In gamma-ray astronomy, a variety of data formats and proprietary software have been traditionally used, often developed for one specific mission or experiment. Especially for ground-based imaging atmospheric Cherenkov telescopes (IACTs), data and software are mostly private to the collaborations operating the telescopes. The next-generation IACT instrument, the Cherenkov Telescope Array (CTA), will be the first ground-based gamma-ray telescope array operated as an open observatory with public observer access. This implies fundamentally different requirements for the data formats and software tools. Open community access is a novelty in this domain, putting a challenge on the implementation of services that make very high energy (VHE) gamma-ray astronomy as accessible as any other waveband.

To work towards open and interoperable data formats for gamma-ray astronomy, we have started this open data format specification. This was started at the PyGamma15 workshop in November 2015, followed by a meeting in April 2016 in Meudon and another meeting in March 2017 in Heidelberg. Version 0.1 of the spec was release in April 2016, version 0.2 in August 2018, version 0.3 in November 2022. You can find more information about this effort in Deil et al. (2017).

The scope of this effort is to cover all high-level data from gamma-ray telescopes, starting at the level of event lists and IRFs, also covering reduced IRFs, maps, spectra, light curves, up to source models and catalogs. Both pointing (IACT) and slewing (Fermi-LAT, HAWC) telescope data is in scope. To a large degree, the formats should also be useful for other astroparticle data, e.g. from cosmic rays or neutrinos.

All of the data formats described here at the moment can be, and in practice mostly are, stored in FITS. Some experimentation with HDF5 and ECSV is ongoing. The data format specifications don’t explicity mention the serialisation format, but rather the key name and data type for each metadata or data field.

The formats described here are partially in use by current instruments (Fermi-LAT, HESS, VERITAS, MAGIC, FACT) as well as in the next-generation Cherenkov Telescope Array CTA. Prototyping and support for many of the formats here is happening in the CTA science tool prototypes Gammapy and ctools. The data formats are also used e.g. in gamma-cat, a collection and source catalog for very-high-energy gamma-ray astronomy.

However, we would like to stress that this is an inofficial effort, the formats described here are work in progress and have not been officially adopted by any of the mentioned instruments. The expectation is that CTA partially adopt these formats, and partially develop new ones in the time frame 2018-2020, and then the other IACTs will use the official format chosen by CTA.

Development of this data format specification happens at https://github.com/open-gamma-ray-astro/gamma-astro-data-formats, information how to contribute is given in CONTRIBUTING.md there. Discussion on major and controversial points should happen on the mailing list (https://lists.nasa.gov/mailman/listinfo/open-gamma-ray-astro). So far, no official decision process exists, so some discussions are stuck because of major differences in opinion. Probably, moving forward, decisions will be made not in this open forum, but rather by the major instruments via their choices, especially CTA.