Search for Lorentz Invariance Violation from stacked Gamma-Ray Burst
  spectral lag data
  A number of works have claimed detections of a turn-over in the spectral lag
data for individual Gamma-Ray Bursts (GRBs), caused by an energy-dependent
speed of light, which could be a possible manifestation of Lorentz invariance
violation (LIV). Here, we stack the spectral lag data from a total of 37 GRBs
(with a total of 91 measurements), to verify if the combined data is consistent
with a unified model consisting of intrinsic astrophysical emission, along with
another contribution due to LIV. We then carry out Bayesian model comparison to
ascertain if this combined spectral lag data shows a preference for an
energy-dependent speed of light, as compared to only an intrinsic astrophysical
emission mechanism. We do not find a decisive evidence for such an
energy-dependent speed of light for two different models of LIV. When we assume
a constant intrinsic lag coupled with an unknown intrinsic scatter, we do not
find any evidence for LIV. However, when we use GRB-dependent parameters to
model the intrinsic emission, we get decisive evidence for LIV violation. We
then carry out a search for LIV Standard Model Extension using this dataset as
well as an independent search using a separate dataset consisting of rest-frame
spectral lags. Finally, none of the models considered here with any of the
aforementioned assumptions provide a good fit to the stacked spectral lag data,
indicating that there is still missing Physics in the model for intrinsic
spectral lags.
