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The Chandra Source Catalog (CSC) is a virtual X-ray astrophysics facility that enables both detailed individual source studies and statistical studies of large samples of X-ray sources detected in ACIS and HRC-I imaging observations obtained by the Chandra X-ray Observatory. The catalog provides carefully-curated, high-quality, and uniformly calibrated and analyzed tabulated positional, spatial, photometric, spectral, and temporal source properties, as well as science-ready X-ray data products. The latter includes multiple types of source- and field-based FITS format products that can be used as a basis for further research, significantly simplifying followup analysis of scientifically meaningful source samples. We discuss in detail the algorithms used for the CSC Release 2 Series, including CSC 2.0, which includes 317,167 unique X-ray sources on the sky identified in observations released publicly through the end of 2014, and CSC 2.1, which adds Chandra data released through the end of 2021 and expands the catalog to 407,806 sources. Besides adding more recent observations, the CSC Release 2 Series includes multiple algorithmic enhancements that provide significant improvements over earlier releases. The compact source sensitivity limit for most observations is ~5 photons over most of the field of view, which is ~2x fainter than Release 1, achieved by co-adding observations and using an optimized source detection approach. A Bayesian X-ray aperture photometry code produces robust fluxes even in crowded fields and for low count sources. The current release, CSC 2.1, is tied to the Gaia-CRF3 astrometric reference frame for the best sky positions for catalog sources., Comment: 66 pages, 17 figures, 16 tables, accepted for publication in The Astrophysical Journal Supplement Series

In a series of three previous papers, formulation and specifics of the representation of World Coordinate Transformations in FITS data have been presented. This fourth paper deals with encoding time. Time on all scales and precisions known in astronomical datasets is to be described in an unambiguous, complete, and self-consistent manner. Employing the well--established World Coordinate System (WCS) framework, and maintaining compatibility with the FITS conventions that are currently in use to specify time, the standard is extended to describe rigorously the time coordinate. World coordinate functions are defined for temporal axes sampled linearly and as specified by a lookup table. The resulting standard is consistent with the existing FITS WCS standards and specifies a metadata set that achieves the aims enunciated above., Comment: FITS WCS Paper IV: Time. 27 pages, 11 tables

Aperture synthesis instruments providing a generally highly uniform sampling of the visibility function often leave an unsampled hole near the origin of the (u,v)-plane. In this paper, originally published in 1979, we first describe the common solution of retrieving the information from scans made with a large single-dish telescope. However, this is not the only means by which short spacing visibility data can be obtained. We propose an alternative technique that employs a short-baseline interferometer to scan the entire primary beam area. The obvious advantage is that a short-baseline pair from the synthesis instrument can be used, ensuring uniformity in instrumental characteristics. This technique is the basis for the mosaicing algorithms now commonly used in aperture synthesis radio astronomy imaging., Comment: 3 pages; IAU Colloquium 49 (Groningen, Netherlands, 1978): "Image Formation from Coherence Functions in Astronomy"

In this study we develop and propose publication metrics, based on an analysis of data from the Chandra bibliographic database, that are more meaningful and less sensitive to observatory-specific characteristics than the traditional metrics. They fall in three main categories: speed of publication; fraction of observing time published; and archival usage. Citation of results is a fourth category, but lends itself less well to definite statements. For Chandra, the median time from observation to publication is 2.36 years; after about 7 years 90% of the observing time is published; after 10 years 70% of the observing time is published more than twice; and the total annual publication output of the mission is 60-70% of the cumulative observing time available, assuming a two year lag between data retrieval and publication., Comment: 22 pages, 8 figures, 3 tables; revised manuscript submitted to PASP

This document provides a complete design description of the Space-Time Coordinate (STC) metadata for the Virtual Observatory. It explains the various components, highlights some implementation considerations, presents a complete set of UML diagrams, and discusses the relation between STC and certain other parts of the Data Model. Two serializations are discussed: XML Schema (STC-X) and String (STC-S); the former is an integral part of this Recommendation.

The first release of the Chandra Source Catalog (CSC) contains ~95,000 X-ray sources in a total area of ~0.75% of the entire sky, using data from ~3,900 separate ACIS observations of a multitude of different types of X-ray sources. In order to maximize the scientific benefit of such a large, heterogeneous data-set, careful characterization of the statistical properties of the catalog, i.e., completeness, sensitivity, false source rate, and accuracy of source properties, is required. Characterization efforts of other, large Chandra catalogs, such as the ChaMP Point Source Catalog (Kim et al. 2007) or the 2 Mega-second Deep Field Surveys (Alexander et al. 2003), while informative, cannot serve this purpose, since the CSC analysis procedures are significantly different and the range of allowable data is much less restrictive. We describe here the characterization process for the CSC. This process includes both a comparison of real CSC results with those of other, deeper Chandra catalogs of the same targets and extensive simulations of blank-sky and point source populations., Comment: To be published in the Astrophysical Journal Supplement Series (Fig. 52 replaced with a version which astro-ph can convert to PDF without issues.)

Although relative errors can readily be calculated, the absolute astrometric accuracy of the source positions in the Chandra Source Catalog (CSC), Version 1.0, is a priori unknown. However, the cross-match with stellar objects from the Sloan Digital Sky Survey (SDSS) offers the opportunity to compare the apparent separations of the cross-matched pairs with the formally calculated errors. The analysis of these data allowed us to derive a value of 0.16" for the residual absolute astrometric error in CSC positions. This error will be added to the published position errors in the CSC from now on, starting with CSC, Version 1.1., Comment: 18 pages, 6 figures, accepted for publication in ApJSuppl

The Chandra Source Catalog (CSC) is a general purpose virtual X-ray astrophysics facility that provides access to a carefully selected set of generally useful quantities for individual X-ray sources, and is designed to satisfy the needs of a broad-based group of scientists, including those who may be less familiar with astronomical data analysis in the X-ray regime. The first release of the CSC includes information about 94,676 distinct X-ray sources detected in a subset of public ACIS imaging observations from roughly the first eight years of the Chandra mission. This release of the catalog includes point and compact sources with observed spatial extents <~ 30''. The catalog (1) provides access to the best estimates of the X-ray source properties for detected sources, with good scientific fidelity, and directly supports scientific analysis using the individual source data; (2) facilitates analysis of a wide range of statistical properties for classes of X-ray sources; and (3) provides efficient access to calibrated observational data and ancillary data products for individual X-ray sources, so that users can perform detailed further analysis using existing tools. The catalog includes real X-ray sources detected with flux estimates that are at least 3 times their estimated 1 sigma uncertainties in at least one energy band, while maintaining the number of spurious sources at a level of <~ 1 false source per field for a 100 ks observation. For each detected source, the CSC provides commonly tabulated quantities, including source position, extent, multi-band fluxes, hardness ratios, and variability statistics, derived from the observations in which the source is detected. In addition to these traditional catalog elements, for each X-ray source the CSC includes an extensive set of file-based data products that can be manipulated interactively., Comment: To appear in The Astrophysical Journal Supplement Series, 53 pages, 27 figures

We investigate the physical properties of the interstellar medium (ISM) in the merging pair of galaxies known as The Antennae (NGC 4038/39), using the deep coadded ~411 ks Chandra ACIS-S data set. The method of analysis and some of the main results from the spectral analysis, such as metal abundances and their variations from ~0.2 to ~20-30 times solar, are described in Paper I (Baldi et al. submitted). In the present paper we investigate in detail the physics of the hot emitting gas, deriving measures for the hot-gas mass (~10^ M_sun), cooling times (10^7-10^8 yr), and pressure (3.5x10^-11-2.8x10^-10 dyne cm^-2). At least in one of the two nuclei (NGC 4038) the hot-gas pressure is significantly higher than the CO pressure, implying that shock waves may be driven into the CO clouds. Comparison of the metal abundances with the average stellar yields predicted by theoretical models of SN explosions points to SNe of Type II as the main contributors of metals to the hot ISM. There is no evidence of any correlation between radio-optical star-formation indicators and the measured metal abundances. Although due to uncertainties in the average gas density we cannot exclude that mixing may have played an important role, the short time required to produce the observed metal masses (<=2 Myr) suggests that the correlations are unlikely to have been destroyed by efficient mixing. More likely, a significant fraction of SN II ejecta may be in a cool phase, in grains, or escaping in hot winds. In each case, any such fraction of the ejecta would remain undetectable with soft X-ray observations., Comment: 29 pages, 6 figures, accepted by the Astrophysical Journal

We present an analysis of the properties of the hot interstellar medium (ISM) in the merging pair of galaxies known as The Antennae (NGC 4038/39), performed using the deep, coadded ~411 ks Chandra ACIS-S data set. These deep X-ray observations and Chandra's high angular resolution allow us to investigate the properties of the hot ISM with unprecedented spatial and spectral resolution. Through a spatially resolved spectral analysis, we find a variety of temperatures (from 0.2 to 0.7 keV) and Nh (from Galactic to 2x10^21 cm^-2). Metal abundances for Ne, Mg, Si, and Fe vary dramatically throughout the ISM from sub-solar values (~0.2) up to several times solar., Comment: 33 pages, 18 figures, revised version accepted by Astrophysical Journal Supplement Series

We explored ways of doing spatial search within a relational database: (1) hierarchical triangular mesh (a tessellation of the sphere), (2) a zoned bucketing system, and (3) representing areas as disjunctive-normal form constraints. Each of these approaches has merits. They all allow efficient point-in-region queries. A relational representation for regions allows Boolean operations among them and allows quick tests for point-in-region, regions-containing-point, and region-overlap. The speed of these algorithms is much improved by a zone and multi-scale zone-pyramid scheme. The approach has the virtue that the zone mechanism works well on B-Trees native to all SQL systems and integrates naturally with current query optimizers - rather than requiring a new spatial access method and concomitant query optimizer extensions. Over the last 5 years, we have used these techniques extensively in our work on SkyServer.sdss.org, and SkyQuery.net., Comment: Original at http://research.microsoft.com/research/pubs/view.aspx?tr_id=736

We have monitored the phase of the main X-ray pulse of the Crab pulsar with the Rossi X-ray Timing Explorer (RXTE) for almost eight years, since the start of the mission in January 1996. The absolute time of RXTE's clock is sufficiently accurate to allow this phase to be compared directly with the radio profile. Our monitoring observations of the pulsar took place bi-weekly (during the periods when it was at least 30 degrees from the Sun) and we correlated the data with radio timing ephemerides derived from observations made at Jodrell Bank. We have determined the phase of the X-ray main pulse for each observation with a typical error in the individual data points of 50 us. The total ensemble is consistent with a phase that is constant over the monitoring period, with the X-ray pulse leading the radio pulse by 0.0102+/-0.0012 period in phase, or 344+/-40 us in time. The error estimate is dominated by a systematic error of 40 us in the radio data, arising from uncertainties in the variable amount of pulse delay due to interstellar scattering and instrumental calibration. The statistical error is 0.00015 period, or 5 us. The separation of the main pulse and interpulse appears to be unchanging at time scales of a year or less, with an average value of 0.4001+/-0.0002 period. There is no apparent variation in these values with energy over the 2-30 keV range. The lag between the radio and X-ray pulses may be constant in phase (rotational) or constant in time (linear pathlength). We are not (yet) able to distinguish between these two interpretations., Comment: 11 pages, 2 figures

[abridged] We investigate the nature of the luminous X-ray source population detected in a (72 ks) Chandra ACIS-S observation of NGC 4038/39, the Antennae galaxies. We derive the average X-ray spectral properties of sources in different luminosity ranges, and we correlate the X-ray positions with radio, IR, and optical HST data. The X-ray sources are predominantly associated with young stellar clusters, indicating that they belong to the young stellar population. Based on both their co-added X-ray spectrum, and on the lack of associated radio emission, we conclude that the Ultra Luminous X-ray sources (ULXs), with Lx>10^{39} erg\s, are not young compact Supernova Remnants (SNR), but accretion binaries. While their spectrum is consistent with those of ULXs studied in nearby galaxies, and interpreted as the counterparts of intermediate mass black-holes (M >10-1000 \msun), comparison with the position of star-clusters suggests that some of the ULXs may be runaway binaries, thus suggesting lower-mass binary systems. The co-added spectrum of the sources in the 3x10^{38}-10^{39} erg\s luminosity range is consistent with those of Galactic black-hole candidates. These sources are also on average displaced from neighboring star clusters. The softer spectrum of the less luminous sources suggests the presence of SNRs or of hot interstellar medium (ISM) in the Chandra source extraction area. Comparison with HI and CO observations shows that most sources are detected in the outskirts of large concentrations of gas. The absorbing columns inferred from these observations would indeed absorb X-rays up to 5 keV, so there may be several hidden X-ray sources. Associated with these obscured regions we find 6 sources with heavily absorbed X-ray spectra and absorption-corrected luminosities in the ULX range., Comment: 18 pages, submitted to ApJ

We report the detailed analysis of the X-ray properties of the discrete sources detected in a long (72 ks) \chandra ACIS-S observation of the Antennae galaxies. We detect 49 sources, down to a detection limit of ~ 10^{38} erg\s; 18 sources have Lx >10^{39} erg\s (Ultra Luminous X-ray sources; ULXs). Six of the 49 sources have an extended component. Two sources show evidence for variability during this observation, and three sources exhibit long term variability in timescales of a few years. The spectra of the discrete sources are diverse, suggesting different emission mechanisms, broadly correlated with the source luminosity: the most luminous sources exhibit harder emission, while the spectra of fainter sources appear softer. Spectra and variability suggest that the ULXs may be binary accretion sources; Supernova remnants or hot ISM in the Chandra beam may be responsible for some of the softer sources., Comment: 27 pages, 9 figures, submitted to ApJS

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

Observations with the Rossi X-ray Timing Explorer and the Jodrell Bank, Parkes, and Green Bank telescopes have enabled us to determine the time delay between radio and X-ray pulses in the two isolated pulsars B1821-24 and B1509-58. For the former we find that the narrow X-ray and radio pulse components are close to being coincident in time, with the radio peak leading by 0.02 period (60 +/- 20 microsec), while the wide X-ray pulse component lags the last of the two wider radio components by about 0.08 period. For the latter pulsar we find, using the standard value for the dispersion measure, that the X-ray pulse lags the radio by about 0.27 period, with no evidence for any energy-dependence in the range 2-100 keV. However, uncertainties in the history of the dispersion measure for this pulsar make a comparison with previous results difficult. It is clear that there are no perceptable variations in either the lag or the dispersion measure at time scales of a year or less., Comment: 20 pages including 6 figures, accepted by Astrophysical Journal

We present the results of observations of the PSR B1509$-$58/MSH 15$-$52 system in X-rays ($2-250$ keV) by the Rossi X-ray Timing Explorer. The spectra of the peak of the pulsed component (radio phase $0.17-0.53$) is fit by a power law of photon index $1.36\pm0.01$, with no evidence of a high energy spectral break seen up to $\sim200$ keV. For the off-pulse spectral component, the spectrum from $2-250$ keV is fit by a power law of photon index $2.215\pm0.005$. An iron emission line at 6.7 keV with an equivalent width of 129 eV improves the fit, but only at a marginal significance. Thermal bremsstrahlung and Raymond-Smith models produce much worse fits to the unpulsed data. The lack of a high energy spectral break in the pulsed emission implies an efficiency of $\geq 3%$ in the conversion of pulsar spindown energy to pulsed X-rays in the system., Comment: Accepted by ApJ Letters, 18 pages, 4 postscript figures

We have monitored the phase of the main X-ray pulse of the Crab pulsar with the Rossi X-ray Timing Explorer (RXTE) for almost eight years, since the start of the mission in January 1996. The absolute time of RXTE's clock is sufficiently accurate to allow this phase to be compared directly with the radio profile. Our monitoring observations of the pulsar took place bi-weekly (during the periods when it was at least 30 degrees from the Sun) and we correlated the data with radio timing ephemerides derived from observations made at Jodrell Bank. We have determined the phase of the X-ray main pulse for each observation with a typical error in the individual data points of 50 microseconds. The total ensemble is consistent with a phase that is constant over the monitoring period, with the X-ray pulse leading the radio pulse by 0.01025 plus or minus 0.00120 period in phase, or 344 plus or minus 40 microseconds in time. The error estimate is dominated by a systematic error of 40 microseconds, most likely constant, arising from uncertainties in the instrumental calibration of the radio data. The statistical error is 0.00015 period, or 5 microseconds. The separation of the main pulse and interpulse appears to be unchanging at time scales of a year or less, with an average value of 0.4001 plus or minus 0.0002 period. There is no apparent variation in these values with energy over the 2-30 keV range. The lag between the radio and X-ray pulses ma be constant in phase (i.e., rotational in nature) or constant in time (i.e., due to a pathlength difference). We are not (yet) able to distinguish between these two interpretations.

We present the calibration and background model for the Proportional Counter Array (PCA) aboard the Rossi X-ray Timing Explorer (RXTE). The energy calibration is systematics limited below 10 keV with deviations from a power law fit to the Crab nebula plus pulsar loss than 1%. Unmodelled variations in the instrument background amount to less than 2% of the observed background below 10 keV and less than 1% between 10 and 20 keV. Individual photon arrival times are accurate to 4.4 microseconds at all times during the mission and to 2.5 microseconds after 29 April 1997. The peak pointing direction of the five collimators is known at few arcsecond precision.

The monitoring of this pulsar is still on-going, and hopefully will be for years to come. It has proved to be possible to fit the timing ephemeris for PSR B1509-59 for the entire monitoring period to date (five and a half years) with a single quintic polynomial. In order to establish its significance, we need at least one more year of monitoring data (for which we have proposed), but if this result is confirmed, it appears that two significant results may emerge: 1) well-determined values for the first and second braking indices; the first is well-established, but the second still needs a longer time base for a significant value 2) there seems to be a periodic variation in the pulse arrival times that can be fit with harmonic sinusoids with periods of approximately 790 and 395 days; if this result holds up on a longer time base it would be very exciting. Note that we are in position now to generate a timing ephemeris on the basis of the x-ray data that is more accurate than the radio ephemeris. In the meantime we have also determined a position for the pulsar that is more accurate than the ones derived from radio and Chandra observations. These results were reported at the 2000 HEAD meeting in Honolulu, HI. Travel was partly out of this grant. The remainder was applied toward the acquisition of a laptop computer that allows independent and fast analysis of all monitoring observations.

The monitoring of the X-ray pulses from the Crab pulsar is still ongoing at the time of this writing, and we hope to be able to continue the campaign for the life of the XTE mission. We have established beyond all doubt that: (1) the X-ray main pulse leads the radio pulse by approximately 300 microseconds, (2) this phase lag is constant and not influenced by glitches, (3) this lag does not depend on X-ray energy, (4) the relative phase of the two X-ray pulses does not vary, and (5) the spectral indices of primary, secondary, and inter-pulse are distinct and constant. At this time we are investigating whether the radio timing ephemeris can be replaced by an x-ray ephemeris and whether any long-time timing ephemeris can be established. If so, it would enable use to study variations in pulse arrival times at a longer time scales. Such a study is easier in x-rays than at radio wavelengths since the dispersion measure plays no role. These results were reported at the 2000 HEAD Meeting in Honolulu, HI. Travel was paid partly out of this grant. The remainder was applied toward the acquisition of a laptop computer that allows independent and fast analysis of all monitoring observations.

Results of the mapping of the blue gas-rich S0 galaxy NGC 5102 in the 21-cm H I line with a spatial resolution of 34 x 37 arcsec (delta(alpha) x Delta(delta)) and a velocity resolution of 12 km/s are presented. The H I distribution has a pronounced central depression of 1.9 kpc radius, and most of the H I is concentrated in a 3.6 kpc wide ring with an average radius of 3.7 kpc, assuming a distance of 4 Mpc for NGC 5102. The maximum azimuthally averaged H I surface density in the ring is 1.4 solar mass/sq pc, comparable to that found in other S0 galaxies. The HI velocity field is quite regular, showing no evidence for large-scale deviations from circular rotation, and the H I is found to rotate in the plane of the stellar disk. Both the H I mass/blue luminosity ratio and the radial H I distribution are similar to those in early-type spirals. The H I may be an old disk or it may have been acquired through capture of a gas-rich smaller galaxy. The recent starburst in the nuclear region, which gave the galaxy its blue color, may have been caused by partial radial collapse of the gas disk, or by infall of a gas-rich dwarf galaxy.

Khoros defines formats for User Interface Specification (UIS) and Program Specification (PS) files. From such files, its code generator, Ghostwriter, creates source files and documentation. The great advantage of the system is that the code fragments that make up part of the PS file are purely generic. All Khoros-related code is created by the code generator; this includes all user interface code. As a matter of fact, both specification files are very generic in nature. Thus, one could imagine using it as the basis for other software systems. A case is made that writing a code generator that would create IRAF-compatible code from the Khoros UIS and PS files is fairly trivial. Another aspect of the Khoros system conventions concerns the way execution commands are generated by the user interface and the actual syntax of those commands. The protocols are such that interoperability at the level of executable modules is readily possible.