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Context : Star formation takes place in cold dense cores in molecular clouds.Earlier observations have found that dense cores exhibit subsonic non-thermalvelocity dispersions. In contrast, CO observations show that the ambientlarge-scale cloud is warmer and has supersonic velocity dispersions. Aims : Weaim to study the ammonia ($\rm NH_3$) molecular line profiles with exquisitesensitivity towards the coherent cores in L1688 in order to study theirkinematical properties in unprecedented detail. Methods : We used $\rm NH_3$(1,1) and (2,2) data from the first data release (DR1) in the Green BankAmmonia Survey (GAS). We first smoothed the data to a larger beam of 1' toobtain substantially more extended maps of velocity dispersion and kinetictemperature, compared to the DR1 maps. We then identified the coherent cores inthe cloud and analysed the averaged line profiles towards the cores. Results :For the first time, we detected a faint (mean $\rm NH_3$(1,1) peak brightness$<$0.25 K in $T_{MB}$), supersonic component towards all the coherent cores inL1688. We fitted two components, one broad and one narrow, and derived thekinetic temperature and velocity dispersion of each component. The broadcomponents towards all cores have supersonic linewidths ($\mathcal{M}_S \ge1$). This component biases the estimate of the narrow dense core component'svelocity dispersion by $\approx$28% and the kinetic temperature by$\approx$10%, on average, as compared to the results from single-componentfits. Conclusions : Neglecting this ubiquitous presence of a broad componenttowards all coherent cores causes the typical single-component fit tooverestimate the temperature and velocity dispersion. This affects the deriveddetailed physical structure and stability of the cores estimated from $\rmNH_3$ observations.

Ubiquitous $\rm NH_3$ supersonic component in L1688 coherent cores