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Multi-epoch Chandra and XMM-Newton observations of the symbiotic system R Aquarii (R Aqr) spanning 22 yr are analysed by means of a reflection model produced by an accretion disc. This methodology helps dissecting the contribution from different components in the X-ray spectra of R Aqr: the soft emission from the jet and extended emission, the heavily-extinguished plasma component of the boundary layer and the reflection contribution, which naturally includes the 6.4 keV Fe fluorescent line. The evolution with time of the different components is studied for epochs between 2000 Sep and 2022 Dec, and it is found that the fluxes of the boundary layer and that of the reflecting component increase as the stellar components in R Aqr approach periastron passage, a similar behaviour is exhibited by the shocked plasma produced by the precessing jet. Using publicly available optical and UV data we are able to study the evolution of the mass-accretion rate $\dot{M}_\mathrm{acc}$ and the wind accretion efficiency $\eta$ during periastron. These exhibit a small degree of variability with median values of $\dot{M}_\mathrm{acc}$=7.3$\times10^{-10}$ M$_\odot$ yr$^{-1}$ and $\eta$=7$\times10^{-3}$. We compare our estimations with predictions from a modified Bondi-Hoyle-Lyttleton accretion scenario., Comment: 17 pages, 15 figures, 4 tables; accepted to MNRAS

We report on the X-ray spectral and spatial evolution of the Symbiotic star R Aqr. Through a multi-epoch observational campaign performed with Chandra between 2017 and 2022, we study the X-ray emission of this binary system, composed of an evolved red giant star and a white dwarf (WD). This analysis is particularly timely as the WD approached the periastron in late 2018/early 2019, thus mass transfer, jet emission and outburst phenomena are to be expected. Through detailed spectral analysis, we detect a significant rise in the soft X-ray (0.5-2 keV) emission of R Aqr, likely linked to jet emission, followed by a decay towards the previous quiescent state. The hard X-ray emission (5-8 keV), is not immediately affected by the periastron passage; the hard component, after maintaining the same flux level between 2017 and 2021, rapidly decays after 2022. Possible explanations for this are a change in the reflection properties of the medium surrounding the binary, obscuration of the central region by material ejected during the periastron passage, or even the partial/complete destruction of the inner regions of the accretion disc surrounding the WD. In addition to this activity in the central region, extended emission is also detected, likely linked to a hot spot in a pre-outburst-emitted jet, which can be observed moving away from the system's central region., Comment: 14 pages, 9 figures, 3 tables. Accepted for publication in ApJ

We present the analysis of archival XMM-Newton and Chandra observations of CH Cyg, one of the most studied symbiotic stars (SySts). The combination of the high-resolution XMM-Newton RGS and Chandra HETG X-ray spectra allowed us to obtain reliable estimates of the chemical abundances and to corroborate the presence of multi-temperature X-ray-emitting gas. Spectral fitting of the medium-resolution XMM-Newton MOS (MOS1+MOS2) spectrum required the use of an additional component not seen in previous studies in order to fit the 2.0-4.0 keV energy range. Detailed spectral modelling of the XMM-Newton MOS data suggests the presence of a reflection component, very similar to that found in active galactic nuclei. The reflection component is very likely produced by an ionised disk (the accretion disk around the white dwarf) and naturally explains the presence of the fluorescent Fe emission line at 6.4 keV while also contributing to the soft and medium energy ranges. The variability of the global X-ray properties of CH Cyg are discussed as well as the variation of the three Fe lines around the 6-7 keV energy range. We conclude that reflection components are needed to model the hard X-ray emission and may be present in most $\beta/\delta$-type SySt., Comment: 12 pages, 7 figures, 5 tables; To appear in MNRAS

We present Chandra ACIS-S imaging spectroscopy results of the extended (1.5''- 8'', 300 pc-1600 pc) hard X-ray emission of NGC 5728, the host galaxy of a Compton thick active galactic nucleus (CT AGN). We find spectrally and spatially-resolved features in the Fe K$\alpha$ complex (5.0-7.5 keV), redward and blueward of the neutral Fe line at 6.4 keV in the extended narrow line region bicone. A simple phenomenological fit of a power law plus Gaussians gives a significance of 5.4$\sigma$ and 3.7$\sigma$ for the red and blue wings, respectively. Fits to a suite of physically consistent models confirm a significance $\geq$3$\sigma$ for the red wing. The significance of the blue wing may be diminished by the presence of rest frame highly ionized Fe XXV and Fe XXVI lines (1.4$\sigma$ - 3.7$\sigma$ range). A detailed investigation of the Chandra ACIS-S point spread function (PSF) and comparison with the observed morphology demonstrates that these red and blue wings are radially extended (~5'', ~1 kpc) along the optical bicone axis. If the wings emission is due solely to redshifted and blueshifted high-velocity neutral Fe K$\alpha$ then the implied line-of-sight velocities are +/- ~0.1c, and their fluxes are consistent with being equal. A symmetric high-velocity outflow is then a viable explanation. This outflow has deprojected velocities ~100 times larger than the outflows detected in optical spectroscopic studies, potentially dominating the kinetic feedback power., Comment: Accepted for publication on ApJ

We present a detailed spectral and imaging analysis of the central $15''$ radius ($\sim 7.5 \text{ kpc}$) region of the merger galaxy NGC 6240 that makes use of all the available \textit{Chandra}-ACIS data ($0.3 - 3 \text{ keV}$ effective exposure of $\sim 190 \text{ ks}$). This region shows extended X-ray structures with lower energy counterparts imaged in CO, [O III] and H$\alpha$ line emission. We find both photo-ionized phases of possible nuclear excitation and thermal shock-excited emission in the different large-scale components: the north-west "loop" detected in H$\alpha$, the region surrounding the two nuclei, the large outflow region to the north-east detected in [O III], and the southern X-ray extensions. The latter could be the ionization cone of the northern nucleus, with the N counterpart being obscured by the galaxy disk. The radial distribution of the X-ray surface brightness suggests a confined hot interstellar medium at $r < 2.5 \text{ kpc}$, with a free-flowing wind at larger radii; if the confinement is magnetic, we estimate B-field values of $\sim 100\,\mu \text{G}$ , similar to those measured in the halo of M82. The thermal gas of the extended halo at $kT \sim 1 \text{ keV}$ absorbs soft X-rays from the AGN, but not the extreme ultraviolet radiation leading to a rapid increase in $F_{\text{[O III]}}/F_{\text{X}}$ beyond $\sim 3 \text{ kpc}$. The $\alpha$ element to Fe abundance ratios of the thermal components in the different regions of the extended X-ray emission are generally compatible with SNe II yields, confirming the importance of the active star formation in NGC 6240., Comment: 36 pages, 16 figures, 4 tables, accepted for publication on ApJ

We present the spatial analysis of five Compton thick (CT) active galactic nuclei (AGNs), including MKN 573, NGC 1386, NGC 3393, NGC 5643, and NGC 7212, for which high resolution Chandra observations are available. For each source, we find hard X-ray emission (>3 keV) extending to ~kpc scales along the ionization cone, and for some sources, in the cross-cone region. This collection represents the first, high-signal sample of CT AGN with extended hard X-ray emission for which we can begin to build a more complete picture of this new population of AGN. We investigate the energy dependence of the extended X-ray emission, including possible dependencies on host galaxy and AGN properties, and find a correlation between the excess emission and obscuration, suggesting a connection between the nuclear obscuring material and the galactic molecular clouds. Furthermore, we find that the soft X-ray emission extends farther than the hard X-rays along the ionization cone, which may be explained by a galactocentric radial dependence on the density of molecular clouds due to the orientation of the ionization cone with respect to the galactic disk. These results are consistent with other CT AGN with observed extended hard X-ray emission (e.g., ESO 428-G014 and the Ma et al. 2020 CT AGN sample), further demonstrating the ubiquity of extended hard X-ray emission in CT AGN., Comment: 29 pages, 22 figures, 7 tables, Accepted for publication in ApJ

RT Cru belongs to the rare class of hard X-ray emitting symbiotics, whose origin is not yet fully understood. In this work, we have conducted a detailed spectroscopic analysis of X-ray emission from RT Cru based on observations taken by the Chandra Observatory using the Low Energy Transmission Grating (LETG) on the High-Resolution Camera Spectrometer (HRC-S) in 2015 and the High Energy Transmission Grating (HETG) on the Advanced CCD Imaging Spectrometer S-array (ACIS-S) in 2005. Our thermal plasma modeling of the time-averaged HRC-S/LETG spectrum suggests a mean temperature of $kT \sim 1.3$ keV, whereas $kT \sim 9.6$ keV according to the time-averaged ACIS-S/HETG. The soft thermal plasma emission component ($\sim1.3$ keV) found in the HRC-S is heavily obscured by dense materials ($> 5 \times 10^{23}$ cm$^{-2}$). The aperiodic variability seen in its light curves could be due to changes in either absorbing material covering the hard X-ray source or intrinsic emission mechanism in the inner layers of the accretion disk. To understand the variability, we extracted the spectra in the "low/hard" and "high/soft" spectral states, which indicated higher plasma temperatures in the low/hard states of both the ACIS-S and HRC-S. The source also has a fluorescent iron emission line at 6.4 keV, likely emitted from reflection off an accretion disk or dense absorber, which was twice as bright in the HRC-S epoch compared to the ACIS-S. The soft thermal component identified in the HRC-S might be an indication of a jet that deserves further evaluations using high-resolution imaging observations., Comment: 28 pages, 18 figures, 4 tables, published in MNRAS

We present a reanalysis of the cumulative ACIS S Chandra data set pointed at the double AGNs of the NGC 6240 merging galaxy, focusing on the hard energy bands containing the hard spectral continuum (5.5-5.9 keV), the redshifted Fe I K alpha line (6.0-6.4 keV), and the redshifted Fe XXV line (6.4-6.7 keV). We have used to the full the Chandra telescope angular resolution, and we have modeled the Chandra PSF by comparing pre-flight calibration model to the data for the two bright AGNs. With two complementary approaches: (1) studying the residuals after PSF subtraction, and (2) producing reconstructed Expectation through Markov Chain Monte Carlo (EMC2) images, we are able to resolve structures extending from 1 kpc to <200 pc in the S AGN. The latter are within the sphere of influence of this BH. We find significant extended emission in both continuum and Fe lines in the 2'' (1 kpc) region surrounding the nuclei, in the region between the N and S AGN, and in a sector of PA 120-210 deg. extending to the SE from the centroid of the S AGN surface brightness. The extended Fe I K alpha emission is likely to originate from fluorescence of X-ray photons interacting with dense molecular clouds, providing a complementary view to recent high-resolution ALMA studies. The non-thermal emission is more prevalent in the region in between the two active X-ray nuclei, and in the N AGN. We do not find strong evidence of X-ray emission associated with the 3rd nucleus recently proposed for NGC 6240., Comment: in press on The Astrophysical Journal

Recent observations of nearby Compton thick (CT) active galactic nuclei (AGNs) with Chandra have resolved hard (>3 keV) X-ray emission extending out from the central supermassive black hole to kiloparsec scales, challenging the long-held belief that the characteristic hard X-ray continuum and fluorescent Fe K lines originate in the inner ~parsec due to the excitation of obscuring material. In this paper we present the results of the most recent Chandra ACIS-S observations of NGC 7212, a CT AGN in a compact group of interacting galaxies, with a total effective exposure of ~150 ks. We find ~20 percent of the observed emission is found outside of the central ~kiloparsec, with ~17 percent associated with the soft X-rays, and ~3 percent with hard X-ray continuum and Fe K line. This emission is extended both along the ionization cone and in the cross-cone direction up to ~3.8 kpc scales. The spectrum of NGC 7212 is best represented by a mixture of thermal and photoionization models that indicate the presence of complex gas interactions. These observations are consistent with what is observed in other CT AGN (e.g., ESO 428-G014, NGC 1068), providing further evidence that this may be a common phenomenon. High-resolution observations of extended CT AGN provide an especially valuable environment for understanding how AGN feedback impacts host galaxies on galactic scales., Comment: 20 pages, 10 figures, 8 tables, Accepted for publication in ApJ

We present the discovery of extended X-ray emission from the planetary nebula (PN) NGC 5189 around the [WO1]-type WD 1330-657 with XMM-Newton. The X-ray-emitting gas fills the cavities detected in the Hubble Space Telescope [O III] narrow-band image and presents a limb-brightened morphology towards the outer edges of the east and west lobes. The bulk of the X-ray emission is detected in the soft (0.3 - 0.7 keV) band with the XMM-Newton EPIC spectra dominated by the C VI Ly$\alpha$ line at 0.37 keV (=33.7 \AA). Spectral analysis resulted in carbon and neon abundances 38 and 6 times their solar values, with a plasma temperature of $kT=0.14\pm0.01$ keV ($T=1.6\times10^{6}$ K) and X-ray luminosity of $L_\mathrm{X}=(2.8\pm0.8)\times10^{32}$ erg s$^{-1}$. NGC 5189 is an evolved and extended PN ($\lesssim$0.70 pc in radius), thus, we suggest that the origin of its X-ray emission is consistent with the born-again scenario in which the central star becomes carbon-rich through an eruptive very late thermal pulse, subsequently developing a fast, carbon-rich wind powering the X-ray emission as suggested for A30 and A78., Comment: 9 pages; 5 figures and 1 table; Accepted to ApJ

We write in response to the call from the 2020 Decadal Survey to submit white papers illustrating the most pressing scientific questions in astrophysics for the coming decade. We propose exploration as the central question for the Decadal Committee's discussions.The history of astronomy shows that paradigm changing discoveries are not driven by well formulated scientific questions, based on the knowledge of the time. They were instead the result of the increase in discovery space fostered by new telescopes and instruments. An additional tool for increasing the discovery space is provided by the analysis and mining of the increasingly larger amount of archival data available to astronomers. Revolutionary observing facilities, and the state of the art astronomy archives needed to support these facilities, will open up the universe to new discovery. Here we focus on exploration for compact objects and multi messenger science. This white paper includes science examples of the power of the discovery approach, encompassing all the areas of astrophysics covered by the 2020 Decadal Survey.

Electromagnetic observations of the sky have been the basis for our study of the Universe for millennia, cosmic ray studies are now entering their second century, the first neutrinos from an astrophysical source were identified three decades ago, and gravitational waves were directly detected only four years ago. Detections of these messengers are now common. Astrophysics will undergo a revolution in the 2020s as multimessenger detections become routine. The 8th Astro2020 Thematic Area is Multimessenger Astronomy and Astrophysics, which includes the identification of the sources of gravitational waves, astrophysical and cosmogenic neutrinos, cosmic rays, and gamma-rays, and the coordinated multimessenger and multiwavelength follow-ups. Identifying and characterizing multimessenger sources enables science throughout and beyond astrophysics. Success in the multimessenger era requires: (i) sensitive coverage of the non-electromagnetic messengers, (ii) full coverage of the electromagnetic spectrum, with either fast-response observations or broad and deep high-cadence surveys, and (iii) improved collaboration, communication, and notification platforms., Comment: Astro2020 White Paper for the 8th Thematic Area of Multimessenger Astronomy and Astrophysics

We report the results of high-resolution subpixel imaging of the hard continuum and Fe K{\alpha} line of the Compton Thick (CT) Active Galactic Nucleus (AGN) ESO 428-G014, observed with Chandra ACIS. While the 3-4 keV emission is dominated by an extended component, a single nuclear point source is prominent in the 4-6 keV range. Instead, two peaks of similar intensity, separated by ~36 pc in projection on the plane of the sky are detected in the Fe K{\alpha} emission. The SE knot could be marginally associated with the heavily obscured hard continuum source. We discuss four possible interpretations of the nuclear morphology. (1) Given the bolometric luminosity and likely black hole (BH) mass of ESO 428-G014, we may be imaging two clumps of the CT obscuring torus in the Fe K{\alpha} line. (2) The Fe K{\alpha} knots may be connected with the fluorescent emission from the dusty bicone, or (3) with the light echo of a nuclear outburst. (4) We also explore the less likely possibility that we may be detecting the rare signature of merging nuclei. Considering the large-scale kpc-size extent of the hard continuum and Fe K{\alpha} emission (Papers I and II), we conclude that the AGN in ESO 428-G014 has been active for at least 104 yrs. Comparison with the models of Czerny et al (2009) suggests high accretion rates during this activity., Comment: ApJ in press

We have analyzed the deep Chandra observation (~155 ks) of the Compton Thick Active Galactic Nucleus (CT AGN) ESO 428-G014, to study in detail the morphology of the diffuse X-ray emission in the inner ~500 pc radius region. Comparing different X-ray energy bands we find localized differences in the absorbing column and in the emission processes. Collisional ionization may be prevalent in the area of most intense optical line emission (Halpha and [OIII]). There is a good correspondence between optical line, radio continuum and soft (<3 keV) X-ray features, consistent with simulations of jet/molecular disk interactions. At all energies >3 keV, the extended emission in the central 1".5 (170 pc) radius circumnuclear region amounts to ~70-30% of the contribution of a point source in that area (or 40-25% of the total counts in the region). Within a 5" radius, the contribution from extended emission overcomes that from a nuclear point source in the 3-4 keV band. This extended emission suggests scattering of nuclear photons by dense molecular clouds in the inner galactic disk of ESO 428-G014. Its presence may adversely bias the torus modeling of spectra from X-ray telescopes with inferior angular resolution than Chandra, such as NuSTAR and XMM-Newton., Comment: ApJ, in press

We present a deep Chandra spectral and spatial study of the kpc-scale diffuse X-ray emission of the Compton thick (CT) AGN ESO428-G014. The entire spectrum is best fit with composite photoionization + thermal models. The diffuse emission is more extended at the lower energies (<3 keV). The smaller extent of the hard continuum and Fe K{\alpha} profiles imply that the optically thicker clouds responsible for this scattering may be relatively more prevalent closer to the nucleus. These clouds must not prevent soft ionizing X-rays from the AGN escaping to larger radii, in order to have photoionized ISM at larger radii. This suggests that at smaller radii there may be a larger population of molecular clouds to scatter the hard X-rays, as in the Milky Way. The diffuse emission is also significantly extended in the cross-cone direction, where the AGN emission would be mostly obscured by the torus in the standard AGN model. Our results suggest that the transmission of the obscuring region in the cross-cone direction is ~10% than in the cone-direction. In the 0.3-1.5 keV band, the ratio of cross-cone to cone photons increases to ~84\%, suggesting an additional soft diffuse emission component, disjoint from the AGN. This could be due to hot ISM trapped in the potential of the galaxy. The luminosity of this component ~5 10^38 erg s^-1 is roughly consistent with the thermal component suggested by the spectral fits in the 170-900 pc annulus., Comment: 38 pages, 12 figure, ApJ in press

We report the discovery of kpc-scale diffuse emission in both the hard continuum (3-6 keV) and in the Fe K alpha line in the Compton Thick (CT) Seyfert galaxy ESO428-G014. This extended hard component contains at least ~24% of the observed 3-8 keV emission, and follows the direction of the extended optical line emission (ionization cone) and radio jet. The extended hard component has ~0.5% of the intrinsic 2-10 keV luminosity within the bi-cones. A uniform scattering medium of density 1 cm-3 would produce this luminosity in a 1kpc path length in the bi-cones. Alternatively, higher column density molecular clouds in the disk of ESO428-G014 may be responsible for these components. The continuum may also be enhanced by the acceleration of charged particles in the radio jet. The steeper spectrum (Gamma ~1.7 +-0.4) of the hard continuum outside of the central 1.5" radius nuclear region suggests a contribution of scattered/fluorescent intrinsic Seyfert emission. Ultrafast nuclear outflows cannot explain the extended F K alpha emission. This discovery suggests that we may need to revise the picture at the base of our interpretation of CT AGN spectra., Comment: Accepted for publication on the Astrophysical Journal Letters

Gravitationally focused wind accretion in binary systems consisting of an evolved star with a gaseous envelope and a compact accreting companion is a possible mechanism to explain mass transfer in symbiotic binaries. We study the mass accretion around the secondary caused by the strong wind from the primary late-type component using global three-dimensional hydrodynamic numerical simulations during quiescence and outburst stages. In particular, the dependence of the mass accretion rate on the mass-loss rate, wind parameters and phases of wind outburst development is considered. For a typical wind from an asymptotic giant branch star with a mass-loss rate of 1e-6 Msun/year and wind speeds of 20-50 km/s, the mass transfer through a focused wind results in efficient infall on to the secondary. Accretion rates onto the secondary of 5-20 per cent of the mass-loss from the primary are obtained during quiescence and outburst periods where the wind velocity and mass-loss rates are varied, about 20-50 per cent larger than in the standard Bondi-Hoyle-Lyttleton approximation. This mechanism could be an important method for explaining observed accretion luminosities and periodic modulations in the accretion rates for a broad range of interacting binary systems., Comment: 11 pages, 8 figures, to be published in MNRAS

We present the size, shape and flux densities at millimeter continuum wavelengths, based on ALMA science verification observations in Band 3 (~94.6 GHz) and Band 6 (~228.7 GHz), from the binary Mira A (o Ceti) and Mira B. The Mira AB system has been observed with ALMA at a spatial resolution of down to ~25 mas. The extended atmosphere of Mira A and the wind around Mira B sources are resolved and we derive the size of Mira A and of the ionized region around Mira B. The spectral indices within Band 3 (between 89-100 GHz) and between Band 3 and Band 6 are also derived. The spectral index of Mira A is found to change from 1.71+-0.05 within Band 3 to 1.54+-0.04 between Band 3 and 6. The spectral index of Mira B is 1.3+-0.2 in Band 3, in good agreement with measurements at longer wavelengths. However it rises to 1.72+-0.11 between the bands. For the first time the extended atmosphere of a star is resolved at these frequencies and for Mira A the diameter is ~3.8x3.2 AU in Band 3 (with brightness temperature Tb~5300 K) and ~4.0x3.6 AU in Band 6 (Tb~2500 K). Additionally, a bright hotspot of ~0.4 AU and with Tb~10000 K is found on the stellar disc of Mira A. The size of the ionized region around the accretion disk of Mira B is found to be ~2.4 AU. The emission around Mira B is consistent with that from a partially ionized wind of gravitationally bound material from Mira A close to the accretion disk of Mira B. The Mira A atmosphere does not fully match predictions, with brightness temperatures in Band 3 significantly higher than expected, potentially due to shock heating. The hotspot is likely due to magnetic activity and could be related to the previously observed X-ray flare of Mira A., Comment: 5 pages, 3 figures, accepted for A&A letters, updated to accepted version April 18

We present new X-ray spectral data for the Seyfert 1 nucleus in NGC 4151 observed with Chandra for 200 ks. A significant ACIS pileup is present, resulting in a non-linear count rate variation during the observation. With pileup corrected spectral fitting, we are able to recover the spectral parameters and find consistency with those derived from unpiled events in the ACIS readout streak and outer region from the bright nucleus. The absorption corrected 2-10 keV flux of the nucleus varied between 6E-11 and 1E-10 erg s^{-1} cm^{-2}. Similar to earlier Chandra studies of NGC 4151 at a historical low state, the photon indices derived from the same absorbed power-law model are \Gamma~0.7-0.9. However, we show that \Gamma is highly dependent on the adopted spectral models. Fitting the power-law continuum with a Compton reflection component gives \Gamma~1.1. By including passage of non-uniform X-ray obscuring clouds, we can reproduce the apparent flat spectral states with \Gamma~1.7, typical for Seyfert 1 AGNs. The same model also fits the hard spectra from previous ASCA "long look" observation of NGC 4151 in the lowest flux state. The spectral variability during our observation can be interpreted as variations in intrinsic soft continuum flux relative to a Compton reflection component that is from distant cold material and constant on short time scale, or variations of partially covering absorber in the line of sight towards the nucleus. An ionized absorber model with ionization parameter \log\xi ~ 0.8-1.1 can also fit the low-resolution ACIS spectra. If the partial covering model is correct, adopting a black hole mass M_{BH} ~ 4.6E+7 Msun we constrain the distance of the obscuring cloud from the central black hole to be r<~9 light-days, consistent with the size of broad emission line region of NGC 4151 from optical reverberation mapping., Comment: Accepted to ApJ. 41 pages, 11 figures, and 4 tables.

Jets have been detected in only a few symbiotic binaries to date, and CH Cyg is one of them. In 2001, a non-relativistic jet was detected in CH Cyg for the first time in X-rays. We carried out coordinated Chandra, HST, and VLA observations in 2008 to study the propagation of this jet and its interaction with the circumbinary medium. We detected the jet with Chandra and HST and determined that the apex has expanded to the South from about 300 AU to about 1400 AU, with the shock front propagating with velocity < 100 km/s. The shock front has significantly slowed down since 2001. Unexpectedly, we also discovered a powerful jet in the NE-SW direction, in the X-ray, optical and radio. This jet has a multi-component structure, including an inner jet and a counter-jet at about 170 AU, and a SW component ending in several clumps extending out to approximately 750 AU. The structure of the jet and the curvature of the outer portion of the SW jet suggest an episodically powered precessing jet, or a continuous precessing jet with occasional mass ejections or pulses. We carried out detailed spatial mapping of the X-ray emission and correlation with the optical and radio emission. X-ray spectra were extracted of the central source, inner NE counter jet, and the brightest clump at a distance of approximately 500 AU from the central source. We discuss the initial results of our analyses, including the multi-component spectral fitting of the jet-components and of the central source., Comment: 15 pages with 4 figures, Accepted by ApJ Letters

From multi-epoch adaptive optics imaging and integral field unit spectroscopy we report the discovery of an expanding and narrowly confined bipolar shell surrounding the helium nova V445 Puppis (Nova Puppis 2000). An equatorial dust disc obscures the nova remnant, and the outflow is characterised by a large polar outflow velocity of 6720 +/- 650 km/s and knots moving at even larger velocities of 8450 +/- 570 km/s. We derive an expansion parallax distance of 8.2 +/- 0.5 kpc and deduce a pre-outburst luminosity of the underlying binary of log L/L_Sun = 4.34 +/- 0.36. The derived luminosity suggests that V445 Puppis probably contains a massive white dwarf accreting at high rate from a helium star companion making it part of a population of binary stars that potentially lead to supernova Ia explosions due to accumulation of helium-rich material on the surface of a massive white dwarf., Comment: Accepted for publication in the Astrophysical Journal

We report high resolution imaging of the nucleus of the Seyfert 1 galaxy NGC 4151 obtained with a 50 ks Chandra HRC observation. The HRC image resolves the emission on spatial scales of 0.5", ~30 pc, showing an extended X-ray morphology overall consistent with the narrow line region (NLR) seen in optical line emission. Removal of the bright point-like nuclear source and image deconvolution techniques both reveal X-ray enhancements that closely match the substructures seen in the Hubble Space Telescope [OIII] image and prominent knots in the radio jet. We find that most of the NLR clouds in NGC 4151 have [OIII] to soft X-ray ratio ~10, despite the distance of the clouds from the nucleus. This ratio is consistent with the values observed in NLRs of some Seyfert 2 galaxies, which indicates a uniform ionization parameter even at large radii and a density decreasing as $r^{-2}$ as expected for a nuclear wind scenario. The [OIII]/X-ray ratios at the location of radio knots show an excess of X-ray emission, suggesting shock heating in addition to photoionization. We examine various mechanisms for the X-ray emission and find that, in contrast to jet-related X-ray emission in more powerful AGN, the observed jet parameters in NGC 4151 are inconsistent with synchrotron emission, synchrotron self-Compton, inverse Compton of CMB photons or galaxy optical light. Instead, our results favor thermal emission from the interaction between radio outflow and NLR gas clouds as the origin for the X-ray emission associated with the jet. This supports previous claims that frequent jet-ISM interaction may explain why jets in Seyfert galaxies appear small, slow, and thermally dominated, distinct from those kpc scale jets in the radio galaxies., Comment: Accepted for publication in ApJ. 28 pages, 9 figures, 3 tables

We study the dynamical effects of gravitational focusing by a binary companion on winds from late-type stars. In particular, we investigate the mass transfer and formation of accretion disks around the secondary in detached systems consisting of an AGB mass-losing star and an accreting companion. The presence of mass outflows is studied as a function of mass loss rate, wind temperature and binary orbital parameters. A 2-dimensional hydrodynamical model is used to study the stability of mass transfer in wind accreting symbiotic binary systems. In our simulations we use an adiabatic equation of state and a modified version of the isothermal approximation, where the temperature depends on the distance from the mass losing star and its companion. We explore the accretion flow between the components and formation of accretion disks for a range of orbital separations and wind parameters. Our results show the formation of stream flow between the stars and accretion disks of various sizes for certain orbital configurations. For a typical slow and massive wind from an AGB star the flow pattern is similar to a RLOF with accretion rates of 10% of the mass loss from the primary. Stable disks with exponentially decreasing density profiles and masses of the order $10^{-4}$ solar masses are formed when wind acceleration occurs at several stellar radii. The formation of tidal streams and accretion disks is found to be weakly dependent on the mass loss from the AGB star. Our simulations of gravitationally focused wind accretion in symbiotic binaries show the formation of stream flows and enhanced accretion rates onto the compact component., Comment: 13 pages, 17 figures. Accepted for publication in ApJ

We study the spectral energy distributions and evolution of a large sample of optically selected quasars from the Sloan Digital Sky Survey (SDSS) that were observed in 323 Chandra images analyzed by the Chandra Multiwavelength Project (ChaMP). Our highest-confidence matched sample includes 1135 X-ray detected quasars in the redshift range 0.23 QSOs detected, we find no evidence for evolution out to z~5 for either the X-ray photon index Gamma or for the ratio of optical/UV to X-ray flux alpha_ox. About 10% of detected QSOs are obscured (Nh>1E22), but the fraction might reach ~1/3 if most non-detections are absorbed. We confirm a significant correlation between alpha_ox and optical luminosity, but it flattens or disappears for fainter AGN alone. Gamma hardens significantly both towards higher X-ray luminosity, and for relatively X-ray loud quasars. These trends may represent a relative increase in non-thermal X-ray emission, and our findings thereby strengthen analogies between Galactic black hole binaries and AGN., Comment: 28 pages, 21 figures. Accepted (26 Aug 2008) for publication in ApJS. Electronic datafiles (for tables 2 and 3) and high resolution figures available at http://hea-www.harvard.edu/CHAMP/

In this paper we report detection of multiple component structures in a Chandra X-ray image obtained in March 2001 of the nearby symbiotic interacting binary system CH Cyg. These components include a compact central source, an arc-like structure or a loop extending to 1.5'' (400 AU) from the central source associated with the 1997 jet activity, and possibly a newly formed jet extending to about 150 AU from the central source. The structures are also visible in VLA and HST images obtained close in time to the Chandra observations. The emission from the loop is consistent with optically thin thermal X-ray emission originating from a shock resulting from interaction of the jet ejecta with the dense circumbinary material. The emission from the central source originates within less then 50 AU region, and is likely associated with the accretion disk around the white dwarf. CH Cyg is only the second symbiotic system with jet activity detected at X-ray wavelengths, and the Chandra high-angular resolution image, combined with the VLA and HST images, provides the closest view of the region of jet formation and interaction with the circumbinary material in a symbiotic binary., Comment: 19 pages, 6 figures, to appear in The Astriphysical Journal, June 2007 issue

We present a brief review of Chandra X-ray Observatory observations of neutron stars. The outstanding spatial and spectral resolution of this great observatory have allowed for observations of unprecedented clarity and accuracy. Many of these observations have provided new insights into neutron star physics. We present an admittedly biased and overly brief overview of these observations, highlighting some new discoveries made possible by the Observatory's unique capabilities. We also include our analysis of recent multiwavelength observations of the putative pulsar and its pulsar-wind nebula in the IC 443 SNR., Comment: 10 pages, 14 figures. Submitted to Astronomy and Space Science. Based on presentation at Isolated Neutron Stars, London, 2006

We report here the Chandra ACIS-S detection of a bright soft X-ray transient in the Mira AB interacting symbiotic-like binary. We resolved the system for the first time in the X-rays. Using Chandra and HST images we determined that the unprecedented outburst is likely associated with the cool AGB star (Mira A), the prototype of Mira-type variables. X-rays have never before been detected from an AGB star, and the recent activity signals that the system is undergoing dramatic changes. The total X-ray luminosity of the system is several times higher than the luminosity estimated using previous XMM and ROSAT observations. The outburst may be caused by a giant flare in Mira A associated with a mass ejection or a jet, and may have long term consequences on the system., Comment: 7 pages, 3 figures, to be published in the Astrophysical Journal Letters

The Chandra Multiwavelength Project (ChaMP) has discovered a jet-like structure associated with a newly recognized QSO at redshift z=1.866. The system was 9.4 arcmin off-axis during an observation of 3C 207. Although significantly distorted by the mirror PSF, we use both a raytrace and a nearby bright point source to show that the X-ray image must arise from some combination of point and extended sources, or else from a minimum of three distinct point sources. We favor the former situation, as three unrelated sources would have a small probability of occurring by chance in such a close alignment. We show that interpretation as a jet emitting X-rays via inverse Compton (IC) scattering on the cosmic microwave background (CMB) is plausible. This would be a surprising and unique discovery of a radio-quiet QSO with an X-ray jet, since we have obtained upper limits of 100 microJy on the QSO emission at 8.46 GHz, and limits of 200 microJy for emission from the putative jet., Comment: 12 pages including 4 figures. Accepted for publication by ApJ Letters

We present the first results of ChaMP X-ray source properties obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. Utilizing only near on-axis, bright X-ray sources (to avoid problems caused by incompleteness and the Eddington bias), we derive the Log(N)-Log(S) relation in soft (0.5-2 keV) and hard (2-8 keV) energy bands. The ChaMP data are consistent with previous results of ROSAT, ASCA and Chandra deep surveys. In particular, our data nicely fill in the flux gap in the hard band between the Chandra Deep Field data and the previous ASCA data. We check whether there is any systematic difference in the source density between cluster and non-cluster fields and also search for field-to-field variations, both of which have been previously reported. We found no significant field-to-field cosmic variation in either test within the statistics (~1 sigma) across the flux levels included in our sample. In the X-ray color-color plot, most sources fall in the location characterized by photon index = 1.5-2 and NH = a few x 10^20 cm^2, suggesting that they are typical broad-line AGNs. There also exist a considerable number of sources with peculiar X-ray colors (e.g., highly absorbed, very hard, very soft). We confirm a trend that on average the X-ray color hardens as the count rate decreases. Since the hardening is confined to the softest energy band (0.3-0.9 keV), we conclude it is most likely due to absorption. We cross-correlate the X-ray sources with other catalogs and describe their properties in terms of optical color, X-ray-to-optical luminosity ratio and X-ray colors., Comment: 26 pages, PDF, including 9 figures. Accepted in Aug 2003 for publication in ApJ. See also an accompanying X-ray paper by Kim et al. (2003) and a follow-up optical paper by Green et al. (2003) in the ChaMP web site http://hea-www.harvard.edu/CHAMP/

The Chandra Multi-wavelength Project (ChaMP) is a wide-area (~14 deg^2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed AGNs, high z clusters of galaxies) at flux levels (fX ~ 10^-15 - 10^-14 erg sec-1 cm-2) intermediate between the Chandra Deep surveys and previous missions. We present the first ChaMP catalog, which consists of 991 near on-axis, bright X-ray sources obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. To assess source reliability and positional uncertainty, we perform a series of simulations and also use Chandra data to complement the simulation study. The false source detection rate is found to be as good as or better than expected for a given limiting threshold. On the other hand, the chance of missing a real source is rather complex, depending on the source counts, off-axis distance (or PSF), and background rate. The positional error (95% confidence level) is usually < 1" for a bright source, regardless of its off-axis distance while it can be as large as 4" for a weak source (~20 counts) at a large off-axis distance (Doff-axis > 8'). We have also developed new methods to find spatially extended or temporary variable sources and those sources are listed in the catalog., Comment: 85 pages, PDF, including 9 tables and 15 figures. Accepted in Aug 2003 for publication in ApJ Supplement. For a full paper, visit the ChaMP web site http://hea-www.harvard.edu/CHAMP/. See also an accompanying X-ray paper by Kim et al. (2003) and a follow-up optical paper by Green et al. (2003)

We describe results from our initial study of the diffuse X-ray emission structures in NGC 5128 (Cen A) using the Chandra X-ray Observatory observations. The high-angular resolution Chandra images reveal multi-scale X-ray struct ures with unprecedented detail and clarity. The large scale structures suggest complex symmetry, including a component possibly associated with the inner radio lobes, and a separate component with an orthogonal symmetry that may be associated with the galaxy as a whole. We detect arc-like soft X-ray structures, extending to ~8 kpc in the direction perpendicular to the jet. These arcs appear to be tracing an ellipse, or a ring seen in projection, and ar e enclosed between the dust and HI emitting gas in the central region of the galaxy, and the optical and HI shell fragments at a distance of about 8-10 kpc. The diffuse X-ray and the optical emission in the arcs could originate in a region of interaction (possibly a shock) between the infalling material from the outer regions of the galaxy, and the cool dust and HI emitting material in the center, or an equatorial outflow resulting from an outburst of nuclear activity ~10^7 years ago., Comment: 8 pages, three color figures, to be published in ApJ September 20, 2002 issue

The VLTI is the ideal instrument for measuring the distances of nearby Cepheids with the Baade-Wesselink method, allowing an accurate recalibration of the Cepheid Period-Luminosity relation. The high accuracy required by such measurement, however, can only be reached taking into account the effects of limb darkening, and its dependence on the Cepheid pulsations. We present here our new method to compute phase- and wavelength-dependent limb darkening profiles, based on hydrodynamic simulation of Classical Cepheid atmospheres., Comment: 3 pages, 2 postscript figures, uses eas.cls LaTeX class file, to appear in the proc. Eurowinter School "Observing with the VLTI", Feb 3-8 2002, Les Houches (France)

This work presents a new method to compute time and wavelength dependent center-to-limb brightness distributions for Classical Cepheids. Our model atmospheres are based on second-order accurate 1-D hydrodynamic calculations, performed in spherical geometry. The brightness intensity distributions, and the resulting limb darkening, are computed through the dynamic atmospheres by using a full set of atomic and molecular opacities. Our results confirm important differences with respect to equivalent hydrostatic models. The amount of limb darkening, and the shape of the limb profiles, show a strong dependence on the pulsational phase of the Cepheid, which cannot be reproduced by static models. Non-linear effects in our hydrodynamic equations add a new level of complexity in the wavelength dependence of our limb profiles, which are affected by the presence of shock-waves traveling through the atmosphere. These effects, already detectable by present-day interferometers, should be taken into consideration when deriving limb darkened diameters for nearby Cepheids with the accuracy required to measure their radial pulsations., Comment: Accepted for publication in the Astrophysical Journal

Sub-arcsecond resolution Chandra observations of Eta Carinae reveal a 40 arcsec X 70 arcsec ring or partial shell of X-ray emission surrounding an unresolved, bright, central source. The spectrum of the central source is strongly absorbed and can be fit with a high-temperature thermal continuum and emission lines. The surrounding shell is well outside the optical/IR bipolar nebula and is coincident with the Outer Shell of Eta Carinae. The X-ray spectrum of the Shell is much softer than that of the central source. The X-ray Shell is irregular and only correlates well with optical features where a bright X-ray knot coincides with a bright feature of the Outer Shell. Implications for the binary model of the central source are discussed., Comment: 6 pages, 6 figures, accepted by ApJ

The launch of the Chandra X-ray Observatory in July 1999 opened a new era in X-ray astronomy. Its unprecedented, <0.5" spatial resolution and low background are providing views of the X-ray sky 10-100 times fainter than previously possible. We have initiated a serendipitous survey (ChaMP) using Chandra archival data to flux limits covering the range between those reached by current satellites and those of the small area Chandra deep surveys. We estimate the survey will cover ~5 sq.deg./year to X-ray fluxes (2-10 keV) in the range 1E(-13)-6E(-16) erg/cm^2/s discovering ~2000 new X-ray sources, ~80% of which are expected to be AGN. The ChaMP has two parts, the extragalactic survey (ChaMP) and the galactic plane survey (ChaMPlane). ChaMP promises profoundly new science return on a number of key questions at the current frontier of many areas of astronomy including (1) locating and studying high redshift clusters and so constraining cosmological parameters (2) defining the true population of AGN, including those that are absorbed, and so constraining the accretion history of the universe, (3) filling in the gap in the luminosity/redshift plane between Chandra deep and previous surveys in studying the CXRB, (4) studying coronal emission from late-type stars and (5) search for CVs and quiescent Low-Mass X-ray Binaries (qLXMBs) to measure their luminosity functions. In this paper we summarize the status, predictions and initial results from the X-ray analysis and optical imaging., Comment: 6 pages, latex file, 4 Postscript FiguresProceedings of New Era of Wide Field Astronomy, Aug 2000, Preston UK

The young pulsar PSR B0540-69 was one of the first targets observed with the Chandra X-Ray Observatory. The high angular resolution of Chandra allows us to resolve the compact nebula surrounding the pulsar. We have determined a position for PSR B0540-69 of R.A. = 05h 40m 11s.221, decl. = -69o 19' 54".98 (J2000) with a 1-sigma radial uncertainty of 0.7". Combining our measurements of the pulsar period with previous measurements covering a span of 12 years, we derive a braking index of 2.08. The spectrum of the pulsed emission is consistent with a power-law with a photon index of 1.83 +/- 0.13. The compact nebula has a softer spectrum with a photon index of 1.85 to 2.26., Comment: 12 pages, accepted by the Astrophysical Journal

Deep Chandra ACIS observations of the region around the putative pulsar, CXOU J061705.3+222117, in the supernova remnant IC443 reveal, for the first time, a ring-like morphology surrounding the pulsar and a jet-like structure oriented roughly north-south across the ring and through the pulsar location. The observations further confirm that (1) the spectrum and flux of the central object are consistent with a rotation-powered pulsar interpretation, (2) the non-thermal surrounding nebula is likely powered by the pulsar wind, and (3) the thermal-dominated spectrum at greater distances is consistent with emission from the supernova remnant. The cometary shape of the nebula, suggesting motion towards the southwest (or, equivalently, flow of ambient medium to the northeast), appears to be subsonic; there is no evidence for a strong bow shock, and the circular ring is not distorted by motion through the ambient medium.