Evidence for an ionized disc in the narrow-line Seyfert 1 galaxy Ark 564

We present simultaneous ASCA and RXTE observations of Ark 564, the brightest known 'narrow-line' Seyfert 1 in the 2–10 keV band. The measured X-ray spectrum is dominated by a steep (Γ≈2.7) power-law continuum extending to at least 20 keV, with imprinted Fe K-line and edge features and an additional 'soft excess' below ∼1.5 keV. The energy of the iron K-edge indicates the presence of highly ionized material, which we identify in terms of reflection from a strongly irradiated accretion disc. The high reflectivity of this putative disc, together with its strong intrinsic O  viii Ly α and O  viii recombination emission, can also explain much of the observed soft excess flux. Furthermore, the same spectral model also provides a reasonable match to the very steep 0.1–2 keV spectrum deduced from ROSAT data. The source is much more rapidly variable than 'normal' Seyfert 1s of comparable luminosity, increasing by a factor of ∼50 per cent in 1.6 h, with no measurable lag between the 0.5–2 keV and 3–12 keV bands, consistent with much of the soft excess flux arising from reprocessing of the primary power-law component in the inner region of the accretion disc. We note, finally, that if the unusually steep power-law component is a result of Compton cooling of a disc corona by an intense soft photon flux, then the implication is that the bulk of these soft photons lie in the unobserved extreme ultraviolet.     

Teleconnections between Ethiopian summer rainfall and sea surface temperature: part II. Seasonal forecasting

A seasonal forecasting system that is capable of skilfully predicting rainfall totals on a regional scale would be of great value to Ethiopia. Here, we describe how a statistical model can exploit the teleconnections described in part 1 of this pair of papers to develop such a system. We show that, in most cases, the predictors selected objectively by the statistical model can be interpreted in the light of physical teleconnections with Ethiopian rainfall, and discuss why, in some cases, unexpected regions are chosen as predictors. We show that the forecast has skill in all parts of Ethiopia, and argue that this method could provide the basis of an operational seasonal forecasting system for Ethiopia.     

On a fractional derivative form of the Green-Ampt infiltration model

The Green-Ampt model for infiltration into homogeneous soils predicts a monotonically decreasing infiltration rate and a wetting front that initially advances as the square root of time. Infiltration in heterogeneous soils, however, can exhibit non-monotonic infiltration rates and wetting front advances that differ from the square root of time (“anomalous diffusion”).Here it is postulated, that if the length scales of the heterogeneities can be assumed to be power law distributed, it may be appropriate to model infiltration in heterogeneous soils in terms of fractional derivatives. Then, by expressing the hydraulic flux as a Caputo fractional derivative (order 0 < α ? 1) of the head, a fractional Green-Ampt infiltration model is obtained. It is shown that solutions of this model predict non-monotonic and anomalous diffusion behaviors consistent with observations in field infiltration trials; a finding that indicates that a non-local moisture flux model, based on fractional derivatives, is a plausible model for describing infiltration into heterogeneous soils.     

Changes in the photosynthetic coefficients α and Pm of Planktothrix rubescens grown on light-dark cycles

The relationship between photosynthesis and irradiance was investigated in a strain of the cyanobacterium Planktothrix rubescens isolated from Lake Zürich, using cultures grown on a 12:12-h light-dark cycle. From the photosynthesis-irradiance (P/I) curves, values of the light-affinity coefficient, f, and the maximum rate of photosynthesis, P_m, were determined: in different cultures f ranged from 0.02-0.08 7mol mg^-1 h^-1 (7mol m^-2 s^-1)^-1 and P_m from 0.6-2.6 7mol mg^-1 h^-1 (rates of O₂ production related to dry biomass). Comparisons made at different phases of the light-dark cycle in individual cultures showed that in the first hour of the light phase f rose by an average of 21 % and P_m by 7 %. In cultures that had been grown in many light-dark cycles, however, the rise occurred only if light was given at the beginning of the notional light phase; cultures kept in darkness showed no rise. It is concluded that the observed increases in f and P_m after dawn were in response to the exogenous irradiance rather than due to endogenous rhythms. These findings are discussed in relation to diel variations in the photosynthetic coefficients of natural populations of P. rubescens in Lake Zürich.