J. Barnstedt

Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo

The Milky Ways halo contains clouds of neutral hydrogen with high radial velocities which do not follow the general rotational motion of the Galaxy. Few distances to these high-velocity clouds are known, so even gross properties such as total mass are hard to determine. As a consequence, there is no generally accepted theory regarding their origin. One idea is that they result from gas that has cooled after being ejected from the Galaxy through fountain-like flows powered by supernovae; another is that they are composed of gas, poor in heavy elements, which is falling onto the disk of the Milky Way from intergalactic space. The presence of molecular hydrogen, whose formation generally requires the presence of dust (and therefore gas, enriched in heavy elements), could help to distinguish between these possibilities. Here we report the discovery of molecular hydrogen absorption in a high-velocity cloud along the line of sight to the Large Magellanic Cloud. We also derive for the same cloud an iron abundance which is half of the solar value. From these data, we conclude that gas in this cloud originated in the disk of the Milky Way.

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 90 nm to 140 nm, the spectral resolution is significantly better than λ /Δ λ  = 10 000, where Δ λ is measured as FWHM of the instrumental profile. The effective area peaks at 1.3 cm2 near 110 nm. 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 ±  0.005 nm.

INTEGRAL observations of the variability of OAO 1657 415

Context. The Galactic Plane Scan (GPS) was one of the core observation programmes of the INTEGRAL satellite. The highly variable accreting pulsar OAO 1657-415 was frequently observed within the GPS. Aims. We investigate the spectral and timing properties of OAO 1657-415 and their variability on short and long time scales in the energy range

HIRDES UV spectrographs

6{-}160

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

keV. Methods. Using standard extraction tools and custom software for extracting INTEGRAL data we analysed energy-resolved light curves with a time resolution of one second – mainly data of the ISGRI instrument. We also analysed phase-averaged broad band spectra – including JEM-X spectra – and pulse-phase resolved spectra of ISGRI. Results. During the time covered by the INTEGRAL observations, the pulse period evolution shows an initial spin-down, which is followed by an equally strong spin-up. In combining our results with historical pulse period measurements (correcting them for orbital variation) and with stretches of continuous observations by BATSE, we find that the long-term period evolution is characterised by a long-term spin-up overlayed by sets of relative spin-down/spin-up episodes, which appear to repeat quasi-periodically on a 4.8 yr time scale. We measure an updated local ephemeris and confirm the previously determined orbital period with an improved accuracy. The spectra clearly change with pulse phase. The spectrum measured during the main peak of the pulse profile is particularly hard. We do not find any evidence of a cyclotron line, wether in the phase-averaged spectrum or in phase-resolved spectra.

The ORFEUS II Echelle spectrum of HD 93521: A reference for interstellar molecular hydrogen

The World Space Observatory Ultraviolet (WSO/UV) is a multi-national project grown out of the needs of the astronomical community to have future access to the ultraviolet range of the spectrum. The development of the WSO/UV S/C and the telescope is headed by the Russian Federal Space Agency (Roscosmos). The mission is scheduled to be launched in 2010 into the L2 orbit. The WSO/UV consists of a single Ultraviolet Telescope, incorporating a primary mirror of 1.7 m diameter feeding UV spectrometer and UV imagers. The UV spectrometer comprises three different single spectrographs, two high resolution echelle spectrographs - the High Resolution Double Echelle Spectrograph (HIRDES) - and a low dispersion long slit instrument. Within the HIRDES the spectral band (102 - 310 nm) is separated into two echelle spectrographs covering the UV range between 174- and 310 nm (UVES) and VacuumUV range between 102 and 176 nm (VUVES) with a very high spectral resolution of > 50000. Each spectrograph encompass a stand alone optical bench structure with a fully redundant high speed MCP detector system, the optomechanics and a network of mechanisms with different functionalities. The fundamental technical concept is based on the heritage of the two previous ORFEUS SPAS missions. The phase B1 development activities are described in this paper under consideration of performance aspects, design drivers, the related trade offs (e.g. mechanical concepts, material selection etc.) and the critical functional and environmental test verification approach. Furthermore the actual state of the other scientific instruments of the WSO/UV (e.g. UV imagers) project is described.

A 30-cm objective grating for far-UV astronomy: theoretical study and laboratory tests

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.

UV MCP Detectors for WSO-UV: Cross Strip Anode and Readout Electronics

During the second flight of the ORFEUS-SPAS mission in November/December 1996, the Echelle spec- trometer was used extensively by the Principal and Guest Investigator teams as one of the two focal plane in- struments of the ORFEUS telescope. The spectrum of HD 93521 was obtained during this mission with a total integration time of 1740 s. This spectrum shows numer- ous sharp interstellar absorption lines. We identied 198 lines of molecular hydrogen including at least 7 lines with a high velocity component. Also most of the 67 identi- ed interstellar metal lines are visible with a high velocity component. We present plots of the complete ORFEUS II Echelle spectrum together with tables of all identied interstel- lar absorption lines including all 14 detectable H I lines. In addition several identied stellar lines, partially with narrow absorption components, and stellar wind lines are given in a separate table.

MCP detector development for WSO-UV

We have performed theoretical determination and experimental calibrations of an objective grating designed for high resolution spectroscopy of astronomical faint sources in the EUV and far-UV wavelength ranges (500-1400 A). First through theoretical calculations we show the feasibility of the concept with an aspheric shape for the grating blank and determine its geometrical parameters. A grating of this large size has been manufactured and tested, associated with a photon counting detector, in a vacuum environment. Finally we demonstrate that a resolving power of 3 x 10(4), a total equivalent effective area of ~5-10 cm(2) can be achieved, together with a very low scattered light level (10(-4)-10(-5) of the peak value).

Low-power readout electronics for micro channel plate detectors with cross-strip anodes

The main instrument of the WSO-UV satellite covers the wavelength range of 102-176 nm and 174-310 nm with two high resolution echelle spectrographs. The essential requirements for the associated detectors are high quantum efficiency, solar blindness, and single photon detection. To achieve this, we are developing microchannel plate (MCP) detectors in sealed tubes. It is planned to use cesium activated gallium nitride as semitransparent photocathode, a stack of two microchannel plates in chevron configuration, and a 33 mm × 44 mm cross strip anode with 64 horizontal and 64 vertical electrodes. This type of anode requires a lower gain of the MCPs ( ≈ 106) compared to other types of anodes. Therefore, it extends the expected lifetime of the detectors to about five to ten years. The challenge for the use of a cross strip anode onboard the WSO-UV satellite is the combination of contradictory constraints on the readout electronics: On the one hand it should be able to handle a maximum count rate of 3·105 s-1 with a spatial resolution better than 15 μm . On the other hand the power consumption is limited to about 8 W. One feasible solution is the so-called Beetle chip. This chip provides 128 input channels with charge-sensitive preamplifiers and shapers. It stores the sampled data temporarily in a ring buffer and multiplexes it to four analogue readout channels. The output is then digitized by four ADCs and processed in a radiation hard FPGA, which also contains the space-wire interface to the satellite bus.