Context : The gas kinetic temperature ( T _ { \mathrm { K } } ) determines the physical and chemical evolution of the Interestellar Medium ( ISM ) .
However , obtaining reliable T _ { \mathrm { K } } estimates usually requires expensive observations including the combination of multi-line analysis and dedicated radiative transfer calculations .

Aims : This work explores the use of HCN and HNC observations , and particularly its I ( HCN ) / I ( HNC ) intensity ratio of their J=1–0 lines , as direct probe of the gas kinetic temperature in the molecular ISM .

Methods : We obtained a new set of large-scale observations of both HCN and HNC ( 1-0 ) lines along the Integral Shape Filament ( ISF ) in Orion .
In combination with ancillary gas and dust temperature measurements , we find a systematic temperature dependence of the observed I ( HCN ) / I ( HNC ) intensity ratio across our maps .
Additional comparisons with chemical models demonstrate that these observed I ( HCN ) / I ( HNC ) variations are driven by the effective destruction and isomerization mechanisms of HNC under low energy barriers .

Results : The observed variations of I ( HCN ) / I ( HNC ) with T _ { \mathrm { K } } can be described with a two-part linear function .
This empirical calibration is then used to create a temperature map of the entire ISF .
Comparisons with similar dust temperature measurements in this cloud , as well as in other regions and galactic surveys , validate this simple technique to obtain direct estimates of the gas kinetic temperature in a wide range of physical conditions and scales with an optimal working range between 15 K \lesssim T _ { \mathrm { K } } \leq 40 K .

Conclusions : Both observations and models demonstrate the strong sensitivity of the I ( HCN ) / I ( HNC ) ratio to the gas kinetic temperature .
Since these lines are easily obtained in observations of local and extragalactic sources , our results highlight the potential use of this observable as new chemical thermometer for the ISM .