Abstract:  Low-mass pre-main sequence (PMS) stars are typically hundreds of times more luminous than main sequence stars in the X-ray band.  Imaging, spectral and variability observations of PMS stars in nearby star forming regions are reviewed with emphasis on a recent Chandra study of the Orion Nebula Cluster.  The enhanced X-ray emission is attributable in most respects to solar-type surface magnetic activity resulting from dynamo processes in the interior where plasma is violently heated to high temperatures in magnetic reconnection events.  However, reconnection in fields extending from the star to the circumstellar disk is possible, particularly in protostars.

Abstract:  We report the detection with Chandra of a Low-Luminosity AGN (LLAGN) in the Low Ionization Emission Line Region (LINER) hosted by Hydra A, a nearby (z=0.0537) powerful FR I radio galaxy with complex radio and optical morphology. In a 20 ks ACIS-S exposure during the calibration phase of the instrument, a point source is detected at energies  2 keV at the position of the compact radio core, embedded in diffuse thermal X-ray emission ( kT ~ 1 keV) at softer energies. The spectrum of the point source is well fitted by a heavily absorbed power law with intrinsic column density N ^int _H  ~ 3 X 10²² cm-2  and photon index  . The intrinsic (absorption-corrected) luminosity is erg s-1. These results provide strong evidence that an obscured AGN is present in the nuclear region of Hydra A. We infer that the optical/UV emission of the AGN is mostly hidden by the heavy intrinsic reddening. In order to balance the photon budget of the nebula, we must either postulate that the ionizing spectrum includes a UV bump or invoke and additional power source (shocks in the cooling flow or interaction with the radio jets). Using an indirect estimate of the black hole mass and the X-ray luminosity, we infer that the accretion rate is low, suggesting that the accretion flow is advection dominated.  Finally, our results support current unification schemes for radio-loud sources, in particular the presence of the putative molecular torus in FR Is. These observations underscore the power of the X-rays and of Chandra in the quest for black holes.

Abstract -  The ACIS front-illuminated CCDs onboard the Chandra X-ray Observatory were damaged in the extreme environment of the Earth's radiation belts, resulting in enhanced charge transfer inefficienty (CTI).  This produces a row dependence in gain, event grade, and energy resolution.  We model the CTI as a function of input photon energy, including the effects of de-trapping (charge trailing), shielding within an event (charge in the leading pixels of the 3x3 event island protect the rest of the island by filling traps), and non-uniform spatial distribution of traps.  This technique cannot fully rrecover the degraded energy resolution, but it reduces the position dependence of gain and grade distributions.  By correcting the grade distributions as well as the event amplitudes, we can improve the instrument's quantum efficiency.  We outline our model for CTI correction and discuss how the corrector can improve astrophysical results derived from ACIS data.

Abstract - We present preliminary results from observations of supernova remnants by the Chandra X-ray Observatory (CXO).  The data include imaging spectroscopy from objects observed with both GTO and GO data.  The high spatial resolution of Chandra is revealing a wealth of small-scale structure in these remnants.  Specifically, we have resolved the remnant of SN1987A, and have discovered fine-scale structure in N103B and G292.0+1.8.

Abstract - We present high resolution Chandra observations of the remnant of SN1987A in the Large Magellanic Cloud.  The high angular resolution of the Chandra X-ray Observatory (CXO) permits us to resolve the X-ray remnant.  We find that the remnant is shell-like in morphology, with X-ray peaks associated with some of the optical hot spots seen in HST images.  The X-ray curve appears to be departing from the linear flux increase observed by ROSAT.  We set an upper limit of 1.5x10³⁴ erg/s on the luminosity of any embedded central source (0.5 - 2 keV).  We also present a high resolution spectrum, showing that the X-ray emission is thermal in origin and is dominated by highly ionized species of O, Ne, Mg, and Si.  The line profile is consistent with a blast wave velocity of about 4000 km/s.

Abstract - We present first results from an X-ray study of the Hubble Deep Field North (HDF-N) and its environs obtained using 166 ks of data collected by the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-ray Observatory. This is the deepest X-ray observation ever reported, and in the HDF-N itself we detect six X-ray sources down to a 0.5--8 keV flux limit of 4E-16 erg cm^-2 s^-1. Comparing these sources with objects seen in multiwavelength HDF-N studies shows positional coincidences with the extremely red object NICMOS J123651.74 +621221.4, an active galactic nucleus (AGN), three elliptical galaxies, and one nearby spiral galaxy. The X-ray emission from the ellipticals is consistent with that expected from a hot interstellar medium, and the spiral galaxy emission may arise from a  `super-Eddington' X-ray binary or ultraluminous supernova remnant. Four of the X-ray sources have been detected at radio wavelengths. We also place X-ray upper limits on AGN candidates found in the HDF-N, and we present the tightest constraints yet on X-ray emission from the SCUBA submillimeter source population. None of the 10 high-significance submillimeter sources reported in the HDF-N and its vicinity is detected with Chandra ACIS. These sources appear to be dominated by star formation or have AGN with Compton-thick tori and little circumnuclear X-ray scattering.

Abstract - We present the first grating-resolution X-ray spectra of the Seyfert 1 galaxy NGC 3783, obtained with the High Energy Transmission Grating Spectrometer on the Chandra X-ray Observatory. These spectra reveal many narrow absorption lines from the H-like and He-like ions of O, Ne, Mg, Si, S and Ar, as  well as FeXVII-FeXXI L-shell lines. We have also identified several weak emission lines, mainly from O and Ne. The absorption lines are blueshifted by a mean velocity of approximately 440\pm200 km/s and are not resolved, indicating a velocity dispersion within the absorbing gas of a few hundred km/s or less. We measure the lines' equivalent widths and compare them with the predictions of photoionization models. The best-fitting model has a microturbulence velocity of 150 km/s and a hydrogen column density of 1.3\times10^22 cm^-2. The  measured blueshifts and inferred velocity dispersions of the X-ray absorption lines are consistent with those of the strongest UV absorption lines observed in this object. However, simple models that propose to strictly unify the X-ray and UV absorbers have difficulty explaining simultaneously the X-ray and UV absorption line strengths.