The nature of nonthermal X-ray filaments near the galactic center

Recent Chandra and XMM-Newton observations reported evidence of two X-ray filaments G359.88−0.08 (SgrA-E) and G359.54+0.18 (the ripple filament) near the Galactic center. The X-ray emission from these filaments has a nonthermal spectrum and coincides with synchrotron emitting radio sources. Here, we report the detection of a new X-ray feature coincident with a radio filament G359.90−0.06 (SgrA-F) and show more detailed VLA, Chandra and BIMA observations of the radio and X-ray filaments. In particular, we show that radio emission from the nonthermal filaments G359.90−0.06 (SgrA-F) and G359.54+0.18 (the ripple) has a steep spectrum whereas G359.88−0.08 (SgrA-E) has a flat spectrum. The X-ray emission from both these sources could be due to synchrotron radiation. However, given that the 20 km s⁻¹ molecular cloud, with its intense 1.2 mm dust emission, lies in the vicinity of SgrA-F, it is possible that the X-rays could be produced by inverse Compton scattering of far-infrared photons from dust by the relativistic electrons responsible for the radio synchrotron emission. The production of X-ray emission from ICS allows an estimate of the magnetic field strength of 0.08 mG within the nonthermal filament. This should be an important parameter for any models of the Galactic center nonthermal filaments.     

XMM-Newton study of soft excess X-ray emission in clusters of galaxies

We discuss the detection of soft excess X-ray emission in a sample of 19 clusters of galaxies observed by XMM-Newton. In 6/19 clusters evidence for a soft X-ray excess is found. Four of these clusters show soft X-ray and O VII line emission from gas with a temperature of 0.2 keV. The centroid of this oxygen line is consistent with the redshift of the cluster. The intensity and spatial extend of the soft excess agrees with previous PSPC measurements. These observations are interpreted as emission from warm-hot intergalactic medium filaments, with density enhancements near the cluster centers, consistent with theoretical predictions. In the other two soft excess clusters a non-thermal origin is consistent with the data.     

An archival study of extended X-ray emission from classical and recurrent novae

I discuss morphology and spectrum of the first resolved and detected classical nova shell in the X-rays – the remnant of GK Persei (1901). The existence of such a nebulosity brings about the possibility of other nova remnants emitting X-rays. I calculate that the X-ray luminosity should be about 10²⁶–10³³ ergs s⁻¹ on the onset of cooling for nova remnants. I have done an archival search on 250 classical and recurrent nova candidates using Chandra, XMM-Newton, ROSAT and ASCA databases. There is no significant extended emission detected which places an upper limit of F_x < × 10⁻¹² erg s⁻¹ cm⁻² (unabsorbed). Only exceptions are GK Per, RR Pic and DQ Her (all observed by Chandra ACIS-S and GK Per also by ROSAT HRI) where the latter two show marginal extended emission in the X-rays associated with emission knots (DQ Her) or an equatorial ring (RR Pic).     

ROSAT observations of galaxy clusters and cosmological implications

Clusters of galaxies are filled with a hot teneous plasma which is emitting soft X-rays over very extended regions. The ROSAT observatory with its high resolution and high sensitivity X-ray telescope allows to image nearby clusters in great detail and is therefore ideally suited to study the structure and morphology of galaxy clusters. In particular the distribution of the intracluster gas can be mapped, some information on the temperature structure of the gas can be obtained, the gravitational mass can be infered, and the dynamical state of the clusters can be investigated. Results of such studies for some nearby clusters as Virgo, Perseus and Coma are presented. Gas mass fractions of the total gravitational mass of the cluster in excess of 10 to 15% are infered from X-ray observations where the cluster emission can be traced out to an Abell radius (3 Mpc). If these values are taken to be representative for the matter composition in the Universe it has interesting consequences for cosmology as for example for the nucleosythesis models and the mean density of the Universe.     

The properties of hard X-ray emitting symbiotic stars

Hard X-ray emitting symbiotic stars are candidates for SN Ia progenitors. The importance of Type Ia SNe as standard candles for cosmology makes the study of their progenitor systems particularly important. Additionally, they provide one of the most promising laboratories for the study of astrophysical jets. Typically, the X-ray emission in these systems is modeled with a collisional plasma model, sometimes with an emission measure distribution taken from a cooling flow model. The lack of any coherent periods in both X-rays and optical wave band strongly suggests that the accreting white dwarfs in the hard X-ray symbiotic stars are non-magnetic. Although relatively few have been discovered to date, but we believe that there are very many of them in our galaxy and could be possible candidates for the Galactic Ridge X-ray Emissions (GRXE).     

Observational appearances of isolated stellar-mass black hole accretion – Theory and observations

General properties of accretion onto isolated stellar-mass black holes in the Galaxy are discussed. An analysis of plasma internal energy growth during the infall is performed. Adiabatic heating of collisionless accretion flow due to magnetic adiabatic invariant conservation is 25% more efficient than in the standard non-magnetized gas case. It is shown that magnetic field line reconnections in discrete current sheets lead to significant nonthermal electron component formation, which leads to a formation of a hard (UV, X-ray, up to gamma), highly variable spectral component in addition to the standard synchrotron optical component first derived by Shvartsman generated by thermal electrons in the magnetic field of the accretion flow. Properties of accretion flow emission variability are discussed. Observation results of two single black hole candidates – gravitational lens MACHO-1999-BLG-22 and radio-loud X-ray source with featureless optical spectrum J1942+10 – in optical band with high temporal resolution are presented and interpreted in the framework of the proposed model.     

On the heating mechanism of magnetic flux loops in the solar atmosphere

Hα filtergrams and magnetograms indicate that bright features (such as plages and granulation boundaries) correspond to areas of strong vertical magnetic fields and dark features (such as fibrils and filaments) are associated with strong horizontal magnetic field. It was suggested by /1/ that there is an excess dissipation of waves, available for heating, in regions of vertical magnetic fields. With this suggestion in mind, we have investigated the physical heating mechanism due to ponderomotive forces exerted by turbulent waves along curved magnetic flux loops. Results show that the temperature difference (ΔT) between the inside and outside of the flux loop can be classified into three parts; ΔT = ΔT₁ + ΔT₂ + ΔT₃; in which ΔT₁ and ΔT₃ represent the heating or cooling effect from the ponderomotive force, and ΔT₂ is the heating effect due to conversion of turbulent energy from the localized plasma. The specific physical mechanism (i.e., the ponderomotive forces exerted by turbulent waves), is used to illustrate solar atmospheric heating via an example leading to the formulation of plages.     

The structure and effects of cooling flows in clusters of galaxies

Rapidly cooling gas is commonly found near the centres of clusters of galaxies. The structure of the resulting gas flows is reviewed. Total gas cooling rates of several hundred M yr⁻¹ have been observed in a number of cases. Thermal instability and the ultimate fate of the cooled gas are discussed. The cooled gas could easily have formed a massive central galaxy.     

太阳黑子的纤维结构

本文采用文献[1]所发展的方法求解非轴对称磁静平衡方程,以解释太阳黑子的纤维结构。结果表明,在黑子半影区中的暗纤维对应着强磁场区。      

Production and evolution of millisecond X-ray and radio pulsars

Observations using the Rossi X-ray Timing Explorer (RXTE) have discovered dozens of accreting neutron stars with millisecond spin periods in low-mass binary star systems. Eighteen are millisecond X-ray pulsars powered by accretion or nuclear burning or both. These stars have magnetic fields strong enough for them to become millisecond rotation-powered (radio) pulsars when accretion ceases. Few, if any, accretion- or rotation-powered pulsars have spin rates higher than 750 Hz. There is strong evidence that the spin-up of some accreting neutron stars is limited by magnetic spin-equilibrium whereas the spin-up of others is halted when accretion ends. Further study will show whether the spin rates of some accretion- or rotation-powered pulsars are or were limited by emission of gravitational radiation.     

Dynamo-related activity in T Tauri stars

New observational evidences indicating that magnetic fields play an active role in T Tauri stars' activity are presented. Periodic light variations displayed by 11 T Tauri stars (TTS) are interpreted in terms of inhomogeneous temperature distribution on their surface. At higher atmospheric levels, magnetic fields may be responsible for the heating of X-ray emitting region while some additional mechanism seems to be necessary to account for the intensity of TTS emission-line spectrum.     

The heart-shaped supernova remnant 3C 391 viewed in multi-bands

Using Chandra X-ray, Spitzer mid-IR, and 1.5 GHz radio data, we examine the spatial structure of SNR 3C 391. The X-ray surface brightness is generally anti-correlative with the IR and radio brightness. The multiband data clearly exhibit a heart-shaped morphology and show the multi-shell structure of the remnant. A previously unseen thin brace-like shell on the south detected at 24 μm is projected outside the radio border and confines the southern faint X-ray emission. The leading 24 μm knot on the SE boundary appears to be partly surrounded by soft X-ray emitting gas. The mid-IR emission is dominated by the contribution of the shocked dust grains, which may have been partly destroyed by sputtering.     

X-ray spectral properties of non-magnetic cataclysmic variables using Chandra HETG

Results from a study of high resolution spectra obtained with the Chandra X-ray observatory for a sample of 6 Cataclysmic Variables (CVs) are presented. A global fit approach has been employed to obtain the spectral characteristics of the sources. The line-rich high-resolution spectra of these sources clearly indicate multi-temperature nature of the emitting plasma. Multi-temperature APEC models describe the spectra very well. Detection of significantly broad emission lines, indicates the presence of high velocity gas in SS Cyg and U Gem during the optical outbursts.     

Three dimensional simulation studies on bright points in the solar corona

The interaction phase of the Converging Flux Model (Priest et al., 1994) for the formation of coronal X-ray bright points is investigated self-consistently by means of magnetohydrodynamical simulations. In these simulations the three-dimensional dynamical evolution of two mutually approaching magnetic structures of opposite polarity is studied. During the ideal phase of the approach a current sheet forms in the region above the polarity inversion line and the low-β coronal plasma is compressed and heated locally. Stronger plasma heating on shorter time scales occurs due to magnetic reconnection as soon as a finite anomalous resistivity leads to a violation of the ideal Ohm's law. Both processes together might account for quasi-stationary soft X-ray emissivity of bright points, flaring of tiny filaments within X-ray bright points, and jet-like plasma flows in the vicinity of bright point features.     

Neutral clouds in a pervasive X-ray gas between the sun and northern HVCs

We present evidence that the soft X-ray distribution observed with the ROSAT PSPC instrument is not adequately explained by the standard Local Hot Bubble model (/1/). We discuss the X-ray absorbing cloud LVC 88 + 36 − 2 embedded in the hot plasma of the Local Hot Bubble, the X-ray shadow of the Draco nebula and other clouds inside and outside the galactic disk, and the X-ray emission associated with halo type objects like the HVC's M I and M II. They populate the distance range from about 60 pc to more than a few kpc and imply the presence of X-ray emitting plasma between the sun and and the outer galactic halo. These observations are consistent with a pervasive X-ray emitting plasma in which neutral clouds are embedded. However, the volume filling factor of this plasma is not known. A model which adequately describes the observed features has been developed and published by Hirth et al. (/28/).

For the first time in the literature we present results of a correlation analysis of X-ray shadows and H I or IR images of a molecular cloud. This is a new technique for the determination of the total column density of hydrogen nuclei for molecular clouds.     

Revealing the hot intergalactic medium with ROSAT

The X-ray background intensity around galaxies and rich clusters of galaxies is investigated in three energy bands using the ROSAT All-Sky Survey maps. It is found that an amplitude of the XRB enhancements surrounding the Abell clusters and high density areas in the Lick galaxy counts depends on photon energy. Excess flux generated in the surrounding of the galaxy concentrations is consistent with the thermal emission by hot gas postulated by hydrodynamic simulations.     

Heating rates in X-ray flares: From solar microflares to stellar events

Soft X-ray solar and stellar flares appear in the coronae of solar-like stars due to abrupt release of energy accumulated in magnetic fields. To build a quantitatively correct model of a flare we need to know how much energy is released in flares of different sizes and strengths. Here we estimate and compare the energy release rate in flares as different as microflares occurring over the quiet Sun and strong stellar events in RS CVn systems. We find one simple scaling law which describes flares differing one from another by 10 orders of magnitude in the amount of emission measure.     

Summary of the workshop on gamma-ray burst afterglows at the 34th COSPAR meeting

A summary is given of the presentations at the COSPAR workshop on γ-ray bursts with some personal commentary on the contributions, the SN/GRB connection, and on the role of magnetic fields in γ-ray bursts and their afterglows. Of special interest were the accumulated arguments for strong collimation and associated reduction in the total required energy for γ-ray bursts. Significant discussion was also devoted to the issues associated with iron and metal lines in X-ray spectra. It is important to note that some of the afterglows seem to require ambient densities 1 g cm⁻³, rather incompatible with a massive star environment. Of associated difficulty is the fact that few, if any, afterglows seem consistent with the r⁻² wind expected for a massive star model. There are reasons to think that if γ-ray bursts are associated with supernovae they are of Type Ic. This suggests that any wind present might be rich in carbon and oxygen, not hydrogen or helium. If γ-ray bursts are narrowly collimated, then the burst is only probing a small portion of any wind, perhaps just that time-dependent and isotropic structure directly along the rotation axis. The characteristics of “hypernovae” may be the result of orientation effects in a mildly inhomogeneous set of progenitors, rather than requiring an excessive total energy or luminosity. The recent event GRB 021004 provided a rich photometric and spectroscopic record and perhaps the most direct evidence yet for the association of a specific γ-ray burst with a massive star progenitor. If the magnetic field plays a significant role in launching a relativistic γ-ray burst jet from within a collapsing star, then the magnetic field may also play a role in the propagation, collimation, and stability of that jet within and beyond the star. The magneto-rotational instability (MRI) can operate under conditions of moderate rotation. This means that the MRI will be at work generating strong fields exponentially rapidly even as the disk of material begins to form and makes a transition from a non-Keplerian to quasi-Keplerian flow in the collapsar and related models.     

Melt temperature fluctuations: Causes and response of the solidification front

It is well understood that temperature oscillations and fluctuations during melt growth give rise to fluctuating concentration of solute in the crystal — so-called solute (or impurity) striations. A major source of the temperature fluctuations is the transition to turbulence in the melt resulting from strong buoyancy and/or surface-driven flows. The interaction of these natural convective flows with any imposed rotation is of significance. The major factors causing flow transitions and particularly those which lead to time-dependent flows in Czochralski and float-zone melts will be briefly reviewed and the likely influence of a low-g environment considered. Theoretical analysis and experimental study of the response of the crystal-melt interface to such temperature fluctuations is reviewed and criteria for maximising crystal homogeneity discussed.     

SIGMA/GRANAT observations of hard X-ray emission from type I X-ray bursters

Unlike black hole candidate systems, accreting neutron stars seem to encounter appreciable difficulties in emitting strong hard X-ray fluxes. However, in the catalogue of the hard X-ray sources detected by SIGMA, three sources are associated with type I X-ray bursters. In this paper, we review the present status of the SIGMA observations of these three X-ray burst sources, namely X 1724-308 in the globular cluster Terzan II, KS 1731-260, and GX 354+0.