ABSTRACT. The Chandra X-Ray Observatory (CXO), the X-ray component of NASA's Great Observatories, was launched on 1999 July 23 by the space shuttle Columbia. After satellite systems activation, the first X-rays focused by the telescope were observed on 1999 August 12. Beginning with the initial observation it was clear that the telescope had survived the launch environment and was operating as expected. Despite an initial surprise due to the discovery that the telescope was far more efficient for concentrating CCD-damaging low-energy protons than had been anticipated, the observatory is performing well and is returning superb scientific data. Together with other space observatories, most notably XMM-Newton, it is clear that we have entered a new era of discovery in high-energy astrophysics.

We present the first high resolution X-ray image of the jet in M 87 using the Chandra X-ray Observatory. There is clear structure in the jet and almost all of the optically bright knots are detected individually. The unresolved core is the brightest X-ray feature but is only 2-3 times brighter than knot A (12.3 ′′ from the core) and the inner knot HST-1 (1.0 ′′ from the core). The X-ray and optical positions of the knots are consistent at the 0.1 ′′ level but the X-ray emission from the brightest knot (A) is marginally upstream of the optical emission peak. Detailed Gaussian fits to the X-ray jet one-dimensional profile show distinct X-ray emission that is not associated with specific optical features. The Xray/optical flux ratio decreases systematically from the core and X-ray emission is not clearly detected beyond 20 ′′ from the core. The X-ray spectra of the core and the two brightest knots, HST-1 and A1, are consistent with a simple power law (S ν ∝ ν −α ) with α = 1.46 ± 0.05, practically ruling out inverse Compton models as the dominant X-ray emission mechanism. The core flux is significantly larger than expected from an advective accretion flow and the spectrum is much steeper, indicating that the core emission may be due to synchrotron emission from a small scale jet. The spectral energy distributions (SEDs) of the knots are well fit by synchrotron models. The spectral indices in the X-ray band, however

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We describe (1) a new test for dark matter and alternate theories of gravitation based on the relative geometries of the X-ray and optical surface brightness distributions and an assumed form for the potential of the optical light, (2) a technique to measure the shapes of the total gravitating matter and dark matter of an ellipsoidal system which is insensitive to the precise value of the temperature of the gas and to modest temperature gradients, and (3) a method to determine the ratio of dark mass to stellar mass that is dependent on the functional forms for the visible star, gas and dark matter distributions, but independent of the distance to the galaxy or the gas temperature.We apply these techniques to X-ray data from the ROSAT Position Sensitive Proportional Counter (PSPC) of the optically-attened elliptical galaxy NGC 720; the optical isophotes have ellipticity 0:40 extending out to 120 00 (10 00 1 kpc assuming a distance of 20h 80 Mpc). The X-ray isophotes are signi cantly elongated, = 0:20 0:30 (90% con dence) for semi-major axis a 100 00 . The major axes of the 1 dbuote@space.mit.edu 2 crc@space.mit.edu { 2 { discuss the viability of this assumption for NGC 720. Milgrom's Modi cation of Newtonian Dynamics (MOND) cannot dispel this manifestation of dark matter. Hence, geometrical considerations, which are essentially independent of gas pressure or temperature, require the presence of an extended, massive dark matter halo in NGC 720.Employing essentially the technique of Buote & Canizares (1992; Buote 1992) we use the shape of the X-ray surface brightness to constrain the shape of the total gravitating matter. The total matter is modeled as either an oblate or prolate spheroid of constant shape and orientation having either a Ferrers ( r n ) or Hernquist density.Assuming the X-ray gas is in hydrostatic equilibrium, we construct a model X-ray gas distribution for various temperature pro les; i.e. isothermal, linear, and polytropic. We determine the ellipticity of the total gravitating matter to be 0:50 0:70. Using the single-temperature model we estimate a total mass (0:41 1:4) 10 12 h 80 M interior to the ellipsoid of semi-major axis 43:6h 80 kpc. Ferrers densities as steep as r 3 do not t the data, but the r 2 and Hernquist models yield excellent ts. We estimate the mass distributions of the stars and the gas and t the dark matter directly. For a given temperature pro le of the gas and functional forms for the visible stars, gas, and dark matter, these models yield a distance-independent and temperature-independent measurement of the ratio of dark mass to stellar mass M DM =M stars . We estimate at minimum M DM =M stars 4 which corresponds to a total mass slightly greater than that derived from the single-temperature models for distance D = 20h 80 Mpc.{ 4 { of the gas. In addition, by exploiting the relative geometries of the X-ray and optical isophotes (and an assumed model for the potential of the optical light) we introduce a test for dark matter and alternate gravity theories that is highly insensit...