An Archival Trifecta

Raffaele D'Abrusco

Recently, the Chandra Data Archive (CDA) has worked on new ways to make archival Chandra data more accessible and more citable. Here’s a brief description of three different projects we hope might interest you.

An Update on Our Obsession with Target Names

As our readers will remember, better naming of the astronomical sources observed by Chandra is one of the keys to making the archive more easily explorable and the data more accessible. In the last three years, our team has replaced thousands of target names for archival observations taken through 2018 that did not correspond to sky coordinates or that did not provide sufficient information to unambiguously identify the source. Starting with Cycle 22, we asked our community of proposers to assign suitable names to their approved targets and to support us in changing these names if needed before the targets were observed, and the response was nothing short of great! Thanks to your help, we managed to change ~85% of unresolvable target names from Cycle 23 (~18% of the total, unique target names) before they were observed, and we aim to exceed that threshold with Cycle 24 targets. So thanks again, and let’s continue to work together for a better, more open archive!

Fantastic DOIs and How To Use Them

It is well known that Chandra observations are identified through their “Observation IDentifier” (ObsID), a unique number assigned to each observation that never changes over the whole data lifetime. But did you know that, since March 2020, each ObsID is also associated with a Digital Object Identifier (DOI) that makes citing its data a breeze? Think of a DOI as a way to turn an ObsID into a link pointing to the actual data associated with the observation. For Chandra ObsIDs, just remember to prepend the fixed prefix, which unambiguously identifies the CDA. If you are writing a LaTeX document using the AASTeX package, for example, simply use the command:


to create a link to the data in the PDF. An explanation of what’s going on behind the curtains can be found here, but if you care about reproducibility of your results and want to make the data used in your publications discoverable (and you should!), our DOIs are the best way to cite Chandra observations. We are also working on minting DOIs for Chandra Source Catalog (CSC) data products as well; look for more information on that project in our next Newsletter update!

Hard Questions, Easy Answers

Have you ever searched for archival Chandra observations covering regions of the sky where all known Cataclysmic Variables, Seyfert 2 galaxies, open stellar clusters, or your favorite category of objects are located? If so, you are aware that with some elbow grease, a few hours, and a lot of cross-matching you can find the answer using our public interfaces and other data repositories. But is this the only path to reach these answers? And more importantly, is it the most convenient one? In the era of instant, seamless access to universal knowledge, addressing these types of inquiries should require very little effort. Our belief is that the full scientific potential of any domain-specific data can be revealed by enhancing it with multi-wavelength knowledge; by thinking of all astronomical sources in a certain region of the sky as attributes of the observation(s) that cover that area, complex search queries can be answered. Less boastfully, the annotation of all Chandra observations with the astronomical objects located in their footprints lets us waltz into a usually arduous terrain of the discovery parameter space and make our archive more easy to explore and access.

About one year ago we started to annotate Chandra archival observations with events observed by the Zwicky Transient Factory (ZTF), a multiwavelength optical synoptic survey that monitors the northern sky and produces a large number of alerts for transient and periodic sources every night. For each new ZTF event, which are also all lightcurve-classified by the specialized broker Alerce, we check if there’s any overlapping observations and nearby CSC sources, collect these facts, and will make them available through the CDA webpage for everyone’s benefit. We also occasionally tweet about it. While the dynamic optical sky offers plenty of opportunities for X-ray aficionados to constrain the energetic phenomena at work, this pilot study allows us to develop more general concepts and tools for the archive annotations. Stay tuned for more about this topic.

The CDA is always working to improve our accessibility, not just through technical developments but also through direct interactions with our users. If you want to know more about these subjects or other aspects of the archive, shoot us an email at