The penetration of solar H Lyman-α radiation into the terrestrial middle atmosphere is studied in detail. The Lyman-α actinic flux is calculated with a Monte Carlo approach including multiple resonance scattering of Lymanα photons within the terrestrial atmosphere and a temperature dependent absorption cross section of molecular oxygen. The dependence of the actinic flux on the temperature prof...

Gamma-ray burst (GRB) progenitors are observationally linked to the death of massive stars. X–ray studies of the GRB afterglows can deepen our knowledge of the ionization status and metal abundances of the matter in the GRB environment. Moreover, the presence of local matter can be inferred through its fingerprints in the X–ray spectrum, i.e. the presence of absorption higher than the Galactic ...

We examine the consistency of Hi properties with the unification scheme for radio galaxies and quasars, and any correlation with the symmetry parameters for a sample of CSS and GPS sources. In our sample, 15 out of 23 galaxies exhibit 21-cm absorption as against 1 out of 9 quasars, which is broadly consistent with the unification scheme. Also there is a tendency for the detection rate as well a...

We present Chandra observations of the radio-quiet Broad Absorption Line (BAL) QSO PG 1254+047. We find that it is a weak X-ray source, with a total of 44 ± 7 photons measured in 36 ksec across the observed energy range 0.7 − 6.0 keV. Its X-ray weakness is consistent with the known correlation between αox and the strength of the UV absorption lines. The spectral energy distribution suggests tha...

The accuracy of models for primary production and light propagation depends on correct assignment of absorption to photosynthetic pigments. The phytoplankton absorption coefficient is comprised of two components: photosynthetic and photoprotective absorption coefficients. A method based on the fluorescent excitation of chlorophyll a is used to quantify the photosynthetic absorption coefficient ...

We have obtained spectra of three QSO/AGNs with the GHRS aboard the Hubble Space Telescope to search for absorption from low column density gas in the halo of the dwarf spheroidal (dSph) galaxy Leo I. The probe sight lines pass 2.1, 3.7, and 8.1 kpc from the center of the galaxy, but no C IV, Si II, or Si IV absorption is found at the velocity of Leo I. The absence of low-ionization species sug...

We present Chandra observations of the radio-quiet Broad Absorption Line (BAL) QSO PG 1254+047. We find that it is a weak X-ray source, with a total of 44 ± 7 photons measured in 36 ksec across the observed energy range 0.7 − 6.0 keV. Its X-ray weakness is consistent with the known correlation between αox and the strength of the UV absorption lines. The spectral energy distribution suggests tha...

In ORFEUS II spectra of the sdO star BD +39 3226 interstellar hydrogen and deuterium is detected. From Ly α profile fitting and a curve of growth analysis of the Lyman series of H i and D i we derive the column densities NH = 1.20 −0.22·10 cm−2 and ND = 1.45 −0.38·10 cm−2. From the analysis of metal absorption lines in ORFEUS and IUE spectra we obtain column densities for 11 elements. In additi...