Polarization of resonance X-ray lines from clusters of galaxies

It is known that resonant scattering can distort the surface brightness profiles of clusters of galaxies in X-ray lines. We demonstrate that the scattered line emission should be polarized and possibly detectable with future X-ray polarimeters. Spectrally resolved mapping of a galaxy cluster in polarized X-rays could provide valuable independent information on the physical conditions, in particular element abundances and the characteristic velocity of small-scale turbulent motions, in the intracluster gas. The expected degree of polarization is of the order of 10 per cent for the richest regular clusters (e.g. Coma) and clusters whose X-ray emission is dominated by a central cooling flow (such as Perseus and M87/Virgo).     

Turbulence in clusters of galaxies and X-ray line profiles

Large-scale bulk motions and hydrodynamic turbulence in the intergalactic gas that fills clusters of galaxies significantly broaden X-ray emission lines. For lines of heavy ions (primarily helium-like and hydrogen-like iron ions), the hydrodynamic broadening is appreciably larger than the thermal broadening. Since clusters of galaxies have a negligible optical depth for resonant scattering in the forbidden and intercombination lines of these ions, these lines are not additionally broadened. At the same time, they are very intense, which allows deviations of the spectrum from the Gaussian spectrum in the line wings to be investigated. The line shape proves to be an important indicator of bulk hydrodynamic processes. Doppler probing of turbulence becomes possible, because the cryogenic detectors of the X-ray observatories now ready for launch and being planned will have a high energy resolution (from 5 eV for ASTRO-E2 to 1–2 eV for Constellation-X and XEUS). We use the spectral representation of a Kolmogorov cascade in the inertial range to calculate the characteristic shapes of radiation lines. Significant deviations in the line profiles from the Gaussian profile (shape asymmetry, additional peaks, sharp breaks in the exponential tails) are expected for large-scale turbulence. The kinematic SZ effect and the X-ray line profiles carry different information about the hydrodynamic velocity distribution in clusters of galaxies and complement each other, allowing the redshift, the peculiar velocity of the cluster, and the bulk velocity dispersion to be separated.     

Width of X-ray lines as a diagnostic of gas motions in cooling flows

The dissipation of turbulent gas motions is one of the likely mechanisms that has been proposed to heat the intracluster medium (ICM) in the cores of clusters and groups of galaxies. We consider the impact of gas motions on the width of the most prominent X-ray emission lines. For heavy elements (like iron), the expected linewidth is much larger than the width due to pure thermal broadening, and the contribution due to turbulent gas motions should be easily detected with the new generation of X-ray microcalorimeters, such as the Spektr-RG calorimeter (SXC). For instance, in the Perseus cluster the turbulent velocity required to balance radiative cooling (as derived by Rebusco et al.) would imply a width of the 6.7 keV Fe line of 10–20 eV, while the pure thermal broadening is ∼4 eV. The radial dependence of the linewidth is sensitive to (i) the radial dependence of the velocity amplitude and (ii) the 'directionality' of the stochastic motions (e.g. isotropic turbulence or predominantly radial gas motions). If the width of several lines, characteristic for different gas temperatures, can be measured, then it should be possible to probe both the 'directionality' and the amplitude of the gas motions. Moreover, a measurement of the width would put a lower limit on the amount of the kinetic energy available for dissipation, giving a constraint on the ICM models.     

The broad emission-line region in AGNs and quasars: The impact of variability studies

Coordinated observations of variability of the continuum and the emission-line luminosities (reverberation mapping) in AGNs and quasars have fundamentally altered our understanding of the broad-line regions in active galaxies. The constraints these observations impose on the models of the BLRs have been demonstrated here by an attempt to model the BLR of NGC5548, the most intensively monitored AGN. Two models of a BLR, with one having a power law radial distribution of density and the other a gaussian radial distribution, are described and the modelled line luminosities and centroid of the line response functions are presented. A self-consistent model is presented for the change in theCiv/Lyα ratio as the continuum luminosity changes. It is shown that BLR gas must be composed of a mixture of optically thin and optically thick gas and the proportion of thick and thin gas alters with the luminosity of the ionizing continuum. The observed centroid or the lag of a line, can be a function of the continuum luminosity. The variability of the profile of theCiv line in the spectrum of NGC5548 is investigated. This profile is extremely robust and is not significantly affected by changes in the ionizing continuum. Future models of the kinematics of the BLR clouds will have to be based on very stable cloud motions and include anisotropic line emission.     

Constraints on turbulent pressure in the X-ray haloes of giant elliptical galaxies from resonant scattering

The dense cores of X-ray emitting gaseous haloes of large elliptical galaxies with temperatures   kT ≲ 0.8  keV  show two prominent Fe  xvii emission features, which provide a sensitive diagnostic tool to measure the effects of resonant scattering. We present here high-resolution spectra of five bright nearby elliptical galaxies, obtained with the reflection grating spectrometers (RGS) on the XMM-Newton satellite. The spectra for the cores of four of the galaxies show the Fe  xvii line at 15.01 Å being suppressed by resonant scattering. The data for NGC 4636 in particular allow the effects of resonant scattering to be studied in detail and to prove that the 15.01 Å line is suppressed only in the dense core and not in the surrounding regions. Using deprojected density and temperature profiles for this galaxy obtained with the Chandra satellite, we model the radial intensity profiles of the strongest resonance lines, accounting for the effects of resonant scattering, for different values of the characteristic turbulent velocity. Comparing the model to the data, we find that the isotropic turbulent velocities on spatial scales smaller than ≈1 kpc are less than 100 km s⁻¹ and the turbulent pressure support in the galaxy core is smaller than 5 per cent of the thermal pressure at the 90 per cent confidence level, and less than 20 per cent at 95 per cent confidence. Neglecting the effects of resonant scattering in spectral fitting of the inner 2 kpc core of NGC 4636 will lead to underestimates of the chemical abundances of Fe and O by ∼10–20 per cent.     

X-ray beaming caused by resonance scattering in the accretion column of magnetic cataclysmic variables

Extremely strong ionized Fe emission lines, with equivalent widths reaching ∼4000 eV, were discovered by ASCA from a few Galactic compact objects, including AX J2315−0592, RX J1802.1+1804 and AX J1842.8−0423. These objects are thought to be binary systems containing magnetized white dwarfs (WDs). A possible interpretation of the strong Fe K line is the line-photon collimation in the WD accretion column, as a result of resonance scattering of line photons. The collimation occurs when the accretion column has a flat shape, and the effect is augmented by the vertical velocity gradient, which reduces the resonant trapping of resonant photons along the magnetic field lines. This effect was quantitatively confirmed with Monte Carlo simulations. Furthermore, with ASCA observations of the polar V834 Centauri, this collimation effect was clearly detected as a rotational modulation of the equivalent width of the Fe K emission line. The extremely strong emission lines mentioned above can be explained consistently by our interpretation. Combining this effect with other X-ray information, the geometry and plasma parameters in the accretion column were determined.     

Fe xxv and Fe xxvi lines from low-velocity, photoionized gas in the X-ray spectra of active galactic nuclei

We have calculated the equivalent widths of the absorption lines produced by Fe  xxv and Fe  xxvi in a Compton-thin, low-velocity photoionized material illuminated by the nuclear continuum in active galactic nuclei. The results, plotted against the ionization parameter and the column density of the gas, are a complement to those presented by Bianchi & Matt for the emission lines from the same ionic species. As an extension to the work by Bianchi & Matt, we also present a qualitative discussion on the different contributions to the He-like iron emission line complex in the regimes where recombination or resonant scattering dominates, providing a useful diagnostic tool to measure the column density of the gas. Future high-resolution missions (e.g. Astro-E2 ) will allow us to fully take advantage of these plasma diagnostics. In the meantime, we compare our results with an up-to-date list of Compton-thick and unobscured (at least at the iron line energy) Seyfert galaxies with emission and/or absorption lines from H- and He-like iron observed with Chandra and XMM–Newton .     

Supersonic motions of galaxies in clusters

We study motions of galaxies in galaxy clusters formed in the concordance Λ cold dark matter cosmology. We use high-resolution cosmological simulations that follow the dynamics of dark matter and gas and include various physical processes critical for galaxy formation: gas cooling, heating and star formation. Analysing the motions of galaxies and the properties of intracluster gas in a sample of eight simulated clusters at z = 0, we study the velocity dispersion profiles of the dark matter, gas and galaxies. We measure the mean velocity of galaxy motions and gas sound speed as a function of radius and calculate the average Mach number of galaxy motions. The simulations show that galaxies, on average, move supersonically with the average Mach number of ≈1.4, approximately independent of the cluster-centric radius. The supersonic motions of galaxies may potentially provide an important source of heating for the intracluster gas by driving weak shocks and via dynamical friction, although these heating processes appear to be inefficient in our simulations. We also find that galaxies move slightly faster than the dark matter particles. The magnitude of the velocity bias,   b _v ≈ 1.1  , is, however, smaller than the bias estimated for subhaloes in dissipationless simulations. Interestingly, we find velocity bias in the tangential component of the velocity dispersion, but not in the radial component. Finally, we find significant random bulk motions of gas. The typical gas velocities are of order ≈20–30 per cent of the gas sound speed. These random motions provide about 10 per cent of the total pressure support in our simulated clusters. The non-thermal pressure support, if neglected, will bias measurements of the total mass in the hydrostatic analyses of the X-ray cluster observations.     

Multisatellite observations of resonant hydromagnetic waves

Most measurements of long period ULF pulsations have come from ground based and single satellite observations. The observations have given strong support to the idea that these waves are resonant standing hydromagnetic waves on geomagnetic field lines. Simultaneous ground-satellite observations provide further details of the pulsation structure and are useful for examining the effect of the ionosphere on the transmission of the waves to the ground. Recently, multisatellite observations have been used to provide further insight into the nature of pulsations and we review the results obtained using this technique. Among the results presented are those from the ISEE 1 and 2 spacecraft which are closely spaced in identical orbits, making it possible to distinguish temporal from spatial structure in waves. The ISEE spacecraft have made measurements of resonant region widths and resonance harmonics. In addition, examples are shown of recent multisatellite observations of the global nature of some pulsations and the localization of Pi2 pulsations in space.     

High resolution radio recombination line observations

Summary Over the last decade sensitive observations of radio recombination line emission using high angular resolution synthesis telescopes have become available. As a result it has now become possible to image the physical parameters deduced from radio recombination lines across individual sources. In the case of HII regions this work has resulted in detailed images of radial velocities, electron temperatures and the abundance of singly ionized helium (Y⁺). Direct observational evidence has been found for pressure broadening and non-LTE effects. Dramatic variations have been found in the ratio of He⁺ to H⁺, from as low as a few percent (the galactic centre) to as high 34% in one region of W3. Detailed images have been obtained of the partially ionized medium (CII and H regions) close to HII regions. Observations of recombination lines at very low frequencies have revealed the existence of very low density ionized gas in all directions in our galaxy. Higher resolution observations have led to a partial understanding of this medium. The first complete velocity field of the ionized gas in the centre of our galaxy has been obtained. Very recently the first images were made of extragalactic radio recombination lines, offering the possibility to study the kinematics of the ionized gas in the central few hundred parsecs of external galaxies.     

MSX mid-infrared imaging of massive star birth environments – I. Ultracompact H ii regions

In solar extreme ultraviolet spectra the He  i and He  ii resonance lines show unusual behaviour and have anomalously high intensities compared with other transition region lines. The formation of the helium resonance lines is investigated through extensive non-local thermal equilibrium radiative transfer calculations. The model atmospheres of Vernazza, Avrett & Loeser are found to provide reasonable matches to the helium resonance line intensities but significantly overestimate the intensities of other transition region lines. New model atmospheres have been developed from emission measure distributions derived by Macpherson & Jordan, which are consistent with SOHO observations of transition region lines other than those of helium. These models fail to reproduce the observed helium resonance line intensities by significant factors. The possibility that non-Maxwellian electron distributions in the transition region might lead to increased collisional excitation rates in the helium lines is studied. Collisional excitation and ionization rates are recomputed for distribution functions with power-law suprathermal tails that may form by the transport of fast electrons from high-temperature regions. Enhancements of the helium resonance line intensities are found, but many of the predictions of the models regarding line ratios are inconsistent with observations. These results suggest that any such departures from Maxwellian electron distributions are not responsible for the helium resonance line intensities.     

Analytical solutions for saturated P Cygni type profiles

Analytical formulae for single P Cygni type saturated resonance line profiles in stellar winds have been derived. The limbdarkening and presence of underlying intrinsic atmospheric profile have been ignored. The Sobolev approximation for radiative transfer has been used and the general velocity law has been specified by widely used β parameter. The analytical formulae for the saturated resonance line profiles can be found for cases when 2β is an integer. The formulae for 2β = 1,2, 3 and 4 have been found by us. Also the formulae for calculating the line profiles in the cases of external and internal sharp truncation (cutoff) of the scattering shell have been given. Some characteristic line profiles have been presented. It has been shown that the turbulence-generated isotropic dominant backscattering of radiation in stellar winds generates wide dark plateaux in the blue wings of spectral lines, and the slopes of plateaux are shaped by turbulence. This revised version was published online in July 2006 with corrections to the Cover Date.     

银河系中球状星团的空间运动

球状星团是银河系中最古老的天体类型之一,其累积光度很大,是银晕中重要的示踪天体。已以发现的银河系球状星团有１４０多个,其中１２０个银心距Ｒ〈４０Ｋｐｃ的星团已被准确地测定了视向速度。根据结数据以及球状星团金属度的统计分析,可以把球状星团次系再进一步分成某些不同的族群。目前已经测定过绝对自行的球状星团只有３８个,尽管这些自行的精度比视向速度和距离的精度差很多,然而,由此可以得出三维的空间速度,在统计     

Observational evidence for AGN fueling: I. The case of NGC 6104—Merging with a companion

We investigate in detail the kinematics and morphology of the Seyfert galaxy NGC 6104 in order to identify the mechanism of gas transportation to the active galactic nucleus (AGN). Our observational data were obtained at the 6-m Special Astrophysical Observatory telescope with the MPFS integral-field spectrograph and the SCORPIO universal device in three modes: direct imaging, a scanning Fabry—Perot interferometer, and long-slit spectroscopy. Images from the HST archive were invoked to study the structure of the circumnuclear region. An analysis of deep images has shown for the first time that NGC 6104 is in the phase of active merging with a companion galaxy. We have been able to study the detailed picture of ionized gas motions up to galactocentric distances of 14 kpc and to construct the stellar velocity field for the inner region. The radial gas motions toward the AGN along the central bar play a significant role at galactocentric distances of 1–5 kpc. In addition, we have detected an outflow of ionized gas from the nucleus that presumably resulted from the intrusion of a radio jet into the ambient interstellar medium. Using diagnostic diagrams, we estimate the contributions from the AGN and star formation to the galactic gas ionization. We estimate the bar pattern speed by the Tremaine-Weinberg method and show that the inner ring observed in the galaxy’s images has a resonant nature. Two possible ring formation scenarios, before and during the interaction with a companion, are discussed.     

Investigation of OH and H<Subscript>2</Subscript>O masers in the star-forming region G 188.946+0.886

We present the results of our observations of the maser radio emission source G188.946+0.886 in hydroxyl (OH) molecular lines with the radio telescope of the Nançay Observatory (France) and in the H₂O line at λ = 1.35 cm with the RT-22 radio telescope at the Pushchino Observatory (Russia). An emission feature in the 1720-MHz satellite line of the OH ground state has been detected for the first time. The radial velocity of the feature, V _LSR = 3.6 km s^?1, has a “blue” shift relative to the range of emission velocities in the main 1665- and 1667-MHz OH lines, which is 8–11 km s^?1. This suggests a probable connection of the observed feature in the 1720-MHz line with the “blue” wing of the bipolar outflow observed in this region in the CO line. We have estimated the magnetic field strength for three features (0.90 and 0.8 mG for 1665 MHz and 0.25 mG for 1720 MHz) from the Zeeman splitting in the 1665- and 1720-MHz lines. No emission and (or) absorption has been detected in the other 1612-MHz satellite OH line. Three cycles of H₂O maser activity have been revealed. The variability is quasi-periodic in pattern. There is a general tendency for the maser activity to decrease. Some clusters of H2O maser spots can form organized structures, for example, chains and other forms.     

Impact of magnetic field on the gas mass fraction of galaxy clusters

Magnetic fields have been observed in galaxy clusters with strengths of the order of  ~ μG. The non-thermal pressure exerted by magnetic fields also contributes to the total pressure in galaxy clusters and can in turn affect the estimates of the gas mass fraction, f_gas. In this paper, we have considered a central magnetic field strength of 5μG, motivated by observations and simulations of galaxy clusters. The profile of the magnetic field has also been taken from the results obtained from simulations and observations. The role of magnetic field has been taken into account in inferring the gas density distribution through the hydrostatic equilibrium condition (HSE) by including the magnetic pressure. We have found that the resultant gas mass fraction is smaller with magnetic field as compared to that without magnetic field. However, this decrease is dependent on the strength and the profile of the magnetic field. We have also determined the total mass using the NFW profile to check for the dependency of f_gas estimates on total mass estimators. From our analysis, we conclude that for the magnetic field strength that galaxy clusters seem to possess, the non-thermal pressure from magnetic fields has an impact of  ≈ 1 % on the gas mass fraction of galaxy clusters. However, with upcoming facilities like Square Kilometre Array (SKA), it can be further expected to improve with more precise observations of the magnetic field strength and profile in galaxy clusters, particularly in the interior region.     

Energy transfer by the scattering of resonant photons

A formal derivation is presented of the energy transfer rate between radiation and matter due to the scattering of an isotropic distribution of resonance line photons. The derivation is developed in the context of the two-level atom in the absence of collisions and radiative transitions to and from the continuum, but includes the full angle-averaged redistribution function for photon scattering. The result is compared with previous derivations, all of which have been based on a Fokker–Planck approximation (FPA) to the radiative transfer equation. A new FPA, including a Kramers–Moyal extension to higher (post-diffusive) orders, is derived to solve the radiative transfer equation, and time-dependent numerical solutions are found. The relaxation of the colour temperature to the matter temperature is computed as the radiation field approaches statistical equilibrium through scattering. The results are discussed in the context of the Wouthuysen–Field mechanism for coupling the 21-cm spin temperature of neutral hydrogen to the kinetic temperature of the gas through Lyα scattering. The evolution of the heating rate is also computed, and shown to diminish as the gas approaches statistical equilibrium.     

A survey of the unusual motions in M17 with a Fabry-Perot monochromator

The results of the most extensive survey ever undertaken of the profiles of the [Oiii] line over M17 are presented. These were obtained with an optically-contacted, pressure-scanned, single-etalon Fabry-Perot monochromator used on the 74-in. Radcliffe reflector. Some very unusual velocity features have been revealed. Several regions were found to be emitting lines split by motions of 20 km s^?1 whereas others produce lines with three or more separate velocity components which stretch up to ?70 km s^?1 from the mean radial velocity of the nebula. Some speculative explanations for these motions are offered.     

On the nature of the X-ray absorption in the Seyfert 2 galaxy NGC 4507

We present results of the ASCA observation of the Seyfert 2 galaxy NGC 4507. The 0.5–10 keV spectrum is rather complex and consists of several components: (i) a hard X-ray power law heavily absorbed by a column density of about 3-10²³ cm⁻², (ii) a narrow Fe Kα line at 6.4 keV, (iii) soft continuum emission well above the extrapolation of the absorbed hard power law and (iv) a narrow emission line at ∼0.9 keV. The line energy, consistent with highly ionized neon (Ne IX ), may indicate that the soft X-ray emission is derived from a combination of resonant scattering and fluorescence in a photoionized gas. Some contribution to the soft X-ray spectrum from thermal emission, as a blend of Fe L lines, by a starburst component in the host galaxy cannot be ruled out with the present data.     

Resonance line scattering from optically thin structures located above the solar limb

We discuss the formation of emission lines by resonance scattering from optically thin structures located above the solar limb. When the scattered radiation is only partially redistributed in frequency and angle, the resulting coherency is sufficient to affect the interpretation of such lines. In particular the apparent Doppler width of the scattered line may be different from the Doppler width in the scattering structure.Mitteilung aus dem Fraunhofer Institut No. 154.