  Recently Wei et al (arXiv:1612.09425) have found evidence for a transition
from positive time lags to negative time lags in the spectral lag data of GRB
160625B. They have fit these observed lags to a sum of two components: an
assumed functional form for intrinsic time lag due to astrophysical mechanisms
and an energy-dependent speed of light due to quadratic and linear Loren tz
invariance violation (LIV) models. Here, we examine the statistical
significance of the evidence for a transition to nega tive time lags. Such a
transition, even if present in GRB 160625B, cannot be due to an energy
dependent speed of light as th is would contradict previous limits by some 3-4
orders of magnitude, and must therefore be of intrinsic astrophysical origin .
We use three different model comparison techniques: a frequentist test and two
information based criteria (AIC and BIC). From the frequentist model comparison
test, we find that the evidence for transition in the spectral lag data is
favored at $3.05\sigma$ and $3.74\sigma$ for the linear and quadratic models
respectively. We find that $\Delta$AIC and $\Delta$BIC have values $\gtrsim$ 10
for the spectral lag transition that was motivated as being due to quadratic
Lorentz invariance vio lating model pointing to "decisive evidence". We note
however that none of the three models (including the model of intr insic
astrophysical emission) provide a good fit to the data.
