To cite the 'sirt' package in publications use:

Robitzsch, A. (2019). sirt: Supplementary Item Response Theory Models. R package version 3.6-21. https://CRAN.R-project.org/package=sirt

Corresponding BibTeX entry:

  @Manual{sirt_3.6-21,
    title = {sirt: Supplementary Item Response Theory Models},
    author = {Alexander Robitzsch},
    year = {2019},
    note = {R package version 3.6-21},
    url = {https://CRAN.R-project.org/package=sirt},
  }