**6. Conclusions**

GMM clustering with MCLUST identified three Gaussian components of *Swift*/BAT and *Fermi*/GBM bursts in the duration-hardness plane. The third component resembled the intermediate duration group identified in previous studies. However, combining components, based on an entropy criterion, identified a short and long duration class only for both samples.

This study highlights the drawbacks of fitting GRB populations with model-based methods. Similar model-based fitting methods, including the log-normal fit procedures applied to GRB duration distributions, have exhibited components thought to be identified incorrectly due to the inherently skewed distribution of long GRB durations [31,36]. Table 1 highlights the diversity of results from model-based studies.

The lack of consensus regarding a definitive number of GRB classes, both in the analysis presented here and in previous studies of GRB catalogues, is a motivator for a modelindependent analysis of GRB light curves. The light curves may also contain more information than the summary data provided by the GRB catalogues. Fourier analysis of the *Swift*/BAT GRB light curves by Jespersen et al. [101] identified two classes of bursts. Following on from the analysis presented in this paper is a wavelet-based feature extraction analysis of GRB light curves from *Swift*/BAT, BATSE and *Fermi*/GBM (Salmon et al. [102], in preparation).

**Author Contributions:** Conceptualization, L.S., L.H. and A.M.-C.; methodology, L.S., L.H. and A.M.-C.; software, L.S.; validation, L.S., L.H. and A.M.-C.; formal analysis, L.S.; investigation, L.S.; resources, L.S., L.H. and A.M.-C.; data curation, L.S.; writing—original draft preparation, L.S.; writing—review and editing, L.H. and A.M.-C.; visualization, L.S.; supervision, L.H. and A.M.-C. ; project administration, L.H. and A.M.-C.; funding acquisition, L.H. and A.M.-C. All authors have read and agreed to the published version of the manuscript.

**Funding:** L.S. acknowledges the Irish Research Council Postgraduate Scholarship No GOIPG/2017/1525. L.H. acknowledges support from Science Foundation Ireland (Grant number 19/FFP/6777) and the EU H2020 (Grant agreement 871158).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** This paper makes use of the *Swift*/BAT catalogue [76], which can be accessed through the online *Swift*/BAT Gamma-ray Burst Catalogue <sup>1</sup> . The *Fermi*/GBM catalogue is accessed through the *Fermi*/GBM data tools [40].

**Acknowledgments:** This research made use of the following Python packages: NumPy [103], Matplotlib [104] and pandas [105,106]. The authors are grateful to Brendan Murphy and Michael Fop for their assistance with MCLUST.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

## **Note**

<sup>1</sup> https://swift.gsfc.nasa.gov/results/batgrbcat/, (accessed on 16 June 2022).
