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 .

We present an analysis of a Chandra ACIS-S observation of the elliptical galaxy NGC 720, to verify the existence of a dark matter halo and to measure its ellipticity. The ACIS-S3 image reveals over 60 point sources distributed throughout the field, most of which were undetected and therefore unaccounted for in previous X-ray studies. For semimajor axes ad150 00 (18:2 h À1 70 kpc), the ellipticity of the diffuse X-ray emission is consistent with a constant value, X % 0:15, which is systematically less than the values of 0.2-0.3 obtained from previous ROSAT PSPC and HRI observations because of the unresolved point sources contaminating the ROSAT values. The Chandra data confirm the magnitude of the $20 position angle (P.A.) twist discovered by ROSAT over this region. However, the twist in the Chandra data is more gradual and occurs at smaller a, also because of the point sources contaminating the ROSAT values. For ae150 00 out to a ¼ 185 00 (22:4 h À1 70 kpc), which is near the edge of the S3 CCD, X and P.A. diverge from their values at smaller a. Possible origins of this behavior at the largest a are discussed. Overall, the ellipticities and P.A. twist for ad150 00 can be explained by the triaxial mass model of NGC 720 published by Romanowsky & Kochanek (which could not produce the abrupt P.A. twist in the ROSAT HRI data). Since the optical image displays no substantial isophote twisting, the X-ray P.A. twist requires a massive dark matter halo if the hot gas is in hydrostatic equilibrium. Furthermore, the values of X obtained by Chandra are too large to be explained if the gravitating mass follows the optical light (M / L Ã ), irrespective of the P.A. twist. The M / L Ã hypothesis is inconsistent with the Chandra ellipticities at the 96% confidence level, assuming oblate symmetry, and at the 98% confidence level for prolate symmetry. Thus, both the P.A. twist and the ellipticities of the Chandra image imply the existence of dark matter, independent of the temperature profile of the gas. This geometric evidence for dark matter cannot be explained by alternative gravity theories, such as the modification of Newtonian dynamics (MOND). To constrain the ellipticity of the dark matter halo, we considered both oblate and prolate spheroidal mass models to bracket the full range of (projected) ellipticities of a triaxial ellipsoid. The dark matter density model, / ða 2 s þ a 2 Þ À1 , provides the best fit to the data and gives ellipticities and 1 errors of ¼ 0:37 AE 0:03 for oblate and ¼ 0:36 AE 0:02 for prolate models. Navarro-FrenkWhite (NFW) and Hernquist models give similar ellipticities for the dark matter. These moderate ellipticities for the dark halo are inconsistent with both the nearly spherical halos predicted if the dark matter is selfinteracting and the highly flattened halos predicted if the dark matter is cold molecular gas. These ellipticities may also be too large to be explained by warm dark matter, but they are consistent with galaxy-sized halos formed in the currently popular ÃCDM paradigm.

We present observations of SS 433 using the Chandra High Energy Transmission Grating Spectrometer. Many emission lines of highly ionized elements are detected with the relativistic blue and red Doppler shifts. The lines are measurably broadened to 1700 km s −1 (FWHM) and the widths do not depend significantly on the characteristic emission temperature, suggesting that the emission occurs in a freely expanding region of constant collimation with opening angle of 1.23 ± 0.06 • . The blue shifts of lines from low temperature gas are the same as those of high temperature gas within our uncertainties, again indicating that the hottest gas we observe to emit emission lines is already at terminal velocity. This velocity is 0.2699 ±0.0007c, which is larger than the velocity inferred from optical emission lines by 2920 ± 440 km s −1 . Fits to the emission line fluxes give a range of temperatures in the jet from 5 × 10 6 to 1 × 10 8 K. We derive the emission measure as a function of temperature for a four component model that fits the line flux data. Using the density sensitive Si XIII triplet, the characteristic electron density is 10 14 cm −3 where the gas temperature is about 1.3 × 10 7 K. Based on an adiabatic expansion model of the jet and a distance of 4.85 kpc, the electron densities drop from ∼ 2 × 10 15 to 4 × 10 13 cm −3 at distances of 2 − 20 × 10 10 cm from the apex of the cone that bounds the flow. The radius of the base of the visible jet is estimated to be ∼ 10 8 cm and the mass outflow rate is 1.5×10 −7 M ⊙ yr −1 . The kinetic power is 3.2 × 10 38 erg s −1 , which is ×1000 larger than the unabsorbed 2-10 keV X-ray luminosity. The bremsstrahlung emission associated with the lines can account for the entire continuum; we see no direct evidence for an accretion disk. The image from zeroth order shows extended emission at a scale of ∼2 ′′ , aligned in the general direction of the radio jets.

We present detailed evidence for a warm absorber in the Seyfert 1 galaxy MCG-6-30-15 and dispute earlier claims for relativistic O line emission. The HETG spectra show numerous narrow, unresolved (FWHM ∼ < 200 km s −1 ) absorption lines from a wide range of ionization states of N, O, Mg, Ne, Si, S, Ar, and Fe. The O VII edge and 1s 2 − 1snp resonance line series to n = 9 are clearly detected at rest in the AGN frame. We attribute previous reports of an apparently highly redshifted O VII edge to the 1s 2 − 1snp (n > 5) O VII resonance lines, and a neutral Fe L absorption complex. The shape of the Fe L feature is nearly identical to that seen in the spectra of several X-ray binaries, and in laboratory data. The implied dust column density agrees with that obtained from reddening studies, and gives the first direct X-ray evidence for dust embedded in a warm absorber. The O VIII resonance lines and weak edge are also detected, and the spectral rollover below ∼ 2 keV is explained by the superposition of numerous absorption lines and edges. We identify, for the first time, a KLL resonance in the O VI photoabsorption cross section, giving a measure of the O VI column density. The O VII (f) emission detected at the systemic velocity implies a covering fraction of ∼ 5% (depending on the observed vs. time-averaged ionizing flux). Our observations show that a dusty warm absorber model is not only adequate to explain all the spectral features ∼ > 0.48 keV ( ∼ < 26 Å) the data require it. This contradicts the interpretation of Branduardi-Raymont et al. (2001) that this spectral region is dominated by highly relativistic line emission from the vicinity of the black hole.