Confirmation of the E srcpeak – E iso ( Amati ) relation from the X - ray flash XRF 050416 A observed by Swift / BAT

We report Swift Burst Alert Telescope (BAT) observations of the X-ray Flash (XRF) XRF 050416A. The fluence ratio between the 15-25 keV and 25-50 keV energy bands of this event is 1.5, thus making it the softest gamma-ray burst (GRB) observed by BAT so far. The spectrum is well fitted by the Band function with E peak of 15.0 +2.3 −2.7 keV. Assuming the redshift of the host galaxy (z = 0.6535), the isotropicequivalent radiated energy Eiso and the peak energy at the GRB rest frame (E peak) of XRF 050416A are not only consistent with the correlation found by Amati et al. and extended to XRFs by Sakamoto et al., but also fillin the gap of this relation around the 30 – 80 keV range of E peak. This result tightens the validity of the E peak – Eiso relation from XRFs to GRBs. We also find that the jet break time estimated using the empirical relation between E peak and the collimation corrected energy Eγ is inconsistent with the afterglow observation by Swift X-ray Telescope. This could be due to the extra external shock emission overlaid around the jet break time or to the non existence of a jet break feature for XRF, which might be a further challenging for GRB jet emission, models and XRF/GRB unification scenarios. Subject headings: Gamma-ray Burst NASA Goddard Space Flight Center, Greenbelt, MD 20771 National Research Council, 2101 Constitution Avenue, NW, TJ2114, Washingtion, DC 20418 Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM, 87545 Department of Physics, University of Maryland, College Park, MD 20742 Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD 21044-3432 Institute of Space and Astronautical Science, JAXA, Kanagawa 229-8510, Japan