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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 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., 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�� 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�� 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�� line. (2) The Fe K�� 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�� 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., ApJ in press

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, Comment: Accepted to ApJ. 41 pages, 11 figures, and 4 tables.

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/

Polaris, the nearest and brightest classical Cepheid, is a single-lined spectroscopic binary with an orbital period of 30 years. Using the High Resolution Channel of the Advanced Camera for Surveys onboard the Hubble Space Telescope (HST) at a wavelength of ~2255\AA, we have directly detected the faint companion at a separation of 0\farcs17. A second HST observation 1.04 yr later confirms orbital motion in a retrograde direction. By combining our two measures with the spectroscopic orbit of Kamper and an analysis of the Hipparcos and FK5 proper motions by Wielen et al., we find a mass for Polaris Aa of 4.5^{+2.2}_{-1.4} M_\odot--the first purely dynamical mass determined for any Cepheid. For the faint companion Polaris Ab we find a dynamical mass of 1.26^{+0.14}_{-0.07} M_\odot, consistent with an inferred spectral type of F6 V and with the flux difference of 5.4 mag observed at 2255\AA. The magnitude difference at the V band is estimated to be 7.2 mag. Continued HST observations will significantly reduce the mass errors, which are presently still too large to provide critical constraints on the roles of convective overshoot, mass loss, rotation, and opacities in the evolution of intermediate-mass stars. Our astrometry, combined with two centuries of archival measurements, also confirms that the well-known, more distant (18") visual companion, Polaris B, has a nearly common proper motion with that of the Aa,Ab pair. This is consistent with orbital motion in a long-period bound system. The ultraviolet brightness of Polaris B is in accordance with its known F3 V spectral type if it has the same distance as Polaris Aa,Ab., Comment: 31 pages, 12 figures, 6 tables; accepted for publication in the Astronomical Journal

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., 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., 7 pages, 3 figures, to be published in the Astrophysical Journal Letters

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., 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 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/

We have investigated the mass loss processes in Long Period Variables, by combining radiative transfer modeling with mid-IR imaging and spectroscopic observations. We find a correlation between variability type and temporal variations in the mass loss rate. Mira variables are more likely to maintain constant mass loss rates over long period of time. Semiregular and Irregular variables are characterized by frequent interruptions in their mass loss rate, on timescales of ~100 yr. High resolution imaging of individual Long Period Variable sources show departures from spherical symmetry. We present the case of o Cet (Mira), in which the presence of a low mass companion plays an active role in shaping the circumstellar environment., Comment: 4 pages, 4 postscript figures, uses Mem. SAIt new mem.cls LaTeX class, to appear in proc. "Convegno Nazionale di Astronomia Infrarossa", December 4-7 2001, Perugia (Italy)

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., 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, 6 pages, latex file, 4 Postscript FiguresProceedings of New Era of Wide Field Astronomy, Aug 2000, Preston UK