  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. However, there is a general movement in science
towards the use of open data and software. In addition, the next-generation
IACT instrument, the Cherenkov Telescope Array (CTA), will be operated as an
open observatory.
  We have created a Github organisation at
https://github.com/open-gamma-ray-astro where we are developing high-level data
format specifications. A public mailing list was set up at
https://lists.nasa.gov/mailman/listinfo/open-gamma-ray-astro and a first
face-to-face meeting on the IACT high-level data model and formats took place
in April 2016 in Meudon (France). This open multi-mission effort will help to
accelerate the development of open data formats and open-source software for
gamma-ray astronomy, leading to synergies in the development of analysis codes
and eventually better scientific results (reproducible, multi-mission).
  This write-up presents this effort for the first time, explaining the
motivation and context, the available resources and process we use, as well as
the status and planned next steps for the data format specifications. We hope
that it will stimulate feedback and future contributions from the gamma-ray
astronomy community.
