Violation of synchrotron line of death by the highly polarized $GRB160802A$

Abstract

$GRB160802A$ is one of the brightest gamma-ray bursts (GRBs) observed with $Fermi$ Gamma-ray Burst Monitor (GBM) in the energy range of $10-1000$ keV, while at the same time it is surprisingly faint at energies $\gtrsim 2$ MeV. An observation with $AstroSat$/CZT Imager (CZTI) also provides the polarisation which helps in constraining different prompt emission models using the novel joint spectra-polarimetric data. We analyze the $Fermi$/GBM data, and find two main bursting episodes that are clearly separated in time, one particularly faint in higher energies and having certain differences in their spectra. The spectrum in general shows a hard-to-soft evolution in both the episodes. Only the later part of the first episode shows intensity tracking behaviour corresponding to multiple pulses. The photon index of the spectrum is hard, and in over 90 per cent cases, cross even the slow cooling limit ($\alpha =-2/3$) of an optically thin synchrotron shock model (SSM). Though such hard values are generally associated with a sub-dominant thermal emission, such a component is not statistically required in our analysis. In addition, the measured polarisation in 100--300\,keV is too high, $\pi =85\pm 29\%$, to be accommodated in such a scenario. Jitter radiation, which allows a much harder index up to $\alpha =+0.5$, in principle can produce high polarisation but only beyond the spectral peak, which in our case lies close to $200-300$ keV during the time when most of the polarisation signal is obtained. The spectro-polarimetric data seems to be consistent with a subphotospheric dissipation process occurring within a narrow jet with a sharp drop in emissivity beyond the jet edge, and viewed along its boundary.

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