A Narrow Band Chandra X-ray Analysis of SNR 3C 391

We present narrow-band and equivalent width (EW) images of the thermal composite supernova remnant (SNR) 3C 391 in the X-ray emission lines of Mg, Si and S using the Chandra ACIS Observational data. The EW images reveal the spatial distribution of the emission of the metal species Mg, Si and S in the remnant. They have a clumpy structure similar to that seen in the broadband diffuse emission, suggesting that they are largely of interstellar origin. We find an interesting finger-like feature protruding outside the southwestern radio border of the remnant, somewhat similar to the jet-like Si structure found in the famous SNR Cas A. This feature may possibly be the debris of the jet of ejecta from an asymmetrical supernova explosion of a massive progenitor star.     

The remnant of Nova Cassiopeiae 1993 (V705 Cassiopeiae)

We have detected radio emission from the remnant of V705 Cas, beginning 221 d after visual maximum. We follow the remnant from first detection while still optically thick through the turn-over to optically thin emission, and finally confirming the absence of emission due to expansion and fading by the end of 1998. While the emission is consistent with thermal bremsstrahlung, as seen in most previous novae, the small-scale structure shows two ridge-like features, which develop significantly from day 585 until our last detection on day 1544. While the development is fairly rapid, it is also much smoother than that seen in V1974 Cyg, in that we can follow individual features as they change over almost 1000 d. Our optical spectra unequivocally indicate some form of axial symmetry, as derived from the velocity structure. This is difficult to reconcile with the observed development in the radio images. We suggest a number of possible explanations.     

Ionization structure in accretion shocks with a composite cooling function

We have investigated the ionization structure of the post-shock regions of magnetic cataclysmic variables, using an analytic density and temperature structure model in which effects caused by bremsstrahlung and cyclotron cooling are considered. We find that in the majority of the shock-heated region where H- and He-like lines of the heavy elements are emitted, the collisional-ionization and corona-condition approximations are justified. We have calculated the line emissivity and ionization profiles for iron as a function of height within the post-shock flow. For low-mass white dwarfs, line emission takes place near the shock. For high-mass white dwarfs, most of the line emission takes place in regions well below the shock and hence it is less sensitive to the shock temperature. Thus, the line ratios are useful to determine the white dwarf masses for the low-mass white dwarfs, but the method is less reliable when the white dwarfs are massive. Line spectra can, however, be used to map the hydrodynamic structure of the post-shock accretion flow.     

Mid-ir imaging polarimetry and circumstellar magnetic fields

We have obtained diffraction-limited images of the mid-infrared emission and polarization patterns in a number of southern-sky objects. By mapping the polarization produced in absorption or emission by aligned aspherical dust grains, we have been able to trace the detailed spatial structure of magnetic fields in the warm circumstellar material of young molecular-outflow sources and HII regions, in the expanding dust shell of the mass-losing star Eta Carinae, and in the inner parsec of the Galactic Centre.     

'Stokes imaging' of the accretion region in magnetic cataclysmic variables — I. Conception and realization

Modelling the polarized cyclotron emission from magnetic cataclysmic variables has been a pivotal technique for determining the structure of the accretion zones on the white dwarf. To date, model solutions have been obtained from trial fits to the intensity and polarization data, which have been constructed from emission regions (for example arcs and spots) put in by hand. These models were all inferred indirectly from arguments based on the polarization and X-ray light curves.   We present a more analytical and objective technique using optimization by a genetic algorithm, Tikhonov regularization and Powell's method that robustly models the details of polarized emission.   To demonstrate the success of this technique, we show the results of several simulations in which we calculated the intensity and polarization curves from arbitrarily shaped emission regions on the surface of a sphere and then applied our code to these curves to recover the original test data. We also show how adding artificial noise affects the outcome of the optimization technique.     

H2O maser flare in NGC 7538 S

We investigate the H₂O maser flare that occurred in the source NGC 7538 S during 1998–2005 in the radial-velocity range from ?57 to ?52 km s^?1. We have found a large number of emission features, suggesting that the medium where the flare occurred is highly fragmented. We have identified four spectral groups of emission features. All groups are most likely associated with the cluster of maser spots located in the center of an elongated structure and related to a massive rotating disk. The observed pattern of variations in the fluxes and radial velocities of the features can be explained by the presence of inhomogeneities in the medium, which can form elongated structures like filaments or chains. The mean extent of this structure is estimated to be 6–8 AU. Two cycles of maser activity have been observed, 1998–2002 and 2003–2005, which may be determined by the cyclic activity of the central object, a massive O-type protostar.     

Thermal cyclotron radiation from hot coronal loops and peculiarities of the polarization structure of solar microwave emission sources: II. Integrated characteristics

We present our calculations of the expected characteristics of the integrated spectrum of thermal cyclotron radiation from a solar active region with a coronal magnetic loop. A hot torus is considered as a three-dimensional loop model. We show that the hot-loop emission can change appreciably the emission characteristics of the active region at centimeter and decimeter wavelengths. At certain loop parameters, the emission frequency spectrum can have a nonmonotonic and complex pattern with several peaks or contain narrow-band cyclotron lines. The polarization structure of the source with a hot loop is also complex and the polarization is repeatedly reversed over the observed frequency range under certain conditions. The revealed spectral-polarization peculiarities are considered from the standpoint of whether some atypical observed properties of the emission sources associated with solar active regions can be explained.     

Neutral hydrogen gas distribution and kinematics in NGC 7479

We have investigated the effects of a bar and an asymmetric spiral structure on the neutral hydrogen distribution and kinematics in the strongly barred spiral galaxy NGC 7479. The strongest 21-cm line emission at 1-kpc resolution comes from the western spiral arm which appears to be slightly inclined to the plane of the main disc. In contrast, the area within the radius of the bar is devoid of 21-cm line emission. The radio continuum emission at 21 cm follows the bar dust lanes, but beyond 3 kpc from the nucleus the radio continuum emission has a peculiar morphology, unlike that of optical and near-infrared images. We did not detect any low surface brightness gas-rich companions in the near neighbourhood of NGC 7479. This leads us to propose that the strong western spiral arm was created in a recent minor merger.     

Observational Characteristics of Radio Emission Related to Multi-polar Magnetic Configuration

We present a, large complex radio burst and its associated fast tune structures observed on 2001 April 10 in the frequency range of 0.65-7.6 GHz. The NoRH radio image observation shows very complex radio source structures which include preexisting, newly emerging, submerging/cancelling polarities and a bipolar, a tripolar (a 'bipolar + remote unipolar'), and a quadrupolar structure. This suggests that the radio burst is generated from a very complicated loop structure. According to the spectral and image observations, we assume that the beginning of this flare was caused by a single bipolar loop configuration with a 'Y-type' re-connection structure. A composite of radio continuum and fast time structures is contained in this flare. The various fast radio emission phenomena include normal and reverse drifting type III bursts, and slowly drifting and no-drift structures. The tripolar configurations may form a double-loop with a 'three-legged' structure, which is an important source of the various types of fast time structures. The two-loop reconnection model can lead simultaneously to electron acceleration and corona heating. We have also analyzed the behaviors of coronal magnetic polarities and the emission processes of different types radio emission qualitatively. Interactions of a bipolar or multi-polar loop are consistent with our observational results. Our observations favor the magnetic reconnection configurations of the 'inverted Y-type' (bipolar) and the 'three-legged' structures (tripolar or quadrupolar).     

A physical parameter method for the design of broad-band X-ray imaging systems to do coronal plasma diagnostics

The technique commonly used for the analysis of data from broad-band X-ray imaging systems for plasma diagnostics is the filter ratio method. This requires the use of two or more broad-band filters to derive temperatures and line-of-sight emission integrals or emission measure distributions as a function of temperature. Here we propose an alternative analytical approach in which the temperature response of the imaging system is matched to some physical parameter which the experimenter wishes to investigate. We have calculated the temperature response of a system designed to measure the total radiated power along the line of sight of any coronal structure. Other examples are discussed.     

High-Resolution Observations in B1-IRS: Ammonia, Ccs and Water Masers

We present a study of the structure and dynamics of the star-forming region B1-IRS (IRAS 03301+3057) using the properties of different molecules at high angular resolution (4). We have used VLA observations of NH₃, CCS, and H₂O masers at 1 cm. CCS emission shows three clumps around the central source, with a velocity gradient from red to blue-shifted velocities towards the protostar, probably due to the interaction with outflowing material. Water maser emission is elongated in the same direction as a reflection nebula detected at 2 m by 2MASS, with the maser spots located in a structure of some hundreds of AU from the central source, possibly tracing a jet. We propose a new outflow model to explain all our observations, consisting of a molecular outflow near the plane of the sky. Ammonia emission is extended and anticorrelated with CCS. We have detected for the first time this anticorrelation at small scales (1400 AU) in a star-forming region.     

Millimeter polarimetry of star-forming regions: magnetic field structure around low-mass stars

We have measured polarization of the 1.1 mm and 0.8 mm continuum emission for 3 pre-T Tauri stars and 2 T Tauri stars. Positive detections were made for NGC 1333 IRAS 4 and IRAS 16293-2422, while L1551 IRS 5 and HL Tau were only marginally detected. For GG Tau we measured a 2 upper limit of 3%. The polarization is interpreted in terms of thermal emission by magnetically aligned dust grains in circumstellar disks or envelopes. We have found a definite geometrical relation between the polarization and other circumstellar structure.     

White-Light Flares: A TRACE/RHESSI Overview

The Transition Region and Coronal Explorer (TRACE) instrument includes a “white light” imaging capability with novel characteristics. Many flares with such white-light emission have been detected, and this paper provides an introductory overview of these data. These observations have 0.5″ pixel size and use the full broad-band response of the CCD sensor; the images are not compromised by ground-based seeing and have excellent pointing stability as well as high time resolution. The spectral response of the TRACE white-light passband extends into the UV, so these data capture, for the first time in images, the main radiative energy of a flare. This initial survey is based on a sample of flares observed at high time resolution for which the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) had complete data coverage, a total of 11 events up to the end of 2004. We characterize these events in terms of source morphology and contrast against the photosphere. We confirm the strong association of the TRACE white-light emissions - which include UV as well as visual wavelengths – with hard X-ray sources observed by RHESSI. The images show fine structure at the TRACE resolution limit, and often show this fine structure to be extended over large areas rather than just in simple footpoint sources. The white-light emission shows strong intermittency both in space and in time and commonly contains features unresolved at the TRACE resolution. We detect white-light continuum emission in flares as weak as GOES C1.6. limited by photon statistics and background solar fluctuations, and support the conclusion of Neidig (1989) that white-light continuum occurs in essentially all flares.     

Flares of the H<Subscript>2</Subscript>O maser in W31(2)

We analyze our monitoring data for the water-vapor maser in the source W31(2), associated with a region of vigorous star formation, a cluster of OB stars. The monitoring was performed with the 22-m radio telescope at Pushchino Radio Astronomy Observatory during 1981–2004. The variability of the H₂O maser in W31(2) was found to be cyclic, with a mean period of 1.9 yr. Two flares were most intense (superflares): in 1985–1986 and 1998–1999. In each activity cycle, we observed up to several short flares, subpeaks. The fluxes of many emission features during the flares were correlated. We also observed successive activation of individual emission features in order of increasing or decreasing radial velocity, suggesting an ordered structure and, hence, a radial-velocity gradient of the medium. There is a clear correlation of the emission peaks of the main components in the spectra at radial velocities of ?1.7, ?1.3, 0.5, and 1.3 km s^?1 with activity cycles and of the emission at V_LSR < ?8 km s^?1 with short flares. During the superflares, the emission in the low-velocity part of the H₂O spectrum and a number of other phenomena related to coherent maser-emission properties were suppressed. The maser spots are assumed to form a compact structure, to have a common pumping source, and to be associated with an accretion flow onto the cluster of OB stars.     

On the superfine structure of solar microwave bursts

Solar microwave bursts with a zebra pattern commonly exhibit a superfine time structure: the zebra stripes consist of separate spike-like pulses. We investigate the superfine structure in the April 21, 2002 event. The emission pulses are shown to exhibit a high periodicity (with a period of about 30 ms); there is a clear correlation between the individual zebra stripes. This structure of the dynamic spectra most likely reflects a periodic injection of electron beams, which generate emission at the double plasma resonance levels.     

Fabry–Perot imaging of NGC 6090 in H2 emission

We have obtained an H₂ v =1–0 S(1) image of a merging galaxy system, NGC 6090, by using a Fabry–Perot imager. The H₂ emission originates between the double nuclei of pre-merger galaxies, and exhibits an arc-like or ring-like structure almost connecting the double nuclei. This structure is similar to that suggested for Arp 220 from the velocity field measured by CO radio emission. The separation of the double nuclei in NGC 6090 is 5–6 arcsec, corresponding to a projected distance of 3–4 kpc. This is much larger than that of Arp 220 and suggests that the molecular gas distribution can form an organized shape between the nuclei, such as a ring, in a rather early phase of merging.     

Seyfert galaxies and the radio–far-infrared correlation

We have observed a sample of 149 Seyfert galaxies and radio-quiet quasars at 13 cm with both a 275-km radio interferometer and the 6-km compact array of the Australia Telescope. The high-resolution observations searched for the presence of compact, high-brightness-temperature radio emission from the active nucleus. The low-resolution observations measured the total radio emission from the galaxy disc and Seyfert core and lobes. From these we draw the following conclusions. (i) Seyfert galaxies that lack compact radio cores display a correlation between radio and far-infrared (FIR) emission similar to the correlation displayed by normal spirals, albeit with greater scatter. The correlation is found to be intrinsic and is not an artefact of the richness effect. (ii) A very different radio–FIR correlation is displayed by those Seyferts that harbour compact radio cores. These tend to be more radio-loud than either normal spirals or the Seyferts that lack compact cores. The compact core emission thus seems to be responsible for the generally poor radio–FIR correlation displayed by Seyfert galaxies. (iii) The radio–FIR correlation is not significantly improved by subtracting off the 0.1-arcsec (20- to 200-pc) compact radio emission from the total radio emission. This suggests that the emission from the active galactic nucleus has significant structure on scales larger than 0.1 arcsec. Perhaps these structures are the 'linear' radio features that have been seen previously in Seyfert nuclei.     

The composition of dust in Jupiter-family comets inferred from infrared spectroscopy

We review the composition of Jupiter-family comet (JFC) dust as inferred from infrared spectroscopy. We find that JFCs have silicate emission features with fluxes roughly 20-25% over the dust continuum (emission strength 1.20-1.25), similar to the weakest silicate features in Oort Cloud (OC) comets. We discuss the grain properties that alter the silicate emission feature (composition, size, and structure/shape), and emphasize that thermal emission from the comet nucleus can have significant influence on the derived silicate emission strength. Recent evidence suggests that grain porosity is the is different between JFCs and OC comets, but more observations and models of silicates in JFCs are needed to determine if a consistent set of grain parameters can explain their weak silicate emission features. Models of 8 m telescope and Spitzer Space Telescope observations have shown that JFCs have crystalline silicates with abundances similar to or less than those found in OC comets, although the crystalline silicate mineralogy of comets 9P/Tempel and C/1995 O1 (Hale-Bopp) differ from each other in Mg and Fe content. The heterogeneity of comet nuclei can also be assessed with mid-infrared spectroscopy, and we review the evidence for heterogeneous dust properties in the nucleus of comet 9P/Tempel. Models of dust formation, mixing in the solar nebula, and comet formation must be able to explain the observed range of Mg and Fe content and the heterogeneity of comet 9P/Tempel, although more work is needed in order to understand to what extent do comets 9P/Tempel and Hale-Bopp represent comets as a whole.     

Chromospheric Models of Solar Type Stars: The Vaughan-Preston Gap

We have built different models for stars of the same spectral type than the Sun but with different levels of chromospheric activity, to study the response of the S index of activity built from the emission of the Ca II H and K lines to changes in the chromospheric structure. We found that the fact that there are many stars with either strong or weak emission, but few with intermediate values of S, the so-called Vaughan–Preston gap, can be due to a discontinuity in the response of the Ca II lines to chromospheric heating. In fact, we are able to reproduce the observed distribution of the number of stars as a function of S.     

Comparison of ground-based and spacecraft observations of the infrared emission from Venus: The nature of thermal contrasts

Comparisons are made between observations of spatial variations in the thermal emission from Venus obtained with ground-based telescopes and those from spacecraft. In particular, we concentrate on measurements of solar-related structure at low and mid-latitudes, limb-darkening, and on the high-contrast polar structure. We conclude that (1) the solar-related emission is predominantly wavenumber 2, although it contains a significant diurnal component; (2) the relative amplitudes of the semidiurnal and diurnal components vary with latitude; (3) thermally excited temperature waves or, alternatively, solar-driven vertical motions of the cloud top are better able to account for the magnitude of the solar-locked emission than brightness temperature contrasts resulting from variations in aerosol microphysical properties; (4) the equatorial limb-darkening shows the top of the main cloud to be diffuse and approximately uniformly mixed with the gas; (5) polar collars are persistent at least for several months but disappear on occasion; and (6) collars have been observed at both poles simultaneously, but whether simultaneous appearance is the exception or the rule is still in question.