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International audience; X-ray emission from solar wind charge exchange (SWCX) produced in interplanetary space contaminates every astrophysical observation, regardless of the line of sight. Unfortunately, the primary SWCX emission lines also happen to be important diagnostics of astrophysical plasmas. Models of SWCX emission are limited by two main uncertainties: the local solar wind fluxes along the line of sight and the charge exchange cross sections. The He cone, a localized density enhancement of helium neutrals, is the only heliospheric SWCX emission feature that is small enough and bright enough to be observationally isolated from the X-ray background and the broader SWCX emission. HaloSat, an X-ray CubeSat mission, has recently completed two series of specialized observations, near and far from the ecliptic plane, during two Earth transits of the He cone. These observations were used to test the predictions of an SWCX emission model against the emission observed at low ecliptic latitude, where the solar wind data are monitored, and at high ecliptic latitude, where the solar wind data are extrapolated. The measured SWCX emission for the set of observations near the ecliptic plane was consistent with the line intensities predicted by the model but underpredicted for the set of observations at high ecliptic latitude near the south ecliptic pole. Additionally, high-temperature Galactic halo emission components are reported for both spectral sets.

Fabrication of boron carbide (B4C) parts through direct ink writing at room temperature has been demonstrated. The 3D printed B4C parts were made from aqueous, thixotropic ink consisting of B4C particles with a solid loading ranging from 50.0 to 59.3 vol%. The porous B4C parts were infiltrated with molten aluminum (Al) to form a dense B4C-Al cermet. Simple cubic samples with varied spacing between B4C filaments were printed to tailor the density profile within the part. The Rockwell hardness of such parts ranges from 20 to 90 Ra (60 kg) depending on the overall density of the B4C structure. Parts were printed with variable spacings between filaments to systematically determine the upper limit for Al infiltration (800 μm). Elemental analysis revealed a homogenous infiltration both between and within the filaments of 3D printed B4C. Keywords: Additive manufacturing, Boron carbide, Cermet, Direct ink writing

Complex shaped boron carbide with carbon (B4C/C) at near-full densities were achieved for the first time using negative additive manufacturing techniques via gelcasting. Negative additive manufacturing involves 3D printing of sacrificial molds used for casting negative copies. B4C powder distributions and rheology of suspensions were optimized to successfully cast complex shapes. In addition to demonstrating scalability of these complex geometries, hierarchically meso-porous structures were also shown to be possible from this technique. Resorcinol-Formaldehyde (RF) polymer was selected as the gelling agent and can also pyrolyze into a carbon aerogel network to act as the sintering aid for B4C. Due to the highly effective distribution of in situ carbon for the B4C matrix, near-full sintered density of 97–98% of theoretical maximum density was achieved. Keywords: Boron carbide, Gelcasting, Negative additive manufacturing, Resorcinol formaldehyde, Sintering

One of the major and unfortunately unforeseen sources of background for the current generation of X-ray telescopes are few tens to hundreds of keV (soft) protons concentrated by the mirrors. One such telescope is the European Space Agency's (ESA) X-ray Multi-Mirror Mission (XMM-Newton). Its observing time lost due to background contamination is about 40\%. This loss of observing time affects all the major broad science goals of this observatory, ranging from cosmology to astrophysics of neutron stars and black holes. The soft proton background could dramatically impact future large X-ray missions such as the ESA planned Athena mission (http://www.the-athena-x-ray-observatory.eu/). Physical processes that trigger this background are still poorly understood. We use a Machine Learning (ML) approach to delineate related important parameters and to develop a model to predict the background contamination using 12 years of XMM observations. As predictors we use the location of satellite, solar and geomagnetic activity parameters. We revealed that the contamination is most strongly related to the distance in southern direction, $Z$, (XMM observations were in the southern hemisphere), the solar wind radial velocity and the location on the magnetospheric magnetic field lines. We derived simple empirical models for the first two individual predictors and an ML model which utilizes an ensemble of the predictors (Extra Trees Regressor) and gives better performance. Based on our analysis, future missions should minimize observations during times associated with high solar wind speed and avoid closed magnetic field lines, especially at the dusk flank region in the southern hemisphere., Comment: 20 pages, 11 figures

HaloSat is a small satellite (CubeSat) designed to map soft X-ray oxygen line emission across the sky in order to constrain the mass and spatial distribution of hot gas in the Milky Way. The goal of HaloSat is to help determine if hot gas gravitationally bound to individual galaxies makes a significant contribution to the cosmological baryon budget. HaloSat was deployed from the International Space Station in July 2018 and began routine science operations in October 2018. We describe the goals and design of the mission, the on-orbit performance of the science instrument, and initial observations., Comment: to appear in the Astrophysical Journal, 11 pages corrected arXiv author list

DXL (Diffuse X-rays from the Local Galaxy) is a sounding rocket mission designed to quantify and characterize the contribution of Solar Wind Charge eXchange (SWCX) to the Diffuse X-ray Background and study the properties of the Local Hot Bubble (LHB). Based on the results from the DXL mission, we quantified and removed the contribution of SWCX to the diffuse X-ray background measured by the ROSAT All Sky Survey (RASS). The "cleaned" maps were used to investigate the physical properties of the LHB. Assuming thermal ionization equilibrium, we measured a highly uniform temperature distributed around kT=0.097 keV+/-0.013 keV (FWHM)+/-0.006 keV (systematic). We also generated a thermal emission measure map and used it to characterize the three-dimensional (3D) structure of the LHB which we found to be in good agreement with the structure of the local cavity measured from dust and gas., Comment: 8 pages, 10 figures, Accepted for publication on ApJ

Most models identify the X-ray bright North Polar Spur (NPS) with a hot interstellar (IS) bubble in the Sco-Cen star-forming region at $\simeq$130 pc. An opposite view considers the NPS as a distant structure associated with Galactic nuclear outflows. Constraints on the NPS distance can be obtained by comparing the foreground IS gas column inferred from X-ray absorption to the distribution of gas and dust along the line of sight. Absorbing columns towards shadowing molecular clouds simultaneously constrain the CO-H$_{2}$ conversion factor. We derived the columns of X-ray absorbing matter NH(abs) from spectral fitting of dedicated XMM-Newton observations towards the NPS southern terminus (l=29{\deg}, b=+5 to +11{\deg}). The IS matter distribution was obtained from absorption lines in stellar spectra, 3D dust maps and emission data, including high spatial resolution CO measurements recorded for this purpose. NH(abs) varies from $\simeq$ 4.3 to $\simeq$ 1.3 x 10$^{21}$ cm$^{-2}$ along the 19 fields. Relationships between X-ray brightness, absorbing column and hardness ratio demonstrate a brightness decrease with latitude governed by increasing absorption. The comparison with absorption data, local and large-scale dust maps rules out a NPS near side closer than 300 pc. The correlation between NH(abs) and the reddening increases with the sightline length from 300 pc to 4 kpc and is the tightest with Planck $\tau_{353}$-based reddening, suggesting a much larger distance. N(H)/E(B-V) $\simeq$ 4.1 x 10$^{21}$ cm$^{-2}$ mag$^{-1}$. NH(abs) absolute values are compatible with HI-CO clouds at -5 $\leq$ V(LSR) $\leq$ +25 to +45 km s$^{-1}$ and a NPS potentially far beyond the Local Arm. A molecular cloud shadow at b=+9deg constrains X$_{CO}$ to $\leq$ 1.0 x 10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s. The average X$_{CO}$ is $\leq$ 0.75 x 10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s., Comment: 15 pages, 26 figures, accepted for publication in Astronomy & Astrophysics

International audience; The Local Leo Cold Cloud (LLCC, at a distance of 11–24 pc) was studied in its relation to the Local Hot Bubble (LHB) and the result suggested that much of the observed $1/4$ keV emission in that direction originates in front of the cloud. This placed a strong constraint on the distribution of X-ray emission within the LHB and called into question the assumption of a uniform distribution of X-ray emitting plasma within the Local Cavity. However, recent work has quantified the contribution of heliospheric solar wind charge exchange (SWCX) emission to the diffuse X-ray background measured by the ROSAT All-Sky Survey (RASS) at $1/4$ keV, and led to the consistency of pressure measurements between the LHB and the local cloud component of the complex of local interstellar clouds (CLICs) surrounding the Sun. In this paper we revisit the LLCC and improve the previous analysis by using higher resolution RASS data, a serendipitous ROSAT pointed observation, a rigorous treatment of the band-averaged X-ray absorption cross section, and models for the heliospheric and magnetospheric SWCX contributions. We find that the foreground emission to the cloud is in excess of the expected heliospheric (interplanetary plus near Earth) SWCX contribution but that it is marginally consistent with the range of possible LHB plasma path lengths between the LLCC and the CLICs given the currently understood plasma emissivity.

We present an optical/NIR imaging survey of the face-on spiral galaxy M83, using data from the Hubble Space Telescope Wide Field Camera 3 (WFC3). Seven fields are used to cover a large fraction of the inner disk, with observations in nine broadband and narrowband filters. In conjunction with a deep Chandra survey and other new radio and optical ground-based work, these data enable a broad range of science projects to be pursued. We provide an overview of the WFC3 data and processing and then delve into one topic, the population of young supernova remnants. We used a search method targeted toward soft X-ray sources to identify 26 new supernova remnants. Many compact emission nebulae detected in [Fe II] 1.644 micron align with known remnants and this diagnostic has also been used to identify many new remnants, some of which are hard to find with optical images. We include 37 previously identified supernova remnants that the data reveal to be, 39 pages, 8 figures. accepted for ApJ

Microlattices make marvelous materials Framework or lattice structures can be remarkably strong despite their very low density. Using a very precise technique known as projection microstereolithography, Zheng et al. fabricated octet microlattices from polymers, metals, and ceramics. The design of the lattices meant that the individual struts making up the materials did not bend under pressure. The materials were therefore exceptionally stiff, strong, and lightweight. Science , this issue p. 1373

We present evidence for the impact of a supernova blast‐wave on a molecular cloud located inside the Local Bubble, in the area of interaction with the Sco‐Cen OB association.The evidence comes from the analysis of the line of sight toward the nearby (170 pc) star HD102065, located behind the tail of a cometary‐shaped, infrared cirrus‐cloud which we believe is only 70 pc away from the Sun. The analysis includes numerous transitions of atoms, ions and molecules, in absorption and in emission, in the UV, the optical and the sub‐millimeter.Several peculiar characteristics of the cloud like the spatial structure of its cold phase, the high negative velocities, the high abundance of atoms in excited states and the highly ionized species associated with the highest velocities, as well as the unusually high abundance of small dust particles, are interpreted as the result of the interaction with a supernova shock wave.This line of sight provides an ideal laboratory to study turbulent flows and mixing layers in the ISM.We present evidence for the impact of a supernova blast‐wave on a molecular cloud located inside the Local Bubble, in the area of interaction with the Sco‐Cen OB association.The evidence comes from the analysis of the line of sight toward the nearby (170 pc) star HD102065, located behind the tail of a cometary‐shaped, infrared cirrus‐cloud which we believe is only 70 pc away from the Sun. The analysis includes numerous transitions of atoms, ions and molecules, in absorption and in emission, in the UV, the optical and the sub‐millimeter.Several peculiar characteristics of the cloud like the spatial structure of its cold phase, the high negative velocities, the high abundance of atoms in excited states and the highly ionized species associated with the highest velocities, as well as the unusually high abundance of small dust particles, are interpreted as the result of the interaction with a supernova shock wave.This line of sight provides an ideal laboratory to study turbulent flows and mixing layers in the...

International audience; Physical importance of the charge-exchange process in the interaction of the cold neutral cloud with surrounding hot fully ionized plasma is investigated. A non-stationary one-dimensional two-component model is presented. It is assumed in the model that the cloud consists of atomic hydrogen. Plasma is assumed to be quasi-neutral and magnetic fields are ignored. We demonstrate distribution of gas-dynamic parameters for plasma and neutral components in the interaction region and investigate time evolution of the interaction interface from the beginning of the interaction between neutral gas and plasma to very large time-intervals.Charge-exchange of atoms with heavy ions is a possible source of the X-ray emission. The charge-exchange rate as function of distance and time is shown. This rate being scaled with the relative abundances of minor heavy ions and with corresponding ion-atom cross-sections can be used to estimate X-ray emissivity for different astrophysical objects

International audience; Three recent results related to the heliosphere and the local interstellar medium (ISM) have provided an improved insight into the distribution and conditions of material in the solar neighborhood. These are the measurement of the magnetic field outside of the heliosphere by Voyager 1, the improved mapping of the three-dimensional structure of neutral material surrounding the Local Cavity using extensive ISM absorption line and reddening data, and a sounding rocket flight which observed the heliospheric helium focusing cone in X-rays and provided a robust estimate of the contribution of solar wind charge exchange emission to the ROSAT All-Sky Survey 1/4 keV band data. Combining these disparate results, we show that the thermal pressure of the plasma in the Local Hot Bubble (LHB) is P/k = 10, 700 cm-3 K. If the LHB is relatively free of a global magnetic field, it can easily be in pressure (thermal plus magnetic field) equilibrium with the local interstellar clouds, eliminating a long-standing discrepancy in models of the local ISM.

The objective of the Di use X-ray emission from the Local Galaxy (DXL) sounding rocket experiment is todistinguish the soft X-ray emission due to the Local Hot Bubble (LHB) from that produced via Solar WindCharge eXchange (SWCX). Enhanced interplanetary helium density in the helium focusing cone provides aspatial variation to the SWCX that can be identi ed by scanning through the focusing cone using an X-rayinstrument with a large grasp. DXL consists of two large proportional counters refurbished from the Aerobeepayload used during the Wisconsin All Sky Survey. The counters utilize P-10 ll gas and are covered by a thinFormvar window (with Cyasorb UV-24 additive) supported on a nickel mesh. DXL's large grasp is 10 cm 2 sr forboth the 14 and 34 keV bands. DXL was successfully launched from White Sands Missile Range, New Mexico onDecember 12, 2012 using a Terrier Mk70 Black Brant IX sounding rocket.The Sheath Transport Observer for the Redistribution of Mass (STORM) instrument is a prototype softX-ray camera also successfully own on the DXL sounding rocket. STORM uses newly developed slumpedmicropore (`lobster eye') optics to focus X-rays onto a position sensitive, chevron con guration, microchannelplate detector. The slumped micropore optics have a 75 cm curvature radius and a polyimide/aluminum lterbonded to its surface. STORM's large eld-of-view makes it ideal for imaging SWCX with exospheric hydrogenfor future missions. STORM represents the rst ight of lobster-eye optics in space.Keywords: solar wind charge exchange| local hot bubble | proportional counter | sounding rocket

SN1957D, located in one of the spiral arms of M83, is one of the small number of extragalactic supernovae that has remained detectable at radio and optical wavelengths during the decades after its explosion. Here we report the first detection of SN1957D in X-rays, as part of a 729 ks observation of M83 with \chandra. The X-ray luminosity (0.3 - 8 keV) is 1.7 (+2.4,-0.3) 10**37 ergs/s. The spectrum is hard and highly self-absorbed compared to most sources in M83 and to other young supernova remnants, suggesting that the system is dominated at X-ray wavelengths by an energetic pulsar and its pulsar wind nebula. The high column density may be due to absorption within the SN ejecta. HST WFC3 images resolve the supernova remnant from the surrounding emission and the local star field. Photometry of stars around SN1957D, using WFC3 images, indicates an age of less than 10**7 years and a main sequence turnoff mass more than 17 solar masses. New spectra obtained with Gemini-South show that the optical spectrum continues to be dominated by broad [O III] emission lines, the signature of fast-moving SN ejecta. The width of the broad lines has remained about 2700 km/s (FWHM). The [O III] flux dropped precipitously between 1989 and 1991, but continued monitoring shows the flux has been almost constant since. In contrast, radio observations over the period 1990-2011 show a decline rate inf the flux proportional to t**-4, far steeper than the rate observed earlier, suggesting that the primary shock has overrun the edge of a pre-SN wind., 28 pages, including 3 tables and 7 figures, accepted for publication in ApJ

International audience; Mixing and aeration characteristics obtained on three full scale loop reactors for 28 configurations (different mixers' location and rotation speed, air flow rates and diffuser grids' location) were analyzed. The propellers' thrust was measured in situ. Observed values vary with the tank geometry, the location of the mixers and the induced axial velocity. In situ thrust values are lower than the nominal ones obtained during shop tests, by 10 to 30 %. For the nominal rotation speed of the propellers, observed thrust-to-power ratios are comprised between 685 and 875 N kW-1. In agreement with previous results, an increase in the oxygen transfer rate with the axial velocity was observed, due to a partial neutralization of the spiral flows formed at the edge of the aerated zones. Aeration efficiency is not decreased, as the additional power consumption due to mixing is offset by the oxygen transfer enhancement. SAE values are comprised between 2.0 and 2.6 kg O2 kWh-1. Such on site results are especially important to be used as input and output data in CFD models.

The Diffuse X-rays from the Local galaxy (DXL) mission is an approved sounding rocket project with a first launch scheduled around December 2012. Its goal is to identify and separate the X-ray emission generated by solar wind charge exchange from that of the local hot bubble to improve our understanding of both. With 1,000 cm2 proportional counters and grasp of about 10 cm2 sr both in the 1/4 and 3/4 keV bands, DXL will achieve in a 5-minute flight what cannot be achieved by current and future X-ray satellites., Comment: 15 Pages, 5 figures. Accepted for publication on Experimental Astronomy

M33 contains a large number of emission nebulae identified as supernova remnants (SNRs) based on the high [S II]:Ha ratios characteristic of shocked gas. Using Chandra data from the ChASeM33 survey with a 0.35-2 keV sensitivity of about 2 x 10**34 ergs/s, we have detected 82 of 137 SNR candidates, yielding confirmation of (or at least strongly support for) their SNR identifications. This provides the largest sample of remnants detected at optical and X-ray wavelengths in any galaxy, including the Milky Way. A spectral analysis of the seven X-ray brightest SNRs reveals that two, G98-31 and G98-35, have spectra that appear to indicate enrichment by ejecta from core-collapse supernova explosions. In general, the X-ray detected SNRs have soft X-ray spectra compared to the vast majority of sources detected along the line of sight to M33. It is unlikely that there are any other undiscovered thermally dominated X-ray SNRs with luminosities in excess of about 4 x 10**35 ergs/s in the portions of M33 covered by the ChASeM33 survey. We have used a combination of new and archival optical and radio observations to attempt to better understand why some objects are detected as X-ray sources and others are not. We have also developed a morphological classification scheme for the optically-identified SNRs, and discuss the efficacy of this scheme as a predictor of X-ray detectability. Finally, we have compared the SNRs found in M33 to those that have been observed in the Galaxy and the Magellanic Clouds. There are no close analogs of Cas A, Kepler's SNR, Tycho's SNR or the Crab Nebula in the regions of M33 surveyed, but we have found an X-ray source with a power law spectrum coincident with a small-diameter radio source that may be the first pulsar-wind nebula recognized in M33., 142 pages, including 8 talbes and 50 figures, to be published in ApJ Supplements

International audience; A model for heliospheric solar wind charge exchange (SWCX) X-ray emission is applied to a series of XMM-Newton observations of the interplanetary focusing cone of interstellar helium. The X-ray data are from three coupled observations of the South Ecliptic Pole (SEP; to observe the cone) and the Hubble Deep Field-North (HDF-N, to monitor global variations of the SWCX emission due to variations in the solar wind (SW)) from the period 2003 November 24 to December 15. There is good qualitative agreement between the model predictions and the data, after the SEP data are corrected using the HDF-N data, with the maximum SWCX flux observed at an ecliptic longitude of ∼72 • , consistent with the central longitude of the He cone. We observe a total excess of 2.1 ± 1.3 line unit (LU) in the O vii line and 2.0 ± 0.9 LU in the O viii line. However, the SWCX emission model, which was adjusted for SW conditions appropriate for late 2003, predicts an excess from the He cone of only 0.5 LU and 0.2 LU, respectively, in the O vii and O viii lines. We discuss the model to data comparison and provide possible explanations for the discrepancies. We also qualitatively reexamine our SWCX model predictions in the 1 4 keV band with data from the ROSAT All-Sky Survey toward the North Ecliptic Pole and SEP, when the He cone was probably first detected in soft X-rays.

We used the FUSE (Far Ultraviolet Spectroscopic Explorer) satellite to observe OVI emission along two sightlines towards the edge of the interaction zone (IZ) between the Loop I superbubble and the Local Bubble. One sightline was chosen because material in the interaction zone blocks distant X-ray emission, and should thus do the same for non-local OVI emission. We measured an OVI intensity of I_shadowed = 2750 +- 550 L.U. along this `Shadowed' sightline, and I_unshadowed = 10800 +- 1200 L.U. along the other sightline. Given these results, very little (< 800 L.U.) of the emission arises from the near side of the interaction zone, which likely has an HI column density of about 4e+20 cm-2 along the `Shadowed' sightline. The OVI emission arising within Loop I (~1e+4 L.U.) is probably associated with gas of n_e ~ 0.1 cm-3 and an emitting pathlength of ~1.2 pc, suggesting it arises at interfaces rather than from gas filling Loop I. In contrast, the CIII emission is similar along both sightlines, indicating that much of the emission likely arises on the near side of the interaction zone., 29 pages, 3 EPS figures, 2 tables. To appear in ApJ 682, 2008 July 20

International audience; We report the results of a search for O VI absorption in the spectra of ~100 hot DA white dwarfs observed by the FUSE satellite. We have carried out a detailed analysis of the radial velocities of interstellar and (where present) stellar absorption lines for the entire sample of stars. In many cases, the velocity differences between the interstellar and photospheric components are below the resolution of the FUSE spectrographs. However, in a significant number of cases the interstellar and photospheric contributions can be separated. Within this group, for those stars where O VI is positively detected, it is possible to unambiguously determine the nature of the O VI component (interstellar or photospheric). In the majority of stars where we find O VI absorption lines, the material is clearly associated with the stellar photosphere and not the interstellar medium. There are a number of lines-of-sight where the gas is interstellar in nature. Mostly these stars are located beyond the boundaries of the local cavity or are at high galactic latitude. Hence, it appears that the Local Bubble contains little hot gas. That component responsible for the O VI absorption may reside at the interface between the cavity and surrounding medium or in that medium itself and is also associated with the soft X-ray background

Aims: We present a uniform catalog of the images and radial profiles of the temperature, abundance, and brightness for 70 clusters of galaxies observed by XMM-Newton. Methods: We use a new "first principles" approach to the modeling and removal of the background components; the quiescent particle background, the cosmic diffuse emission, the soft proton contamination, and the solar wind charge exchange emission. Each of the background components demonstrate significant spectral variability, several have spatial distributions that are not described by the photon vignetting function, and all except for the cosmic diffuse emission are temporally variable. Because these backgrounds strongly affect the analysis of low surface brightness objects, we provide a detailed description our methods of identification, characterization, and removal. Results: We have applied these methods to a large collection of XMM-Newton observations of clusters of galaxies and present the resulting catalog. We find significant systematic differences between the Chandra and XMM-Newton temperatures., Accepted for publication in A&A, 55 pages with 42 figures

We present an overview of the Chandra ACIS Survey of M33 (ChASeM33): A Deep Survey of the Nearest Face-on Spiral Galaxy. The 1.4 Ms survey covers the galaxy out to $R \approx 18\arcmin (\approx 4$ kpc). These data provide the most intensive, high spatial resolution assessment of the X-ray source populations available for the confused inner regions of M33. Mosaic images of the ChASeM33 observations show several hundred individual X-ray sources as well as soft diffuse emission from the hot interstellar medium. Bright, extended emission surrounds the nucleus and is also seen from the giant \hii regions NGC 604 and IC 131. Fainter extended emission and numerous individual sources appear to trace the inner spiral structure. The initial source catalog, arising from $\sim$~2/3 of the expected survey data, includes 394 sources significant at the $3\sigma$ confidence level or greater, down to a limiting luminosity (absorbed) of $\sim$1.6\ergs{35} (0.35 -- 8.0 keV). The hardness ratios of the sources separate those with soft, thermal spectra such as supernova remnants from those with hard, non-thermal spectra such as X-ray binaries and background active galactic nuclei. Emission extended beyond the Chandra point spread function is evident in 23 of the 394 sources. Cross-correlation of the ChASeM33 sources against previous catalogs of X-ray sources in M33 results in matches for the vast majority of the brighter sources and shows 28 ChASeM33 sources within 10\arcsec of supernova remnants identified by prior optical and radio searches. This brings the total number of such associations to 31 out of 100 known supernova remnants in M33., Comment: accepted for publication ApJS, full resolution images and complete tables available at http://hea-www.harvard.edu/vlp_m33_public/

We report the discovery of an eclipsing X-ray binary in M101, the first such system to be discovered outside the Local Group. Based on a sequence of 25 Chandra observations that sample a wide range of orbital phases, we find a period of 32.688 $\pm$ 0.002 hours, which we interpret as an orbital period. The folded light curve exhibits an eclipse lasting about 8 hours, suggesting a compact orbit in an nearly edge-on configuration. The X-ray binary has an average luminosity of $L_X(0.3-8 {\rm keV}) \approx 1.3\times10^{38}$ erg/sec, with only one out of the 25 observations significantly lower in flux than the average light curve.The presence of the eclipse and the $\sim$ 1.4-day orbital period suggests that this source is an analog of the well studied eclipsing X-ray binary pulsars Her X-1 or LMC X-4. Combining the Chandra data and the HST ACS/WFC images, we have identified several possible optical counterparts, including an O5-O3 star with V = 25.0. Follow-up optical monitoring observations should be able to identify the donor and further constrain the orbital properties., 24 pages, 9 figures, 2 tables, ApJ in press

As with other mixed morphology remnants, W44's projected center is bright in thermal X-rays. It has an obvious radio shell, but no discernable X-ray shell. X-ray bright knots dot W44's image. The Chandra data show that the remnant's hot, bright projected center is metal-rich and that the bright knots are regions of comparatively elevated elemental abundances. The neon abundance is elevated, suggesting that the center is rich in ejecta. Furthermore, some of the emitting iron atoms appear to be underionized with respect to the other ions, providing the first X-ray evidence for dust destruction in a supernova remnant. We use the Chandra data to test the following explanations for W44's X-ray bright center: 1.) entropy mixing from thermal conduction or bulk mixing, 2.) cloud evaporation, and 3.) a metallicity gradient, possibly due to dust destruction and ejecta enrichment. In these tests, we assume that the remnant has evolved beyond the adiabatic evolutionary stage, which explains the X-ray dimness of the shell. The entropy mixed model spectrum was found to be a good match to the Chandra spectrum. The bright knots have similar levels of ionization as the surrounding regions, challenging the evaporating clouds model. While both of these models are known to predict centrally bright X-ray morphologies, their predictions fall short of the observed brightness gradient. The resulting brightness gap can be largely filled in by emission from the extra metals in and near the remnant's projected center. The preponderance of evidence suggests that W44's remarkable morphology can be attributed to dust destruction and ejecta enrichment within an entropy mixed, adiabatic phase supernova remnant., Accepted for publication in the August 20, 2004 issue of ApJ. 22 pages (in well spaced layout), references, captions, and figures