The influence of coronal EUV irradiance on the emission in the He I 10830 A and D3 multiplets

Two of the most attractive spectral windows for spectropolarimetric investigations of the physical properties of the plasma structures in the solar chromosphere and corona are the ones provided by the spectral lines of the He I 10830 A and 5876 A (or D3) multiplets, whose polarization signals are sensitive to the Hanle and Zeeman effects. However, in order to be able to carry out reliable diagnostics, it is crucial to have a good physical understanding of the sensitivity of the observed spectral line radiation to the various competing driving mechanisms. Here we report a series of off-the-limb non-LTE calculations of the He I D3 and 10830 A emission profiles, focusing our investigation on their sensitivity to the EUV coronal irradiation and the model atmosphere used in the calculations. We show in particular that the intensity ratio of the blue to the red components in the emission profiles of the He I 10830 A multiplet turns out to be a good candidate as a diagnostic tool for the coronal irradiance. Measurements of this observable as a function of the distance to the limb and its confrontation with radiative transfer modeling might give us valuable information on the physical properties of the solar atmosphere and on the amount of EUV radiation at relevant wavelengths penetrating the chromosphere from above.Comment: 19 pages, 11 figures (pre-print format). Accepted for publication in Ap

Polarization of the \lya Halos Around Sources Before Cosmological Reionization

In Loeb & Rybicki (1999; paper I) it was shown that before reionization, the scattering of \lya photons from a cosmological source might lead to a fairly compact ($\sim 15''$) \lya halo around the source. Observations of such halos could constrain the properties of the neutral intergalactic medium (IGM), and in particular yield the cosmological density parameters of baryons and matter on scales where the Hubble flow is unperturbed. Paper I did not treat the polarization of this scattered radiation, but did suggest that the degree of such polarization might be large. In this Letter we report on improved calculations for these \lya halos, now accounting for the polarization of the radiation field. The polarization is linear and is oriented tangentially to the projected displacement from the center of the source. The degree of polarization is found to be 14% at the core radius, where the intensity has fallen to half of the central value. It rises to 32% and 45% at the radii where the intensity has fallen to one-tenth and one-hundreth of the central intensity, respectively. At larger radii the degree of polarization rises further, asymptotically to 60%. Such high values of polarization should be easily observable and provide a clear signature of the phenomenon of \lya halos surrounding sources prior to reionization.Comment: 8 pages, 2 Postscript figures, accepted by Astrophysical Journal Letters; some typos corrected; added two paragraphs at the end of section 3 concerning detectability of Lyman alpha halo

The spectrum of the Broad Line Region and the high-energy emission of powerful blazars

High-energy emission (from the X-ray through the gamma-ray band) of Flat Spectrum Radio Quasars is widely associated with the inverse Compton (IC) scattering of ambient photons, produced either by the accretion disk or by the Broad Line Region, by high-energy electrons in a relativistic jet. In the modelling of the IC spectrum one usually adopts a simple black-body approximation for the external radiation field, though the real shape is probably more complex. The knowledge of the detailed spectrum of the external radiation field would allow to better characterize the soft-medium X-ray IC spectrum, which is crucial to address several issues related to the study of these sources. Here we present a first step in this direction, calculating the IC spectra expected by considering a realistic spectrum for the external radiation energy density produced by the BLR, as calculated with the photoionization code CLOUDY. We find that, under a wide range of the physical parameters characterizing the BLR clouds, the IC spectrum calculated with the black-body approximation reproduces quite well the exact spectrum for energies above few keV. In the soft energy band, instead, the IC emission calculated using the BLR emission shows a complex shape, with a moderate excess with respect to the approximate spectrum, which becomes more important for decreasing values of the peak frequency of the photoionizing continuum. We also show that the high-energy spectrum shows a marked steepening, due to the energy dependence of the scattering cross section, above a characteristic energy of 10-20 GeV, quasi independent on the Lorentz factor of the jet.Comment: 10 pages, 9 figures, accepted for publication in MNRA

Magnification Ratio of the Fluctuating Light in Gravitational Lens 0957+561

Radio observations establish the B/A magnification ratio of gravitational lens 0957+561 at about 0.75. Yet, for more than 15 years, the optical magnfication ratio has been between 0.9 and 1.12. The accepted explanation is microlensing of the optical source. However, this explanation is mildly discordant with (i) the relative constancy of the optical ratio, and (ii) recent data indicating possible non-achromaticity in the ratio. To study these issues, we develop a statistical formalism for separately measuring, in a unified manner, the magnification ratio of the fluctuating and constant parts of the light curve. Applying the formalism to the published data of Kundi\'c et al. (1997), we find that the magnification ratios of fluctuating parts in both the g and r colors agrees with the magnification ratio of the constant part in g-band, and tends to disagree with the r-band value. One explanation could be about 0.1 mag of consistently unsubtracted r light from the lensing galaxy G1, which seems unlikely. Another could be that 0957+561 is approaching a caustic in the microlensing pattern.Comment: 12 pages including 1 PostScript figur

A New Kinetic Equation for Compton Scattering

A kinetic equation for Compton scattering is given that differs from the Kompaneets equation in several significant ways. By using an inverse differential operator this equation allows treatment of problems for which the radiation field varies rapidly on the scale of the width of the Compton kernel. This inverse operator method describes, among other effects, the thermal Doppler broadening of spectral lines and continuum edges, and automatically incorporates the process of Compton heating/cooling. It is well adapted for inclusion into a numerical iterative solution of radiative transfer problems. The equivalent kernel of the new method is shown to be a positive function and with reasonable accuracy near the intitial frequency, unlike the Kompaneets kernel, which is singular and not wholly positive. It is shown that iterates of the inverse operator kernel can be easily calculated numerically, and a simple summation formula over these iterates is derived that can be efficiently used to compute Comptonized spectra. It is shown that the new method can be used for initial value and other problems with no more numerical effort than the Kompaneets equation, and that it more correctly describes the solution over times comparable to the mean scattering time.Comment: 27 pages, 5 figures, to be published in ApJ. Minor changes, including one reference correcte

Parallel Implementation of the PHOENIX Generalized Stellar Atmosphere Program

We describe the parallel implementation of our generalized stellar atmosphere and NLTE radiative transfer computer program PHOENIX. We discuss the parallel algorithms we have developed for radiative transfer, spectral line opacity, and NLTE opacity and rate calculations. Our implementation uses a MIMD design based on a relatively small number of MPI library calls. We report the results of test calculations on a number of different parallel computers and discuss the results of scalability tests.Comment: To appear in ApJ, 1997, vol 483. LaTeX, 34 pages, 3 Figures, uses AASTeX macros and styles natbib.sty, and psfig.st

Scattered Lyman-alpha Radiation Around Sources Before Cosmological Reionization

The spectra of the first galaxies and quasars in the Universe should be strongly absorbed shortward of their rest-frame Lyman-alpha wavelength by neutral hydrogen (HI) in the intervening intergalactic medium. However, the Lyman-alpha line photons emitted by these sources are not eliminated but rather scatter until they redshift out of resonance and escape due to the Hubble expansion of the surrounding intergalactic HI. We calculate the resulting brightness distribution and the spectral shape of the diffuse Lyman-alpha line emission around high redshift sources, before the intergalactic medium was reionized. Typically, the Lyman-alpha photons emitted by a source at z=10 scatter over a characteristic angular radius of order 15 arcseconds around the source and compose a line which is broadened and redshifted by about a thousand km/s relative to the source. The scattered photons are highly polarized. Detection of the diffuse Lyman-alpha halos around high redshift sources would provide a unique tool for probing the neutral intergalactic medium before the epoch of reionization. On sufficiently large scales where the Hubble flow is smooth and the gas is neutral, the Lyman-alpha brightness distribution can be used to determine the cosmological mass densities of baryons and matter.Comment: 21 pages, 5 Postscript figures, accepted by ApJ; figures 1--3 corrected; new section added on the detectability of Lyman alpha halos; conclusions update

Polarization in the prompt emission of gamma-ray bursts and their afterglows

Synchrotron is considered the dominant emission mechanism in the production of gamma-ray burst photons in the prompt as well as in the afterglow phase. Polarization is a characteristic feature of synchrotron and its study can reveal a wealth of information on the properties of the magnetic field and of the energy distribution in gamma-ray burst jets. In this paper I will review the theory and observations of gamma-ray bursts polarization. While the theory is well established, observations have prove difficult to perform, due to the weakness of the signal. The discriminating power of polarization observations, however, cannot be overestimated.Comment: 16 pages, 9 figures, accepted for publication in the New Journal of Physics focus issue on Gamma Ray Burst

Magnetic Reconnection with Radiative Cooling. I. Optically-Thin Regime

Magnetic reconnection, a fundamental plasma process associated with a rapid dissipation of magnetic energy, is believed to power many disruptive phenomena in laboratory plasma devices, the Earth magnetosphere, and the solar corona. Traditional reconnection research, geared towards these rather tenuous environments, has justifiably ignored the effects of radiation on the reconnection process. However, in many reconnecting systems in high-energy astrophysics (e.g., accretion-disk coronae, relativistic jets, magnetar flares) and, potentially, in powerful laser plasma and z-pinch experiments, the energy density is so high that radiation, in particular radiative cooling, may start to play an important role. This observation motivates the development of a theory of high-energy-density radiative magnetic reconnection. As a first step towards this goal, we present in this paper a simple Sweet--Parker-like theory of non-relativistic resistive-MHD reconnection with strong radiative cooling. First, we show how, in the absence of a guide magnetic field, intense cooling leads to a strong compression of the plasma in the reconnection layer, resulting in a higher reconnection rate. The compression ratio and the layer temperature are determined by the balance between ohmic heating and radiative cooling. The lower temperature in the radiatively-cooled layer leads to a higher Spitzer resistivity and hence to an extra enhancement of the reconnection rate. We then apply our general theory to several specific astrophysically important radiative processes (bremsstrahlung, cyclotron, and inverse-Compton) in the optically thin regime, for both the zero- and strong-guide-field cases. We derive specific expressions for key reconnection parameters, including the reconnection rate. We also discuss the limitations and conditions for applicability of our theory.Comment: 31 pages, 1 figur

DOPING: a New Non-parametric Deprojection Scheme

We present a new non-parametric deprojection algorithm DOPING (Deprojection of Observed Photometry using and INverse Gambit), that is designed to extract the three dimensional luminosity density distribution $\rho$, from the observed surface brightness profile of an astrophysical system such as a galaxy or a galaxy cluster, in a generalised geometry, while taking into account changes in the intrinsic shape of the system. The observable is the 2-D surface brightness distribution of the system. While the deprojection schemes presented hitherto have always worked within the limits of an assumed intrinsic geometry, in DOPING, geometry and inclination can be provided as inputs. The $\rho$ that is most likely to project to the observed brightness data is sought; the maximisation of the likelihood is performed with the Metropolis algorithm. Unless the likelihood function is maximised, $\rho$ is tweaked in shape and amplitude, while maintaining positivity, but otherwise the luminosity distribution is allowed to be completely free-form. Tests and applications of the algorithm are discussed.Comment: 8 pages; to be published in IJMP(D) (Feb, 2008 issue), Vol 17, No. 2, as part of proceedings for the 6th International Workshop on Data Analysis in Astronomy, ``Modelling and Simulations in Science'