M. H. Ulrich

The ultraviolet spectrum of NGC 4151 from 1978 to 1990 : general characteristics and evolution

From the examination of all the low-dispersion IUE spectra of NGC 4151 taken since the launch of IUE in 1978, including the latest data taken in 1990, we follow the evolution of this active nucleus on time scales ranging from days to 10 years. We concentrate our analysis on the campaigns of observations during which the nucleus was not continuously in a minimum state.

Optical spectroscopy of BL Lac objects: New redshifts and mis-identified sources

We are carrying out a program of high signal to noise optical spectroscopy of BL Lacs with unknown or tentative redshift. Here we report some preliminary results. New redshifts are measured for PKS0754+100 (z=0.266) and 1ES0715-259 (z=0.464) . From lineless spectra of PG1553+113 and PKS1722+119 we set a lower limit of z>0.3 for both sources. In two cases (UM493 and 1620+103) stellar spectra indicate a wrong classification.

3C 273: A REVIEW OF RECENT RESULTS

Abstract3C 273 is the most extensively studied quasar both from the ground and from space. Recent satellite observations have given important information on the overall electromagnetic spectrum of 3C 273 in the γ-ray, X-ray, and UV ranges. The most salient results are: (i) the energy per decade of frequency emitted by 3C 273 is nearly constant between 6000 Å and 500 MeV and is ∼20 × 1046 erg s-1 for H = 50 km s-1 Mpc-1; (ii) there is no absorption in the soft X-ray range in contrast to the X-ray spectrum of Seyfert nuclei; (iii) the optical and UV spectra cannot be fitted by power-law spectra only, and the energy distribution in this range suggests that a substantial fraction of the energy in the UV is emitted as back-body radiation at ≈20 000 K. If the peculiar shape of the UV spectrum is indeed caused by black-body radiation, then an estimate of the energy emitted under this form is 2.5 × 1046erg s-1, corresponding to an optically thick disk of ∼1016 cm in diameter.The UV spectrum of 3C 273 shows absorption lines at zero redshift caused by interstellar matter in the disk and halo of our Galaxy. The strength of C iv λ 1550 in absorption indicates the presence of a hot outer region in the halo. Extragalactic objects with mostly continuous UV spectra, such as 3C 273, are very promising UV sources which allow us to observe the absorbing material over the entire line of sight throughout the galactic halo.

Observational Evidence for Accretion Disks in Galactic Nuclei

The electromagnetic spectrum of quasars and Seyfert 1 galaxies shows an excess of energy at λ > 4000 A over the extrapolation of the infrared/optical spectrum: This ”blue bump” is generally interpreted as thermal radiation by gas at T > 3.104 °K. But is it an accretion disk which emits this thermal radiation? To answer this question geometrical and kinematical information on the dense gas in the vicinity of the black hole is needed. We examine the most important observations in favor or against the presence of an accretion disk: the photoelectric X-ray absorption, the energy budget in the broad line region, and the profiles of the Balmer lines. The optical polarization and the occurrence of Lyman edge absorptions are also reviewed.

Optical imaging and spectroscopy of BL Lac objects

We present optical images and spectroscopy for a dozen of BL Lac objects. Most of these objects were not previously studied and we give for the first time the properties of their host galaxies. The properties of the new host galaxies are generally consistent with those derived in previous optical studies. We found a case (1101-23) where the external isophotes of the galaxy are clearly boxy. In addition we gathered spectroscopy for several BL Lac objects with unknown redshift and for companion galaxies. This allowed us to derive a tentative redshift for two new BL Lacs and to investigate the environment around PKS 0829+04. These data complement existing data available in the literature on host galaxies of BL Lacs and their (close) environments.

Simultaneous EXOSAT-IUE observations of NGC 4151

NGC 4151 was observed four times in Nov. 83. The results indicate that: a) there exists a correlation between the X-ray and UV fluxes on the long term; b) the soft X-ray excess between 0.1 and 1 keV is probably steeper than expected from the leaky absorber model by Holt et al (1980); c)the spectral fit to the ME data, after correction for a soft component, yields α=1,73±0.27, NH=(15.2±2.2)×1022 cm−2, E.W.(Fe line)=0.208±0.084 keV, and does not require a strong overabundance of Fe in the absorber. The relationship between NH and the strength of the broad emission lines is commented.

Ultraviolet, Optical and X-Ray Monitoring of the Continuum of NGC 4151

The nucleus of the Seyfert galaxy NGC 4151 has been extensively monitored with the International Ultraviolet Explorer (IUE) in the wave- length range 1200–3000 A, a region of the spectrum where the contamination by the stellar component of the nuclear light is negligible. A first detailed account of the data collected from February 1978 to January 1979 is given in Penston et al (1981), and three more papers devoted to the continuum, the emission and the absorption lines based on all IUE data up to May 1980 are in preparation. This note is a summary of the paper on the continuum, which includes also optical and X-ray observations (Perola et al, 1981).