Immo Appenzeller

The ORFEUS II Echelle Spectrometer: Instrument description, performance and data reduction

During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spectrometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane instruments of the ORFEUS telescope. We present the in-flight performance and the principles of the data reduction for this instrument. The wavelength range is 90u2009nm to 140u2009nm, the spectral resolution is significantly better than λ /Δ λ u2009=u200910u2009000, where Δ λ is measured as FWHM of the instrumental profile. The effective area peaks at 1.3u2009cm2 near 110u2009nm. The background is dominated by straylight from the Echelle grating and is about 15% in an extracted spectrum for spectra with a rather flat continuum. The internal accuracy of the wavelength calibration is better than ± u20090.005u2009nm.

ORFEUS II Far-Ultraviolet Observations of 3C 273: Interstellar and Intergalactic Absorption Lines*

We present the first intermediate-resolution (λ/Δλ = 3000) spectrum of the bright quasi-stellar object 3C 273 at wavelengths between 900 and 1200 A. Observations were performed with the Berkeley spectrograph aboard the ORFEUS II mission. We detect Lyβ counterparts to intergalactic Lyα features identified by Morris and coworkers at cz = 19,900, 1600, and 1000 km s−1; counterparts to other putative Lyα clouds along the sight line are below our detection limit. The strengths of the two very low redshift Lyβ features, which are believed to arise in Virgo intracluster gas, exceed preflight expectations (Weymann and coworkers), suggesting that the previous determination of the cloud parameters may underestimate the true column densities. A curve-of-growth analysis sets a minimum H I column density of 4 × 1014 cm−2 for the 1600 km s−1 cloud. We find marginally significant evidence for Galactic H2 along the sight line, with a total column density of about 1015 cm−2. We detect the stronger interstellar O VI doublet member unambiguously; the weaker member is blended with other features. If the Doppler b-value for O VI is comparable to that determined for N V by Sembach and collaborators, then the O VI column density is (7 ± 2) × 1014 cm−2, significantly above the only previous estimate, by Davidsen. The O VI/N V ratio is about 10, consistent with the low end of the range observed in the disk, as shown in the compilation by Hurwitz & Bowyer. Additional interstellar species detected for the first time toward 3C 273 (at modest statistical significance) include P II, Fe III, Ar I, and S III.

ORFEUS II Far-Ultraviolet Observations of 3C 273: The Intrinsic Spectrum

Using the Berkeley spectrometer of the ORFEUS-SPAS II mission, we observed the spectrum of the bright, low-redshift QSO 3C 273 in the wavelength range 900-1200 A (780-1040 A in the QSOs rest frame). The QSOs spectrum is dominated by broad emission blends of the resonance lines of O VI, C III, N III, and S VI. Only relatively weak emission is detected at the wavelengths of the higher Lyman lines, and no significant Lyman discontinuity is present at the QSOs redshift. The reddening-corrected underlying smooth continuum shows (for the epoch of the ORFEUS observations) a turnover of the νFν spectrum at about 2.5×1015 Hz. While this turnover frequency is in the general range expected for active galactic nucleus accretion disks, it is lower than predicted for QSOs as luminous as 3C 273 by standard accretion disk models.

Medium-resolution far-ultraviolet spectroscopy of PKS 2155-304

Using the Berkeley spectrometer of the Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) we observed the 87-117 nm UV spectrum of the BL Lac object PKS 2155-304 with about 0.5 A resolution. In addition to the expected interstellar lines we detected higher quantum number counterparts of the intergalactic Lyman alpha lines discovered earlier with IUE and the Hubble Space Telescope (HST) in the direction of PKS 2155-304. The Lyman discontinuities indicate for three of the redshifted clouds a combined H I column density of 2-5 x 10(exp 16)/sq cm, while the column density for another cloud appears to be well below 5 x 10(exp 15)/sq cm. No siginificant O VI absorption in the galactic halo toward PKS 2155-304 could be detected from our data. Assuming that saturation effects are negligible for these weak features, we obtain for the O VI column density toward PKS 2155-304 a 3 sigma upper limit of 2.7 x 10(exp 14)/sq cm.

ORFEUS Observation of the Central Star of NGC 6543

During the ORFEUS-SPAS (Orbiting Retrievable Far and Extreme Ultraviolet Spectrometer on the Shuttle Pallet Satellite) mission STS-51, flown in September 1993, we observed the central star of the planetary nebula NGC 6543 in the far ultraviolet (90 nm to 115 nm) wavelength region using the University of California, Berkeley spectrometer with a spectral resolution of 0.03 nm.