Chandra Source Catalog Release 2 (CSC 2.1)

Introduction

The Chandra Source Catalog version 2.1 (CSC 2.1) was released on April 2^nd 2024. It includes measured properties for 407,806 unique compact and extended X-ray sources and more than 1.3 million individual detections observed with either ACIS or HRC-I observations released publicly prior to the end of 2021. The total sky coverage is ~730 square degrees. CSC 2.1 consists of two types of data products: database tables (e.g. positions, significance, fluxes, hardness ratios, and variability indices) and file-based data products (e.g. images, PHA spectra, light curves, and sensitivity maps). Figure 1 shows the location of the observations in CSC 2.1.

Figure 1: Detections in CSC 2.1

[Thumbnail image: Aitoff projection of the sky in equatorial coordinates showing the location of stacks in CSC 2.1 as circles. The source color indicates the number of observations in each stack, and ranges from blue through yellow to red. Most stacks are blue, indicating 1 to 4 observations, although a few stacks are red, with more than 30 observations in the stack. The size of the circle indicates the number of detections in each stack, and most have 100 or less detections, but a few have 1000 or more (most of these are around the center of the image, indicating the galactic center).]

[Version: full-size]

[Print media version: Aitoff projection of the sky in equatorial coordinates showing the location of stacks in CSC 2.1 as circles. The source color indicates the number of observations in each stack, and ranges from blue through yellow to red. Most stacks are blue, indicating 1 to 4 observations, although a few stacks are red, with more than 30 observations in the stack. The size of the circle indicates the number of detections in each stack, and most have 100 or less detections, but a few have 1000 or more (most of these are around the center of the image, indicating the galactic center).]

Figure 1: Detections in CSC 2.1

CSC 2.1 detections shown in galactic coordinates, so the center of the image shows the Galactic plane. Each circle represents an observation stack, with the size of the circle indicating the total number of observations included in the stack and the color the number of detections in each stack.

Structure of CSC 2.1

CSC 2.1 is hierarchical. Source/detection properties are provided at three different levels: master level, stack level, and per-observation level. This hierarchy results from the fact that source detection is performed upon stacking individual Chandra observations to improve the signal to noise ratio. The detected source position and the associated source region are used to compute the photometric, spectral, and variability properties of the source, both in the stack detection and in the individual observations contributing to the stack. However, observations are stacked together only if they have pointings within 1 arcmin of each other. Consequently, a single source can be detected in more than one stack. The master level properties are the best possible estimate of source properties, given the information contained in all contributing stacks. A full description of the catalog organization can be found in the Catalog Organization page.

Figure 2: Master Level

[There is a dotted outline to represent the covered region of the sky. There are four sources, labelled 1 (green), 2 (yellow), 3 (red), and 4 (blue).]

[Print media version: There is a dotted outline to represent the covered region of the sky. There are four sources, labelled 1 (green), 2 (yellow), 3 (red), and 4 (blue).]

Figure 2: Master Level

The master level properties are the best-possible estimate of source properties given the information contained in all contributing stacks.

Figure 3: Stack Level

[There are two areas of overlapping sky, labelled "stack 1" and "stack 2". Sources 1, 2, and 4 are only present in one of the stacks (source 1 is in stack 2 and sources 2 and 4 are in stack 1). Source 3 is found in both stacks and so there are two separate stack detections.]

[Print media version: There are two areas of overlapping sky, labelled "stack 1" and "stack 2". Sources 1, 2, and 4 are only present in one of the stacks (source 1 is in stack 2 and sources 2 and 4 are in stack 1). Source 3 is found in both stacks and so there are two separate stack detections.]

Figure 3: Stack Level

Overlapping observations are stacked if their pointing is within 1 arcminute of each other.

A source may be detected in multiple stacks.

Figure 4: Observation Level

[There are three separate observations (obs 1, obs 2, and obs 3) where obs 1 and obs 2 are part of stack 1 and obs 3 is in stack 2. There are now per-observation detections for source 3 (as it is found in both stacks) and per-observation detections for sources 2 and 4, as they are observed in observations 1 and 2.]

[Print media version: There are three separate observations (obs 1, obs 2, and obs 3) where obs 1 and obs 2 are part of stack 1 and obs 3 is in stack 2. There are now per-observation detections for source 3 (as it is found in both stacks) and per-observation detections for sources 2 and 4, as they are observed in observations 1 and 2.]

Figure 4: Observation Level

A source may be detected in multiple Chandra observations.