VTSCat: The VERITAS Catalog of Gamma-Ray Observations

VTSCat is inspired by various movements for open data, among them the initiative for data formats in gamma-ray astronomy ³⁶ (Deil et al. 2017). It is built on an early version of the gamma-ray catalog gamma-cat ³⁷ and profited significantly from the input of the gamma-cat developers. All VTSCat data can be accessed file-by-file 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:

• 1.  
  high-level data such as spectral flux points, light curves, spectral fits in human- and machine-readable yaml and ecsv file formats, ³⁸
• 2.  
  table data such as upper limits from dark matter searches or results on the extragalactic background light in the ecsv file format,
• 3.  
  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 ³⁹ 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 ⁴⁰ file provides an overview of all data products, figures, source names, and citations. For this example, the data files are:

• 1.  
  Observation data: VER-000168.yaml Observational details and results like observation time, detection significance, and spectral models.
• 2.  
  Spectral flux points: VER-000168-sed-1.ecsv Spectral flux points including errors and upper limits as published in Figure 2 of Abeysekara et al. (2018).
• 3.  
  Light-curve data: VER-000168-lc.ecsv Light-curve data (integrated gamma-ray fluxes versus time) as published in Figure 3 of Abeysekara et al. (2018).
• 4.  
  FITS data: VER-000168-signif-skymap.fits Statistical significance sky map as published in Figure 1 of Abeysekara et al. (2018) https://github.com/VERITAS-Observatory/VERITAS-VTSCat/blob/main/2018/2018ApJ861L20A/VER-000168-signif-skymap.fits.

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) = (037257, −004588).

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 versus 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.

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. ⁴¹ 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.

We thank Antara Basu-Zych and Alan Smale for their assistance in archiving the VERITAS catalog at HEASARC.

This research is supported by grants from the U.S. Department of Energy Office of Science, the U.S. National Science Foundation and the Smithsonian Institution, by NSERC in Canada, and by the Helmholtz Association in Germany. This research used resources provided by the Open Science Grid, which is supported by the National Science Foundation and the U.S. Department of Energy's Office of Science, and resources of the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under Contract No. DE-AC02-05CH11231. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the instrument. The material is based upon work supported by NASA under award number 80GSFC21M0002.

Facility: VERITAS. -