Evidence of two spectral breaks in the prompt emission of Gamma Ray Bursts. (arXiv:1903.02555v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Ravasio_M/0/1/0/all/0/1">M. E. Ravasio</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ghirlanda_G/0/1/0/all/0/1">G. Ghirlanda</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nava_L/0/1/0/all/0/1">L. Nava</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ghisellini_G/0/1/0/all/0/1">G. Ghisellini</a>
The long-lasting tension between the observed spectra of Gamma Ray Bursts
(GRBs) and the predicted synchrotron emission spectrum might be solved if
electrons do not completely cool. Evidence for incomplete cooling was recently
found in Swift GRBs with prompt observations down to 0.1 keV and in one bright
Fermi burst, GRB 160625B. Here we systematically search for evidence of
incomplete cooling in the spectra of the ten brightest short and long GRBs
observed by Fermi. We find that in 8/10 long GRBs there is compelling evidence
of a low energy break (below the peak energy) and good agreement with the
photon indices of the synchrotron spectrum (-2/3 and -3/2, respectively) below
the break and between the break and the peak energy. Interestingly, none of the
ten short GRBs analysed shows a break but the low energy spectral slope is
consistent with -2/3. In a standard scenario, these results imply a very low
magnetic field in the emission region (B’ ~ 10 G in the comoving frame), at odd
with expectations.
The long-lasting tension between the observed spectra of Gamma Ray Bursts
(GRBs) and the predicted synchrotron emission spectrum might be solved if
electrons do not completely cool. Evidence for incomplete cooling was recently
found in Swift GRBs with prompt observations down to 0.1 keV and in one bright
Fermi burst, GRB 160625B. Here we systematically search for evidence of
incomplete cooling in the spectra of the ten brightest short and long GRBs
observed by Fermi. We find that in 8/10 long GRBs there is compelling evidence
of a low energy break (below the peak energy) and good agreement with the
photon indices of the synchrotron spectrum (-2/3 and -3/2, respectively) below
the break and between the break and the peak energy. Interestingly, none of the
ten short GRBs analysed shows a break but the low energy spectral slope is
consistent with -2/3. In a standard scenario, these results imply a very low
magnetic field in the emission region (B’ ~ 10 G in the comoving frame), at odd
with expectations.
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