Multiplicity in the Release 2 CSC: Stacks, Bundles, and Blocks
To improve the limiting sensitivity of release 2 of the CSC in regions of the sky that have multiple overlapping observations, source detection is performed on stacked (co-added) observations. This brings some added complexity that is described in detail at various places in the documentation, but we summarize the key concepts here for convenience:
Observations taken with the same instrument (ACIS or HRC-I) and whose pointing direction is co-aligned within 1 arcminute are grouped in stacks. Formally, the observations are matched using a tree clustering algorithm with complete linkage. This means that the pointing direction of each observation in the stack is co-aligned with the pointing direction of every other observation in the stack within 1 arcminute, and each stack includes the maximum number of observations for which this condition is satisfied.
Source detection is performed on the co-added stacks (termed detect stacks) rather than on the individual observations. Once a source is detected, per-detection properties are evaluated using the subset of observations for which the source falls on the valid per-observation pixel mask—essentially, the pixels for which the 'ideal' (i.e., only including spacecraft dither motion) exposure is greater than ~90% of the peak exposure. The subset of observations for which this is true is termed the valid stack for the detection. See Figure 1.
Groups of detections whose source regions and/or associated background regions overlap are collected into bundles. Detections in a bundle are typically processed together as a set to ensure that a consistent set of properties is determined. For example, aperture photometry is performed on a bundle basis to ensure that photon counts in the overlap regions of multiple detections are accounted for correctly. Bundles are redefined at several stages during the processing.
Aperture photometry measures in release 2 of the CSC are derived using a Bayesian X-ray aperture photometry algorithm that computes the flux probability density functions (PDFs) for each individual detection (observation) of a source. These PDFs are then evaluated using a Bayesian Blocks algorithm to group the observations into equivalence classes, called blocks, such that for all observations in a single block, the multi-band fluxes are consistent with no variation. There are two types of blocks, based on how the Bayesian Blocks algorithm groups the observations. The first is flux-ordered blocks, where only the multi-band aperture photometry fluxes are considered. In flux-ordered blocks, multiple consistent observations of a variable source will be combined to improve S/N, even if they are separated by observations of the source in another state. For time-ordered blocks, only consecutive observations can be grouped together, which is appropriate when computing temporal variability measures. In the catalog tables we report aperture photometry measures (and derived measures such as hardness ratios) for the flux-ordered block with the highest exposure time (as well as global average fluxes). Additionally, properties for all flux-ordered and time-ordered blocks are reported in each source-specific blocks3 FITS data product that can be downloaded via CSCview. See the discussion of the Source Properties pipeline and of relevant table columns and data products for more details.
Detect, Valid, and Likely Stacks of Three ObsIDs
Catalog Entries: Master Sources, Stacked Observation Detections, and Per-Observation Detections
Because the size of the Chandra PSF varies by roughly two orders of magnitude between the center and edge of the field of view, in the CSC we differentiate between detections and sources. Detections are the blobs of photon counts that we see on the detector image, whereas sources are our interpretation of the detections as distinct X-ray sources on the sky. There is potentially a many-to-many relationship between detections and sources. A single source may correspond to multiple detections because the source is included in more than one observation that includes the same region of the sky. Similarly, a single detection may correspond to multiple sources; this typically occurs when a single far off-axis detection (large PSF) is resolved into multiple sources by another observation where the same position on the sky is located closer to the telescope optical-axis (small PSF).
The catalog is split into three main tables: the Master Sources Table, the Stacked Observation Detections Table, and the Per-Observation Detections Table. In the Master Sources Table, each row describes a source, whereas in the latter two tables, each row describes a detection. In all three tables, the columns contain information about properties that describe the source or detection, as appropriate.
The Master Sources Table contains 'best estimate' sources properties for each unique X-ray source in the catalog. These properties are typically determined by analyzing simultaneously the detections from the set of individual observations that comprise the flux-ordered Bayesian block with the highest exposure time.
The Stacked Observation Detections Table contains detection properties based on observational data extracted independently from each stack of Chandra pointings in which the source is detected; because a source may be detected in multiple stacks, this table may contain multiple entries corresponding to a single entry in the Master Sources Table.
The Per-Observation Detections Table contains detection properties based on observational data extracted independently from each individual observation (Chandra pointing) in which the corresponding stacked observation detection is located; because a stack may include multiple observations, this table may contain multiple entries corresponding to a single entry in the Stacked Observation Detections Table.
The Column Descriptions pages describe how each master source, stacked observations detection, and per-observation detection property is determined.
Additionally, the Stacked Observation Detections Table and Per-Observation Detections Table report a number of stack- and observation-specific properties, such as pointing and instrument information, that are common for all detections identified in that stack/observation.
The catalog includes a number of FITS format science-ready data products that can be accessed using several of the catalog user interfaces. These include full-field products at the stack and observation level, such as photon event lists, background maps, bad pixel files, and exposure maps, as well as detection and source-specific data products, including source region photon event lists, per-band PSF files, instrumental responses, low-resolution (PHA) spectra, light curves, and aperture photometry PDFs, among others; see the Data Products section for the full list of CSC file-based data products.
Each individual stack-level detection in the catalog is classified as either uniquely or ambiguously linked to a master source ('match_type' equals 'u' or 'a').
A detection that is uniquely linked to a master source only a positional match for that single master source. The detections properties contribute to the master source properties recorded in the Master Sources Table.
A detection that is ambiguously linked to a master source could positionally match more than one master source. Because the detection's photon events cannot be assigned unambiguously to a specific master source, they contribute to the properties of the linked master sources recorded in the Master Sources Table only in a very limited way. For example, they can provide a photometric upper limit for all of the linked master sources at the epoch(s) of the stack observation(s), but cannot contribute to knowledge of the master source position.
Note that even in the case of uniquely linked per-stack detections, if an ACIS observation is piled-up (estimated pile-up fraction exceeds ~10%), its per-observation detection properties will not contribute to the corresponding stacked observation detection or master source property UNLESS all other ACIS observations of that source are also piled-up.
New in CSC 2.0 are nondetections, which are linked to a master source with 'match_type' = 'n'. If a master source position falls on a stacked observation for which there is no corresponding detection that can be linked to the master source, then nondetection regions are created at the master source position for each of the observations that comprise that stacked observation. These nondetection regions are sized based on the local PSF (90% ECF) and are used to compute aperture photometry upper limits for the source brightness at the epochs of each of the observations.
Figure 2 demonstrates that all "master source/stacked observation detection/per-observation detection associations" in the catalog, whether ambiguous, unique, or non-detection, are transparent to the user. This feature allows a request of any combination of master source properties, stacked detection properties and per-observation detection properties in a single query to the database, so the user is not restricted to searching a single table at a given time, and can understand how all observations of a single source contribute to a master source entry.
Linking Individual Detections with a Master Source
The Catalog Processing section describes in detail how the catalog is created from individual Chandra data sets.