Exact optics: a unification of optical telescope design

A strong emission line at 2.8935 μm discovered by Rubin et al. in an ISO SWS02 spectrum of the Orion Nebula is identified as the     multiplet of O  i . Line formation is due to de-excitation cascades following UV-pumping of high ³S^o and ³D^o terms and occurs in the O  i zone immediately behind the hydrogen ionization front. This cascade mechanism also accounts for permitted O  i triplet lines in the optical spectrum of the Nebula, as shown by Grandi. An escape probability treatment of the O  i cascades accounts for the strength of the λ 2.89-μm line and suggests interesting diagnostic possibilities for the optical lines.     

Toy Stars in two dimensions

Toy Stars are gas masses where the compressibility is treated without approximations but gravity is replaced by a force which, for any pair of masses, is along their line of centres and proportional to their separation. They provide an invaluable resource for testing the suitability of numerical codes for astrophysical gas dynamics. In this paper, we derive the equations for both small-amplitude oscillations and non-linear solutions for rotating and pulsating Toy Stars in two dimensions, and show that the solutions can be reduced to a small number of ordinary differential equations. We compare the accurate solutions of these equations with Smoothed Particle Hydrodynamics (SPH) simulations. The two-dimensional Toy Star solutions are found to provide an excellent benchmark for SPH algorithms, highlighting many of the strengths and also some weaknesses of the method.     

The infrared emission lines of H2

The photodissociation regions located between ionized regions and molecular clouds are studied by using a one-dimensional model where molecular H₂ are formed on the dust grains, and destructed by photodissociation. The escape probability method is used for the line transfer. The excitation of infrared emission lines of H₂ by UV fluorescence in M17, by shock heating in Orion KL and mainly by UV fluorescence in NGC 2023 are discussed.     

Emission Line Profiles of Warped Disks in the Kerr Spacetime

The calculations of emission line pro?les of a warped disk in the Kerr spacetime have been discussed in this paper. The main content consists of two parts. In the ?rst part, the geodesic motion in the Kerr spacetime and the equations of motion in integral form, as well as the method to obtain the solution with the Weierstrass’ elliptic functions are presented. Using the Weierstrass’ elliptic functions, the Boyer-Lindquist (B-L) coordinates and the affine parameter o are semi-analytically expressed as the functions of a parameter p. Then a code named ynogk (Yunnan Observatory Geodesic Kerr) is introduced based on the above discussions to calculate very fast the null geodesic in the Kerr spacetime. As an application of ynogk, the emission line pro?les of warped disks are investigated in detail in the second part of this paper. Here the structure model of warped disks is based on the results of Bardeen and Petterson in 1975, and the line pro?les are computed with the ray tracing method. Finally, the discussions and conclusions of the calculated results are presented. It is indicated that the line pro?les are dependent mainly on the inclination and azimuthal angles of the observer, as well as the index of emissivity, and that they may have a triple-peaked even multi-peaked structure, different from the double-peaked structure in the line pro?les of a standard thin accretion disk.     

A Monte Carlo simulation of the intergalactic absorption and the detectability of the Lyman continuum from distant galaxies

We have made a Monte Carlo simulation of the intergalactic absorption in order to model the Lyman continuum absorption, which is required to estimate the escape fraction of the Lyman continuum from distant galaxies. To input into the simulation, we derive an empirical distribution function of the intergalactic absorbers which reproduces recent observational statistics of the Lyman α forest, Lyman limit systems (LLSs) and damped Lyman α systems (DLAs) simultaneously. In particular, we assume a common functional form of the number evolution along the redshift for all types of absorbers. The Lyman series transmissions in our simulation reproduce the observed redshift evolution of the transmissions excellently, and the Lyman continuum transmission also agrees with an observed estimation which is still quite rare in the literature. The probability distribution of the Lyman α opacity in our simulation is lognormal with a tail towards a large opacity. This tail is produced by DLAs. The probability distribution of the Lyman continuum opacity in our simulation also shows a broad tail towards a large opacity. This tail is produced by LLSs. Because of the rarity of LLSs, we have a chance to have a clean line of sight in the Lyman continuum even for   z ∼ 4  with a probability of about 20 per cent. Our simulation expects a good correlation between the Lyman continuum opacity and the Lyman α opacity, which may be useful to estimate the former from the latter for an individual line of sight.     

The difference PDF of 21-cm fluctuations: a powerful statistical tool for probing cosmic reionization

A new generation of radio telescopes are currently being built with the goal of tracing the cosmic distribution of atomic hydrogen at redshifts 6–15 through its 21-cm line. The observations will probe the large-scale brightness fluctuations sourced by ionization fluctuations during cosmic reionization. Since detailed maps will be difficult to extract due to noise and foreground emission, efforts have focused on a statistical detection of the 21-cm fluctuations. During cosmic reionization, these fluctuations are highly non-Gaussian and thus more information can be extracted than just the one-dimensional function that is usually considered, i.e. the correlation function. We calculate a two-dimensional function that if measured observationally would allow a more thorough investigation of the properties of the underlying ionizing sources. This function is the probability distribution function (PDF) of the difference in the 21-cm brightness temperature between two points, as a function of the separation between the points. While the standard correlation function is determined by a complicated mixture of contributions from density and ionization fluctuations, we show that the difference PDF holds the key to separately measuring the statistical properties of the ionized regions.     

CORA — emission line fitting with Maximum Likelihood

The advent of pipeline‐processed data both from space‐ and ground‐based observatories often disposes of the need of full‐fledged data reduction software with its associated steep learning curve. In many cases, a simple tool doing just one task, and doing it right, is all one wishes. In this spirit we introduce CORA, a line fitting tool based on the maximum likelihood technique, which has been developed for the analysis of emission line spectra with low count numbers and has successfully been used in several publications. CORA uses a rigorous application of Poisson statistics. From the assumption of Poissonian noise we derive the probability for a model of the emission line spectrum to represent the measured spectrum. The likelihood function is used as a criterion for optimizing the parameters of the theoretical spectrum and a fixed point equation is derived allowing an efficient way to obtain line fluxes. As an example we demonstrate the functionality of the program with an X‐ray spectrum of Capella obtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra observatory and choose the analysis of the Ne IX triplet around 13.5 Å.     

Failed disc winds: a physical origin for the soft X-ray excess?

The origin of the soft X-ray excess emission observed in many type-1 active galactic nuclei (AGN) has been an unresolved problem in X-ray astronomy for over two decades. We develop the model proposed by Gierliński & Done, which models the soft excess with heavily smeared, ionized, absorption, by including the emission that must be associated with this absorption. We show that, rather than hindering the ionized absorption model, the addition of the emission actually helps this model reproduce the soft excess. The emission fills in some of the absorption trough, while preserving the sharp rise at ∼1 keV, allowing the total model to reproduce the soft excess curvature from a considerably wider range of model parameters. We demonstrate that this model is capable of reproducing even the strongest soft X-ray excesses by fitting it to the XMM–Newton EPIC PN spectrum of PG1211+143, with good results. The addition of the emission reduces the column density required to fit these data by a factor of ∼2 and reduces the smearing velocity from ∼0.28c to ∼0.2c. Gierliński & Done suggested a tentative origin for the absorption in the innermost, accelerating, region of an accretion disc wind, and we highlight the advantages of this interpretation in comparison to accretion disc reflection models of the soft excess. Associating this material with a wind off the accretion disc results in several separate problems however, namely, the radial nature, and the massive implied mass-loss rate, of the wind. We propose an origin in a 'failed wind', where the central X-ray source is strong enough to overionize the wind, removing the acceleration through line absorption before the material reaches escape velocity, allowing the material to fall back to the disc at larger radii.     

恒星线性非绝热非径向脉动问题的退耦化处理方法

本文发展了一种解恒星线性非绝热非径向脉动问题的退耦化方法。这个方法把非绝热非径向脉动问题的六阶线性微分方程,分解为由一个代数方程联系起来的一个四阶线性微分方程和另一个二阶线性微分方程进行数值求解。这样的一个退耦处理,有利于克服以前在数值解这类问题时常常遇到的收敛域小和收敛速度慢等困难,并且为数值解方程时所采用的Henyey方法提供了一个自然和方便的初始猜测解。     

Probing the wind–wind collision in γ2 Velorum with high-resolution Chandra X-ray spectroscopy: evidence for sudden radiative braking and non-equilibrium ionization

We present a new analysis of an archived Chandra HETGS X-ray spectrum of the WR+O colliding wind binary γ² Velorum. The spectrum is dominated by emission lines from astrophysically abundant elements: Ne, Mg, Si, S and Fe. From a combination of broad-band spectral analysis and an analysis of line flux ratios we infer a wide range of temperatures in the X-ray-emitting plasma (∼4–40 MK). As in the previously published analysis, we find the X-ray emission lines are essentially unshifted, with a mean FWHM of  1240 ± 30 km s⁻¹  . Calculations of line profiles based on hydrodynamical simulations of the wind–wind collision predict lines that are blueshifted by a few hundred  km s⁻¹  . The lack of any observed shift in the lines may be evidence of a large shock-cone opening half-angle (>85°), and we suggest this may be evidence of sudden radiative braking. From the R and G ratios measured from He-like forbidden-intercombination-resonance triplets we find evidence that the Mg  xi emission originates from hotter gas closer to the O star than the Si  xiii emission, which suggests that non-equilibrium ionization may be present.     

3D simulations of rotationally-induced line variability from a classical T Tauri star with a misaligned magnetic dipole

We present 3D simulations of rotationally induced line variability arising from complex circumstellar environment of classical T Tauri stars (CTTS) using the results of the 3D magnetohydrodynamics (MHD) simulations of Romanova et al., who considered accretion on to a CTTS with a misaligned dipole magnetic axis with respect to the rotational axis. The density, velocity and temperature structures of the MHD simulations are mapped on to the radiative transfer grid, and corresponding line source function and the observed profiles of neutral hydrogen lines (Hβ, Paβ and Brγ) are computed using the Sobolev escape probability method. We study the dependency of line variability on inclination angles ( i ) and magnetic axis misalignment angles (Θ). We find the line profiles are relatively insensitive to the details of the temperature structure of accretion funnels, but are influenced more by the mean temperature of the flow and its geometry. By comparing our models with the Paβ profiles of 42 CTTS observed by Folha & Emerson, we find that models with a smaller misaligngment angle  (Θ < ∼15°)  are more consistent with the observations which show that majority of Paβ are rather symmetric around the line centre. For a high inclination system with a small dipole misalignment angle  (Θ≈ 15°)  , only one accretion funnel (on the upper hemisphere) is visible to an observer at any given rotational phase. This can cause an anticorrelation of the line equivalent to the width in the blue wing  ( v < 0)  and that in the red wing  ( v > 0)  over half of a rotational period, and a positive correlation over the other half. We find a good overall agreement of the line variability behaviour predicted by our model and those from observations.     

Explicit Symplectic Algorithms For Time‐Transformed Hamiltonians

By Hamiltonian manipulation we demonstrate the existence of separable time‐transformed Hamiltonians in the extended phase‐space. Due to separability explicit symplectic methods are available for the solution of the equations of motion. If the simple leapfrog integrator is used, in case of two‐body motion, the method produces an exact Keplerian ellipse in which only the time‐coordinate has an error. Numerical tests show that even the rectilinear N‐body problem is feasible using only the leapfrog integrator. In practical terms the method cannot compete with regularized codes, but may provide new directions for studies of symplectic N‐body integration. This revised version was published online in July 2006 with corrections to the Cover Date.     

Wind signatures in the X-ray emission-line profiles of the late-O supergiant ζ Orionis

X-ray line-profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line profiles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line profile provides information about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper, we apply several analysis techniques to the emission lines in the Chandra High Energy Transmission Grating Spectrometer (HETGS) spectrum of the late-O supergiant ζ Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blueshifts and profile asymmetry, as well as broadening in the X-ray emission lines of ζ Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for ζ Ori are very similar to those for the earlier O star, ζ Pup, which we have previously shown to be well fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in ζ Ori, as compared to ζ Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both ζ Ori and ζ Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from ζ Ori can be understood within the framework of the standard wind-shock scenario for hot stars.     

Multi-density models of the Broad Line Region in Active Galactic Nuclei: a comparative study with single-cloud models

Nearly ten years ago Kwan and Krolik (1979, 1981) published the firstsuccessful photoionization model of the Broad Line Region of Active Galactic Nuclei, the so-called ‘Standard Model’. Since then several efforts have been made to obtain better results using more sophisticated models. Anopen issue is that photoionization models are generally computed startingfrom the assumption that the emission line spectrum is produced by a single-slab cloud with a ‘standard’ hydrogen density n_H = 10^9.5 cm^-3, but it seems more likely that a range of densities is present in the Broad Line Region. Purpose of this paper is to review the results given by single-cloud models using the most recent photoionization code, Cloudy 84, and to investigate if the addition of one or more components with different densities does affect the line ratios. To this aim we compute the emission line ratios produced by photoionized single-slab clouds for a wide range of hydrogen densities (n_H = 10^9.5−10¹³ cm⁻³), ionization parameters (U = 10⁻⁴− 10^−0.5) and column densities (N_H = 10²³−10²⁵ cm⁻²). Two or more populations of clouds are then combined and the resulting emission line spectrum is compared with a sample of mean observed line ratios. We find that the addition to the standard component of clouds having different densities and located at different positions from the central source introduces many changes in the line ratios, and that these changes are in the direction of a better agreement with the observed emission line ratios.     

Inclination of Broad Line Region in Narrow Line and Broad Line Seyfert 1 Galaxies

The sizes of the Broad Line Region (BLR) of some Seyfert 1 galax-ies and nearby quasars can be determined by the reverberation mapping method.Combining with the observed FWHM of Hβ emission line and assuming that themotion of BLR clouds is virialized, the black hole masses of these objects have beenestimated. However, this method strongly depends on the poorly-understood geom-etry and inclination of the BLR. On the other hand, a tight correlation between theblack hole mass and the bulge velocity dispersion was recently found for both activeand nearby inactive galaxies. This may provide another method, independent of theBLR geometry, for estimating the black hole mass. Using this method for estimatingthe black hole mass and combining with the measured BLR size and FWHM of Hβemission line, we derived the BLR inclination angles for 20 Seyfert I galaxies underthe assumption that the BLR is disk-like. The derived inclination angles agree wellwith those derived previously by fitting the UV continuum and Hβ emission lineprofiles. Adopting a relation between the FWHMs of [OⅢ]λ5007 forbidden line andthe stellar velocity dispersion, we also estimated the BLR inclinations for 50 nar-row line Seyfert 1 galaxies (NLSls). We found that the inclinations of broad LineSeyfert 1 galaxies (BLS1s) are systematically greater than those of NLS1s, whichseldom exceed 30. This may be an important factor that leads to the differencesbetween NLS1s and BLS1s if the BLR of NLS1s is really disk-like.     

Multiband photometric detection of a huge flare on the M9 dwarf 2MASSW J1707183+643933

We present simultaneous UV , G , R and I monitoring of 19 M dwarfs that reveal a huge flare on the M9 dwarf with an amplitude in the UV of at least 6 mag. This is one of the strongest detections ever of an optical flare in an M star and one of the first in an ultracool dwarf (spectral types later than about M7). Four intermediate-strength flares  (Δ m _UV < 4 mag)  were found in this and three other targets. For the whole sample we deduce a flare probability of 0.013 (rate of  0.022 h⁻¹  ), and  0.049 (0.090 h⁻¹)  for 2M1707+64 alone. Deviations of the flare emission from a blackbody is consistent with strong  Hα  line emission. We also confirm our previously found rotation period for 2M1707+64 and determine it more precisely to be  3.619 ± 0.015 h  .     

Neutron-loaded outflows in gamma-ray bursts

The force-free limit of magnetohydrodynamics (MHD) is often a reasonable approximation to model black hole and neutron star magnetospheres. We describe a general relativistic force-free (GRFFE) formulation that allows general relativistic magnetohydrodynamic (GRMHD) codes to directly evolve the GRFFE equations of motion. Established, accurate and well-tested conservative GRMHD codes can simply add a new inversion piece of code to their existing code, while continuing to use all the already-developed facilities present in their GRMHD code. We show how to enforce the   E · B = 0  constraint and energy conservation, and we introduce a simplified general model of the dissipation of the electric field to enforce the   B ²− E ² > 0  constraint. We also introduce a simplified yet general method to resolve current sheets, without much reconnection, over many dynamical times. This formulation is incorporated into an existing GRMHD code ( harm ), which is demonstrated to give accurate and robust GRFFE results for Minkowski and black hole space–times.