with the synchrotron. radio index of 0.9 +/- 0.2, the X-rays are thought to arise when the radio-emitting relativistic electrons make inverse-Compton scattering off the cosmic microwave photons. Comparison of the X-ray and radio fluxes yields the lobe magnetic field intensity of 2-4 mu G. The relativistic electrons in the lobes are inferred to have an energy density close to that of the magnetic fields.

The BL Lac object PKS 0548-322 was observed in X-rays with Ginga five times between 1990 January and 1991 March. The 2-20 keV flux varied by a factor of 2.5, and the spectral energy index a showed a significant change of between 0.8+/-0.02 and 1.03+/-0.04. No simple relation has been found between the flux and the spectral index. An upper limit for the equivalent width of the iron K line is 83 eV at 90% confidence. Based on the spectral results, we propose a simple scheme to interpret the variation of the X-ray spectrum in terms of the synchrotron emission. This gives new constraints on the properties of the emission region.

We present initial results from ASCA on the spectrum of the Cosmic X-ray Background from 0.4 to 10 keV. About 250 ks of deep survey data has been collected from the performance verification phase of ASCA. About 110 ks of dark-earth data are used to assess the internal background. A single power-law describes the X-ray background very well in the 1 to 10 keV with no evidence for a steepening in the 1 to 3 keV range. At 1 keV, the intensity of the X-ray background is 9.6 keV s(-1) cm(-2) sr-l keV(-1). Our data clearly show an excess above the extrapolation of the single power law model below 1 keV. At least part of this excess can be accounted for with a thermal component as there is evidence for O VII and O VIII emission.

The BL Lac object PKS 0548-322 was observed in X-rays with Ginga five times between 1990 January and 1991 March. The 2-20 keV flux varied by a factor of 2.5, and the spectral energy index a showed a significant change of between 0.8+/-0.02 and 1.03+/-0.04. No simple relation has been found between the flux and the spectral index. An upper limit for the equivalent width of the iron K line is 83 eV at 90% confidence. Based on the spectral results, we propose a simple scheme to interpret the variation of the X-ray spectrum in terms of the synchrotron emission. This gives new constraints on the properties of the emission region.

We present initial results from ASCA on the spectrum of the Cosmic X-ray Background from 0.4 to 10 keV. About 250 ks of deep survey data has been collected from the performance verification phase of ASCA. About 110 ks of dark-earth data are used to assess the internal background. A single power-law describes the X-ray background very well in the 1 to 10 keV with no evidence for a steepening in the 1 to 3 keV range. At 1 keV, the intensity of the X-ray background is 9.6 keV s(-1) cm(-2) sr-l keV(-1). Our data clearly show an excess above the extrapolation of the single power law model below 1 keV. At least part of this excess can be accounted for with a thermal component as there is evidence for O VII and O VIII emission.

X-ray properties of the radio galaxy 3C 390.3 were investigated using the EXOSAT and Ginga satellites. Long-term, large-amplitude X-ray intensity changes were detected over a period extending from 1984 through 1991, and high-quality X-ray spectra were obtained especially with Ginga. The X-ray continuum spectra were described with power-law model with photon slope in the range 1.5-1.8, and the slope flattened as the 2-20 keV luminosity decreased by 40%. There was a first detection of the iron emission line from this source at the 90% confidence level. An upper limit was derived on the thermal X-ray component. X-ray emission mechanisms and possible origins of the long-term variation are discussed.

The spectra and images of the nearby jet galaxy NGC 4258 (M106) obtained with ASCA indicate presence of several distinct X-ray emission components. The emission above 3 keV is pointlike and coincident in position with the optical nucleus, exhibiting a hard (photon index approximately 1.78) and absorbed (N(H) approximately 1.5 x 10(23) cm-2) spectrum. This provides clear evidence that NGC 4258 hosts an obscured active nucleus of low luminosity, about 4 x 10(40) ergs s-1 in 2-10 keV after removing the absorption. Iron K-line emission with an equivalent width 0.25 +/- 0.10 keV was detected. The emission below 1 keV is dominated by an extended (approximately 4') thin-thermal component with a temperature approximately 0.5 keV exhibiting atomic emission lines, possibly associated with the jet. There exists a third continuum component with an intermediate spectral hardness, which is brightest at approximately 1' south-east of the nucleus.

The BL Lacertae object H0323+022 was observed in X-rays with Ginga on three occasions, in 1987 July, 1988 August, and 1991 August. In the 1987 observation, the source was relatively bright, and variable by a factor of 3 on time scales of several hours. The observed 2-20 keV energy flux ranged between 2.2 and 4.1 x 10(-11) ergs CM-2 s-1 during the variation. The X-ray spectra were well described by a power-law model without intrinsic photoelectric absorption, and the photon index varied between 1.3 and 1.9 as the intensity changed. The spectrum was much better correlated with the time derivative of the X-ray intensity than the intensity, itself, in such a way that the spectrum hardened when the intensity increased. A detailed analysis of this spectrum-intensity correlation indicates that variation in the higher-energy band (4.1-10.5 keV) precedes that in the lower-energy band (1.2-4.1 keV) by roughly 0.05 d. This supports the idea that X-ray emission from H0323+022 is of synchrotron origin. In the 1988 and 1991 observations, the source was rather faint and only marginally detected at a 2-20 keV flux level between 0.6 and 1.1 x 10(-11) ergs CM-2 S-1. On these two occasions no intensity variations were detected, and soft spectra with a photon index of more than 2.0 are indicated.

We report preliminary results of an ASCA observation of 3C 273. There is no spectral variability within the ASCA observation, in contrast to the longer term behavior found with EXOSAT and Ginga which established significant changes in the spectral index of the power-law continuum on timescales of a week or more, uncorrelated with luminosity. Comparison of the ASCA data with a simultaneous ROSAT/Ginga observation in 1990 December shows that while the shape of the spectrum above approximately 1 keV is essentially the same, the flux in the soft component, below approximately 1 keV, is considerably weaker relative to the hard component in the ASCA observation. The 2-10 keV luminosity is at least a factor of approximately 1.5 greater in the ASCA observation than in the ROSAT/Ginga observation whilst the ratio of 2-10 keV luminosity to 0.4-1.0 keV luminosity increases by a factor of approximately 1.2. Thus the spectral shape of the hard component appears to be independent of the relative magnitude of the soft component and is an important constraint on emission and reprocessing models. We find no significant Fe K line-emission in this observation. The data also highlight very well some of the outstanding problems in the instrumental responses.

X-ray properties of the radio galaxy 3C 390.3 were investigated using the EXOSAT and Ginga satellites. Long-term, large-amplitude X-ray intensity changes were detected over a period extending from 1984 through 1991, and high-quality X-ray spectra were obtained especially with Ginga. The X-ray continuum spectra were described with power-law model with photon slope in the range 1.5-1.8, and the slope flattened as the 2-20 keV luminosity decreased by 40%. There was a first detection of the iron emission line from this source at the 90% confidence level. An upper limit was derived on the thermal X-ray component. X-ray emission mechanisms and possible origins of the long-term variation are discussed.

The spectra and images of the nearby jet galaxy NGC 4258 (M106) obtained with ASCA indicate presence of several distinct X-ray emission components. The emission above 3 keV is pointlike and coincident in position with the optical nucleus, exhibiting a hard (photon index approximately 1.78) and absorbed (N(H) approximately 1.5 x 10(23) cm-2) spectrum. This provides clear evidence that NGC 4258 hosts an obscured active nucleus of low luminosity, about 4 x 10(40) ergs s-1 in 2-10 keV after removing the absorption. Iron K-line emission with an equivalent width 0.25 +/- 0.10 keV was detected. The emission below 1 keV is dominated by an extended (approximately 4') thin-thermal component with a temperature approximately 0.5 keV exhibiting atomic emission lines, possibly associated with the jet. There exists a third continuum component with an intermediate spectral hardness, which is brightest at approximately 1' south-east of the nucleus.

The BL Lacertae object H0323+022 was observed in X-rays with Ginga on three occasions, in 1987 July, 1988 August, and 1991 August. In the 1987 observation, the source was relatively bright, and variable by a factor of 3 on time scales of several hours. The observed 2-20 keV energy flux ranged between 2.2 and 4.1 x 10(-11) ergs CM-2 s-1 during the variation. The X-ray spectra were well described by a power-law model without intrinsic photoelectric absorption, and the photon index varied between 1.3 and 1.9 as the intensity changed. The spectrum was much better correlated with the time derivative of the X-ray intensity than the intensity, itself, in such a way that the spectrum hardened when the intensity increased. A detailed analysis of this spectrum-intensity correlation indicates that variation in the higher-energy band (4.1-10.5 keV) precedes that in the lower-energy band (1.2-4.1 keV) by roughly 0.05 d. This supports the idea that X-ray emission from H0323+022 is of synchrotron origin. In the 1988 and 1991 observations, the source was rather faint and only marginally detected at a 2-20 keV flux level between 0.6 and 1.1 x 10(-11) ergs CM-2 S-1. On these two occasions no intensity variations were detected, and soft spectra with a photon index of more than 2.0 are indicated.

We report preliminary results of an ASCA observation of 3C 273. There is no spectral variability within the ASCA observation, in contrast to the longer term behavior found with EXOSAT and Ginga which established significant changes in the spectral index of the power-law continuum on timescales of a week or more, uncorrelated with luminosity. Comparison of the ASCA data with a simultaneous ROSAT/Ginga observation in 1990 December shows that while the shape of the spectrum above approximately 1 keV is essentially the same, the flux in the soft component, below approximately 1 keV, is considerably weaker relative to the hard component in the ASCA observation. The 2-10 keV luminosity is at least a factor of approximately 1.5 greater in the ASCA observation than in the ROSAT/Ginga observation whilst the ratio of 2-10 keV luminosity to 0.4-1.0 keV luminosity increases by a factor of approximately 1.2. Thus the spectral shape of the hard component appears to be independent of the relative magnitude of the soft component and is an important constraint on emission and reprocessing models. We find no significant Fe K line-emission in this observation. The data also highlight very well some of the outstanding problems in the instrumental responses.

We present the results of Ginga observations of three narrow emission line X-ray galaxies, NGC 7172, NGC 7582 and ESO 103-G35. In each of these galaxies the column density along the line of sight to the active nucleus is extremely high (i.e. n(H) greater-than-or-similar-to 10(23) cm-2). A comparison of the optical depth in the iron K-edge with the low-energy absorption in the X-ray spectrum indicates an apparent iron abundance in excess of the solar value in these sources. Measurements of the iron K-edge energy show that the bulk of the X-ray-absorbing gas is not strongly photoionized, with only ESO 103-G35 exhibiting an edge energy significantly above that for neutral iron. In the case of NGC7582, the line-of-sight column density may have increased by a factor of approximately 3 over an interval of about 4 yr. The implications of these results for current models of the state and distribution of gaseous material in low-luminosity active galactic nuclei are briefly discussed.

We present the results of Ginga observations of three narrow emission line X-ray galaxies, NGC 7172, NGC 7582 and ESO 103-G35. In each of these galaxies the column density along the line of sight to the active nucleus is extremely high (i.e. n(H) greater-than-or-similar-to 10(23) cm-2). A comparison of the optical depth in the iron K-edge with the low-energy absorption in the X-ray spectrum indicates an apparent iron abundance in excess of the solar value in these sources. Measurements of the iron K-edge energy show that the bulk of the X-ray-absorbing gas is not strongly photoionized, with only ESO 103-G35 exhibiting an edge energy significantly above that for neutral iron. In the case of NGC7582, the line-of-sight column density may have increased by a factor of approximately 3 over an interval of about 4 yr. The implications of these results for current models of the state and distribution of gaseous material in low-luminosity active galactic nuclei are briefly discussed.

We present a detailed study of the hard X-ray properties of the BL Lacertae object PKS 2155-304 based on measurements made in 1988 and 1989 with the Large Area Counter (LAC) on board the Ginga satellite. The source exhibited a high degree of variability with a dynamic range of a factor 7 in the 2-6 keV band. The fastest amplitude variation was a factor 2 decline in the intensity in this band within 4 hours. The spectrum is characterized by a break which occurs at approximately 4 keV. Spectral fits to the data integrated in 6400 s time bins reveal that, in common with previous observations of BL Lacertae objects, the spectral slope is generally anticorrelated with intensity in the sense that the spectrum hardens as the intensity increases. However, the tracks of sequential points in the index-intensity plane are occasionally seen to differ during the rise and decay stages of individual flares. Furthermore, during one, or possibly two, flaring episodes the spectral index is observed to correlate with intensity variations. The X-ray properties of PKS 2155-304 are most readily interpreted in terms of direct synchrotron radiation originating within a relativistic jet, however, the X-ray emitting region is probably more complex than the geometries assumed in most standard jet models. Further development of time-dependent theoretical models will be necessary to explain some of the details of the present observations.

We present a detailed study of the hard X-ray properties of the BL Lacertae object PKS 2155-304 based on measurements made in 1988 and 1989 with the Large Area Counter (LAC) on board the Ginga satellite. The source exhibited a high degree of variability with a dynamic range of a factor 7 in the 2-6 keV band. The fastest amplitude variation was a factor 2 decline in the intensity in this band within 4 hours. The spectrum is characterized by a break which occurs at approximately 4 keV. Spectral fits to the data integrated in 6400 s time bins reveal that, in common with previous observations of BL Lacertae objects, the spectral slope is generally anticorrelated with intensity in the sense that the spectrum hardens as the intensity increases. However, the tracks of sequential points in the index-intensity plane are occasionally seen to differ during the rise and decay stages of individual flares. Furthermore, during one, or possibly two, flaring episodes the spectral index is observed to correlate with intensity variations. The X-ray properties of PKS 2155-304 are most readily interpreted in terms of direct synchrotron radiation originating within a relativistic jet, however, the X-ray emitting region is probably more complex than the geometries assumed in most standard jet models. Further development of time-dependent theoretical models will be necessary to explain some of the details of the present observations.

Two bright quasars NRAO 140 (z = 1.258) and 4C 34.47 (z = 0.206) have been studied with Ginga. The observed spectra in the energy range 2-20 keV, in the observer rest frame, are well described by power-law models. The measured 2-10 keV luminosities and energy indices are L(x) = 3.8 x 10(46) ergs s-1 and alpha = 0.72 +/- 0.11 for NRAO 140, and L(x) = 5.3 x 10(44) ergs s-1 and alpha = 0.64(-0.14)+0.17 for 4C 34.47 (H-0 = 100 km s-1 Mpc-1; q0 = 0.5); the intensity variation was less than about 10% during the approximately 1 day observation for each source. Upper limits, in the source rest frame, for iron line emission are EW < 200 and EW < 120 eV for NRAO 140 and 4C 34.47, respectively. The featureless X-ray spectra, whose slopes are flatter than those of BL Lac objects, and the high X-ray luminosities in excess of the extension of the IR-optical continuum are consistent with the synchrotron self-Compton emission accompanied by a moderate beaming toward us.

Two bright quasars NRAO 140 (z = 1.258) and 4C 34.47 (z = 0.206) have been studied with Ginga. The observed spectra in the energy range 2-20 keV, in the observer rest frame, are well described by power-law models. The measured 2-10 keV luminosities and energy indices are L(x) = 3.8 x 10(46) ergs s-1 and alpha = 0.72 +/- 0.11 for NRAO 140, and L(x) = 5.3 x 10(44) ergs s-1 and alpha = 0.64(-0.14)+0.17 for 4C 34.47 (H-0 = 100 km s-1 Mpc-1; q0 = 0.5); the intensity variation was less than about 10% during the approximately 1 day observation for each source. Upper limits, in the source rest frame, for iron line emission are EW < 200 and EW < 120 eV for NRAO 140 and 4C 34.47, respectively. The featureless X-ray spectra, whose slopes are flatter than those of BL Lac objects, and the high X-ray luminosities in excess of the extension of the IR-optical continuum are consistent with the synchrotron self-Compton emission accompanied by a moderate beaming toward us.