No Arabic abstract
We have observed a radio-quiet QSO Kaz 102 (z=0.136) with ASCA as a part of our program of complete spectral characterization of hard X-ray selected AGNs. We found that Kaz 102 shows unusual spectral properties. A simple power-law with absorption in our galaxy gave a satisfactory description of the spectrum. However, it showed a very hard photon index of Gamma=1.0 with no sign of deep absorption or a prominent spectral feature. We further explored the Compton reflection with Fe K-alpha line and warm absorber models for hardening the spectrum. Both gave statistically satisfactory fits. However, the Compton reflection model requires a very low metal abundance (0.03-0.07 in solar units).The warm absorber model with no direct component is preferred and gave a very high ionization parameter xi=200. If this is the case, the values of xi, warm absorber column density, and variability over about 10 years may suggest that the warm absorber resides in the broad-line region and crosses the line of sight to the central X-ray source.
We report on the X-ray observation of the radio selected supernova SN1979C carried out with ASCA in December 1997. The supernova of type II$_{L}$ was first observed in the optical and occurred in the weakly barred, almost face on spiral galaxy NGC 4321 (M100) which is at a distance of 17.1 Mpc, and contains at least three other supernovae discovered in this century. No point source was detected at the radio position of SN1979C in a 3 diameter half power response circle in a 27.3 ks SIS exposure. The background and galaxy subtracted SN signal had a 3$sigma$ upper limit to the count rate of 1.2$times 10^{-3}$ cps in the full ASCA SIS band (0.4-10.0 keV). These measurements give the first ever x-ray flux limit of a Type II$_{L}$ SN in the higher energy band ($geq$ 2 keV) which is an important diagnostic of the {it outgoing} shock wave ploughing through the circumstellar medium.
We report the results of hard X-ray observations of Abell 496 (A496), a nearby relaxed cluster, using the Rossi X-ray Timing Explorer (RXTE). The 3-20 keV spectrum of this cluster is well-modeled by a thermal component of kT ~ 4.1 keV plus a cooling flow with mass accretion rate of dot{M} ~ 285 M_{odot} yr^{-1}. The spectrum is equally well-modeled by a multi-temperature plasma component with a Gaussian temperature distribution of mean temperature 3.8 keV and sigma_{kT} ~ 0.9 keV. The metallicity is found to be approximately 1/3 solar; however, the Ni/Fe ratio is about 3.6. No significant nonthermal emission at hard X-rays was detected for this cluster. We discuss the implications of the models presented here and compare them with the temperature profiles derived for this cluster using the Advanced Satellite for Cosmology and Astrophysics (ASCA). Our results are inconsistent with declining temperature profiles.
We present an analysis of the rosat and asca spectra of 21 broad line AGN (QSOs) with $zsim 1$ detected in the 2-10 keV band with the asca gis. The summed spectrum in the asca band is well described by a power-law with $Gamma=1.56pm0.18$, flatter that the average spectral index of bright QSOs and consistent with the spectrum of the X-ray background in this band. The flat spectrum in the asca band could be explained by only a moderate absorption ($sim 10^{22} rm cm^{-2}$) assuming the typical AGN spectrum ie a power-law with $Gamma$=1.9. This could in principle suggest that some of the highly obscured AGN, required by most X-ray background synthesis models, may be associated with normal blue QSOs rather than narrow-line AGN. However, the combined 0.5-8 keV asca-rosat spectrum is well fit by a power-law of $Gamma=1.7pm0.2$ with a spectral upturn at soft energies. It has been pointed out that such an upturn may be an artefact of uncertainties in the calibration of the ROSAT or ASCA detectors. Nevertheless if real, it could imply that the above absorption model suggested by the asca data alone is ruled out. Then a large fraction of QSOs could have ``concave spectra ie they gradually steepen towards softer energies. This result is in agreement with the bepposax hardness ratio analysis of $sim$ 100 hard X-ray selected sources.
We report the results from an ASCA X-ray observation of the powerful Broad Line Radio Galaxy, 3C109. The ASCA spectra confirm our earlier ROSAT detection of intrinsic X-ray absorption associated with the source. The absorbing material obscures a central engine of quasar-like luminosity. The luminosity is variable, having dropped by a factor of two since the ROSAT observations 4 years before. The ASCA data also provide evidence for a broad iron emission line from the source, with an intrinsic FWHM of ~ 120,000 km/s. Interpreting the line as fluorescent emission from the inner parts of an accretion disk, we can constrain the inclination of the disk to be $> 35$ degree, and the inner radius of the disk to be $< 70$ Schwarzschild radii. Our results support unified schemes for active galaxies, and demonstrate a remarkable similarity between the X-ray properties of this powerful radio source, and those of lower luminosity, Seyfert 1 galaxies.
We present the first results of the Chandra and optical follow-up observations of hard X-ray sources detected in the ASCA Medium Sensitivity Survey (AMSS). Optical identifications are made for five objects. Three of them show either weak or absent optical narrow emission lines and are at low redshift <z>~0.06. One of them is a broad line object at z=0.910 and one is a z=0.460 object with only narrow lines. All the narrow line objects show strong evidence for absorption in their X-ray spectra. Their line ratios are consistent with a Seyfert II/LINER identification as are the line widths. The three low redshift objects have the colors of normal galaxies and apparently the light is dominated by stars. This could be due to the extinction of the underlying nuclear continuum by the same matter that absorbs X-rays and/or due to the dilution of the central source by starlight. These results suggest that X-ray sources that appear as ``normal galaxies in optical and near-IR bands significantly contribute to the hard X-ray background. This population of objects has a high space density and probably dominates the entire population of active galaxies.