Asymmetry of compton scattering by relativistic electrons with an isotropic velocity distribution: Astrophysical implications

The angular distribution of low-frequency radiation after a single scattering by relativistic electrons with an isotropic velocity distribution differs markedly from the Rayleigh angular function. In particular, the scattering by an ensemble of ultrarelativistic electrons is described by the law p=1?cosα, where α is the scattering angle. Thus, photons are mostly scattered backward. We discuss some consequences of this fact for astrophysical problems. We show that a hot atmosphere of scattering electrons is more reflective than a cold one: the fraction of incident photons reflected after a single scattering can be larger than that in the former case by up to 50%. This must affect the photon exchange between cold accretion disks and hot coronae (or advective flows) near relativistic compact objects, as well as the rate of cooling (through multiple inverse-Compton scattering of the photons supplied from outside) of optically thick clouds of relativistic electrons in compact radio sources. Scattering asymmetry also causes the spatial diffusion of photons to proceed more slowly in a hot plasma than in a cold one, which affects the shapes of Comptonization spectra and the time delay in the detection of soft and hard radiation from variable X-ray sources.     

Spectral variability of the X-ray pulsar Hercules X-1

We present the results of our comparative timing and spectral analysis of the high and low (off) states in the X-ray pulsar Her X-1 based on data from the ART-P telescope onboard the Granat observatory. A statistically significant (several mCrab) persistent flux with a simple power-law spectrum was detected during the low state. The spectral slope changed from observation to observation by almost a factor of 2. Pulsations were detected only during the high state of the source, when its flux was a factor of ～25 larger than the low-state flux. The spectral shape of Her X-1 in its high state was complex, with the parameters depending on pulse phase.     

Parametric determination of the inclination of Keplerian circumstellar discs from spectropolarimetric profiles of scattered lines

Polarimetric line profiles arising from the Doppler redistribution of monochromatic stellar line radiation, Thomson scattered in a Keplerian rotating circumstellar disc are presented. It is shown that analysis of the scattered line profiles at different wavelengths which, due to Doppler redistribution, sample different disc regions allows the disc inclination to be determined.     

Transport of ionizing radiation in terrestrial-like exoplanet atmospheres

The propagation of ionizing radiation through model atmospheres of terrestrial-like exoplanets is studied for a large range of column densities and incident photon energies using a Monte Carlo code we have developed to treat Compton scattering and photoabsorption. Incident spectra from parent star flares, supernovae, and gamma-ray bursts are modeled and compared to energetic particles in importance. Large irradiation events with fluences of 10⁶-10⁹ erg cm⁻² at the conventional habitable zone can occur at a rate from many per day (flares from young low-mass parent stars) to ∼100 per Gyr (supernovae and gamma-ray bursts). We find that terrestrial-like exoplanets with atmospheres thinner than about 100 g cm⁻² block nearly all X-rays, but transmit and reprocess a significant fraction of incident γ-rays, producing a characteristic, flat surficial spectrum. Thick atmospheres (?100 g cm⁻²) efficiently block even γ-rays, but nearly all the incident energy is redistributed into diffuse UV and visible aurora-like emission, increasing the effective atmospheric transmission by many orders of magnitude. Depending on the presence of molecular UV absorbers and atmospheric thickness, up to 10% of the incident energy can reach the surface as UV reemission. For the Earth, between 2×10⁻³ and 4×10⁻² of the incident flux reaches the ground in the biologically effective 200-320 nm range, depending on O₂/O₃ shielding. For atmospheres thicker than ∼50 g cm⁻² in the case of pure Rayleigh scattering and ∼100 g cm⁻² in the case of O₂/O₃ absorption, the UV reemission exceeds the surficial transmitted ionizing radiation. We also discuss the effects of angle of incidence and derive a modified two-stream approximation solution for the UV transfer. Finally, we suggest that transient atmospheric ionization layers can be frequently created at altitudes lower than the equilibrium layers that result from steady irradiation and winds from the parent star. We suggest that these events can produce frequent fluctuations in atmospheric ionization levels and surficial UV fluxes on terrestrial-like planets.     

The first World Atlas of the artificial night sky brightness

We present the first World Atlas of the zenith artificial night sky brightness at sea level. Based on radiance-calibrated high-resolution DMSP satellite data and on accurate modelling of light propagation in the atmosphere, it provides a nearly global picture of how mankind is proceeding to envelop itself in a luminous fog. Comparing the Atlas with the United States Department of Energy (DOE) population density data base, we determined the fraction of population who are living under a sky of given brightness. About two-thirds of the World population and 99 per cent of the population in the United States (excluding Alaska and Hawaii) and European Union live in areas where the night sky is above the threshold set for polluted status. Assuming average eye functionality, about one-fifth of the World population, more than two-thirds of the United States population and more than one half of the European Union population have already lost naked eye visibility of the Milky Way. Finally, about one-tenth of the World population, more than 40 per cent of the United States population and one sixth of the European Union population no longer view the heavens with the eye adapted to night vision, because of the sky brightness.     

Rayleigh Scattering and Laser Spot Elongation Problems at ALFA

This article describes the qualitative effects of LGS spot elongation and Rayleigh scattering on ALFA wavefront sensor images.An analytical model of Rayleigh scattering and a numerical model of laser plume generation at the altitude of the Na-layer were developed. These models, integrated into ageneral AO simulation, provide the sensor sub-apertureimages. It is shown that the centroid measurement accuracyis affected by these phenomena. The simulation was made both for the ALFA system and for the VLT Nasmyth Adaptive Optics System (NAOS).     

A reanalysis of the Apollo light scattering observations, and implications for lunar exospheric dust

Conspicuous excess brightness, exceeding that expected from coronal and zodiacal light (CZL), was observed above the lunar horizon in the Apollo 15 coronal photographic sequence acquired immediately after orbital sunset (surface sunrise). This excess brightness systematically faded as the Command Module moved farther into shadow, eventually becoming indistinguishable from the CZL background. These observations have previously been attributed to scattering by ultrafine dust grains (radius ∼0.1 microns) in the lunar exosphere, and used to obtain coarse estimates of dust concentration at several altitudes and an order-of-magnitude estimate of ∼10⁻⁹ g cm⁻² for the column mass of dust near the terminator, collectively referred to as model “0”.We have reanalyzed the Apollo 15 orbital sunset sequence by incorporating the known sightline geometries in a Mie-scattering simulation code, and then inverting the measured intensities to retrieve exospheric dust concentration as a function of altitude and distance from the terminator. Results are presented in terms of monodisperse (single grain size) dust distributions. For a grain radius of 0.10 microns, our retrieved dust concentration near the terminator (∼0.010 cm⁻³) is in agreement with model “0” at z=10 km, as is the dust column mass (∼3–6×10⁻¹⁰ g cm⁻²), but the present results indicate generally larger dust scale heights, and much lower concentrations near 1 km (<0.08 cm⁻³ vs. a few times 0.1 cm⁻³ for model “0"). The concentration of dust at high altitudes (z>50 km) is virtually unconstrained by the measurements. The dust exosphere extends into shadow a distance somewhere between 100 and 200 km from the terminator, depending on the uncertain contribution of CZL to the total brightness. These refined estimates of the distribution and concentration of exospheric dust above the lunar sunrise terminator should place new and more rigorous constraints on exospheric dust transport models, as well as provide valuable support for upcoming missions such as the Lunar Atmosphere and Dust Environment Explorer (LADEE).