W. Zheng, G. A. Kriss, R. C. Telfer, J. P. Grimes & A. F. Davidsen
Center for Astrophysical Sciences, The Johns Hopkins University
The spectral features in the sub--Ly region of AGN spectra provide critical insights into the physical processes around the central engine, and they become accessible to IUE and HST at intermediate redshift. O'Brien, Gondhalekar, & Wilson (1988) analyzed the IUE data and found a gradual steepening of the quasar continuum from the near--infrared to far--UV band. The Hubble Space Telescope (HST), with its superb light--collecting ability, has greatly extended our probe of the distant universe. The HST Faint Object Spectrograph (FOS) database is superior to the IUE database in terms of S/N level and spectral resolution. During the last five years, a significant number of FOS spectra of quasars have been accumulated. As part of an archival study, we have constructed a composite spectrum of quasars based on 149 FOS spectra of 80 quasars. It reveals some spectral features which are not noticed in individual spectra, particularly in the far--UV range.
Archival FOS spectra are retrieved from the Space Telescope Science Institute in the form of calibrated data. The target selection criteria are: (1) redshift ; (2) no broad absorption line is present. Our sample includes 41 spectra with grating G160L, 5 with G130H, 40 with G190H, 49 with G270H, and 14 with G400H. The G160L and G130H spectra are checked for proper background subtraction and are reprocessed if necessary.
The FOS spectra are corrected for Galactic extinction, and then shifted to the quasar rest frame. Data points in the window near the significant airglow lines at 1216Å and 1304Å are replaced by values linearly interpolated from neighboring parts of the spectrum. If a spectrum contains an obvious flux discontinuity due to an external Lyman--limit system, only the undepressed portion of the spectrum is used. Each spectrum is resampled into 0.1Å bins, and then normalized to match the average flux of a template spectrum which covers the whole spectral region. The initial template is generated by adding all the spectra with equal weights. The spectra are subsequently combined pixel by pixel, weighted by where f is the flux and e the error. A combination process with this weighting is similar to count summation of raw data. Only the data points with average are used. The resultant spectrum serves as the new template spectrum, and the normalization and combining processes are repeated to reduce discontinuities until a smooth final spectrum is produced. Figure 1 displays the number of merged spectra vs. wavelength.
The composite quasar spectrum is shown in Figure 2. The spectral region near 1200Å has the highest S/N level, per 0.1Å, and the region near 700Å has . The flux depression shortward of the Ly emission due to Ly forest--line absorption is small, at a level not more than 10%. The continuum can be approximated with a broken power law. The power--law index () is longward of 1000Å and at shorter wavelengths. The break point of the power law appears to be near 1000Å. The change in the continuum shape around 1000Å is so significant and abrupt that it cannot be explained by reddening or intervening absorption, commonly known as the Lyman Valley (Møller & Jakobsen 1990).
The continuum shape in the merged spectrum matches the characteristics of a Comptonized spectrum of an accretion disk with Lyman--limit absorption. The thermal emission from a geometrically thin, optically thick accretion disk around a central black hole, plus an underlying power law, may well fit the continuum shape longward of 1000Å (Sun & Malkan 1989). The continuum at shorter wavelengths may be modified by electron scattering in a corona above the disk (Czerny & Zbyszewska 1991). Comptonization produces a power--law high--energy tail and smears out any Lyman--limit discontinuity feature intrinsic to the disk. Such a feature is significant in the spectra of several quasars, but not detected in most objects.
A number of emission lines are present. In addition to the major emission features marked in Figure 2, some weak features, such as C III 977 and N III 991, can be measured because of the high S/N level. The emission feature around 690Å may be O III+N III arising from Bowen fluorescence (Eastman & MacAlpine 1985). The presence of far--UV emission lines beyond O VI suggests a high temperature ( K) in the region emitting these lines as their excitation energies are greater than 12 eV.
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