VTSCat: The VERITAS Catalog of Gamma-Ray Observations

The ground-based gamma-ray observatory VERITAS (Very Energetic Radiation Imaging Telescope Array System) is sensitive to photons of astrophysical origin with energies in the range between $\approx 85$ GeV to $\approx 30$ TeV. The instrument consists of four 12-m diameter imaging Cherenkov telescopes operating at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona. VERITAS started four-telescope operations in 2007 and collects about 1100 hours of good-weather data per year. The VERITAS collaboration has published over 100 journal articles since 2008 reporting on gamma-ray observations of a large variety of objects: Galactic sources like supernova remnants, pulsar wind nebulae, and binary systems; extragalactic sources like star forming galaxies, dwarf-spheroidal galaxies, and highly-variable active galactic nuclei. This note presents VTSCat: the catalog of high-level data products from all VERITAS publications.


ABSTRACT
The ground-based gamma-ray observatory VERITAS (Very Energetic Radiation Imaging Telescope Array System a) , (Holder et al. 2006)) is sensitive to photons of astrophysical origin with energies in the range between ≈ 85 GeV to ≈ 30 TeV. The instrument consists of four 12-m diameter imaging Cherenkov telescopes operating at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona. VERITAS started four-telescope operations in 2007 and collects about 1100 hours of good-weather data per year. The VERITAS collaboration has published over 100 journal articles since 2008 reporting on gamma-ray observations of a large variety of objects: Galactic sources like supernova remnants, pulsar wind nebulae, and binary systems; extragalactic sources like star forming galaxies, dwarf-spheroidal galaxies, and highly-variable active galactic nuclei. This note presents VTSCat: the catalog of high-level data products from all VERITAS publications.
Keywords: high-energy astrophysics -gamma-ray telescopes -catalogs VTSCAT VTSCat is inspired by various movements for open data, among them the initiative for data formats in gamma-ray astronomy 1 (Deil et al. 2017). It is built on an a) https://veritas.sao.arizona.edu/ 1 see https://gamma-astro-data-formats.readthedocs.io/en/latest/ early version of the gamma-ray catalog gamma-cat 2 and profited significantly from the input of the gamma-cat developers. All VTSCat data can be accessed file-byfile on GitHub (https://github.com/VERITAS-Observatory/VERITAS-VTSCat) or downloaded via Zenodo (VERITAS Collaboration et al. (2022), downloadable as a compressed file). The light curves and spectral results in VTSCat are also available through NASA's High Energy Astrophysics Science Archive Research Center (HEASARC; https://heasarc.gsfc.nasa.gov/W3Browse/all/verimaster.html), thus providing an interface more familiar to astronomers (see next section).
The VTSCat data collection contains: • high-level data such as spectral flux points, light curves, spectral fits in humanand machine-readable yaml and ecsv file formats 3 , • table data such as upper limits from dark matter searches or results on the extragalactic background light in the ecsv file format, • sky maps (wherever available) in the FITS file format.
The physical units are explicitly given in each file; the ecsv -files follow standards which allow them to be read by common tools such as the astropy python library (Astropy Collaboration et al. 2022).
The data files are organized in VTSCat by year and publication, using the ADS bibcodes as reference identifiers. Objects are identified by a running integer (labeled source id in data files) following the scheme developed by gamma-cat. The description files for objects can be found in the sources subdirectory and include the most relevant names for a given object (common name in the field, VERITAS object identifier, primary identifier by SIMBAD), and the object coordinates.
For illustration, the data entry for the VERITAS publication Abeysekara et al. (2018) is discussed in the following (see the directory 2018/2018ApJ...861L..20A on the repository to follow the discussion below). A README file provides an overview of all data products, figures, source names, and citations. For this example, the data files are: • Observation data: VER-000168.yaml Observational details and results like observation time, detection significance, and spectral models. The yaml datatypes and spectral models used are explained in the documents Formats and Models.md and SpectralModels.md. The data types and units used in ecsv files are described at the top of each data file; for FITS files this information is provided in the FITS header. Preview PNG images are included for all lightcurves and spectral energy distribution plots.
Translation of the data in the publications to a uniform catalog was a complex process, requiring significant manual effort and occasional addition of information. For example, we note the addition of the source name VER J1746-286 that refers to the diffuse emission of seven fields near the Galactic Center (Adams et al. 2021) with an average position of (l, b) = (0 • .37257, −0 • .04588).

VTSCAT AT HEASARC
Most of the information contained in VTSCat is also archived and available at the HEASARC, which is familiar to a large part of the community of high-energy astrophysicists. Users can take advantage of the HEASARC through multiband data searches and VERITAS data can be included in these searches. The main catalog at the HEASARC will be a source catalog including all sources detected with VERITAS. The SEDs and light curve data for individual sources will be in individual files linked to the main source catalog. As in VTSCat, the objects and sources are identified by their source identifiers (IDs). All data fields along with their units are described in detail at https://heasarc.gsfc.nasa.gov/W3Browse/all/verimaster.html. Following VTSCat, the data files wrapped in FITS format contain lightcurve data (integrated gamma-ray fluxes vs time) and spectral flux points (including errors and upper limits). The data types and units for these data products are described in the FITS header.

OUTLOOK
VTSCat will be updated regularly with new publications of the VERITAS collaboration. Future structural updates of VTSCat may include multi-wavelength data presented in VERITAS publications including observations by e.g., radio, optical, or X-ray instruments, and may also allow to use tools like the SED builder provided through SSDC 4 . The effort of disseminating all VERITAS results in digital format will aid in multiwavelength analysis and input to the modeling of sources of high-energy gamma rays.