We present high resolution X-ray spectra of the X-ray bright classical T Tauri star, TW Hydrae, covering the wavelength range of 1.5-25Å. The differential emission measure derived from fluxes of temperature-sensitive emission lines shows a plasma with a sharply peaked temperature distribution, peaking at log T = 6.5. Abundance anomalies are apparent, with iron very deficient relative to oxygen, while neon is enhanced relative to oxygen. Density-sensitive line ratios of Ne ix and O vii indicate densities near log n e = 13. A flare with rapid (∼ 1 ks) rise time was detected during our 48 ksec observation; however, based on analysis of the emission-line spectrum during quiescent and flaring states, the derived plasma parameters do not appear strongly time-dependent. The inferred plasma temperature distribution and densities are consistent with a model in which the bulk of the X-ray emission from TW Hya is generated via mass accretion from its circumstellar disk. Assuming accretion powers the X-ray emission, our results for log n e suggest an accretion rate of ∼ 10 −8 M ⊙ yr −1 .

The CIAO (Chandra Interactive Analysis of Observations) software package was first released in 1999 following the launch of the Chandra X-ray Observatory and is used by astronomers across the world to analyze Chandra data as well as data from other telescopes. From the earliest design discussions, CIAO was planned as a generalpurpose scientific data analysis system optimized for X-ray astronomy, and consists mainly of command line tools (allowing easy pipelining and scripting) with a parameter-based interface layered on a flexible data manipulation I/O library. The same code is used for the standard Chandra archive pipeline, allowing users to recalibrate their data in a consistent way.We will discuss the lessons learned from the first six years of the software's evolution. Our initial approach to documentation evolved to concentrate on recipe-based "threads" which have proved very successful. A multidimensional abstract approach to data analysis has allowed new capabilities to be added while retaining existing interfaces. A key requirement for our community was interoperability with other data analysis systems, leading us to adopt standard file formats and an architecture which was as robust as possible to the input of foreign data files, as well as re-using a number of external libraries. We support users who are comfortable with coding themselves via a flexible user scripting paradigm, while the availability of tightly constrained pipeline programs are of benefit to less computationally-advanced users. As with other analysis systems, we have found that infrastructure maintenance and re-engineering is a necessary and significant ongoing effort and needs to be planned in to any long-lived astronomy software.

Details of the design, fabrication, ground and flight calibration of the High Energy Transmission Grating, HETG, on the Chandra X-ray Observatory are presented after five years of flight experience. Specifics include the theory of phased transmission gratings as applied to the HETG, the Rowland design of the spectrometer, details of the grating fabrication techniques, and the results of ground testing and calibration of the HETG. For nearly six years the HETG has operated essentially as designed, although it has presented some subtle flight calibration effects.

The Orion Trapezium is one of the youngest and closest star-forming regions within our Galaxy. With a dynamic age of $3  10 5 yr, it harbors a number of very young hot stars, which likely are on the zero-age main sequence (ZAMS). We analyzed high-resolution X-ray spectra in the wavelength range of 1.5-25 Å of three of its X-ray-brightest members ( 1 Ori A, C, and E) obtained with the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra X-Ray Observatory. We measured X-ray emission lines, calculated differential emission measure distributions (DEMs), and fitted broadband models to the spectra. The spectra from all three stars are very rich in emission lines, specifically from highly ionized Fe, which includes emission from Fe xvii to Fe xxv ions. A complete line list is included. This is a mere effect of high temperatures rather than an overabundance of Fe, which in fact turns out to be underabundant in all three Trapezium members. Similarly there is a significant underabundance in Ne and O as well, whereas Mg, Si, S, Ar, and Ca appear close to solar. The DEM derived from over 80 emission lines in the spectrum of  1 Ori C indicates three peaks located at 7.9, 25, and 66 MK. The emission measure varies over the 15.4 day wind period of the star. For the two phases observed, the low-temperature emission remains stable, while the high-temperature emission shows significant differences. The line widths seem to show a similar bifurcation, where we resolve some of the soft X-ray lines with velocities up to 850 km s À1 (all widths are stated as halfwidth at half-maximum), whereas the bulk of the lines remain unresolved with a confidence limit of 110 km s À1 . The broadband spectra of the other two stars can be fitted with several collisionally ionized plasma model components within a temperature range of 4.3-46.8 MK for  1 Ori E and 4.8-42.7 MK for  1 Ori A. The high-temperature emissivity contributes over 70% to the total X-ray flux. None of the lines are resolved for  1 Ori A and E with a confidence limit of 160 km s À1 . The influence of the strong UV radiation field on the forbidden line in the He-like triplets allows us to set an upper limit on distance of the line-emitting region from the photosphere. The bulk of the X-ray emission cannot be produced by shock instabilities in a radiation-driven wind and are likely the result of magnetic confinement in all three stars. Although confinement models cannot explain all the results, the resemblance of the unresolved lines and of the DEM with recent observations of active coronae in II Peg and AR Lac during flares is quite obvious. Thus we speculate that the X-ray production mechanism in these stars is similar, with the difference that the Orion stars may be in a state of almost continuous flaring driven by the wind. We clearly rule out major effects due to X-rays from a possible companion. The fact that all three stars appear to be magnetic and are near zero age on the main sequence also raises the issue of whether the Orion stars are ...

We have obtained high-resolution X-ray spectra of the coronally active binary II Pegasi (HD 224085), covering the wavelength range of 1.5È25For the Ðrst half of our 44 ks observation, the source was in Ó. a quiescent state with constant X-ray Ñux, after which it Ñared, reaching twice the quiescent Ñux in 12 ks, then decreased. We analyze the emission-line spectrum and continuum during quiescent and Ñaring states. The di †erential emission measure derived from lines Ñuxes shows a hot corona with a continuous distribution in temperature. During the nonÑare state, the distribution peaks near log T \ 7.2, and when Ñaring, it peaks near 7.6. High-temperature lines are enhanced slightly during the Ñare, but most of the change occurs in the continuum. Coronal abundance anomalies are apparent, with iron very deÐcient relative to oxygen and signiÐcantly weaker than expected from photospheric measurements, while neon is enhanced relative to oxygen. We Ðnd no evidence of appreciable resonant scattering optical depth in line ratios of iron and oxygen. The Ñare light curve is consistent with solar two-ribbon Ñare models but with a very long reconnection time constant of about 65 ks. We infer loop lengths of about 0.05 to about 0.25 stellar radii in the Ñare, if the Ñare emission originated from a single, low-density loop.

Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique opportunity to study accretion onto a neutron star from clumpy winds of O/B stars and to disentangle the complex accretion geometry of these systems. In Chandra-HETGS spectroscopy at orbital phase ∼0.25, when our line of sight towards the source does not pass through the large-scale accretion structure such as the accretion wake, we observe changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in overall absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars. We detect line features from high and low ionization species of silicon, magnesium and neon whose strengths and presence depend on the overall level of absorption. They imply a co-existence of cool and hot gas phases in the system that we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.

We investigate the structure of the wind in the neutron star X-ray binary system Vela X-1 by analyzing its flaring behavior. Vela X-1 shows constant flaring, with some flares reaching fluxes of more than 3.0 Crab between 20−60 keV for several 100 s, while the average flux is around 250 mCrab. We analyzed all archival INTEGRAL data, calculating the brightness distribution in the 20−60 keV band, which, as we show, closely follows a log-normal distribution. Orbital resolved analysis shows that the structure is strongly variable, explainable by shocks and a fluctuating accretion wake. Analysis of RXTE ASM data suggests a strong orbital change of N H . Accreted clump masses derived from the INTEGRAL data are on the order of 5×10 19 −10 21 g. We show that the lightcurve can be described with a model of multiplicative random numbers. In the course of the simulation we calculate the power spectral density of the system in the 20−100 keV energy band and show that it follows a red-noise power law. We suggest that a mixture of a clumpy wind, shocks, and turbulence can explain the measured mass distribution. As the recently discovered class of supergiant fast X-ray transients (SFXT) seems to show the same parameters for the wind, the link between persistent HMXB like Vela X-1 and SFXT is further strengthened.

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