Motivations

The target names of Chandra observations are of primary importance to A) characterize the scientific goal of the observations, B) unequivocally locate them in space and time, and C) to make the observations discoverable through the archive interfaces after they become public. Target names that do not satisfy either one of the previous conditions (hereinafter “bad” target names) can negatively affect how observations in the archive are discovered and retrieved and, in turn, jeopardize the optimal scientific exploitation of the legacy of the Chandra mission.

In preparation of the 5-th general reprocessing of the archive (Repro-V), the Chandra Data Archive (CDA) team initiated a major effort to clean up the archive from bad target names by reviewing all archival public observations (see Applicability below for details) observed before January 1 2017 to select target names requiring enhancements (Selection of bad target names) and determineing suitable replacements (Replacement of bad target names). Starting in October 2021, regular migrations of bad target names of observations taken after January 1 2017 will occur every year to carry on the clean-up operation by replacing bad target names of recent observations.

The target names of affected observations are replaced with the new target names in the Chandra Observation Catalog (OCat) and across all the VO protocols that serve Chandra observation metadata. The headers of the Chandra data products containing the target name values in the OBJECT keyword are updated through reprocessing of the observations. No other parameter that can be used to identify observations (ObsIDs, target coordinates) is modified. So far, two migrations affecting different parts of the archive have occurred:

• Bad target names of archival observations taken before 01/01/2017 were modifried on 09/15/2020; the associated observations were reprocessed during Repro V.
• Bad target names of archival observations taken after or on 01/01/2017 and before 01/01/2018 were modified on 10/04/2021; the associated observations were reprocessed shortly thereafter.
• Bad target names of archival observations taken after or on 01/01/2018 and before 01/01/2019 were modified on 11/01/2022; the associated observations were reprocessed shortly thereafter.

 

Applicability

All OR (GO, GTO, TOO, DDT, CAL) archival observations taken through 2017 and all CCTs observed as of 10/27/21 have been reviewed. Proprietary observations taken after 12/31/2017 have not been modified yet as a courtesy towards their observers who might still be working on the data.

 

Selection of bad target names

Archival observations have been considered for target name replacement if their target names qualify for one of the following categories:

• not informative: target names that do not unequivocally determine the identity of the target for being too generic (like "PROTO CLUSTER", "NEWMAGNETAR"), or not descriptive of the actual target but linked to the type of observation (i.e. observations whose target names contain the string "TOO")
• containing typos, as in the case of "1FLG" instead of "1FGL" for Fermi Large Area Telescope First Source Catalog
• lacking a substring: many target names do not contain catalog identifiers that are necessary to correctly determine the object that has been observed
• not resolvable into coordinates, i.e. not associable to sky coordinates by neither the NED and Sesame name resolving services

 

Target names of observations associated with the same deep fields/surveys/regions, whose current coverage has grown over time through the accumulation of multiple programs, have also been reviewed for naming consistency. It has been verified that all observations of the same field are discoverable with a single search based on the most common and/or resolvable name (when available) of the field/survey/region. When not, the differences in the target names of observations of the same field were reconciled to minimize the number of target name changes needed.

 

Replacement of bad target names

The determination of the replacement target names for some of the bad target names proved to be non trivial and required manual work, including surveying the linked scientific literature, inspection of the proposal abstract, comparison with existing catalogs of astronomical sources and, in a few cases, contacting the original PIs of the observations. For not obvious cases, the new target name has been selected to reflect as closely as possible the original intention of the observer. Other criteria that were applied to determine the replacement target names are described below:

• When an observation is selected for target name replacement, all other ObsIDs with same target name have been reviewed (this rule only applies to compact sources, non transients sources);
• When a section of a target name that - globally - does not resolve into coordinates, is resolvable, the target name has not been modified;
• Parts of the original target names to be replaced, containing valuable information about the targets have been retained in the new target name (for example, "cold front" for observations of a cluster of galaxies);
• While "resolvable" new target names have been prioritized over "not resolvable" designations for observations with “bad” target names, non-resolvable new target names deemed to be sufficiently recognizable, based on their common usage in the literature, have been used as replacements (provided that their observations are discoverable through "string-based" searches).

 

Lists of replaced bad target names

The final list of Observation IDentifiers (ObsIDs) for Chandra archival observations whose target names have been replaced, the old target name and the new target names can be downloaded as a file below for the Repro V and the annual migrations: s

• Repro-V migration: 5181 distinct observations (for 2212 single bad target names) got their target names replaced. The final list of bad target names migrated can be downloaded in ASCII or FITS formats.
• 2021 annual migration: 202 distinct observations (for 155 unique target names) got their target names replaced. The final list of bad target names migrated can be downloaded in ASCII or FITS formats.
• 2022 annual migration: 266 distinct observations (for 198 unique target names) got their target names replaced. The final list of bad target names migrated can be downloaded in ASCII or FITS formats.
• 2023 annual migration: 212 distinct observations (for 151 unique target names) got their target names replaced. The final list of bad target names migrated can be downloaded in ASCII or FITS formats.
• 2024 annual migration: 199 distinct observations (for 105 unique target names) got their target names replaced. The final list of bad target names migrated can be downloaded in ASCII or FITS formats.

 

Additional resources

In order to resolve into sky coordinates, target names need to follow naming conventions agreed upon by the International Astronomical Union (IAU). Chandra archive users can find useful information about the syntactic rules for acceptable designations of astronomical sources here. More detailed information about “subcomponents” in the names of astronomical sources are available at this link. A web-based resource dictionary of the nomenclature of celestial objects is hosted by CDS at this link.