Many different physical processes have been suggested to explain the prompt
gamma-ray emission in gamma-ray bursts (GRBs). Although there are examples of
both bursts with photospheric and synchrotron emission origins, these distinct
spectral appearances have not been generalized to large samples of GRBs. Here,
we search for signatures of the different emission mechanisms in the full Fermi
Gamma-ray Space Telescope GBM catalogue. We use Gaussian Mixture Models to
cluster bursts according to their parameters from the Band function (α,
β, and Epk​) as well as their fluence and T90​. We find five
distinct clusters. We further argue that these clusters can be divided into
bursts of photospheric origin (2/3 of all bursts, divided into 3 clusters) and
bursts of synchrotron origin (1/3 of all bursts, divided into 2 clusters). For
instance, the cluster that contains predominantly short bursts is consistent of
photospheric emission origin. We discuss several reasons that can determine
which cluster a burst belongs to: jet dissipation pattern and/or the jet
content, or viewing angle.Comment: Accepted for publication in MNRA