  HNCO and SiO are well known shock tracers and have been observed in nearby
galaxies, including the nearby (D=3.5 Mpc) starburst galaxy NGC 253. The
simultaneous detection of these two species in regions where the star formation
rate is high may be used to study the shock history of the gas. We perform a
multi-line molecular study using these two shock tracers (SiO and HNCO) with
the aim of characterizing the gas properties. We also explore the possibility
of reconstructing the shock history in NGC 253's Central Molecular Zone (CMZ).
Six SiO transitions and eleven HNCO transitions were imaged at high resolution
$1''.6$ (28 pc) with the Atacama Large Millimeter/submillimeter Array (ALMA) as
part of the ALCHEMI Large Programme. Both non-LTE radiative transfer analysis
and chemical modelling were performed in order to characterize the gas
properties, and to investigate the chemical origin of the emission. The non-LTE
radiative transfer analysis coupled with Bayesian inference shows clear
evidence that the gas traced by SiO has different densities and temperatures
than that traced by HNCO, with an indication that shocks are needed to produce
both species. Chemical modelling further confirms such a scenario and suggests
that fast and slow shocks are responsible for SiO and HNCO production,
respectively, in most GMCs. We are also able to infer the physical
characteristics of the shocks traced by SiO and HNCO for each GMC. Radiative
transfer and chemical analysis of the SiO and HNCO in the CMZ of NGC 253 reveal
a complex picture whereby most of the GMCs are subjected to shocks. We
speculate on the possible shock scenarios responsible for the observed emission
and provide potential history and timescales for each shock scenario. Higher
spatial resolution observations of these two species are required in order to
quantitatively differentiate between scenarios.
