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We present a detailed analysis of GRB 151006A, the first GRB detected byAstrosat CZT Imager (CZTI). We study the long term spectral evolution byexploiting the capabilities of \emph{Fermi} and \emph{Swift} satellites atdifferent phases, which is complemented by the polarization measurement withthe CZTI. While the light curve of the GRB in different energy bands show asimple pulse profile, the spectrum shows an unusual evolution. The first phaseexhibits a hard-to-soft (HTS) evolution until $\sim16-20$\,s, followed by asudden increase in the spectral peak reaching a few MeV. Such a dramatic changein the spectral evolution in case of a single pulse burst is reported for thefirst time. This is captured by all models we used namely, Band function,Blackbody+Band and two blackbodies+power law. Interestingly, the \emph{Fermi}Large Area Telescope (LAT) also detects its first photon ($>100$\,MeV) duringthis time. This new injection of energy may be associated with either thebeginning of afterglow phase, or a second hard pulse of the prompt emissionitself which, however, is not seen in the otherwise smooth pulse profile. Byconstructing Bayesian blocks and studying the hardness evolution we find a goodevidence for a second hard pulse. The \emph{Swift} data at late epochs($>T_{90}$ of the GRB) also shows a significant spectral evolution consistentwith the early second phase. The CZTI data (100--350\,keV), though having lowsignificance ($1\sigma$), show high values of polarization in the two epochs($77\%$ to $94\%$), in agreement with our interpretation.

Surprise in simplicity: an unusual spectral evolution of a single pulse GRB 151006A