Chandra Source Catalog Release 2.0

Observation Stacking

Exploiting the unique resolution and very low background of Chandra data, the limiting sensitivity of the catalog was improved significantly by stacking (co-adding) multiple observations of the same field prior to source detection. This is a significant departure from CSC 1.1 where sources were detected in each observation separately and their properties combined. To minimize the impact of the variation of the Chandra point spread function (PSF) across the field, source detection in CSC 2.0 is constrained to run on stacks of observations that have pointings co-located within 60″, and that were obtained using the same instrument (ACIS or HRC-I).

Improved Background Maps

CSC 2.0 uses a new Voronoi-tessellation background tool (mkvtbkg) to create improved background maps prior to source detection. These background maps perform better than those used in earlier releases in regions where the background intensity is changing rapidly (e.g., in areas of extended emission near the centers of galaxies) and at large off-axis angles.

Improved Two-step Compact Source Detection

A new method is used that can reliably detect compact sources down to roughly 5 net counts over much of the field-of-view. Improved background maps (see below) allow compact source detection in areas with diffuse emission and near the edge of fields. As for CSC 1.1, source detection is performed first using the CIAO wavelet source detection tool (wavdetect), but in CSC 2.0 the tool parameters are updated to identify fainter candidate detections, albeit with an unacceptably high false detection rate. Candidate compact detections superimposed on regions with bright extended emission are identified as a side-effect of running the mkvtbkg tool (in CSC 1.1, these regions were excluded from the catalog).

The combined set of candidate detections are further evaluated by a second step in processing (new to CSC 2.0). A new maximum likelihood estimator (mle) uses the Sherpa modeling and fitting package to fit the local, energy band dependent PSF model to each candidate detection to evaluate the likelihood that the detection is real. Candidate detections included in the CSC 2.0 are classified as either TRUE or MARGINAL in the catalog, by comparing their likelihood estimates with a pair of thresholds that calibrate the permissible false detection rates.

Improved PSF Modeling

Since the Chandra PSF is highly position dependent, the local PSF model is calculated separately for each energy band for each detection position prior to use by the mle tool. Fitting the local PSF to the photon splash of the detection improves source astrometry, particularly for larger off-axis angles where PSF asymmetries can bias centroid-based position determinations. Each PSF model includes ~50,000 counts, which is adequate for many types of studies.

Inclusion of Bright Extended Sources

In addition to background determination and identification of candidate compact detections, mkvtbkg can identify regions of extended emission. This capability allows bright, extended sources to be included in the CSC for the first time. Such sources are identified using bounding convex hull polygons in release 2.0 of the catalog. However, sets of quasi-surface brightness polygons with multiple intensity thresholds are available (as FITS format data products) for end users who wish to perform more detailed analysis of detected extended sources. Extended sources are considered to be an "alpha" release in CSC 2.0, and may not satisfy the same rigorous level of quality assurance that the compact sources meet.

Better Handling of Edge Effects

Statistical characterization of CSC 1.1 concluded that there was a significantly higher false source rate for detections near the edges of the (stacked) fied-of-view, in the gaps between ACIS back-illuminated and front-illuminated CCDs (on the ACIS-S array), and on readout streaks associated with saturated, bright sources, compared to the rest of the field. Detections in such regions are excluded from CSC 2.0 through the application of a pixel mask that removes those pixels from consideration.

Improved Limiting Sensitivity Information

CSC 2.0 includes multi-band limiting sensitivity maps so that users can identify regions that are included in/excluded from the catalog. The limiting sensitivity for a point source to satisfy the catalog TRUE or MARGINAL detection thresholds in each energy band are computed on a fine-grained (~3.22″×3.22″) scale (c.f. ~30″×30″ scale for CSC 1.1).

Improved Evaluation of Source and Detection Properties

Like earlier releases, CSC 2.0 includes numerous raw measurements for each detected source as well as scientifically useful properties derived from the observations in which the source is detected. All reported measurements and properties have associated two-sided confidence intervals.

Detection properties are provided at both the stacked-observation detection level and the per-observation detection level in CSC 2.0. The stacked-observation level allows composite properties to be reported from the co-added observations for detections that would otherwise not be visible or have poor S/N in individual observations, while for higher S/N detections the per-observation properties facilitate analysis of variable sources.

In CSC 2.0, reported positions include 95% error ellipses for brighter sources and circular errors for faint sources. Multi-band aperture photometry will be determined using an updated Bayesian approach to compute photometric probability density functions that are subsequently used directly for computing such quantities as cross-band hardness ratios and temporal variability measures. This avoids some inconsistencies present in CSC 1.1 where these properties were computed independently. If a source is detected in one or more stacked-observations, photometric upper limits that are useful for temporal variability analyses are calculated for any stacked-observations and individual observations in which the source is not detected.

CSC 2.0 uses a Bayesian Blocks analysis to identify multiple detections of the same source that have consistent multi-band aperture photometry. Such detections are analyzed simultaneously to improve S/N when determining detection properties. The properties for the longest exposure Bayesian Block are promoted to master source properties, but the properties for all blocks are recorded in an end-user FITS format data product.

Spectral fits computed using multiple models (e.g., absorbed power-law, black body, and—new in CSC 2.0—bremsstrahlung) are provided for brighter sources. The minimum counts required to compute spectral fits remains 150 counts, same as CSC 1.1, but the analysis now includes simultaneous fits to multiple detections of the same source totaling at least 150 counts.

Additional Data Products

In addition to the tabulated properties, CSC 2.0 provides numerous FITS format data products that are immediately usable for further analysis. These products include full field event lists, multi-band images, exposure maps, limiting sensitivity maps, merged source lists, and extended source polygons. Source region data products include per-source-region event lists, multi-band images, exposure maps, pulse-invariant spectra, spectral response matrices, aperture photometry probability density functions, Bayesian block properties, position-error MCMC draws, and optimally-binned light curves.