Ensuring the Scientific Integrity of the Chandra Data Archive: The Role of Archival Reprocessings

Raffaele D'Abrusco on behalf of the DPAOps team

The scientific community relying on Chandra X-ray Observatory data is characterized by its productivity, diversity, and inventiveness. To ensure this community can always shed the brightest light on the high-energy Universe, they must have access to the best and most current version of any Chandra observation. A robust system is in place for individual users: the user-friendly, straightforward, and fully configurable CIAO command, chandra_repro. Thanks to the efforts of the Chandra X-ray Center (CXC) Calibration team, the Calibration DataBase Scientist, and the Science Data System team (supported by the Data System Software team), users downloading archival data can reprocess their observations to apply the latest, most appropriate processing software and calibration files to the downloaded data at their discretion.

However, ensuring data quality is not solely the responsibility of individual users. As the adage goes, a strong team is only moving at the speed of its slowest member. Accordingly, the CXC Data Processing and Archive Operations (DPAOps) team is responsible for maintaining the overall quality and scientific readiness of the archive, and the team does so through proactive archival reprocessing runs.

Simply defined, a reprocessing run is the CXC systematic method for bringing all datasets in the archive into the best possible shape to facilitate its long-term maintenance, assure its scientific validity, and enable its optimal readiness for future investigations. Reprocessings are not undertaken lightly; while infrequent, they are time-consuming and computationally intensive projects triggered by specific, significant advancements or necessary corrective actions. Over the more than twenty-six years of Chandra operations, the DPAOps team has performed only five major reprocessings (affecting 80% or more of all observations) and two partial ones (affecting less than 30%). Despite their rarity, the mission has spent approximately 30% of its operational life with an ongoing reprocessing running in the background. The longevity of the mission means that the average number of reprocessings per observation is 3.4, with some of the oldest observations having gone through as many as five full reprocessings. Regardless of the number of reprocessings it has already gone through during its life in the archive, every observation also undergoes rigorous human inspection during the Validation & Verification (V&V) stage of the reprocessing, where expert data operators can capture (and often correct) known observation-specific issues.

As the ongoing Period D of the fifth major reprocessing of the Chandra archive winds down, which covers all observations taken between July 2002 and the end of 2024, this is a good time to review the crucial mechanism of archival reprocessing. The four usual, broad motivations that can trigger a major reprocessing are:

A) New Calibration Files that, when applied, would result in a measurable improvement in the quality of a non-negligible subset of observations. These progresses are usually the result of better modeling of detector effects, contamination build-up, or the evolution in instrument-specific response.

B) Updates to Software and Pipelines that either fix existing software issues, allow the creation of new, valuable data products, or globally improve the quality of data for all or a large fraction of potentially affected observations. These changes are often the result of lessons learned from routine data processing or feedback from users.

C) Use of Definitive Ephemeris, which provides the most accurate estimation of the spacecraft position at the time of the observations, rather than the predictive versions employed in the initial data processing. The definitive ephemeris ensures the optimal accuracy of all timing information employed during the processing of Chandra observations.

D) Preparation for a New Catalog, which often requires an archival reprocessing to make sure that the foundational data used for the detections of sources is uniform, consistent, and of the highest possible quality.

Given the substantial planning and effort required to execute a reprocessing, the CXC DPAOps team typically waits until multiple of the drivers listed above accumulate to warrant the undertaking. While these main drivers, reflecting the improving characterization of the behavior of instruments and detectors and the evolution of data processing techniques, have recurred multiple times over the mission’s life, other, more occasional innovations have also contributed to triggering reprocessings. A prime example is the nearly complete Reprocessing V (Repro V), a multi-year project during which, for the first time, all observations ever taken by Chandra—from first light in 1999 through the end of 2024—were reprocessed. A major factor that justified Repro V was the adoption of Digital Object Identifiers (DOIs) as dataset identifiers for every existing observation in the archive. Furthermore, this expansive run saw the replacement of unresolvable target names with more informative alternatives for thousands of observations, significantly enhancing the searchability and discoverability of archival observations.

Reprocessings are neither identical nor monolithic in their execution. Their scope and extent vary horizontally and vertically depending on the specific nature of the updates being applied:

Ultimately, these reprocessings are the most significant and work-intensive forward-moving projects entirely performed by the DPAOps team. While they are mostly invisible to the external user community—except for the occasional courtesy emails sent to observers of recent observations that are reprocessed less than three years after their initial distribution—they form the bedrock of the Chandra archive's reliability and its enduring scientific legacy.