Makoto Tashiro

This paper summarizes the design and performance of the hard X-ray detector constructed for the ASTRO-E satellite. The detector utilizes the GSO/BGO well-type phoswich counters in a compound-eye configuration to achieve an extremely low background level of a few x 10(-5) counts s(-1) cm(-2)keV(-1) [1]. The GSO scintillators installed in the BGO active shield wells are sensitive to 30-600 keV photons, while the 2-mm-thick silicon PIN diodes, placed in front of each GSO crystal, cover the 10-60 keV energy band with a spectral resolution of similar to3.5-keV full-width at half-maximum. The design goals, of both low background and high energy resolution, in the hard X-ray bands were verified through the preflight calibration experiments.

This paper summarizes the design and performance of the hard X-ray detector constructed for the ASTRO-E satellite. The detector utilizes the GSO/BGO well-type phoswich counters in a compound-eye configuration to achieve an extremely low background level of a few x 10(-5) counts s(-1) cm(-2)keV(-1) [1]. The GSO scintillators installed in the BGO active shield wells are sensitive to 30-600 keV photons, while the 2-mm-thick silicon PIN diodes, placed in front of each GSO crystal, cover the 10-60 keV energy band with a spectral resolution of similar to3.5-keV full-width at half-maximum. The design goals, of both low background and high energy resolution, in the hard X-ray bands were verified through the preflight calibration experiments.

Three uninterrupted, long (lasting respectively 7, 10, and 10 days) ASCA observations of the well-studied TeV-bright blazars Mrk 421, Mrk 501, and PKS 2155-304 all show continuous strong X-ray flaring. Despite the relatively faint intensity states in two of the three sources, there was no identifiable quiescent period in any of the observations. Structure function analysis shows that all blazars have a characteristic timescale of similar to1 day, comparable to the recurrence time and to the timescale of the stronger flares. On the other hand, examination of these flares in more detail reveals that each of the strong flares is not a smooth increase and decrease but exhibits substructures of shorter flares having timescales of similar to 10 ks. We verify via simulations that in order to explain the observed structure function, these shorter flares ("shots") are unlikely to be fully random, but in some way are correlated with each other. The energy dependent cross-correlation analysis shows that interband lags are not universal in TeV blazars. This is important since in the past only positive detections of lags were reported. In this work, we determine that the sign of a lag may differ from flare to flare; significant lags of both signs were detected from several flares, while no significant lag was detected from others. However, we also argue that the nature of the underlying component can affect these values. The facts that all flares are nearly symmetric and that fast variability shorter than the characteristic timescale is strongly suppressed, support the scenario where the light crossing time dominates the variability timescales of the day-scale flares.

Three uninterrupted, long (lasting respectively 7, 10, and 10 days) ASCA observations of the well-studied TeV-bright blazars Mrk 421, Mrk 501, and PKS 2155-304 all show continuous strong X-ray flaring. Despite the relatively faint intensity states in two of the three sources, there was no identifiable quiescent period in any of the observations. Structure function analysis shows that all blazars have a characteristic timescale of similar to1 day, comparable to the recurrence time and to the timescale of the stronger flares. On the other hand, examination of these flares in more detail reveals that each of the strong flares is not a smooth increase and decrease but exhibits substructures of shorter flares having timescales of similar to 10 ks. We verify via simulations that in order to explain the observed structure function, these shorter flares ("shots") are unlikely to be fully random, but in some way are correlated with each other. The energy dependent cross-correlation analysis shows that interband lags are not universal in TeV blazars. This is important since in the past only positive detections of lags were reported. In this work, we determine that the sign of a lag may differ from flare to flare; significant lags of both signs were detected from several flares, while no significant lag was detected from others. However, we also argue that the nature of the underlying component can affect these values. The facts that all flares are nearly symmetric and that fast variability shorter than the characteristic timescale is strongly suppressed, support the scenario where the light crossing time dominates the variability timescales of the day-scale flares.

A long (155 hr) ASCA observation was performed of two ultraluminous compact X-ray sources, source 1 and source 2, in the spiral galaxy IC 342. While source 1, which was in a hard spectral state, varied little, source 2, which was in its soft spectral state, varied significantly on a timescale of about 1 day. The rms variation amplitude amounts to 5% in the 2-10 keV band but is less than 4% in 0.7- 2 keV. The variation involves statistically significant changes in the parameters describing multicolor disk blackbody emission from this source. The variation is possibly periodic, with a period of either 31 +/- 2 or 41 +/- 3 hr. Both are consistent with the orbital period of a semidetached binary formed by a black hole and a main-sequence star of several tens of solar masses. These results reinforce the interpretation of these X-ray objects in terms of accreting massive stellar black holes.

A long (155 hr) ASCA observation was performed of two ultraluminous compact X-ray sources, source 1 and source 2, in the spiral galaxy IC 342. While source 1, which was in a hard spectral state, varied little, source 2, which was in its soft spectral state, varied significantly on a timescale of about 1 day. The rms variation amplitude amounts to 5% in the 2-10 keV band but is less than 4% in 0.7-2 keV. The variation involves statistically significant changes in the parameters describing multicolor disk blackbody emission from this source. The variation is possibly periodic, with a period of either 31 ± 2 or 41 ± 3 hr. Both are consistent with the orbital period of a semidetached binary formed by a black hole and a main-sequence star of several tens of solar masses. These results reinforce the interpretation of these X-ray objects in terms of accreting massive stellar black holes.

The ASTRO-E hard X-ray defector utilizes GSO(Gd2SiO5:Ce 0.5% mol)-BGO(Bi4Ge3O12) well-type phoswich counters [1] in compound-eye configuration to achieve an extremely low background level of about a few times 10(-5) counts s(-1) cm(-2) keV(-1). The GSO scintillators placed at the bottom of the BGO well observe photons in the energy range 30-600 keV. To cover the lower energy range of 10-60 keV, silicon p-i-n diodes of 2 mm in thickness and 21.5 x 21.5 mm(2) in size were newly developed and placed in front of the GSO scintillators. The p-i-n diode exhibits complex spectral responses, including subpeak and low energy tail components. To examine the origin of these components, we measured the spatially resolved response of the p-i-n diode and confirmed that the subpeak and the low energy tall are related to the electrode structures. and electric, fields in the p-i-n. diode, respectively.

The ASTRO-E hard X-ray defector utilizes GSO(Gd2SiO5:Ce 0.5% mol)-BGO(Bi4Ge3O12) well-type phoswich counters [1] in compound-eye configuration to achieve an extremely low background level of about a few times 10(-5) counts s(-1) cm(-2) keV(-1). The GSO scintillators placed at the bottom of the BGO well observe photons in the energy range 30-600 keV. To cover the lower energy range of 10-60 keV, silicon p-i-n diodes of 2 mm in thickness and 21.5 x 21.5 mm(2) in size were newly developed and placed in front of the GSO scintillators. The p-i-n diode exhibits complex spectral responses, including subpeak and low energy tail components. To examine the origin of these components, we measured the spatially resolved response of the p-i-n diode and confirmed that the subpeak and the low energy tall are related to the electrode structures. and electric, fields in the p-i-n. diode, respectively.

Two ASCA observations were made of two ultraluminous compact X-ray sources (ULXs) in the spiral galaxy IC 342. In the 1993 observation, source 2 showed a 0.5-10 keV luminosity of 6 x 10(39) ergs s(-1) (assuming a distance of 4.0 Mpc) and a hard power-law spectrum of photon index similar to1.4. As already reported, source 1 was similar to3 times brighter on that occasion and exhibited a soft spectrum represented by a multicolor disk model with an inner- disk temperature of similar to 1.8 keV. The second observation, made in 2000 February, revealed that source 1 had made a transition into a hard spectral state, while source 2 made a transition into a soft spectral state. The ULXs are therefore inferred to exhibit two distinct spectral states, and they sometimes make transitions between them. These results significantly reinforce the scenario that describes ULXs as mass-accreting black holes.

The X-ray properties of the BL Lacertae object OJ 287, observed with ASCA on 1997 April 26 and November 17, are reported. The 0.5-10 keV flux was lower than that obtained in previous X-ray observations, and no evidence of intensity variations was found during each observation. The obtained flux densities at 1 keV, 0.22-0.26 mu Jy, exceed the extrapolations from lower frequency synchrotron continua, which were measured in nearly the same period as the present ASCA observations. The X-ray spectra acquired with the GIS and SIS were consistently described with a single power-law model modified by the Galactic absorption, and the derived photon indices, 1.5-1.6, are flatter than those observed so far. These results strongly suggest that the X-ray spectra observed in 1997 arise via an inverse Compton process alone. The X-ray spectra obtained in 1994 (Idesawa et al. 1997, AAA 68.159.334), exhibiting a steeper slope than those in 1997, is thought to be contaminated by a "synchrotron soft tail".

Two ASCA observations were made of two ultraluminous compact X-ray sources (ULXs) in the spiral galaxy IC 342. In the 1993 observation, source 2 showed a 0.5-10 keV luminosity of 6 x 10(39) ergs s(-1) (assuming a distance of 4.0 Mpc) and a hard power-law spectrum of photon index similar to1.4. As already reported, source 1 was similar to3 times brighter on that occasion and exhibited a soft spectrum represented by a multicolor disk model with an inner- disk temperature of similar to 1.8 keV. The second observation, made in 2000 February, revealed that source 1 had made a transition into a hard spectral state, while source 2 made a transition into a soft spectral state. The ULXs are therefore inferred to exhibit two distinct spectral states, and they sometimes make transitions between them. These results significantly reinforce the scenario that describes ULXs as mass-accreting black holes.

The X-ray properties of the BL Lacertae object OJ 287, observed with ASCA on 1997 April 26 and November 17, are reported. The 0.5-10 keV flux was lower than that obtained in previous X-ray observations, and no evidence of intensity variations was found during each observation. The obtained flux densities at 1 keV, 0.22-0.26 mu Jy, exceed the extrapolations from lower frequency synchrotron continua, which were measured in nearly the same period as the present ASCA observations. The X-ray spectra acquired with the GIS and SIS were consistently described with a single power-law model modified by the Galactic absorption, and the derived photon indices, 1.5-1.6, are flatter than those observed so far. These results strongly suggest that the X-ray spectra observed in 1997 arise via an inverse Compton process alone. The X-ray spectra obtained in 1994 (Idesawa et al. 1997, AAA 68.159.334), exhibiting a steeper slope than those in 1997, is thought to be contaminated by a "synchrotron soft tail".

A follow-up X-ray study was made of the west lobe of the radio galaxy Fornax A (NGC 1316) that was based on new ASCA observations made in 1997 for 98 ks and that incorporated the previous observation in 1994 for 39 ks. The 0.7-10 keV spectrum of the emission can be described by a power law with an energy index of 0.74 +/- 0.10 which agrees with the synchrotron radio index of 0.9 +/- 0.2. Therefore, the X-rays are reconfirmed to arise via the inverse Compton scattering of the cosmic microwave photons, as Kaneda et al. and Feigelson et al. concluded. The surface brightness of the inverse Compton X-rays exhibits a relatively flat distribution over the west lobe, indicative of an approximately spherical emissivity distribution with a radius of similar to 11 ' (75 kpc). In contrast, the 1.4 GHz radio image by Ekers et al. exhibits a rim-brightened surface brightness, consistent with a shell-like emissivity distribution whose inner and outer boundaries are 4 ' and 11 ', respectively. These morphological differences between radio and X-rays suggest that the relativistic electrons are distributed homogeneously over the lobe volume, whereas the magnetic field is amplified toward the lobe rim region.

A follow-up X-ray study was made of the west lobe of the radio galaxy Fornax A (NGC 1316) that was based on new ASCA observations made in 1997 for 98 ks and that incorporated the previous observation in 1994 for 39 ks. The 0.7-10 keV spectrum of the emission can be described by a power law with an energy index of 0.74 +/- 0.10 which agrees with the synchrotron radio index of 0.9 +/- 0.2. Therefore, the X-rays are reconfirmed to arise via the inverse Compton scattering of the cosmic microwave photons, as Kaneda et al. and Feigelson et al. concluded. The surface brightness of the inverse Compton X-rays exhibits a relatively flat distribution over the west lobe, indicative of an approximately spherical emissivity distribution with a radius of similar to 11 ' (75 kpc). In contrast, the 1.4 GHz radio image by Ekers et al. exhibits a rim-brightened surface brightness, consistent with a shell-like emissivity distribution whose inner and outer boundaries are 4 ' and 11 ', respectively. These morphological differences between radio and X-rays suggest that the relativistic electrons are distributed homogeneously over the lobe volume, whereas the magnetic field is amplified toward the lobe rim region.

The high redshift (z = 0.997) blazar B2 1308+326 was observed contemporaneously at x-ray, optical and radio wavelengths in June 1996. The x-ray observations were performed with ASCA. The ASCA results were found to be consistent with reanalysed data from two earlier ROSAT observations. The combined ASCA and ROSAT data reveal an xray spectrum that is best fit by a broken power law with absorber model with photon spectral indices of Gamma (soft) = 3.4(-1.1)(+5.1) and Gamma (hard) = 1.63(-0.09)(+0.10) and a break energy at 1.1(-0.4)(+0.4) keV in the rest-frame of the blazar. The break in the x-ray spectrum is interpreted, from the shape of the simultaneous broadband spectral energy distribution, to be the emerging importance of inverse Compton (IC) emission which dominates the ASCA spectrum. The faint optical state reported for these observations (m(V) = 18.3 +/- 0.25) is incompatible with the high synchrotron flux previously detected by ROSAT. The TC emission detected by both ROSAT and ASCA was not significantly affected by the large change in the synchrotron component. Mg II emission was detected with an equivalent width (W-lambda) of similar to 15 Angstrom, significantly different from previously reported values. The small and variable W-lambda in B2 1308+326 may be due to the highly variable continuum and not intrinsically weak lines in the source. A lower limit on the Doppler boost factor calculated from the contemporaneous data is consistent with expectations for highly polarised quasars and higher than expected for BL Lacs. Absorption at a level of N-H = 3.0(-0.6)(+2.3) x 10(20) cm(-2) was detected which is in excess of the Galactic value of NH 1.1 x 10(20) cm(-2), indicating the possible presence of a foreground absorber. A gravitational microlensing scenario cannot therefore be ruled out for this blazar. No significant variability on timescales of hours was detected in the optical or x-ray data. B2 1308+326 could be a typical radio-selected BL Lac in terms of peak synchrotron frequency and optical and radio variability but its high bolometric luminosity, variable line emission and high Doppler boost factor make it appear more like a quasar than a BL Lac. It is suggested that B2 1308+326 be considered as the prototype of this class of composite source.

The high redshift (z = 0.997) blazar B2 1308+326 was observed contemporaneously at x-ray, optical and radio wavelengths in June 1996. The x-ray observations were performed with ASCA. The ASCA results were found to be consistent with reanalysed data from two earlier ROSAT observations. The combined ASCA and ROSAT data reveal an xray spectrum that is best fit by a broken power law with absorber model with photon spectral indices of Gamma (soft) = 3.4(-1.1)(+5.1) and Gamma (hard) = 1.63(-0.09)(+0.10) and a break energy at 1.1(-0.4)(+0.4) keV in the rest-frame of the blazar. The break in the x-ray spectrum is interpreted, from the shape of the simultaneous broadband spectral energy distribution, to be the emerging importance of inverse Compton (IC) emission which dominates the ASCA spectrum. The faint optical state reported for these observations (m(V) = 18.3 +/- 0.25) is incompatible with the high synchrotron flux previously detected by ROSAT. The TC emission detected by both ROSAT and ASCA was not significantly affected by the large change in the synchrotron component. Mg II emission was detected with an equivalent width (W-lambda) of similar to 15 Angstrom, significantly different from previously reported values. The small and variable W-lambda in B2 1308+326 may be due to the highly variable continuum and not intrinsically weak lines in the source. A lower limit on the Doppler boost factor calculated from the contemporaneous data is consistent with expectations for highly polarised quasars and higher than expected for BL Lacs. Absorption at a level of N-H = 3.0(-0.6)(+2.3) x 10(20) cm(-2) was detected which is in excess of the Galactic value of NH 1.1 x 10(20) cm(-2), indicating the possible presence of a foreground absorber. A gravitational microlensing scenario cannot therefore be ruled out for this blazar. No significant variability on timescales of hours was detected in the optical or x-ray data. B2 1308+326 could be a typical radio-selected BL Lac in terms of peak synchrotron frequency and optical and radio variability but its high bolometric luminosity, variable line emission and high Doppler boost factor make it appear more like a quasar than a BL Lac. It is suggested that B2 1308+326 be considered as the prototype of this class of composite source.

We conducted a multifrequency campaign for the TeV blazar Markarian 421 in 1998 April. The campaign started from a pronounced high-amplitude flare recorded by BeppoSAX and Whipple; the ASCA observation started 3 days later. In the X-ray data, we detected multiple flares, occurring on timescales of about 1 day. ASCA data clearly reveal spectral variability. The comparison of the data from ASCA, the Extreme Ultraviolet Explorer, and the Rossi X-Ray Timing Explorer indicates that the variability amplitudes in the low-energy synchrotron component are larger at higher photon energies. In TeV gamma -rays, large intraday variations-which were correlated with the X-ray flux-were observed when results from three Cerenkov telescopes were combined. The rms variability of TeV gamma -rays was similar to that observed in hard X-rays, above 10 keV. The X-ray light curve reveals flares that are almost symmetric for most cases, implying that the dominant timescale is the light crossing time through the emitting region. The structure function analysis based on the continuous X-ray light curve of 7 days indicates that the characteristic timescale is similar to0.5 days. The analysis of ASCA light curves in various energy bands appears to show both soft (positive) and hard (negative) lags. These may not be real, as systematic effects could also produce these lags, which are all much smaller than an orbit. If the lags of both signs are red, these imply that the particle acceleration and X-ray cooling timescales are similar.

We conducted a multifrequency campaign for the TeV blazar Markarian 421 in 1998 April. The campaign started from a pronounced high-amplitude flare recorded by BeppoSAX and Whipple; the ASCA observation started 3 days later. In the X-ray data, we detected multiple flares, occurring on timescales of about 1 day. ASCA data clearly reveal spectral variability. The comparison of the data from ASCA, the Extreme Ultraviolet Explorer, and the Rossi X-Ray Timing Explorer indicates that the variability amplitudes in the low-energy synchrotron component are larger at higher photon energies. In TeV gamma -rays, large intraday variations-which were correlated with the X-ray flux-were observed when results from three Cerenkov telescopes were combined. The rms variability of TeV gamma -rays was similar to that observed in hard X-rays, above 10 keV. The X-ray light curve reveals flares that are almost symmetric for most cases, implying that the dominant timescale is the light crossing time through the emitting region. The structure function analysis based on the continuous X-ray light curve of 7 days indicates that the characteristic timescale is similar to0.5 days. The analysis of ASCA light curves in various energy bands appears to show both soft (positive) and hard (negative) lags. These may not be real, as systematic effects could also produce these lags, which are all much smaller than an orbit. If the lags of both signs are red, these imply that the particle acceleration and X-ray cooling timescales are similar.

Studies were made of ASCA spectra of seven ultraluminous compact X-ray sources in nearby spiral galaxies: M33 X-8, M81 X-6, IC 342 source 1, Dwingeloo 1 X-1, NGC 1313 source B, and two sources in NGC 4565. With the 0.5-10 keV luminosities in the range 10(39)-10(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 seven sources have been described successfully with so-called multicolor disk blackbody emission arising from optically thick standard accretion disks around black holes. Except for 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 similar to 100 M.. 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.

Studies were made of ASCA spectra of seven ultraluminous compact X-ray sources in nearby spiral galaxies: M33 X-8, M81 X-6, IC 342 source 1, Dwingeloo 1 X-1, NGC 1313 source B, and two sources in NGC 4565. With the 0.5-10 keV luminosities in the range 10(39)-10(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 seven sources have been described successfully with so-called multicolor disk blackbody emission arising from optically thick standard accretion disks around black holes. Except for 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 similar to 100 M.. 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.

The TeV blazar PKS 2155 - 304 was monitored with the X-ray satellite ASCA in 1994 May as part of a multiwavelength campaign from the radio to X-ray bands. At the beginning of the two-day continuous observation, we detected a lame flare, in which the 2-10 keV flux changed by a factor of 2 on a timescale of 3 x 10(4) s. During the hare, the increase in the hard X-ray flux clearly preceded that observed in the soft X-rays, with the spectral evolution tracking a "clockwise loop" in the flux versus photon index plane. Ascribing the energy-dependent variability to differential synchrotron cooling of relativistic electrons, we estimate the magnetic field B in the emission region. We tested two different methods of comparing the time series in various X-ray bands: (1) fitting the light curves to a Gaussian function and searching for the time shift of the peak of the flare, and (2) calculating the discrete correlation function. Both methods yielded a consistent solution of B similar to 0.1 G; We also found that the flare amplitude becomes larger as the photon energy increases, while the duration of the flare stays roughly constant throughout the ASCA energy band (0.7-7.5 keV). In the framework of the time-dependent synchrotron self-Compton model in a homogeneous region, we consider a flare where the maximum Lorentz factor (gamma(max)) of the injected electrons increases uniformly throughout the emission volume. The temporal evolution of spectra as well as the light curves were reproduced with the physical parameters self-consistently determined from seven observables. We obtained B similar to 0.1-0.2 G and a region size R similar to 10(-2) pc for relativistic beaming with a Doppler factor of delta similar to 20-30. We discuss the significance of light-travel time effects.

We present data for 18 blazars observed with ASCA, half of which were also observed contemporaneously with EGRET as parts of multi-wavelength campaigns. The ASCA X-ray spectra of High-energy peaked BL Lacs (HBLs) are soft, and they form the highest energy tail of the low energy (synchrotron) component. The X-ray spectra of the quasar-hosted blazars (QHBs) are hard and consistent with the lowest energy end of the high energy (Compton) component. For Low-energy peaked BL Lacs (LBLs), the X-ray spectra are intermediate. We find that the radiation process responsible for the HE peak for HBLs can be explained solely by Synchrotron-Self-Compton (SSC) emission. For many QHBs, on the other hand, the gamma-rays cannot be solely due to the SSC mechanism. We consider an alternative scenario for QHBs where the SSC component dominates in the X-ray band, but it is below the observed gamma-ray spectrum. We infer the magnetic field B to be 0.1 - 1 Gauss, and Lorentz factors gamma(b) of electrons radiating at the peak of the nu F(nu) spectrum of similar to 10(3) for QHBs. This is much lower than gamma(b) similar to 10(5) for HBLs. (C) 2000 COSPAR. Published by Elsevier Science Ltd.

The ASCA detections of inverse-Compton (IC) X-rays from lobes of radio galaxies are reported. Significant IC X-rays are observed from radio lobes of Fornax A (Kaneda et al. 1995; Feigelson et al. 1995), Centaurus B (PKS 1343-601) and NGC 612. The energy density and magnetic field intensity in the lobes are sorted out by comparing the IC X-ray flux with the synchrotron radio flux. The results from Centaurus B strongly suggest particle domination in the lobes; meanwhile, X-ray and radio image analysis shows an outward increase in the magnetic energy density. The X-ray image and spectrum observed from NGC: 612 reveal a heavily absorbed low luminosity core (nucleus) emission, together with an extended emission associated with the lobes. The extended source luminosity is rather close to the expected value assuming the equipartition state, although the possibility of particle dominant lobes cannot be excluded. (C) 2000 COSPAR. Published by Elsevier Science Ltd.

The TeV blazar PKS 2155 - 304 was monitored with the X-ray satellite ASCA in 1994 May as part of a multiwavelength campaign from the radio to X-ray bands. At the beginning of the two-day continuous observation, we detected a lame flare, in which the 2-10 keV flux changed by a factor of 2 on a timescale of 3 x 10(4) s. During the hare, the increase in the hard X-ray flux clearly preceded that observed in the soft X-rays, with the spectral evolution tracking a "clockwise loop" in the flux versus photon index plane. Ascribing the energy-dependent variability to differential synchrotron cooling of relativistic electrons, we estimate the magnetic field B in the emission region. We tested two different methods of comparing the time series in various X-ray bands: (1) fitting the light curves to a Gaussian function and searching for the time shift of the peak of the flare, and (2) calculating the discrete correlation function. Both methods yielded a consistent solution of B similar to 0.1 G; We also found that the flare amplitude becomes larger as the photon energy increases, while the duration of the flare stays roughly constant throughout the ASCA energy band (0.7-7.5 keV). In the framework of the time-dependent synchrotron self-Compton model in a homogeneous region, we consider a flare where the maximum Lorentz factor (gamma(max)) of the injected electrons increases uniformly throughout the emission volume. The temporal evolution of spectra as well as the light curves were reproduced with the physical parameters self-consistently determined from seven observables. We obtained B similar to 0.1-0.2 G and a region size R similar to 10(-2) pc for relativistic beaming with a Doppler factor of delta similar to 20-30. We discuss the significance of light-travel time effects.

We present data for 18 blazars observed with ASCA, half of which were also observed contemporaneously with EGRET as parts of multi-wavelength campaigns. The ASCA X-ray spectra of High-energy peaked BL Lacs (HBLs) are soft, and they form the highest energy tail of the low energy (synchrotron) component. The X-ray spectra of the quasar-hosted blazars (QHBs) are hard and consistent with the lowest energy end of the high energy (Compton) component. For Low-energy peaked BL Lacs (LBLs), the X-ray spectra are intermediate. We find that the radiation process responsible for the HE peak for HBLs can be explained solely by Synchrotron-Self-Compton (SSC) emission. For many QHBs, on the other hand, the gamma-rays cannot be solely due to the SSC mechanism. We consider an alternative scenario for QHBs where the SSC component dominates in the X-ray band, but it is below the observed gamma-ray spectrum. We infer the magnetic field B to be 0.1 - 1 Gauss, and Lorentz factors gamma(b) of electrons radiating at the peak of the nu F(nu) spectrum of similar to 10(3) for QHBs. This is much lower than gamma(b) similar to 10(5) for HBLs. (C) 2000 COSPAR. Published by Elsevier Science Ltd.

The ASCA detections of inverse-Compton (IC) X-rays from lobes of radio galaxies are reported. Significant IC X-rays are observed from radio lobes of Fornax A (Kaneda et al. 1995; Feigelson et al. 1995), Centaurus B (PKS 1343-601) and NGC 612. The energy density and magnetic field intensity in the lobes are sorted out by comparing the IC X-ray flux with the synchrotron radio flux. The results from Centaurus B strongly suggest particle domination in the lobes; meanwhile, X-ray and radio image analysis shows an outward increase in the magnetic energy density. The X-ray image and spectrum observed from NGC: 612 reveal a heavily absorbed low luminosity core (nucleus) emission, together with an extended emission associated with the lobes. The extended source luminosity is rather close to the expected value assuming the equipartition state, although the possibility of particle dominant lobes cannot be excluded. (C) 2000 COSPAR. Published by Elsevier Science Ltd.

The edge-on spiral galaxy NGC 4565 was observed for similar to 35 ks with ASCA in the 0.5-10 keV energy band. The X-ray emission was dominated by two bright sources, which could be identified with two point-like X-ray sources seen in the ROSAT HRI image. The observed 0.5-10 keV fluxes of these sources, 1.7 x 10(-12) erg s(-1) cm(-2) and 0.7 x 10(-12) erg s(-1) cm(-2), imply bolometric luminosities of 1.2 x 10(40) erg s(-1) and 4.6 x 10(39) erg s(-1), respectively. They exhibit similar spectra, which can be explained by emission from optically thick accretion disks with inner-disk temperature of 1.4-1.6 keV. One of them, coincident in position with the nucleus, shows too low absorption to be the active nucleus seen through the galaxy disk. Their spectra and high luminosities suggest that they are both mass-accreting black-hole binaries. However, the black-hole mass required by the Eddington limit is rather high (greater than or equal to 50M.), and the observed disk temperature is too high to be compatible with the high black-hole mass. Several attempts have been made to solve these problems.