The flat synchrotron spectra of partially self-absorbed jets revisited

Flat radio spectra with large brightness temperatures at the core of active galactic nuclei and X-ray binaries are usually interpreted as the partially self-absorbed bases of jet flows emitting synchrotron radiation. Here we extend previous models of jets propagating at large angles to our line of sight to self-consistently include the effects of energy losses of the relativistic electrons due to the synchrotron process itself and the adiabatic expansion of the jet flow. We also take into account energy gains through self-absorption. Two model classes are presented. The ballistic jet flows, with the jet material travelling along straight trajectories, and adiabatic jets. Despite the energy losses, both scenarios can result in flat emission spectra; however, the adiabatic jets require a specific geometry. No re-acceleration process along the jet is needed for the electrons. We apply the models to observational data of the X-ray binary Cygnus X-1. Both models can be made consistent with the observations. The resulting ballistic jet is extremely narrow with a jet opening angle of only 5 arcsec. Its energy transport rate is small compared to the time-averaged jet power and therefore suggests the presence of non-radiating protons in the jet flow. The adiabatic jets require a strong departure from energy equipartition between the magnetic field and the relativistic electrons. These models also imply a jet power of two orders of magnitude higher than the Eddington limiting luminosity of a  10-M_⊙  black hole. The models put strong constraints on the physical conditions in the jet flows on scales well below achievable resolution limits.     

On the Multiplicity of the O-Star Cyg OB2 #8a and its Contribution to the γ-ray Source 3EG J2033+4118

We present the results of an intensive spectroscopic campaign in the optical waveband revealing that Cyg OB2 #8A is an O6+O5.5 binary system with a period of about 21.9 days. Cyg OB2 #8A is a bright X-ray source, as well as a non-thermal radio emitter. We discuss the binarity of this star in the framework of a campaign devoted to the study of non-thermal emitters, from the radio waveband to γ-rays. In this context, we attribute the non-thermal radio emission from this star to a population of relativistic electrons, accelerated by the shock of the wind-wind collision. These relativistic electrons could also be responsible for a putative γ-ray emission through inverse Compton scattering of photospheric UV photons, thus contributing to the yet unidentified EGRET source 3EG J2033+4118. Based partly on data Obtained at the Observatoire de Haute-Provence, France.     

X-Ray Line Transfer in Plasmas with Large Velocity Gradients

An astrophysically relevant experiment is compared to the output of a multidimensional radiation transfer code in which populations and radiation are self-consistently treated. Experimental Al Ly α spectra obtained with a very high-resolution spectrometer are presented as quantitative evidence of dot plasma non-planar expansion. Analysis of these spectra using the code is performed, in particular examining the effects of velocity gradients in directions other than that of the primary expansion. These calculations are found to be in good agreement with the experimental results. Usage of the Ly α doublet as a planarity diagnostic is discussed.     

Energy-dependent variability from accretion flows

We develop a formalism to calculate energy-dependent fractional variability (rms) in accretion flows. We consider rms spectra resulting from radial dependences of the level of local variability (as expected from the propagation of disturbances in accretion flows) assuming the constant shape of the spectrum emitted at a given radius. We consider the cases when the variability of the flow is either coherent or incoherent between different radial zones. As an example of local emission, we consider blackbody, Wien and thermal Comptonization spectra. In addition to numerical results, we present a number of analytical formulae for the resulting rms. We also find an analytical formula for the disc Wien spectrum, which we find to be a very good approximation to the disc blackbody. We compare our results to the rms spectrum observed in an ultrasoft state of GRS 1915+105.     

The X-ray spectrum of Cyg X-2

The spectra of disc accreting neutron stars generally show complex curvature, and individual components from the disc, boundary layer and neutron star surface cannot be uniquely identified. Here we show that much of the confusion over the spectral form derives from inadequate approximations for Comptonization and for the iron line. There is an intrinsic low-energy cut-off in Comptonized spectra at the seed photon energy. It is very important to model this correctly in neutron star systems as these have expected seed photon temperatures (from either the neutron star surface, inner disc or self-absorbed cyclotron) of ≈1 keV, clearly within the observed X-ray energy band. There is also reflected continuum emission which must accompany the observed iron line, which distorts the higher energy spectrum. We illustrate these points by a reanalysis of the Ginga spectra of Cyg X-2 at all points along its Z track, and show that the spectrum can be well fitted by models in which the low-energy spectrum is dominated by the disc, while the higher energy spectrum is dominated by Comptonized emission from the boundary layer, together with its reflected spectrum from a relativistically smeared, ionized disc.