$E_{\rm peak}$ values $< 20$ keV observed by GBM are unlikely to be constrained because there is insufficient data to constrain the low-energy index. In this case, the spectrum would appear similar to a simple power law with an index similar to the index of the Band high-energy index. Figure 27$b$ in @GoldsteinCat presents a similar set of simulations with BATSE with similar findings. Due to the narrower energy band of BATSE (20-2000 keV) with respect to GBM, there was also an apparent decrease in accuracy if $E_{\rm peak}$ existed above the detector band, although the loss of accuracy was not as dramatic compared to the low-$E_{\rm peak}$ case. The interpretation for the high-$E_{\rm peak}$ case is that as long as the low-energy index is reasonably constrained and some curvature is measurable within the detector band, then it is possible to estimate $E_{\rm peak}$, albeit with less accuracy and constraint. It is in part to avoid the issue of the interplay between fitting [$E_{\rm peak}$]{} and $\beta$ in the Band function that we choose to perform our pulse spectral evolution simulations with the COMP function, as discussed above.

![Plot of the fraction of Band function fits that contain the simulated 